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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/Attic/jdbc.sgml,v 1.36 2002/03/22 19:20:11 petere Exp $
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<chapter id="jdbc">
<title><acronym>JDBC</acronym> Interface</title>
<note>
<title>Author</title>
<para>
Originally written by Peter T. Mount (<email>peter@retep.org.uk</email>),
the original author of the <acronym>JDBC</acronym> driver.
</para>
</note>
<para>
<acronym>JDBC</acronym> is a core <acronym>API</acronym> of Java 1.1 and later.
It provides a standard set of
interfaces to <acronym>SQL</acronym>-compliant databases.
</para>
<para>
<productname>PostgreSQL</> provides a <firstterm>type
4</firstterm> <acronym>JDBC</acronym> Driver. Type 4 indicates
that the driver is written in Pure Java, and communicates in the
database system's own network protocol. Because of this, the driver
is platform independent; once compiled, the driver can be used on
any system.
</para>
<para>
This chapter is not intended as a complete guide to
<acronym>JDBC</acronym> programming, but should help to get you
started. For more information refer to the standard
<acronym>JDBC</acronym> <acronym>API</acronym> documentation.
Also, take a look at the examples included with the source. The
basic example is used here.
</para>
<sect1 id="jdbc-setup">
<title>Setting up the <acronym>JDBC</acronym> Driver</title>
<sect2 id="jdbc-build">
<title>Getting the Driver</title>
<para>
Precompiled versions of the driver can be downloaded from
the <ulink
url="http://jdbc.postgresql.org"><productname>PostgreSQL</productname>
<acronym>JDBC</acronym> web site</ulink>.
</para>
<para>
Alternatively you can build the driver from source. Although you
should only need to do this if you are making changes to the source
code.
</para>
<para>
Starting with <productname>PostgreSQL</productname> version 7.1,
the <acronym>JDBC</acronym> driver is built using
<application>Ant</application>, a special tool for building
Java-based packages. You should download
<application>Ant</application> from the <ulink
url="http://jakarta.apache.org/ant/index.html"><application>Ant</application>
web site</ulink> and install it before proceeding. Precompiled
<application>Ant</application> distributions are typically set up
to read a file <filename>.antrc</filename> in the current user's
home directory for configuration. For example, to use a different
<acronym>JDK</acronym> than the default, this may work:
<programlisting>
JAVA_HOME=/usr/local/sun-jdk1.3
JAVACMD=$JAVA_HOME/bin/java
</programlisting>
</para>
<para>
To build the driver, add the <option>--with-java</option> option to your
<filename>configure</filename> command line, e.g.,
<screen>
<prompt>$</prompt> <userinput>./configure --prefix=<replaceable>xxx</replaceable> --with-java ...</userinput>
</screen>
This will build and install the driver along with the rest of the
<productname>PostgreSQL</productname> package when you issue the
<literal>make/gmake</literal> and <literal>make/gmake install</literal>
commands. If you only want to build the driver and not the rest
of <productname>PostgreSQL</productname>, change into the
directory <filename
class="directory">src/interfaces/jdbc</filename> and issue the
respective <literal>make/gmake</literal> command there. Refer to the
<productname>PostgreSQL</productname> installation instructions
for more information about the configuration and build process.
</para>
<para>When building the driver from source the jar file that is created
will be named <filename>postgresql.jar</filename>. The build will
create this file in the <filename>src/interfaces/jdbc/jars</filename>
directory. The resulting driver will be built for the version of
Java you are running. If you build with a 1.1 JDK you will build
a version that supports the jdbc1 specification, if you build with a
Java2 JDK (i.e. JDK1.2 or JDK1.3) you will build a version that
supports the jdbc2 specification.
</para>
<note>
<para>
Do not try to build the driver by calling <command>javac</command>
directly, as the driver uses some dynamic loading techniques for
performance reasons, and <command>javac</command> cannot cope.
Do not try to run <command>ant</command> directly either, because
some configuration information is communicated through the
makefiles. Running <command>ant</command> directly without
providing these parameters will result in a broken driver.
</para>
</note>
</sect2>
<sect2 id="jdbc-classpath">
<title>Setting up the Class Path</title>
<para>
To use the driver, the jar archive (named
<filename>postgresql.jar</filename> if you built from source, otherwise
it will likely be named <filename>jdbc7.2-1.1.jar</filename> or
<filename>jdbc7.2-1.2.jar</filename> for the jdbc1 and jdbc2 versions
respectively)
needs to be included in the
class path, either by putting it in the <envar>CLASSPATH</envar>
environment variable, or by using flags on the
<command>java</command> command line. By default, the jar archive
is installed in the directory <filename
class="directory">/usr/local/pgsql/share/java</filename>. You may
have it in a different directory if you used the
<option>--prefix</option> option when you ran
<filename>configure</filename>, or if you are using a binary distribution
that places it in some different location.
</para>
<informalexample>
<para>
For instance, I have an application that uses the
<acronym>JDBC</acronym> driver to access a large database
containing astronomical objects. I have the application and the
<acronym>JDBC</acronym> driver installed in the <filename
class="directory">/usr/local/lib</filename> directory, and the
Java <acronym>JDK</acronym> installed in
<filename>/usr/local/jdk1.3.1</filename>. To run the
application, I would use:
<programlisting>
export CLASSPATH=/usr/local/lib/finder.jar<co id="co.jdbc-finder">:/usr/local/pgsql/share/java/postgresql.jar:.
java Finder
</programlisting>
<calloutlist>
<callout arearefs="co.jdbc-finder">
<para>
<filename>finder.jar</filename> contains the Finder application.
</para>
</callout>
</calloutlist>
</para>
<para>
Loading the driver from within the application is covered in
<xref linkend="jdbc-use">.
</para>
</informalexample>
</sect2>
<sect2 id="jdbc-prepare">
<title>Preparing the Database for <acronym>JDBC</acronym></title>
<para>
Because Java only uses TCP/IP connections, the
<application>PostgreSQL</application> server must be configured to
accept TCP/IP connections. This can be done by setting
<literal>tcpip_socket = true</literal> in the
<filename>postgresql.conf</filename> file or by supplying the
<option>-i</option> option flag when starting
<command>postmaster</command>.
</para>
<para>
Also, the client authentication setup in the
<filename>pg_hba.conf</filename> file may need to be configured.
Refer to the <citetitle>Administrator's Guide</citetitle> for
details. The <acronym>JDBC</acronym> Driver supports trust,
ident, password, md5, and crypt authentication methods.
