EJB Handles

There is another model provided for getting to EJBs.  This method uses "handles".

Each home or bean object has a handle.  You can ask for this handle from the home or bean object.  Later on, you can get back to the object by using the handle.

The handle doesn't just work during the running program, it even works if your program exits and is restarted, or if the handle is used from another program.  The handle itself is like a permanent object.  Of course, if your program exits, it has to have a way to save the handle.  This can be done simply by writing the handle to a file (or any another such method of passing or preserving bytes.)  The handle derives from java.io.Serializable, so it can be written to files.

The code to obtain a handle from an EJB object and to get an EJB object back from a handle is fairly straightforward.  The following code fragment shows how to obtain a handle from a bean object called "stock" and to write it into a file:

Handle handle = stock.getHandle();
ObjectOutputStream p = new ObjectOutputStream(
       new FileOutputStream("C:\\SaveHandle.dat"));
p.writeObject( handle );
p.close();

Once the handle has been saved, it can be read back from the file and object created from it:

ObjectInputStream p = new ObjectInputStream(
         new FileInputStream( "C:\\SaveHandle.dat"));
Handle handle = (Handle) p.readObject();
Stock stock = (Stock) handle.getEJBObject();
// Use PortableRemoteObject in 1.3 or when available.

Note that the object returned by getEJBObject() has to be cast into the correct type.

An important item to remember about handles is that session beans are discarded after a certain timeout.  Therefore, if you use a session bean's handle after that certain timeout, you will not get the bean back.  Instead, an exception will be thrown.  (The timeout value can usually be changed, see server documentation for this.)

Exercise:  Modify your session bean client to save a handle to the (stateful) StockQuotes bean.  Write another program that retrieves the handle and gets the bean.  Confirm that the "stock" member variable of the bean is the same as when saved.  Obtain the price from this "recovered" bean.  Try using your second bean within the timeout period, and after the timeout period.