Sun GlassFish Enterprise Server v2.1.1 Developer's Guide

Creating Applications That Use the JDBC API

An application that uses the JDBC API is an application that looks up and connects to one or more databases. This section covers these topics:

Sharing Connections

When multiple connections acquired by an application use the same JDBC resource, the connection pool provides connection sharing within the same transaction scope. For example, suppose Bean A starts a transaction and obtains a connection, then calls a method in Bean B. If Bean B acquires a connection to the same JDBC resource with the same sign-on information, and if Bean A completes the transaction, the connection can be shared.

Connections obtained through a resource are shared only if the resource reference declared by the Java EE component allows it to be shareable. This is specified in a component’s deployment descriptor by setting the res-sharing-scope element to Shareable for the particular resource reference. To turn off connection sharing, set res-sharing-scope to Unshareable.

For general information about connections and JDBC URLs, see Chapter 3, JDBC Resources, in Sun GlassFish Enterprise Server v2.1.1 Administration Guide.

Obtaining a Physical Connection From a Wrapped Connection

The DataSource implementation in the Enterprise Server provides a getConnection method that retrieves the JDBC driver’s SQLConnection from the Enterprise Server’s Connection wrapper. The method signature is as follows:

public java.sql.Connection getConnection(java.sql.Connection con) 
throws java.sql.SQLException

For example:

InitialContext ctx = new InitialContext();
com.sun.appserv.jdbc.DataSource ds = (com.sun.appserv.jdbc.DataSource) 
   ctx.lookup("jdbc/MyBase");
Connection con = ds.getConnection();
Connection drivercon = ds.getConnection(con);
// Do db operations.
// Do not close driver connection.
con.close(); // return wrapped connection to pool.

Marking Bad Connections

The DataSource implementation in the Enterprise Server provides a markConnectionAsBad method. A marked bad connection is removed from its connection pool when it is closed. The method signature is as follows:

public void markConnectionAsBad(java.sql.Connection con)

For example:

com.sun.appserv.jdbc.DataSource ds=
   (com.sun.appserv.jdbc.DataSource)context.lookup("dataSource");
Connection con = ds.getConnection();
Statement stmt = null;
try{
   stmt = con.createStatement();
   stmt.executeUpdate("Update");
}
catch (BadConnectionException e){
   dataSource.markConnectionAsBad(con) //marking it as bad for removal
}
finally{
   stmt.close();	
   con.close(); //Connection will be destroyed during close.
}

Using Non-Transactional Connections

You can specify a non-transactional database connection in any of these ways:

Check the Non-Transactional Connections box on the JDBC Connection Pools page in the Admin Console. The default is unchecked. For more information, click the Help button in the Admin Console.
Specify the --nontransactionalconnections option in the asadmin create-jdbc-connection-pool command. For more information, see the Sun GlassFish Enterprise Server v2.1.1 Reference Manual.
Use the DataSource implementation in the Enterprise Server, which provides a getNonTxConnection method. This method retrieves a JDBC connection that is not in the scope of any transaction. There are two variants.
```
public java.sql.Connection getNonTxConnection() throws java.sql.SQLException
```
```
public java.sql.Connection getNonTxConnection(String user, String password) 
   throws java.sql.SQLException
```
Create a resource with the JNDI name ending in __nontx. This forces all connections looked up using this resource to be non transactional.

Typically, a connection is enlisted in the context of the transaction in which a getConnection call is invoked. However, a non-transactional connection is not enlisted in a transaction context even if a transaction is in progress.

The main advantage of using non-transactional connections is that the overhead incurred in enlisting and delisting connections in transaction contexts is avoided. However, use such connections carefully. For example, if a non-transactional connection is used to query the database while a transaction is in progress that modifies the database, the query retrieves the unmodified data in the database. This is because the in-progress transaction hasn’t committed. For another example, if a non-transactional connection modifies the database and a transaction that is running simultaneously rolls back, the changes made by the non-transactional connection are not rolled back.

Here is a typical use case for a non-transactional connection: a component that is updating a database in a transaction context spanning over several iterations of a loop can refresh cached data by using a non-transactional connection to read data before the transaction commits.

Using JDBC Transaction Isolation Levels

For general information about transactions, see Chapter 16, Using the Transaction Service and Chapter 12, Transactions, in Sun GlassFish Enterprise Server v2.1.1 Administration Guide. For information about last agent optimization, which can improve performance, see Transaction Scope.

Not all database vendors support all transaction isolation levels available in the JDBC API. The Enterprise Server permits specifying any isolation level your database supports. The following table defines transaction isolation levels.

Table 15–1 Transaction Isolation Levels


Transaction Isolation Level	Description
`TRANSACTION_READ_UNCOMMITTED`	Dirty reads, non-repeatable reads, and phantom reads can occur.
`TRANSACTION_READ_COMMITTED`	Dirty reads are prevented; non-repeatable reads and phantom reads can occur.
`TRANSACTION_REPEATABLE_READ`	Dirty reads and non-repeatable reads are prevented; phantom reads can occur.
`TRANSACTION_SERIALIZABLE`	Dirty reads, non-repeatable reads and phantom reads are prevented.

Note that you cannot call setTransactionIsolation() during a transaction.

You can set the default transaction isolation level for a JDBC connection pool. For details, see Creating a Connection Pool.

To verify that a level is supported by your database management system, test your database programmatically using the supportsTransactionIsolationLevel() method in java.sql.DatabaseMetaData, as shown in the following example:

InitialContext ctx = new InitialContext();
DataSource ds = (DataSource)
ctx.lookup("jdbc/MyBase");
Connection con = ds.getConnection();
DatabaseMetaData dbmd = con.getMetaData();
if (dbmd.supportsTransactionIsolationLevel(TRANSACTION_SERIALIZABLE)
{ Connection.setTransactionIsolation(TRANSACTION_SERIALIZABLE); }

For more information about these isolation levels and what they mean, see the JDBC API specification.

Note –

Applications that change the isolation level on a pooled connection programmatically risk polluting the pool, which can lead to errors.

Allowing Non-Component Callers

You can allow non-Java-EE components, such as servlet filters, lifecycle modules, and third party persistence managers, to use this JDBC connection pool. The returned connection is automatically enlisted with the transaction context obtained from the transaction manager. Standard Java EE components can also use such pools. Connections obtained by non-component callers are not automatically closed at the end of a transaction by the container. They must be explicitly closed by the caller.

You can enable non-component callers in the following ways:

Check the Allow Non Component Callers box on the JDBC Connection Pools page in the Admin Console. The default is false. For more information, click the Help button in the Admin Console.
Specify the --allownoncomponentcallers option in the asadmin create-jdbc-connection-pool command. For more information, see the Sun GlassFish Enterprise Server v2.1.1 Reference Manual.
Create a JDBC resource with a __pm suffix.