</para>
</sect2>
</sect1>
<sect1 id="jdbc-use">
<title>Using the Driver</title>
<sect2 id="jdbc-import">
<title>Importing <acronym>JDBC</acronym></title>
<para>
Any source that uses <acronym>JDBC</acronym> needs to import the
<literal>java.sql</literal> package, using:
<programlisting>
import java.sql.*;
</programlisting>
<important>
<para>
Do not import the <literal>org.postgresql</literal> package. If
you do, your source will not compile, as
<command>javac</command> will get confused.
</para>
</important>
</para>
</sect2>
<sect2 id="jdbc-load">
<title>Loading the Driver</title>
<para>
Before you can connect to a database, you need to load the
driver. There are two methods available, and it depends on your
code which is the best one to use.
</para>
<para>
In the first method, your code implicitly loads the driver using the
<function>Class.forName()</function> method.
For <productname>PostgreSQL</>, you would use:
<programlisting>
Class.forName("org.postgresql.Driver");
</programlisting>
This will load the driver, and while loading, the driver will automatically
register itself with <acronym>JDBC</acronym>.
<note>
<para>
The <function>forName()</function> method can throw a
<classname>ClassNotFoundException</classname> if the driver is
not available.
</para>
</note>
</para>
<para>
This is the most common method to use, but restricts your code to
use just <productname>PostgreSQL</productname>. If your code may
access another database system in the future, and you do not use
any <productname>PostgreSQL</productname>-specific extensions, then
the second method is advisable.
</para>
<para>
The second method passes the driver as a parameter to the
<acronym>JVM</acronym> as it starts, using the <option>-D</option>
argument. Example:
<programlisting>
java -Djdbc.drivers=org.postgresql.Driver example.ImageViewer
</programlisting>
In this example, the <acronym>JVM</acronym> will attempt to load
the driver as part of its initialization. Once done, the
<classname>ImageViewer</classname> is started.
</para>
<para>
Now, this method is the better one to use because it allows your
code to be used with other database packages without recompiling
the code. The only thing that would also change is the connection
<acronym>URL</acronym>, which is covered next.
</para>
<para>
One last thing: When your code then tries to open a
<classname>Connection</classname>, and you get a <errorname>No
driver available</errorname> <classname>SQLException</classname>
being thrown, this is probably caused by the driver not being in
the class path, or the value in the parameter not being correct.
</para>
</sect2>
<sect2 id="jdbc-connect">
<title>Connecting to the Database</title>
<para>
With <acronym>JDBC</acronym>, a database is represented by a
<acronym>URL</acronym> (Uniform Resource Locator). With
<application>PostgreSQL</application>, this takes one of the
following forms:
<itemizedlist>
<listitem>
<synopsis>
jdbc:postgresql:<replaceable class="parameter">database</replaceable>
</synopsis>
</listitem>
<listitem>
<synopsis>
jdbc:postgresql://<replaceable class="parameter">host</replaceable>/<replaceable class="parameter">database</replaceable>
</synopsis>
</listitem>
<listitem>
<synopsis>
jdbc:postgresql://<replaceable class="parameter">host</replaceable>:<replaceable class="parameter">port</replaceable>/<replaceable class="parameter">database</replaceable>
</synopsis>
</listitem>
</itemizedlist>
where:
<variablelist>
<varlistentry>
<term>
<replaceable class="parameter">host</replaceable>
</term>
<listitem>
<para>
The host name of the server. Defaults to <literal>localhost</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<replaceable class="parameter">port</replaceable>
</term>
<listitem>
<para>
The port number the server is listening on. Defaults to the
<productname>PostgreSQL</productname> standard port number (5432).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<replaceable class="parameter">database</replaceable>
</term>
<listitem>
<para>
The database name.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
To connect, you need to get a <classname>Connection</classname> instance from
<acronym>JDBC</acronym>. To do this,
you would use the <function>DriverManager.getConnection()</function> method:
<programlisting>
Connection db = DriverManager.getConnection(url, username, password);
</programlisting>
</para>
</sect2>
<sect2 id="jdbc-disconnect">
<title>Closing the Connection</title>
<para>
To close the database connection, simply call the
<function>close()</function> method to the <classname>Connection</classname>:
<programlisting>
db.close();
</programlisting>
</para>
</sect2>
</sect1>
<sect1 id="jdbc-query">
<title>Issuing a Query and Processing the Result</title>
<para>
Any time you want to issue <acronym>SQL</acronym> statements to
the database, you require a <classname>Statement</classname> or
<classname>PreparedStatement</classname> instance. Once you have
a <classname>Statement</classname> or
<classname>PreparedStatement</classname>, you can use issue a
query. This will return a <classname>ResultSet</classname>
instance, which contains the entire result. <xref
linkend="jdbc-query-example"> illustrates this process.
</para>
<example id="jdbc-query-example">
<title>Processing a Simple Query in <acronym>JDCB</acronym></title>
<para>
This example will issue a simple query and print out the first
column of each row using a <classname>Statement</classname>.
<programlisting>
Statement st = db.createStatement();
ResultSet rs = st.executeQuery("SELECT * FROM mytable where columnfoo = 500");
while(rs.next()) {
System.out.print("Column 1 returned ");
System.out.println(rs.getString(1));
}
rs.close();
st.close();
</programlisting>
</para>
<para>
This example will issue the same query as before using
a <classname>PreparedStatement</classname>
and a bind value in the query.
<programlisting>
int foovalue = 500;
PreparedStatement st = db.prepareStatement("SELECT * FROM mytable where columnfoo = ?");
st.setInt(1, foovalue);
ResultSet rs = st.executeQuery();
while(rs.next()) {
System.out.print("Column 1 returned ");
System.out.println(rs.getString(1));
}
rs.close();
st.close();
</programlisting>
</para>
</example>
<sect2>
<title>Using the <classname>Statement</classname> or <classname>
PreparedStatement</classname> Interface</title>
<para>
The following must be considered when using the
<classname>Statement</classname> or
<classname>PreparedStatement</classname> interface:
<itemizedlist>
<listitem>
<para>
You can use a single <classname>Statement</classname> instance
as many times as you want. You could create one as soon as you
open the connection and use it for the connection's
lifetime. But you have to remember that only one
<classname>ResultSet</classname> can exist per
<classname>Statement</classname> or
<classname>PreparedStatement</classname> at a given time.
</para>
</listitem>
<listitem>
<para>
If you need to perform a query while processing a
<classname>ResultSet</classname>, you can simply create and
use another <classname>Statement</classname>.
</para>
</listitem>
<listitem>
<para>
If you are using threads, and several are using the database,
you must use a separate <classname>Statement</classname> for
each thread. Refer to <xref linkend="jdbc-thread"> if you are
thinking of using threads, as it covers some important points.
