Swing (Java)

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This article is about the GUI toolkit for Java. For other uses, see Swing.
Example Swing widgets in Java 5.0+ for the X Window System. Font rendering may vary on different platforms.
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Example Swing widgets in Java 5.0+ for the X Window System. Font rendering may vary on different platforms.

Swing is a GUI toolkit for Java. It is one part of the Java Foundation Classes (JFC). Swing includes graphical user interface (GUI) widgets such as text boxes, buttons, split-panes, and tables.

Swing widgets provide more sophisticated GUI components than the earlier Abstract Window Toolkit. Since they are written in pure Java, they run the same on all platforms, unlike the AWT which is tied to the underlying platform's windowing system. Swing supports pluggable look and feel – not by using the native platform's facilities, but by roughly emulating them. This means you can get any supported look and feel on any platform. The disadvantage of heavyweight components is slower execution. The advantage is uniform behavior on all platforms.

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[edit] History

The Internet Foundation Classes (IFC) were a graphics library for Java originally developed by Netscape Communications Corporation and first released on December 16, 1996.

On April 2, 1997, Sun Microsystems and Netscape Communications Corporation announced their intention to combine IFC with other technologies to form the Java Foundation Classes. In addition to the components originally provided by IFC, Swing introduced a mechanism that allowed the look and feel of every component in an application to be altered without making substantial changes to the application code. The introduction of support for a pluggable look and feel allowed Swing components to emulate the appearance of native components while still retaining the benefits of platform independence. This feature also makes it easy to have an individual application's appearance look very different from other native programs.

Originally distributed as a separately downloadable library, Swing has been included as part of the Java Standard Edition since release 1.2. The Swing classes are contained in the javax.swing package hierarchy.

[edit] Architecture

Swing is a platform independent, component-oriented, extensible, customizable, configurable, loosely-coupled, light-weight UI, event dispatching, MVC GUI framework, with a single-threaded programming model, for the Java system.

  • Platform independence: Swing is platform independent both in terms of its expression (Java) and its implementation (non-native universal rendering of widgets).
  • Extensible: Swing is a highly partitioned architecture which allows for the 'plugging' of various custom implementations of specified framework interfaces: Users can provide their own custom implementation(s) of these components to override the default implementations. In general, Swing users can extend the framework by: extending existing (framework) classes; providing alternative implementations of core components.
  • Component-Oriented: Swing is a component based framework. The distinction between objects and components is a fairly subtle point: concisely, a component is a well-behaved object with a known/specified characteristic pattern of behaviour. Swing objects asynchronously fire events, have 'bound' properties, and respond to a well known set of commands (specific to the component.) Specifically, Swing components are Java Beans components, compliant with the Java Beans Component Architecture specifications.
  • Customizable: Given the programmatic rendering model of the Swing framework, fine control over the details of rendering of a component is possible in Swing. As a general pattern, the visual representation of a Swing component is a composition of a standard set of elements, such as a 'border', 'inset', decorations, etc. Typically, users will programmatically customize a standard Swing component (such as a JTable) by assigning specific Borders, Colors, Backgrounds, etc., as the properties of the component. The core component will then use these property (settings) to determine the appropriate renderers to use in painting its various aspects. However, it is also completely possible to create unique GUI controls with highly customized visual representation. (Swing components support transparent rendering.)
  • Configurable: Swing's heavy reliance on runtime mechanisms and indirect composition patterns allow it to respond at runtime to fundamental changes in its settings. For example, a Swing-based application can change its look and feel at runtime (from say MacOS look and feel to a Windows XP look and feel). Further, users can provide their own look and feel implementation, which allows for uniform changes in the look and feel of existing Swing applications without any programmatic change to the application code.
  • Lightweight UI: The magic of Swing's configurability is also due to the fact that it does NOT use the native host OS's GUI controls for representation, but rather 'paints' its controls programmatically, through the use of Java 2D apis. Thus, a Swing component does NOT have a corresponding native OS GUI 'peer', and is free to render itself in anyway that is possible with the graphics APIs.

However, at its core, every Swing component relies on an AWT container, since (Swing's) JComponent extends (AWT's) Container. This allows Swing to plug into the host OS's GUI management framework, including the crucial device/screen mappings and user interactions (such as key presses, mouse movements, etc.) Swing simply 'transposes' its own (OS agnostic) semantics over the underlying (OS specific) components. So, for example, every Swing component paints its rendition on the graphic device in response to a call to component.paint(), which is defined in (AWT) Container. But unlike AWT components, which delegated the painting to their OS-native "heavy weight" widget, Swing components are responsible for their own rendering.

Further, this transposition and decoupling is not merely visual, and extends to the Swing's management of, and application of its own OS-indepenent semantics for, events fired within its component containment hierarchies.

Overall, the Swing Architecture delegates the task of mapping the various flavors of OS GUI semantics onto a simple, but generalized pattern to the AWT container. Then, building on that generalized platform, it establishes its own rich and complex GUI semantics, in form of the JComponent model.

