<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 7. Storing Binary Data</title><meta content="DocBook XSL Stylesheets V1.78.1" name="generator"><link rel="home" href="index.html" title="The PostgreSQL™ JDBC Interface"><link rel="up" href="index.html" title="The PostgreSQL™ JDBC Interface"><link rel="prev" href="callproc.html" title="Chapter 6. Calling Stored Functions"><link rel="next" href="escapes.html" title="Chapter 8. JDBC escapes"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">Chapter 7. Storing Binary Data</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="callproc.html">Prev</a> </td><th align="center" width="60%"> </th><td align="right" width="20%"> <a accesskey="n" href="escapes.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="binary-data"></a>Chapter 7. Storing Binary Data</h1></div></div></div><a name="N10734" class="indexterm"></a><a name="N1073C" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span>™ provides two distinct ways to
store binary data. Binary data can be stored in a table using
the data type <span class="type">bytea</span> or by using the Large Object
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
<span class="type">oid</span> in your table.
</p><p>
In order to determine which method is appropriate you
need to understand the limitations of each method. The
<span class="type">bytea</span> data type is not well suited for storing very
large amounts of binary data. While a column of type
<span class="type">bytea</span> can hold up to 1 GB of binary data, it would
require a huge amount of memory 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 reference 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 can view
and/or modify any Large Object, even if they don't have
permissions to view/update the row containing the Large Object reference.
</p><p>
Version 7.2 was the first release of the <acronym class="acronym">JDBC</acronym> driver
that supports the <span class="type">bytea</span> data type. The introduction of
this functionality in 7.2 has introduced a change in behavior
as compared to previous releases. Since 7.2, the methods
<code class="function">getBytes()</code>, <code class="function">setBytes()</code>,
<code class="function">getBinaryStream()</code>, and
<code class="function">setBinaryStream()</code> operate on
the <span class="type">bytea</span> data type. In 7.1 and earlier, these methods operated
on the <span class="type">oid</span> data type associated with Large Objects.
It is possible to revert the driver back to the old 7.1 behavior
by setting the property <code class="literal">compatible</code> on
the <code class="classname">Connection</code> object to the value
<code class="literal">7.1</code>. More details on connection properties are
available in <a class="xref" href="connect.html#connection-parameters" title="Connection Parameters">the section called “Connection Parameters”</a>.
</p><p>
To use the <span class="type">bytea</span> data type you should simply use
the <code class="function">getBytes()</code>, <code class="function">setBytes()</code>,
<code class="function">getBinaryStream()</code>, or
<code class="function">setBinaryStream()</code> methods.
</p><p>
To use the Large Object functionality you can use either the
<code class="classname">LargeObject</code> class provided by the
<span class="productname">PostgreSQL</span>™ <acronym class="acronym">JDBC</acronym> driver, or by
using the <code class="function">getBLOB()</code> and
<code class="function">setBLOB()</code> methods.
</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>
You must access Large Objects within an <acronym class="acronym">SQL</acronym>
transaction block. You can start a transaction block by calling
<code class="function">setAutoCommit(false)</code>.
</p></div><p>
<a class="xref" href="binary-data.html#binary-data-example" title="Example 7.1. Processing Binary Data in JDBC">Example 7.1, “Processing Binary Data in <acronym class="acronym">JDBC</acronym>”</a> contains some examples on
how to process binary data using the <span class="productname">PostgreSQL</span>™
<acronym class="acronym">JDBC</acronym> driver.
</p><div class="example"><a name="binary-data-example"></a><p class="title"><b>Example 7.1. Processing Binary Data in <acronym class="acronym">JDBC</acronym></b></p><div class="example-contents"><p>
For example, suppose you have a table containing the file names of
images and you also want to store the image in a <span class="type">bytea</span>
column:
</p><pre class="programlisting">
CREATE TABLE images (imgname text, img bytea);
</pre><p>
</p><p>
To insert an image, you would use:
</p><pre class="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, (int)file.length());
ps.executeUpdate();
ps.close();
fis.close();
</pre><p>
Here, <code class="function">setBinaryStream()</code> transfers a set number
of bytes from a stream into the column of type <span class="type">bytea</span>.
This also could have been done using the <code class="function">setBytes()</code>
method if the contents of the image was already in a
<code class="classname">byte[]</code>.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
The length parameter to <code class="function">setBinaryStream</code> must be
correct. There is no way to indicate that the stream is of unknown
length. If you are in this situation, you must read the stream yourself
into temporary storage and determine the length. Now with the correct
length you may send the data from temporary storage on to the driver.
</p></div><p>
</p><p>
Retrieving an image is even easier. (We use
<code class="classname">PreparedStatement</code> here, but the
<code class="classname">Statement</code> class can equally be used.)
</p><pre class="programlisting">
PreparedStatement ps = conn.prepareStatement("SELECT img FROM images WHERE imgname = ?");
ps.setString(1, "myimage.gif");
ResultSet rs = ps.executeQuery();
while (rs.next()) {
byte[] imgBytes = rs.getBytes(1);
// use the data in some way here
}
rs.close();
ps.close();
</pre><p>
</p><p>
Here the binary data was retrieved as an
<code class="classname">byte[]</code>. You could have used a
<code class="classname">InputStream</code> object instead.
</p><p>
Alternatively you could be storing a very large file and want to use
the <code class="classname">LargeObject</code> <acronym class="acronym">API</acronym> to
store the file:
</p><pre class="programlisting">
CREATE TABLE imageslo (imgname text, imgoid oid);
</pre><p>
</p><p>
To insert an image, you would use:
</p><pre class="programlisting">
// All LargeObject API calls must be within a transaction block
conn.setAutoCommit(false);
// Get the Large Object Manager to perform operations with
LargeObjectManager lobj = ((org.postgresql.PGConnection)conn).getLargeObjectAPI();
// Create a new large object
long oid = lobj.createLO(LargeObjectManager.READ | LargeObjectManager.WRITE);
// Open the large object for writing
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.setLong(2, oid);
ps.executeUpdate();
ps.close();
fis.close();
// Finally, commit the transaction.
conn.commit();
</pre><p>
</p><p>
Retrieving the image from the Large Object:
</p><pre class="programlisting">
// All LargeObject API calls must be within a transaction block
conn.setAutoCommit(false);
// Get the Large Object Manager to perform operations with
LargeObjectManager lobj = ((org.postgresql.PGConnection)conn).getLargeObjectAPI();
PreparedStatement ps = conn.prepareStatement("SELECT imgoid FROM imageslo WHERE imgname = ?");
ps.setString(1, "myimage.gif");
ResultSet rs = ps.executeQuery();
while (rs.next()) {
// Open the large object for reading
long oid = rs.getLong(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();
// Finally, commit the transaction.
conn.commit();
</pre><p>
</p></div></div><br class="example-break"></div><div class="navfooter"><hr><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="callproc.html">Prev</a> </td><td align="center" width="20%"> </td><td align="right" width="40%"> <a accesskey="n" href="escapes.html">Next</a></td></tr><tr><td valign="top" align="left" width="40%">Chapter 6. Calling Stored Functions </td><td align="center" width="20%"><a accesskey="h" href="index.html">Home</a></td><td valign="top" align="right" width="40%"> Chapter 8. <acronym class="acronym">JDBC</acronym> escapes</td></tr></table></div></body></html>