</para>
</listitem>
<listitem>
<para>
When you are done using the <classname>Statement</classname>
or <classname>PreparedStatement</classname>
you should close it.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2>
<title>Using the <classname>ResultSet</classname> Interface</title>
<para>
The following must be considered when using the
<classname>ResultSet</classname> interface:
<itemizedlist>
<listitem>
<para>
Before reading any values, you must call
<function>next()</function>. This returns true if there is a
result, but more importantly, it prepares the row for
processing.
</para>
</listitem>
<listitem>
<para>
Under the <acronym>JDBC</acronym> specification, you should
access a field only once. It is safest to stick to this rule,
although at the current time, the
<productname>PostgreSQL</productname> driver will allow you to
access a field as many times as you want.
</para>
</listitem>
<listitem>
<para>
You must close a <classname>ResultSet</classname> by calling
<function>close()</function> once you have finished using it.
</para>
</listitem>
<listitem>
<para>
Once you make another query with the
<classname>Statement</classname> used to create a
<classname>ResultSet</classname>, the currently open
<classname>ResultSet</classname> instance is closed
automatically.
</para>
</listitem>
<listitem>
<para>
<classname>ResultSet</classname> is currently read only.
You can not update data through the <classname>ResultSet</classname>.
If you want to update data you need to do it the old fashioned way
by issuing a <acronym>SQL</acronym> update statement. This is
in conformance with the <acronym>JDBC</acronym> specification
which does not require drivers to provide this functionality.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="jdbc-update">
<title>Performing Updates</title>
<para>
To change data (perform an insert, update, or delete)
you use the <function>executeUpdate()</function> method.
<function>executeUpdate()</function> is similar to the
<function>executeQuery()</function> used to issue a select,
however it doesn't return a <classname>ResultSet</classname>,
instead it returns the number of records affected by the insert,
update, or delete statement.
</para>
<example id="jdbc-delete-example">
<title>Simple Delete Example</title>
<para>
This example will issue a simple delete and print out the number
of rows deleted.
<programlisting>
int foovalue = 500;
PreparedStatement st = db.prepareStatement("DELETE FROM mytable where columnfoo = ?");
st.setInt(1, foovalue);
int rowsDeleted = st.executeUpdate();
System.out.println(rowsDeleted + " rows deleted");
st.close();
</programlisting>
</para>
</example>
</sect1>
<sect1 id="jdbc-ddl">
<title>Creating and Modifying Database Objects</title>
<para>
To create, modify or drop a database object like a table or view
you use the <function>execute()</function> method.
<function>execute</function> is similar to the
<function>executeQuery()</function> used to issue a select,
however it doesn't return a result.
</para>
<example id="jdbc-drop-table-example">
<title>Drop Table Example</title>
<para>
This example will drop a table.
<programlisting>
Statement st = db.createStatement();
ResultSet rs = st.executeQuery("DROP TABLE mytable");
st.close();
</programlisting>
</para>
</example>
</sect1>
<sect1 id="jdbc-binary-data">
<title>Storing Binary Data</title>
<para>
<application>PostgreSQL</application> provides two distinct ways to
store binary data. Binary data can be stored in a table using
<application>PostgreSQL's</application> binary data type
<type>bytea</type>, or by using the <firstterm>Large Object</firstterm>
feature which stores the binary data in a separate table in a special
format, and refers to that table by storing a value of type
<type>OID</type> in your table.
</para>
<para>
In order to determine which method is appropriate you
need to understand the limitations of each method. The
<type>bytea</type> data type is not well suited for storing very
large amounts of binary data. While a column of type
<type>bytea</type> can hold upto 1Gig of binary data, it would
require a huge amount of memory (<acronym>RAM</acronym>) to
process such a large value. The Large Object method for
storing binary data is better suited to storing very large values,
but it has its own limitations. Specifically deleting a row
that contains a Large Object does not delete the Large Object.
Deleting the Large Object is a separate operation that needs to
be performed. Large Objects also have some security
issues since anyone connected to the database case view
and/or modify any Large Object, even if they don't have
permissions to view/update the row containing the Large Object.
</para>
<para>
7.2 is the first release of the <acronym>JDBC</acronym> Driver
that supports the <type>bytea</type> data type. The introduction of
this functionality in 7.2 has introduced a change in behavior
as compared to previous releases. In 7.2 the methods
<function>getBytes()</function>, <function>setBytes()</function>,
<function>getBinaryStream()</function>, and
<function>setBinaryStream()</function> operate on
the <type>bytea</type> data type. In 7.1 these methods operated
on the <type>OID</type> data type associated with Large Objects.
It is possible to revert the driver back to the old 7.1 behavior
by setting the <parameter>compatible</parameter> property on
the <classname>Connection</classname> to a value of
<literal>7.1</literal>
</para>
<para>
To use the <type>bytea</type> data type you should simply use
the <function>getBytes()</function>, <function>setBytes()</function>,
<function>getBinaryStream()</function>, or
<function>setBinaryStream()</function> methods.
</para>
<para>
To use the Large Object functionality you can use either the
<classname>LargeObject</classname> <acronym>API</acronym>
provided by the <application>PostgreSQL</application>
<acronym>JDBC</acronym> Driver, or by using the
<function>getBLOB()</function> and <function>setBLOB()</function>
methods.
</para>
<important>
<para>
For <application>PostgreSQL</application>, you must access Large
Objects within an <acronym>SQL</acronym> transaction. You would
open a transaction by using the
<function>setAutoCommit()</function> method with an input
parameter of <literal>false</literal>.
</para>
</important>
<note><para>In a future release of the
<acronym>JDBC</acronym> Driver, the <function>getBLOB()</function>
and <function>setBLOB()</function> methods may no longer
interact with Large Objects and will instead work on
<type>bytea</type> data types. So it is recommended that you
use the <classname>LargeObject</classname> <acronym>API</acronym>
if you intend to use Large Objects.
</para></note>
<example id="jdbc-binary-data-example">
<title>Binary Data Examples</title>
<para>
For example, suppose you have a table containing the file name of
an image and you also want to store the image in a <type>bytea</type>
column:
<programlisting>
CREATE TABLE images (imgname text, img bytea);
</programlisting>
</para>
<para>
To insert an image, you would use:
<programlisting>
File file = new File("myimage.gif");
FileInputStream fis = new FileInputStream(file);
PreparedStatement ps = conn.prepareStatement("INSERT INTO images VALUES (?, ?)");
ps.setString(1, file.getName());
ps.setBinaryStream(2, fis, file.length());
ps.executeUpdate();
ps.close();
fis.close();
</programlisting>
Here, <function>setBinaryStream()</function> transfers a set number
of bytes from a stream into the column of type <type>bytea</type>.
This also could have been done using the <function>setBytes()</function>
method if the contents of the image was already in a
<classname>byte[]</classname>.
</para>
<para>
Retrieving an image is even easier. (We use
<classname>PreparedStatement</classname> here, but the
<classname>Statement</classname> class can equally be used.)
