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PostgreSQL™ provides two distinct ways to store binary data. Binary data can be stored in a table using the data type bytea 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 oid in your table.
In order to determine which method is appropriate you need to understand the limitations of each method. The bytea data type is not well suited for storing very large amounts of binary data. While a column of type bytea 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.
Version 7.2 was the first release of the JDBC driver
that supports the bytea 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
getBytes(), setBytes(),
getBinaryStream(), and
setBinaryStream() operate on
the bytea data type. In 7.1 and earlier, these methods operated
on the oid data type associated with Large Objects.
It is possible to revert the driver back to the old 7.1 behavior
by setting the property compatible on
the Connection object to the value
7.1. More details on connection properties are
available in the section called “Connection Parameters”.
To use the bytea data type you should simply use
the getBytes(), setBytes(),
getBinaryStream(), or
setBinaryStream() methods.
To use the Large Object functionality you can use either the
LargeObject class provided by the
PostgreSQL™ JDBC driver, or by
using the getBLOB() and
setBLOB() methods.
You must access Large Objects within an SQL
transaction block. You can start a transaction block by calling
setAutoCommit(false).
Example 7.1, “Processing Binary Data in JDBC” contains some examples on how to process binary data using the PostgreSQL™ JDBC driver.
Example 7.1. Processing Binary Data in JDBC
For example, suppose you have a table containing the file names of images and you also want to store the image in a bytea column:
CREATE TABLE images (imgname text, img bytea);
To insert an image, you would use:
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();
Here, setBinaryStream() transfers a set number
of bytes from a stream into the column of type bytea.
This also could have been done using the setBytes()
method if the contents of the image was already in a
byte[].
The length parameter to setBinaryStream 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.
Retrieving an image is even easier. (We use
PreparedStatement here, but the
Statement class can equally be used.)
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();
Here the binary data was retrieved as an
byte[]. You could have used a
InputStream object instead.
Alternatively you could be storing a very large file and want to use
the LargeObject API to
store the file:
CREATE TABLE imageslo (imgname text, imgoid oid);
To insert an image, you would use:
// 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();
Retrieving the image from the Large Object:
// 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();