AWT, SWT, Swing: Java GUI Clean Up (2)

Implementations

The above comparison is mainly conducted in API level. Let's continue the comparison with focus on implementation details. In all the difference between Swing and SWT/AWT is that Swing is purely implemented in Java, while SWT and AWT is a mixture of Java and JNI. Of course, their target is same, to provide a cross-platform APIs. However to achieve this, SWT and AWT has to sacrifice some components and some features so that they can provide a universal APIs.

AWT

An AWT component is usually a component class which holds a reference with a peer interface type. This reference points to a native peer implementation. Take java.awt.Label for example, its peer interface is LabelPeer. LabelPeer is platform independent. On every platform, AWT provides different peer class which implements LabelPeer. On Windows, the peer class is WlabelPeer, which implement label functionalities by JNI calls. These JNI methods are coded in C or C++. They do the actual work, interacting with a native label. Let's look at the figure. You can see that AWT components provide a universal public API to the application by AWT component class and AWT peers. A component class and its peer interface are identical across platform. Those underlying peer classes and JNI codes are different.

SWT

SWT implementation also utilize JNI methodology. But the detail is different from that of AWT. SWT evangelists often became furious when they heard people describing SWT as another AWT. Steve Northover, the father of SWT, once complained about this.

Yes, they are different. Let's delve into SWT code. In SWT, the only identical part on every platform is the component interface. That is class and method definition signature. All the underlying codes are different from platform to platform. SWT provides an OS class for every platform. This class encapsulates many native APIs by JNI methods. And then SWT component class glues these JNI method together to provide a meaning functionality.

For example, on Windows, text field selection can be conducted by only one system call. This system call is implemented in the Windows OS class as an native method. So there is only one JNI call in the setSelection method of Text on Windows.

However, on motif platform, text selection involves two native calls. Again SWT implements these two calls in the motif OS class. So the component class on motif needs to call these two calls to achieve text selection.

By now, you can see the major difference between SWT and AWT is that they use different peer code to wipe out the differences. SWT uses java code, or java peer to glue system calls implemented by JNI. However, AWT put these code in native peers, which complicates the situation. I think SWT's method is more clever.

Swing

When it comes to Swing, everything becomes clear and straight forward. Except the top containers, Swing implementation depends on nothing of individual platform. It has all the controls and resources. What Swing needs is event inputs to drive the system, and graphics, fonts and colors which are inherited from the top AWT containers. Ordinary Swing components can be seen as a logical area on AWT containers. They do not have a peer registered. All swing components added to a same top container share its AWT peer to acquire system resources, such as font, graphics etc. Swing has its own component data structure stored in JVM space. It manages drawing process, event dispatching and component layout totally by itself.

Resource Management

Because both AWT and SWT holds reference to native components, they must release them in a correct manner to avoid memory leaks and JVM crashes. AWT takes most of the resource management task to the system, relieving developers from tedious resource management. However this complicates the AWT implementation. Once it is implemented, developers has less opportunities to make errors and crash their applications.

SWT follows another way. In essence, SWT let the developers to manage those resources by themselves. There's a famous rule there. That is those who create the component should release it as well. Thus developers have to explicitly and carefully call the dispose method on every component or resource he has created. This greatly simplifies the SWT implementation model. But it puts the developers at the risk that they might easily crash their applications due to incorrect coding.

Emulation difference

Both Swing and SWT uses emulation in their implementation. SWT emulate those components which are missing from one platform. The difference is that SWT's emulation is much more like those of AWT Canvas. SWT has a Composite class which has a counterpart peer in the operating system. It gets all the resources it needs such as graphics object, font or color from its own peer. It gets all the events directly from the operating systems process it. However, swing component does not have a counterpart peer. It is only logical area of the top container. The resources it acquires from itself are in fact borrowed from those top containers' peer. As to event, swing event is not the event generated from the underlying system. It is in fact a pseudo event which is generated when the top container processes AWT event. We'll detail it later in the event parts.

Graphical Layer Architecture

The other difference is that swing components have its own separate z-order system from AWT components. As I mentioned above, swing components share a same peer with the top AWT container. Therefore, swing components have same z-order with the top container. SWT and AWT components each have a different z-order from the top container. So if AWT components and Swing components are mixed together, Swing components will probably be hidden by AWT components, because z-order values of AWT components are higher than the top container, while Swing components have the same z-order value with the top container. When operating system begin to update the UI, top container and swing components are always painted earlier than those of AWT. When they finished painting, AWT components will wipe out what swing has painted. So it is not encouraged to mix swing and AWT components together. If there are floating swing components such as menu, AWT component probably can hide menus.

Layout Manager

Not all of the elements of the three are different. Layout manager is an exception. What is layout manager? When developing a gui application, developers need to re-position or resize components when the container is resized. In traditional language, this is usually achieved by listening to resizing events. The code snippets usually scattered around and mess up the code. Java introduces the idea of wrapping layout codes together and name it Layout Manager. When a layout manager object is set to the container, it is automatically connected to resizing events. When resizing happens, layout method of the manger is called to re-position or reshape its children components.

AWT, SWT and Swing agree on this infrastructure. However every one has its own specific layout managers. Because AWT and Swing share a common parent class java.awt.Component, therefore AWT and Swing layout manger can work interchangeably.

(To be continued ...)