<programlisting>
PreparedStatement ps = con.prepareStatement("SELECT img FROM images WHERE imgname=?");
ps.setString(1, "myimage.gif");
ResultSet rs = ps.executeQuery();
if (rs != null) {
while(rs.next()) {
byte[] imgBytes = rs.getBytes(1);
// use the stream in some way here
}
rs.close();
}
ps.close();
</programlisting>
</para>
<para>
Here the binary data was retrieved as an
<classname>byte[]</classname>. You could have used a
<classname>InputStream</classname> object instead.
</para>
<para>
Alternatively you could be storing a very large file and want to use
the <classname>LargeObject</classname> <acronym>API</acronym> to
store the file:
<programlisting>
CREATE TABLE imagesLO (imgname text, imgOID OID);
</programlisting>
</para>
<para>
To insert an image, you would use:
<programlisting>
// All LargeObject API calls must be within a transaction
conn.setAutoCommit(false);
// Get the Large Object Manager to perform operations with
LargeObjectManager lobj = ((org.postgresql.Connection)conn).getLargeObjectAPI();
//create a new large object
int oid = lobj.create(LargeObjectManager.READ | LargeObjectManager.WRITE);
//open the large object for write
LargeObject obj = lobj.open(oid, LargeObjectManager.WRITE);
// Now open the file
File file = new File("myimage.gif");
FileInputStream fis = new FileInputStream(file);
// copy the data from the file to the large object
byte buf[] = new byte[2048];
int s, tl = 0;
while ((s = fis.read(buf, 0, 2048)) > 0)
{
obj.write(buf, 0, s);
tl += s;
}
// Close the large object
obj.close();
//Now insert the row into imagesLO
PreparedStatement ps = conn.prepareStatement("INSERT INTO imagesLO VALUES (?, ?)");
ps.setString(1, file.getName());
ps.setInt(2, oid);
ps.executeUpdate();
ps.close();
fis.close();
</programlisting>
</para>
<para>
Retrieving the image from the Large Object:
<programlisting>
// All LargeObject API calls must be within a transaction
conn.setAutoCommit(false);
// Get the Large Object Manager to perform operations with
LargeObjectManager lobj = ((org.postgresql.Connection)conn).getLargeObjectAPI();
PreparedStatement ps = con.prepareStatement("SELECT imgOID FROM imagesLO WHERE imgname=?");
ps.setString(1, "myimage.gif");
ResultSet rs = ps.executeQuery();
if (rs != null) {
while(rs.next()) {
//open the large object for reading
int oid = rs.getInt(1);
LargeObject obj = lobj.open(oid, LargeObjectManager.READ);
//read the data
byte buf[] = new byte[obj.size()];
obj.read(buf, 0, obj.size());
//do something with the data read here
// Close the object
obj.close();
}
rs.close();
}
ps.close();
</programlisting>
</para>
</example>
</sect1>
<sect1 id="jdbc-ext">
<title><application>PostgreSQL</application> Extensions to the
<acronym>JDBC</acronym> <acronym>API</acronym></title>
<para>
<productname>PostgreSQL</productname> is an extensible database
system. You can add your own functions to the backend, which can
then be called from queries, or even add your own data types. As
these are facilities unique to <productname>PostgreSQL</productname>,
we support them from Java, with a set of extension
<acronym>API</acronym>'s. Some features within the core of the
standard driver actually use these extensions to implement Large
Objects, etc.
</para>
<sect2>
<title>Accessing the Extensions</title>
<para>
To access some of the extensions, you need to use some extra
methods in the <classname>org.postgresql.Connection</classname>
class. In this case, you would need to case the return value of
<function>Driver.getConnection()</function>. For example:
<programlisting>
Connection db = Driver.getConnection(url, username, password);
// ...
// later on
Fastpath fp = ((org.postgresql.Connection)db).getFastpathAPI();
</programlisting>
</para>
<sect3>
<title>Class <classname>org.postgresql.Connection</classname></title>
<synopsis>
public class Connection extends Object implements Connection
java.lang.Object
|
+----org.postgresql.Connection
</synopsis>
<para>
These are the extra methods used to gain access to
<productname>PostgreSQL</productname>'s extensions. Methods
defined by <classname>java.sql.Connection</classname> are not
listed.
</para>
<sect4>
<title>Methods</title>
<itemizedlist>
<listitem>
<synopsis>
public Fastpath getFastpathAPI() throws SQLException
</synopsis>
<para>
This returns the Fastpath <acronym>API</acronym> for the
current connection. It is primarily used by the Large Object
<acronym>API</acronym>.
</para>
<para>
The best way to use this is as follows:
<programlisting>
import org.postgresql.fastpath.*;
...
Fastpath fp = ((org.postgresql.Connection)myconn).getFastpathAPI();
</programlisting>
where <varname>myconn</> is an open <classname>Connection</> to <productname>PostgreSQL</productname>.
</para>
<formalpara>
<title>Returns:</title>
<para>
Fastpath object allowing access to functions on the
<productname>PostgreSQL</productname> backend.
</para>
</formalpara>
<formalpara>
<title>Throws:</title>
<para>
SQLException by Fastpath when initializing for first time
</para>
</formalpara>
</listitem>
<listitem>
<para>
<synopsis>
public LargeObjectManager getLargeObjectAPI() throws SQLException
</synopsis>
This returns the Large Object <acronym>API</acronym> for the
current connection.
</para>
<para>
The best way to use this is as follows:
<programlisting>
import org.postgresql.largeobject.*;
...
LargeObjectManager lo = ((org.postgresql.Connection)myconn).getLargeObjectAPI();
</programlisting>
where <varname>myconn</> is an open <classname>Connection</> to
<productname>PostgreSQL</productname>.
</para>
<formalpara>
<title>Returns:</title>
<para>
<classname>LargeObject</classname> object that implements the <acronym>API</acronym>
</para>
</formalpara>
<formalpara>
<title>Throws:</title>
<para>
<classname>SQLException</classname> by <classname>LargeObject</classname> when initializing for first time
</para>
</formalpara>
</listitem>
<listitem>
<para>
<synopsis>
public void addDataType(String type, String name)
</synopsis>
This allows client code to add a handler for one of
<productname>PostgreSQL</productname>'s more unique data types. Normally, a data type not
known by the driver is returned by <literal>ResultSet.getObject()</literal> as a
<classname>PGobject</> instance. This method allows you to write a class
that extends <classname>PGobject</>, and tell the driver the type name, and
class name to use. The down side to this, is that you must
call this method each time a connection is made.
</para>
<para>
The best way to use this is as follows:
<programlisting>
...
((org.postgresql.Connection)myconn).addDataType("mytype","my.class.name");
...
