RMI, and Java GUIs

4-3-2002

Comments

Everyone should be filling out and turning in group evaluations. These are largely for your protection. It can be used to point out where there are problems in group mechanics.

I had planned on talking about sequence diagrams today, but I’m going to push that back a week and discuss some Java topics instead.

Remote Method Invocation

Java makes virtually all network communication easier than it is in C/C++. This is particularly true when using RMI.

With RMI you can treat objects on other computers as if they were local on your own computer and call methods on them accordingly. Of course, there is a lag when you do this, but you don’t have to explicitly worry about the socketing and whatnot.

Remote InterfacesThe entity you get for an object remotely is

actually not the object itself, but an interface that object implements. The interface must extend java.rmi.Remote.

The implementation class should also extend java.rmi.server.UnicastRemoteObject.

Note that this implies that the implementation can have more functionality than the remote interface does.

All remote methods can throw java.rmi.RemoteException.

Passing

Different types of objects are passed differently Remote objects - any object that extends

java.rmi.Remote is passed as a remote object. You get a skeleton that does network communication.

Serializable objects - If an object is not Remote it must be serializable and then it is passed by value.

Primitives - No pass by reference of primitives

Registering and LookupOnce you have a remote object, it can

pass you others, but getting the first one takes a different approach.

An object can register itself with the rmi registry using the rebind method of java.rmi.Naming. (You have to start a local registry with rmiregistry first.)

Objects can get a remote reference to a registered object using the lookup method of java.rmi.Naming.

Compiling

Once you have written your code you first compile it with a normal Java compiler. After that you have to do another step to create stub and skeleton classes that do most of the work behind RMI. You to this is the RMI compiler, rmic. Just run rmic specifying the name of the implementation class.

Java GUIs

One of the greatest things about Java, from an application building standpoint, is how easy it is to make a GUI in it.

Since 5 out of 8 of the groups HAVE to do this to implement anything we will take a few minutes to discuss it.

Java has two graphics libraries, AWT and Swing. They work very similarly.

AWT

The AWT library uses inheritance extensively. Components of the GUI are represented by the component class. Container is a subclass of Component that can have other components added to it. Each of these has other subclasses representing buttons, windows, text fields, labels, etc.

Layouts

To make Java GUIs portable, they use layout managers to position and size various components. You can place things at specific locations on the screen, but that isn’t very portable.

Every container has a layout manager it uses to place the components in it. By nesting containers and layout managers you can get great flexibility.

Some Layout Managers

Here are some of the layout managers in AWT. FlowLayout - places one component after

another like text on a page. GridLayout - places components on a regular

grid. BorderLayout - Objects can be placed a north,

south, east, west, or center. GridBagLayout - A powerful layout manager

for placing objects in an irregular grid.

Events

No GUI is complete unless it can react to the user. User interactions with a GUI are called events and we want to be able to create code to “handle” events.

In Java (since version 1.1) we do this be attaching event listeners to GUI components. When an event happens for an object all the appropriate listeners for that object are called.

Events and Listeners

There are a fair number of event types and listeners.

The listeners include ActionListener, MouseListener, KeyListener, and FocusListener. Each listener type has certain methods that it must define.

Action events are some of the most common and happen when you do things like click buttons.

Swing

Swing is actually based on AWT, but provides a look and feel that is machine independent. It has pretty much all the same objects but the names typically start is J (i.e. JButton and JFrame).

Swing also has other classes AWT didn’t. These include JTable, JTree, JColorChooser, and JToolBar.

AWT or Swing for project?