An Introduction to JDBC, Part 3

Batch Updates

The original JDBC standard was not very efficient for loading large amounts of information into a database. Even if you use a PreparedStatement, your program still executes a separate query for each piece of data inserted. If your software inserts 10,000 rows into the database, it can introduce a substantial performance bottleneck.

The new addBatch( ) method of Statement allows you to lump multiple update statements as a unit and execute them at once. Call addBatch( ) after you create the statement, and before execution:

con.setAutoCommit(false); // If some fail, we want to rollback the rest

Statement stmt = con.createStatement(  );

 

stmt.addBatch(

"INSERT INTO CUSTOMERS VALUES (1, "J Smith", "617 555-1323");

stmt.addBatch(

"INSERT INTO CUSTOMERS VALUES (2, "A Smith", "617 555-1132");

stmt.addBatch(

"INSERT INTO CUSTOMERS VALUES (3, "C Smith", "617 555-1238");

stmt.addBatch(

"INSERT INTO CUSTOMERS VALUES (4, "K Smith", "617 555-7823");

 

int[] upCounts = stmt.executeBatch(  );

con.commit(  );

Notice that we turn transaction auto-commit off before creating the batch. This is because we want to roll back all the SQL statements if one or more of them fail to execute properly (a more detailed discussion of transaction handling may be found later in this chapter, in the section "Transactions"). After calling addBatch( ) multiple times to create our batch, we call executeBatch( ) to send the SQL statements off to the database to be executed as a batch. Batch statements are executed in the order they are added to the batch. executeBatch( ) returns an array of update counts, in which each value in the array represents the number of rows affected by the corresponding batch statement. If you need to remove the statements from a pending batch job, you can call clearBatch( ), as long as you call it before calling executeBatch( ).

Note that you can use only SQL statements that return an update count (e.g., CREATE, DROP, INSERT, UPDATE, DELETE) as part of a batch. If you include a statement that returns a result set, such as SELECT, you get a SQLException when you execute the batch. If one of the statements in a batch can't be executed for some reason, executeBatch( ) throws a BatchUpdateException. This exception, derived from SQLException, contains an array of update counts for the batch statements that executed successfully before the exception was thrown. If we then call rollback( ), the components of the batch transaction that did execute successfully will be rolled back.

The addBatch( ) method works slightly differently for PreparedStatement and CallableStatement objects. To use batch updating with a PreparedStatement, create the statement normally, set the input parameters, and then call the addBatch( ) method with no arguments. Repeat as necessary and then call executeBatch( ) when you're finished:

con.setAutoCommit(false); // If some fail, we want to rollback the rest

PreparedStatement stmt = con.prepareStatement(

 "INSERT INTO CUSTOMERS VALUES (?,?,?)");

 

stmt.setInt(1,1);

stmt.setString(2, "J Smith");

stmt.setString(3, "617 555-1323");

stmt.addBatch(  );

 

stmt.setInt(1,2);

stmt.setString(2, "A Smith");

stmt.setString(3, "617 555-1132");

stmt.addBatch(  );

 

int[] upCounts = stmt.executeBatch(  );

con.commit(  );

This batch functionality also works with CallableStatement objects for stored procedures. The catch is that each stored procedure must return an update count and may not take any OUT or INOUT parameters.

BLOBs and CLOBs

As users began to increase the volume of data stored in databases, vendors introduced support for Large Objects (LOBs). The two varieties of LOBs, binary large objects (BLOBs) and character large objects (CLOBs), store large amounts of binary or character data, respectively.

Support for LOB types across databases varies. Some don't support them at all, and most have unique type names (BINARY, LONG RAW, and so forth). JDBC 1.0 makes programs retrieve BLOB and CLOB data using the getBinaryStream( ) or getAsciiStream( ) methods. (A third method, getUnicodeStream( ), has been deprecated in favor of the new getCharacterStream( ) method, which returns a Reader.)

In JDBC 2.0, the ResultSet interface includes getBlob( ) and getClob( ) methods, which return Blob and Clob objects, respectively. The Blob and Clob objects themselves allow access to their data via streams (the getBinaryStream( ) method of Blob and the getCharacterStream( ) method of Clob) or via direct-read methods (the getBytes( ) method of Blob and the getSubString( ) method of Clob).

To retrieve the data from a CLOB, simply retrieve the Clob object and call the getCharacterStream( ) method:

String s;

Clob clob = blobResultSet.getBlob("CLOBFIELD");

BufferedReader clobData = new BufferedReader(clob.getCharacterStream(  ));

while((s = clobData.readLine(  )) != null)

  System.out.println(s);

In addition, you can set Blob and Clob objects when you are working with a PreparedStatement, using the setBlob( ) and setClob( ) methods. While the API provides update methods for streams, there are no updateBlob( ) or updateClob( ) methods, and the Blob interface provides no mechanism for altering the contents of a Blob already stored in the database (although some drivers support updating of BLOB and CLOB types via the setBinaryStream( ) and setCharacterStream( ) methods of PreparedStatement). Note that the lifespan of a Blob or Clob object is limited to the transaction that created it.

JDBC driver support for BLOB and CLOB types varies wildly. Some vendors don't support any LOB functionality at all, and others (including Oracle) have added extensions to allow manipulation of LOB data. Check your driver documentation for more details.

In the next installments, learn about metadata and then transactions using JDBC within the J2EE framework.

William Crawford, Jim Farley is a coauthor of Java Enterprise in a Nutshell, 2nd Edition, and has been developing web-based enterprise applications since 1995. He is currently Principal Software Architect at Perceptive Informatics, Inc. Massachusetts, provider of software and services to the pharmaceutical industry.

David Flanagan is the author of a number of O'Reilly books, including Java in a Nutshell, Java Examples in a Nutshell, Java Foundation Classes in a Nutshell, JavaScript: The Definitive Guide, and JavaScript Pocket Reference.

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