</programlisting>
where <varname>myconn</varname> is an open <classname>Connection</> to
<productname>PostgreSQL</productname>. The handling class must
extend <classname>org.postgresql.util.PGobject</classname>.
</para>
</listitem>
</itemizedlist>
</sect4>
</sect3>
<sect3>
<title>Class <classname>org.postgresql.Fastpath</classname></title>
<synopsis>
public class Fastpath extends Object
java.lang.Object
|
+----org.postgresql.fastpath.Fastpath
</synopsis>
<para>
<classname>Fastpath</classname> is an <acronym>API</acronym> that
exists within the <application>libpq</application> C interface, and allows a client machine
to execute a function on the database backend. Most client code
will not need to use this method, but it is provided because the
Large Object <acronym>API</acronym> uses it.
</para>
<para>
To use, you need to import the
<classname>org.postgresql.fastpath</classname> package, using the
line:
<programlisting>
import org.postgresql.fastpath.*;
</programlisting>
Then, in your code, you need to get a
<classname>FastPath</classname> object:
<programlisting>
Fastpath fp = ((org.postgresql.Connection)conn).getFastpathAPI();
</programlisting>
This will return an instance associated with the database
connection that you can use to issue commands. The casing of
<classname>Connection</classname> to
<classname>org.postgresql.Connection</classname> is required, as
the <function>getFastpathAPI()</function> is an extension method,
not part of <acronym>JDBC</acronym>. Once you have a
<classname>Fastpath</classname> instance, you can use the
<function>fastpath()</function> methods to execute a backend
function.
</para>
<formalpara>
<title>See Also:</title>
<para>
<classname>FastpathFastpathArg</classname>, <classname>LargeObject</classname>
</para>
</formalpara>
<sect4>
<title>Methods</title>
<itemizedlist>
<listitem>
<synopsis>
public Object fastpath(int fnid,
boolean resulttype,
FastpathArg args[]) throws SQLException
</synopsis>
<para>
Send a function call to the <productname>PostgreSQL</productname> backend.
</para>
<formalpara>
<title>Parameters:</title>
<para>
<parameter>fnid</> - Function id
<parameter>resulttype</> - True if the result is an integer, false
for
other results
<parameter>args</> - <classname>FastpathArguments</classname> to pass to fastpath
</para>
</formalpara>
<formalpara>
<title>Returns:</title>
<para>
null if no data, Integer if an integer result, or byte[]
otherwise
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public Object fastpath(String name,
boolean resulttype,
FastpathArg args[]) throws SQLException
</synopsis>
<para>
Send a function call to the <productname>PostgreSQL</productname> backend by name.
</para>
<note>
<para>
The mapping for the procedure name to function id needs to
exist, usually to an earlier call to <function>addfunction()</function>. This is
the preferred method to call, as function id's can/may change
between versions of the backend. For an example of how this
works, refer to org.postgresql.LargeObject
</para>
</note>
<formalpara>
<title>Parameters:</title>
<para>
<parameter>name</> - Function name
<parameter>resulttype</> - True if the result is an integer, false
for
other results
<parameter>args</> - <classname>FastpathArguments</classname> to pass to fastpath
</para>
</formalpara>
<formalpara>
<title>Returns:</title>
<para>
null if no data, Integer if an integer result, or byte[]
otherwise
</para>
</formalpara>
<formalpara>
<title>See Also:</title>
<para><classname>LargeObject</classname></para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public int getInteger(String name,
FastpathArg args[]) throws SQLException
</synopsis>
<para>
This convenience method assumes that the return value is an Integer
</para>
<formalpara>
<title>Parameters:</title>
<para>
<parameter>name</parameter> - Function name
<parameter>args</parameter> - Function arguments
</para>
</formalpara>
<formalpara>
<title>Returns:</title>
<para>integer result</para>
</formalpara>
<formalpara>
<title>Throws:</title>
<para>
<classname>SQLException</classname> if a database-access error occurs or no result
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public byte[] getData(String name,
FastpathArg args[]) throws SQLException
</synopsis>
<para>
This convenience method assumes that the return value is binary
data.
</para>
<formalpara>
<title>Parameters:</title>
<para>
<parameter>name</parameter> - Function name
<parameter>args</parameter> - Function arguments
</para>
</formalpara>
<formalpara>
<title>Returns:</title>
<para>byte[] array containing result</para>
</formalpara>
<formalpara>
<title>Throws:</title>
<para>
<classname>SQLException</classname> if a database-access error occurs or no result
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public void addFunction(String name,
int fnid)
</synopsis>
<para>
This adds a function to our look-up table. User code should
use the <function>addFunctions</function> method, which is based upon a query,
rather than hard coding the oid. The oid for a function is not
guaranteed to remain static, even on different servers of the
same version.
</para>
</listitem>
<listitem>
<synopsis>
public void addFunctions(ResultSet rs) throws SQLException
</synopsis>
<para>
This takes a <classname>ResultSet</classname> containing two columns. Column 1
contains the function name, Column 2 the oid. It reads the
entire <classname>ResultSet</classname>, loading the values into the function table.
</para>
<important>
<para>
Remember to <function>close()</function> the
<classname>ResultSet</classname> after calling this!
</para>
</important>
<note>
<title>Implementation note about function name look-ups</title>
<para>
<productname>PostgreSQL</productname> stores the function id's and their corresponding
names in the <classname>pg_proc</> table. To speed things up locally,
instead of querying each function from that table when
required, a <classname>Hashtable</classname> is used. Also, only the function's
required are entered into this table, keeping connection
times as fast as possible.
</para>
<para>
The <classname>org.postgresql.LargeObject</classname> class
performs a query upon its start-up, and passes the returned
<classname>ResultSet</classname> to the
<function>addFunctions()</function> method here. Once this
has been done, the Large Object <acronym>API</acronym> refers
to the functions by name.
</para>
<para>
Do not think that manually converting them to the OIDs will
work. OK, they will for now, but they can change during
development (there was some discussion about this for V7.0),
so this is implemented to prevent any unwarranted headaches
in the future.
</para>
</note>
<formalpara>
<title>See Also:</title>
<para>
<classname>LargeObjectManager</classname>
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public int getID(String name) throws SQLException
</synopsis>
<para>
This returns the function id associated by its name If
<function>addFunction()</function> or <function>addFunctions()</function> have not been called for this
name, then an <classname>SQLException</classname> is thrown.
</para>
</listitem>
</itemizedlist>
</sect4>
</sect3>
<sect3>
<title>Class <classname>org.postgresql.fastpath.FastpathArg</classname></title>
<synopsis>
public class FastpathArg extends Object
java.lang.Object
|
+----org.postgresql.fastpath.FastpathArg
</synopsis>
<para>
Each fastpath call requires an array of arguments, the number and
type dependent on the function being called. This class
implements methods needed to provide this capability.
