public blobtest(String[] args) throws IOException, ClassNotFoundException, SQLException, FileNotFoundException 
{
    String url = args[0];
    String usr = args[1];
    String pwd = args[2];
    // Load the driver
    Class.forName("org.postgresql.Driver");
    // Connect to database
    System.out.println("Connecting to Database URL = " + url);
    db = DriverManager.getConnection(url, usr, pwd);
    // This is required for all LargeObject calls
    System.out.println("Connected... First turn off autoCommit()");
    db.setAutoCommit(false);
    System.out.println("Now creating a statement");
    s = db.createStatement();
    // Now run tests using postgresql's own Large object api
    // NOTE: The methods shown in this example are _NOT_ JDBC, but are
    // an implementation of the calls found in libpq. Unless you need to
    // use this functionality, look at the jdbc tests on how to access blobs.
    ownapi();
    // Now run tests using JDBC methods
    //jdbcapi(db,s);
    // Finally close the database
    System.out.println("Now closing the connection");
    s.close();
    db.close();
}

 public static  void instructions() 
{
    System.err.println("java example.blobtest jdbc-url user password [debug]");
    System.err.println("\nExamples:\n");
    System.err.println("java -Djdbc.driver=org.postgresql.Driver example.blobtest jdbc:postgresql:test postgres password\nThis will run the tests on the database test on the local host.\n");
    System.err.println("java -Djdbc.driver=org.postgresql.Driver example.blobtest jdbc:postgresql:test postgres password debug\nThis is the same as above, but will output debug information.\n");
    System.err.println("This example tests the binary large object api of the driver.\nThis allows images or java objects to be stored in the database, and retrieved\nusing both postgresql's own api, and the standard JDBC api.");
}

 public  void jdbc2api(int oid) throws IOException, SQLException 
{
	System.out.println("Testing JDBC2 Blob interface:");
	jdbc2api_cleanup();
	System.out.println("Creating Blob on large object "+oid);
	s.executeUpdate("create table basic (a oid)");
	System.out.println("Inserting row");
	s.executeUpdate("insert into basic values ("+oid+")");
	System.out.println("Selecting row");
	ResultSet rs = s.executeQuery("select a from basic");
	if(rs!=null) {
	    while(rs.next()) {
		System.out.println("Fetching Blob");
		Blob b = rs.getBlob("a");
		System.out.println("Blob.length() = "+b.length());
		System.out.println("Characters 400-500:");
		System.out.write(b.getBytes(400l,100));
		System.out.println();
	    }
	    rs.close();
	}
	System.out.println("Cleaning up");
	jdbc2api_cleanup();
}

 public static  void main(String[] args) 
{
    System.out.println("PostgreSQL blobtest v7.0 rev 1\n");
    if(args.length< 3) {
      instructions();
      System.exit(1);
    }
    // This line outputs debug information to stderr. To enable this, simply
    // add an extra parameter to the command line
    if(args.length >3)
      DriverManager.setLogStream(System.err);
    // Now run the tests
    try {
      blobtest test = new blobtest(args);
    } catch(Exception ex) {
      System.err.println("Exception caught.\n"+ex);
      ex.printStackTrace();
    }
}

 public  void ownapi() throws IOException, SQLException, FileNotFoundException 
{
    System.out.println("\n----------------------------------------------------------------------\nTesting postgresql large object api\n----------------------------------------------------------------------\n");
    // Internally, the driver provides JDBC compliant methods to access large
    // objects, however the unique methods available to postgresql makes
    // things a little easier.
    System.out.println("Gaining access to large object api");
    lobj = ((org.postgresql.Connection)db).getLargeObjectAPI();
    int oid = ownapi_test1();
    ownapi_test2(oid);
    // Now call the jdbc2api test
    jdbc2api(oid);
    // finally delete the large object
    ownapi_test3(oid);
    System.out.println("\n\nOID="+oid);
}

Now this is an extension to JDBC, unique to postgresql. Here we fetch an PGlobj object, which provides us with access to postgresql's large object api.