Example: Using connections with concurrency

A Runnable or Callable can use the connections that its creating component obtained using java:comp resource references.

For more information on resource references, refer to the References topic. The following is an example of a task that uses connections correctly:

class GoodTask implements Callable<Object> 
{
   DataSource ds;
   public Task() throws NamingException
   {   
      // Cache a connection factory or datasource
      // as class instance data.
      InitialContext ic = new InitialContext();
      // It is assumed that the created Java EE component has this
      // resource reference defined in its deployment descriptor.
      ds = (DataSource)ic.lookup("java:comp/env/jdbc/myDataSource");
   }
   // When the asynchronous bean method is called, get a connection,
   // Use it, then close it.
   public Object call() throws SQLException
   {
      try (Connection c = ds.getConnection()) {
             // Use the connection now.
             return someResult;
      }
   }
} 

The following example of a task that uses connections incorrectly:

class BadTask implements Callable<Object> 
{
   DataSource ds;
   // Do not do this. We cannot cache connections across method calls.
   Connection c;

   public BadTask() throws NamingException, SQLException
   {
      // Cache a connection factory or datasource as
      // class instance data.
      InitialContext ic = new InitialContext();
      ds = (DataSource)ic.lookup("java:comp/env/jdbc/myDataSource");
      // Here, you broke the rules.
      c = ds.getConnection();
   }

   // Now when the method is called, illegally use the cached connection
   // and we likely see J2C related exceptions at run time.
   public Object call()
   {
      // Use the connection now.
      return someResult;
   }
} 

Related:

Asynchronous beans

References in application deployment descriptor files