</para>
<para>
For an example on how to use this, refer to the
<classname>org.postgresql.LargeObject</classname> package.
</para>
<formalpara>
<title>See Also:</title>
<para>
<classname>Fastpath</classname>, <classname>LargeObjectManager</classname>, <classname>LargeObject</classname>
</para>
</formalpara>
<sect4>
<title>Constructors</title>
<itemizedlist>
<listitem>
<synopsis>
public FastpathArg(int value)
</synopsis>
<para>
Constructs an argument that consists of an integer value
</para>
<formalpara>
<title>Parameters:</title>
<para>
value - int value to set
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public FastpathArg(byte bytes[])
</synopsis>
<para>
Constructs an argument that consists of an array of bytes
</para>
<formalpara>
<title>Parameters:</title>
<para>
bytes - array to store
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public FastpathArg(byte buf[],
int off,
int len)
</synopsis>
<para>
Constructs an argument that consists of part of a byte array
</para>
<formalpara>
<title>Parameters:</title>
<para>
<variablelist>
<varlistentry>
<term><parameter>buf</></term>
<listitem>
<simpara>source array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>off</parameter></term>
<listitem>
<simpara>offset within array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>len</parameter></term>
<listitem>
<simpara>length of data to include</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public FastpathArg(String s)
</synopsis>
<para>
Constructs an argument that consists of a String.
</para>
</listitem>
</itemizedlist>
</sect4>
</sect3>
</sect2>
<sect2>
<title>Geometric Data Types</title>
<para>
<productname>PostgreSQL</productname> has a set of data types that
can store geometric features into a table. These include single
points, lines, and polygons. We support these types in Java with
the org.postgresql.geometric package. It contains classes that
extend the org.postgresql.util.PGobject class. Refer to that
class for details on how to implement your own data type handlers.
</para>
<programlisting>
Class org.postgresql.geometric.PGbox
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGbox
public class PGbox extends PGobject implements Serializable,
Cloneable
This represents the box data type within <productname>PostgreSQL</productname>.
Variables
public PGpoint point[]
These are the two corner points of the box.
Constructors
public PGbox(double x1,
double y1,
double x2,
double y2)
Parameters:
x1 - first x coordinate
y1 - first y coordinate
x2 - second x coordinate
y2 - second y coordinate
public PGbox(PGpoint p1,
PGpoint p2)
Parameters:
p1 - first point
p2 - second point
public PGbox(String s) throws SQLException
Parameters:
s - Box definition in <productname>PostgreSQL</productname> syntax
Throws: SQLException
if definition is invalid
public PGbox()
Required constructor
Methods
public void setValue(String value) throws SQLException
This method sets the value of this object. It should be
overridden, but still called by subclasses.
Parameters:
value - a string representation of the value of the
object
Throws: SQLException
thrown if value is invalid for this type
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two boxes are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGbox in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGcircle
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGcircle
public class PGcircle extends PGobject implements Serializable,
Cloneable
This represents <productname>PostgreSQL</productname>'s circle data type, consisting of a point
and a radius
Variables
public PGpoint center
This is the center point
double radius
This is the radius
Constructors
public PGcircle(double x,
double y,
double r)
Parameters:
x - coordinate of center
y - coordinate of center
r - radius of circle
public PGcircle(PGpoint c,
double r)
Parameters:
c - PGpoint describing the circle's center
r - radius of circle
public PGcircle(String s) throws SQLException
Parameters:
s - definition of the circle in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
public PGcircle()
This constructor is used by the driver.
Methods
public void setValue(String s) throws SQLException
Parameters:
s - definition of the circle in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two circles are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGcircle in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGline
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGline
public class PGline extends PGobject implements Serializable,
Cloneable
This implements a line consisting of two points. Currently line is
not yet implemented in the backend, but this class ensures that when
it's done were ready for it.
Variables
public PGpoint point[]
These are the two points.
Constructors
public PGline(double x1,
double y1,
double x2,
double y2)
Parameters:
x1 - coordinate for first point
y1 - coordinate for first point
x2 - coordinate for second point
y2 - coordinate for second point
public PGline(PGpoint p1,
PGpoint p2)
Parameters:
p1 - first point
p2 - second point
public PGline(String s) throws SQLException
Parameters:
s - definition of the line in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
public PGline()
required by the driver
Methods
public void setValue(String s) throws SQLException
Parameters:
s - Definition of the line segment in <productname>PostgreSQL</productname>'s
syntax
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two lines are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGline in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGlseg
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGlseg
public class PGlseg extends PGobject implements Serializable,
Cloneable
This implements a lseg (line segment) consisting of two points
Variables
public PGpoint point[]
These are the two points.
Constructors
public PGlseg(double x1,
double y1,
double x2,
double y2)
Parameters:
x1 - coordinate for first point
y1 - coordinate for first point
x2 - coordinate for second point
y2 - coordinate for second point
public PGlseg(PGpoint p1,
PGpoint p2)
Parameters:
p1 - first point
p2 - second point
public PGlseg(String s) throws SQLException
Parameters:
s - Definition of the line segment in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
public PGlseg()
required by the driver
Methods
public void setValue(String s) throws SQLException
Parameters:
s - Definition of the line segment in <productname>PostgreSQL</productname>'s
syntax
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two line segments are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGlseg in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGpath
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGpath
public class PGpath extends PGobject implements Serializable,
Cloneable
This implements a path (a multiply segmented line, which may be
closed)
Variables
public boolean open
True if the path is open, false if closed
public PGpoint points[]
The points defining this path
Constructors
public PGpath(PGpoint points[],
boolean open)
Parameters:
points - the PGpoints that define the path
open - True if the path is open, false if closed
public PGpath()
Required by the driver
public PGpath(String s) throws SQLException
Parameters:
s - definition of the path in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
Methods
public void setValue(String s) throws SQLException
Parameters:
s - Definition of the path in <productname>PostgreSQL</productname>'s syntax
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two pathes are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
This returns the path in the syntax expected by
<productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
public boolean isOpen()
This returns true if the path is open
public boolean isClosed()
This returns true if the path is closed
public void closePath()
Marks the path as closed
public void openPath()
Marks the path as open
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGpoint
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGpoint
public class PGpoint extends PGobject implements Serializable,
Cloneable
This implements a version of java.awt.Point, except it uses double
to represent the coordinates.
It maps to the point data type in <productname>PostgreSQL</productname>.
Variables
public double x
The X coordinate of the point
public double y
The Y coordinate of the point
Constructors
public PGpoint(double x,
double y)
Parameters:
x - coordinate
y - coordinate
public PGpoint(String value) throws SQLException
This is called mainly from the other geometric types, when a
point is embedded within their definition.
Parameters:
value - Definition of this point in <productname>PostgreSQL</productname>'s
syntax
public PGpoint()
Required by the driver
Methods
public void setValue(String s) throws SQLException
Parameters:
s - Definition of this point in <productname>PostgreSQL</productname>'s syntax
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two points are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGpoint in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
public void translate(int x,
int y)
Translate the point with the supplied amount.
Parameters:
x - integer amount to add on the x axis
y - integer amount to add on the y axis
public void translate(double x,
double y)
Translate the point with the supplied amount.
Parameters:
x - double amount to add on the x axis
y - double amount to add on the y axis
public void move(int x,
int y)
Moves the point to the supplied coordinates.
Parameters:
x - integer coordinate
y - integer coordinate
public void move(double x,
double y)
Moves the point to the supplied coordinates.
Parameters:
x - double coordinate
y - double coordinate
public void setLocation(int x,
int y)
Moves the point to the supplied coordinates. refer to
java.awt.Point for description of this
Parameters:
x - integer coordinate
y - integer coordinate
See Also:
Point
public void setLocation(Point p)
Moves the point to the supplied java.awt.Point refer to
java.awt.Point for description of this
Parameters:
p - Point to move to
See Also:
Point
<!-- **************************************************************** -->
Class org.postgresql.geometric.PGpolygon
java.lang.Object
|
+----org.postgresql.util.PGobject
|
+----org.postgresql.geometric.PGpolygon
public class PGpolygon extends PGobject implements Serializable,
Cloneable
This implements the polygon data type within <productname>PostgreSQL</productname>.
Variables
public PGpoint points[]
The points defining the polygon
Constructors
public PGpolygon(PGpoint points[])
Creates a polygon using an array of PGpoints
Parameters:
points - the points defining the polygon
public PGpolygon(String s) throws SQLException
Parameters:
s - definition of the polygon in <productname>PostgreSQL</productname>'s syntax.
Throws: SQLException
on conversion failure
public PGpolygon()
Required by the driver
Methods
public void setValue(String s) throws SQLException
Parameters:
s - Definition of the polygon in <productname>PostgreSQL</productname>'s syntax
Throws: SQLException
on conversion failure
Overrides:
setValue in class PGobject
public boolean equals(Object obj)
Parameters:
obj - Object to compare with
Returns:
true if the two polygons are identical
Overrides:
equals in class PGobject
public Object clone()
This must be overridden to allow the object to be cloned
Overrides:
clone in class PGobject
public String getValue()
Returns:
the PGpolygon in the syntax expected by <productname>PostgreSQL</productname>
Overrides:
getValue in class PGobject
</programlisting>
</sect2>
<sect2>
<title>Large Objects</title>
<para>
Large objects are supported in the standard
<acronym>JDBC</acronym> specification. However, that interface is
limited, and the <acronym>API</acronym> provided by <productname>PostgreSQL</productname> allows for random
access to the objects contents, as if it was a local file.
</para>
<para>
The org.postgresql.largeobject package provides to Java the <application>libpq</application>
C interface's large object <acronym>API</acronym>. It consists of
two classes, <classname>LargeObjectManager</classname>, which deals with creating,
opening and deleting large objects, and <classname>LargeObject</classname> which deals
with an individual object.
</para>
<sect3>
<title>Class <classname>org.postgresql.largeobject.LargeObject</classname></title>
<synopsis>
public class LargeObject extends Object
java.lang.Object
|
+----org.postgresql.largeobject.LargeObject
</synopsis>
<para>
This class implements the large object interface to
<productname>PostgreSQL</productname>.
</para>
<para>
It provides the basic methods required to run the interface, plus
a pair of methods that provide <classname>InputStream</classname> and <classname>OutputStream</classname>
classes for this object.
</para>
<para>
Normally, client code would use the methods in
<classname>BLOB</classname> to access large objects.
</para>
<para>
However, sometimes lower level access to Large Objects is
required, that is not supported by the <acronym>JDBC</acronym>
specification.
</para>
<para>
Refer to org.postgresql.largeobject.LargeObjectManager on how to
gain access to a Large Object, or how to create one.
</para>
<formalpara>
<title>See Also:</title>
<para><classname>LargeObjectManager</classname></para>
</formalpara>
<sect4>
<title>Variables</title>
<variablelist>
<varlistentry>
<term>public static final int SEEK_SET</term>
<listitem>
<para>Indicates a seek from the beginning of a file</para>
</listitem>
</varlistentry>
<varlistentry>
<term>public static final int SEEK_CUR</term>
<listitem>
<para>Indicates a seek from the current position</para>
</listitem>
</varlistentry>
<varlistentry>
<term>public static final int SEEK_END</term>
<listitem>
<para>Indicates a seek from the end of a file</para>
</listitem>
</varlistentry>
</variablelist>
</sect4>
<sect4>
<title>Methods</title>
<itemizedlist>
<listitem>
<synopsis>
public int getOID()
</synopsis>
<para>
Returns the OID of this <classname>LargeObject</classname>
</para>
</listitem>
<listitem>
<synopsis>
public void close() throws SQLException
</synopsis>
<para>
This method closes the object. You must not call methods in
this object after this is called.
</para>
</listitem>
<listitem>
<synopsis>
public byte[] read(int len) throws SQLException
</synopsis>
<para>
Reads some data from the object, and return as a byte[] array
</para>
</listitem>
<listitem>
<synopsis>
public int read(byte buf[],
int off,
int len) throws SQLException
</synopsis>
<para>
Reads some data from the object into an existing array
</para>
<formalpara>
<title>Parameters:</title>
<para>
<variablelist>
<varlistentry>
<term>buf</term>
<listitem>
<simpara>destination array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>off</term>
<listitem>
<simpara>offset within array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>len</term>
<listitem>
<simpara>number of bytes to read</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
</formalpara>
</listitem>
<listitem>
<synopsis>
public void write(byte buf[]) throws SQLException
</synopsis>
<para>
Writes an array to the object
</para>
</listitem>
<listitem>
<synopsis>
public void write(byte buf[],
int off,
int len) throws SQLException
</synopsis>
<para>
Writes some data from an array to the object
</para>
<formalpara>
<title>Parameters:</title>
<para>
<variablelist>
<varlistentry>
<term>buf</term>
<listitem>
<simpara>destination array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>off</term>
<listitem>
<simpara>offset within array</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>len</term>
<listitem>
<simpara>number of bytes to write</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
</formalpara>
</listitem>
<!--
public void seek(int pos,
int ref) throws SQLException
Sets the current position within the object.
This is similar to the fseek() call in the standard C
library.It allows you to have random access to the large object.
Parameters:
pos - position within object
ref - Either SEEK_SET, SEEK_CUR or SEEK_END
Throws: SQLException
if a database-access error occurs.
public void seek(int pos) throws SQLException
Sets the current position within the object.
This is similar to the fseek() call in the standard C
library.It allows you to have random access to the large object.
Parameters:
pos - position within object from begining
Throws: SQLException
if a database-access error occurs.
public int tell() throws SQLException
Returns:
the current position within the object
Throws: SQLException
if a database-access error occurs.
public int size() throws SQLException
This method is inefficient, as the only way to find out the
size of the object is to seek to the end, record the current position,
then return to the original position.
A better method will be found in the future.
Returns:
the size of the large object
Throws: SQLException
if a database-access error occurs.
public InputStream getInputStream() throws SQLException
Returns an InputStream from this object.
This InputStream can then be used in any method that
requires an InputStream.
Throws: SQLException
if a database-access error occurs.
public OutputStream getOutputStream() throws SQLException
Returns an OutputStream to this object
This OutputStream can then be used in any method that
requires an OutputStream.
Throws: SQLException
if a database-access error occurs.
-->
</itemizedlist>
</sect4>
</sect3>
<sect3>
<title>Class <classname>org.postgresql.largeobject.LargeObjectManager</classname></title>
<synopsis>
public class LargeObjectManager extends Object
java.lang.Object
|
+----org.postgresql.largeobject.LargeObjectManager
</synopsis>
<para>
This class implements the large object interface to
<productname>PostgreSQL</productname>. It provides methods that
allow client code to create, open and delete large objects from
the database. When opening an object, an instance of
<classname>org.postgresql.largeobject.LargeObject</classname> is
returned, and its methods then allow access to the object.
</para>
<para>
This class can only be created by org.postgresql.Connection. To
get access to this class, use the following segment of code:
<programlisting>
import org.postgresql.largeobject.*;
Connection conn;
LargeObjectManager lobj;
// ... code that opens a connection ...
lobj = ((org.postgresql.Connection)myconn).getLargeObjectAPI();
</programlisting>
</para>
<para>
Normally, client code would use the <classname>BLOB</classname>
methods to access large objects. However, sometimes
lower level access to Large Objects is required, that is not
supported by the <acronym>JDBC</acronym> specification.
</para>
<para>
Refer to org.postgresql.largeobject.LargeObject on how to
manipulate the contents of a Large Object.
</para>
<sect4>
<title>Variables</title>
<variablelist>
<varlistentry>
<term>public static final int WRITE</term>
<listitem>
<simpara>This mode indicates we want to write to an object.</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>public static final int READ</term>
<listitem>
<simpara>This mode indicates we want to read an object.</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>public static final int READWRITE</term>
<listitem>
<simpara>This mode is the default. It indicates we want read and write access to a large object.</simpara>
</listitem>
</varlistentry>
</variablelist>
</sect4>
<sect4>
<title>Methods</title>
<itemizedlist>
<listitem>
<synopsis>
public LargeObject open(int oid) throws SQLException
</synopsis>
<para>
This opens an existing large object, based on its OID. This
method assumes that READ and WRITE access is required (the
default).
</para>
</listitem>
<listitem>
<synopsis>
public LargeObject open(int oid,
int mode) throws SQLException
</synopsis>
<para>
This opens an existing large object, based on its OID, and
allows setting the access mode.
</para>
</listitem>
<listitem>
<synopsis>
public int create() throws SQLException
</synopsis>
<para>
This creates a large object, returning its OID.
It defaults to READWRITE for the new object's attributes.
</para>
</listitem>
<listitem>
<synopsis>
public int create(int mode) throws SQLException
</synopsis>
<para>
This creates a large object, returning its OID, and sets the
access mode.
</para>
</listitem>
<listitem>
<synopsis>
public void delete(int oid) throws SQLException
</synopsis>
<para>
This deletes a large object.
</para>
</listitem>
<listitem>
<synopsis>
public void unlink(int oid) throws SQLException
</synopsis>
<para>
This deletes a large object. It is identical to the delete
method, and is supplied as the C <acronym>API</acronym> uses
<quote>unlink</quote>.
</para>
</listitem>
</itemizedlist>
</sect4>
</sect3>
</sect2>
</sect1>
<sect1 id="jdbc-thread">
<title>Using the driver in a multithreaded or a servlet environment</title>
<para>
A problem with many <acronym>JDBC</acronym> drivers is that only
one thread can use a <classname>Connection</classname> at any one
time -- otherwise a thread could send a query while another one is
receiving results, and this would be a bad thing for the database
engine.
</para>
<para>
The <productname>PostgreSQL</productname> <acronym>JDBC</acronym> Driver
is thread safe.
Consequently, if your application uses multiple threads then you do
not have to worry about complex algorithms to ensure that only one
uses the database at any time.
</para>
<para>
If a thread attempts to use the connection while another one is
using it, it will wait until the other thread has finished its
current operation. If it is a regular <acronym>SQL</acronym>
statement, then the operation consists of sending the statement and
retrieving any <classname>ResultSet</classname> (in full). If it
is a <classname>Fastpath</classname> call (e.g., reading a block
from a <classname>LargeObject</classname>) then it is the time to
send and retrieve that block.
</para>
<para>
This is fine for applications and applets but can cause a
performance problem with servlets. With servlets you can have a
heavy load on the connection. If you have several threads
performing queries then each but one will pause, which may not be what
you are after.
</para>
<para>
To solve this, you would be advised to create a pool of
connections. When ever a thread needs to use the database, it asks
a manager class for a <classname>Connection</classname>. The
manager hands a free connection to the thread and marks it as
busy. If a free connection is not available, it opens one. Once
the thread has finished with it, it returns it to the manager who
can then either close it or add it to the pool. The manager would
also check that the connection is still alive and remove it from
the pool if it is dead.
</para>
<para>
So, with servlets, it is up to you to use either a single
connection, or a pool. The plus side for a pool is that threads
will not be hit by the bottle neck caused by a single network
connection. The down side is that it increases the load on the
server, as a backend process is created for each
<classname>Connection</classname>. It is up to you and your
applications requirements.
</para>
</sect1>
<sect1 id="jdbc-reading">
<title>Further Reading</title>
<para>
If you have not yet read it, I'd advise you read the
<acronym>JDBC</acronym> <acronym>API</acronym> Documentation
(supplied with Sun's <acronym>JDK</acronym>), and the
<acronym>JDBC</acronym> Specification. Both are available from
<ulink
url="http://java.sun.com/products/jdbc/index.html">http://java.sun.com/products/jdbc/index.html</ulink>.
</para>
<para>
<ulink
url="http://jdbc.postgresql.org">http://jdbc.postgresql.org</ulink>
contains updated information not included in this document, and
also includes precompiled drivers.
</para>
</sect1>
</chapter>
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