(The interested reader is encouraged to review the source of Container.java and JComponent.java classes for further insights into the nature of the interface between Swing's lightweight components and AWT's heavyweight widgets.)

  • Loosely-Coupled/MVC: The Swing library makes heavy use of the Model/View/Controller software design pattern, which conceptually decouples the data being viewed from the user interface controls through which it is viewed. Because of this, most Swing components have associated models (which are specified in terms of Java interfaces), and the programmer can use various default implementations or provide their own. The framework provides default implementations of model interfaces for all of its concrete components.

Typically, Swing component model objects are responsible for providing a concise interface defining events fired, and accessible properties for the (conceptual) data model for use by the associated JComponent. Given that the overall MVC pattern is a loosely-coupled collaborative object relationship pattern, the model provides the programmatic means for attaching event listeners to the data model object. Typically, these events are model centric (ex: row inserted event in a table model) and are mapped by the JComponent specialization into a meaningful event for the GUI component.

For example, the JTable has a model called TableModel that describes an interface for how a table would access tabular data. A default implementation of this operates on a two-dimensional array.

The view component of a Swing JComponent is the object used to graphically 'represent' the conceptual GUI control. A distinction of Swing, as a GUI framework, is in its reliance on programmatically rendered gui controls (as opposed to the use of native host OS's GUI controls). (This distinction is a source of complications when mixing AWT controls, which use native controls, with Swing controls in a GUI.)

It must be noted that the typical use of Swing framework does not require the creation of custom models, as the framework provides a set of default implementations that are transparently, by default, associated with the corresponding JComponent child class in the Swing library. In general, only complex components such as tables and collection views may require customization of the default models provided.

(However, to get a good sense of the potential that the Swing architecture makes possible, consider the hypotethical situation where custom models for tables and lists are wrappers over DAO and/or EJB services.)

Finally, in terms of visual composition and management, Swing favors relative layouts (which specify the positional relationships between components) as opposed to absolute layouts (which specify the exact location and size of components). This bias towards 'fluid' visual ordering is a direct legacy of the development of Swing from the ashes of AWT (framework) and the associated assumptions regarding the Applet operating environment that framed the design and development of the original Java gui toolkit.

(Conceptually, this view of the layout management is quite similar to that which informs the rendering of HTML content in browsers, and addresses the same set of concerns that motivated the former.)

[edit] Look and feel

Swing allows one to specialize the look and feel of widgets, by modifying the default (via runtime parameters), deriving from an existing one, by creating one from scratch, or, beginning with J2SE 5.0, by using the skinnable Synth Look and Feel, which is configured with an XML property file. The look and feel can be changed at runtime, and early demonstrations of Swing would frequently provide a way to do this.

[edit] Relationship to AWT

Since early versions of Java, a portion of the Abstract Windowing Toolkit (AWT) has provided platform independent APIs for user interface components. In AWT, each component is rendered and controlled by a native peer component specific to the underlying windowing system.

By contrast, Swing components are often described as lightweight because they do not require allocation of native resources in the operating system's windowing toolkit. The AWT components are referred to as heavyweight components.

Much of the Swing API is generally a complementary extension of the AWT rather than a direct replacement. In fact, every Swing lightweight interface ultimately exists within an AWT heavyweight component because all of the top-level components in Swing (JApplet, JDialog, JFrame, and JWindow) extend an AWT top-level container. The core rendering functionality used by Swing to draw its lightweight components is provided by Java2D, another part of JFC. However, the use of both lightweight and heavyweight components within the same window is generally discouraged due to Z-order incompatibilities.

[edit] Relationship to SWT

The Standard Widget Toolkit (SWT) is a competing toolkit originally developed by IBM and now maintained by the Eclipse Foundation. SWT's implementation has more in common with the heavyweight components of AWT. This confers benefits such as more accurate fidelity with the underlying native windowing toolkit, at the cost of an increased exposure to the native platform in the programming model.

The advent of SWT has given rise to a great deal of division among Java desktop developers with many strongly favouring either SWT or Swing. Sun's development on Swing continues to focus on look and feel (PLAF) fidelity with each platform's windowing toolkit in the approaching Java SE 7 release (as of December 2006). In the meantime there are other sources of high-fidelity PLAFs, many of which are collected on the javootoo site.

There has been significant debate and speculation about the performance of SWT versus Swing; SWT's heavy dependence on JNI makes it slower when the GUI component and Java need to communicate data, but faster at rendering when the data model has been loaded into the GUI.

SWT serves the Windows platform very well but is considered by some to be less effective as a technology for cross-platform development. By using the high-level features of each native windowing toolkit, SWT returns to the issues seen in the mid 90's (with toolkits like zApp, Zinc, XVT and IBM/Smalltalk) where toolkits attempted to mask differences in focus behaviour, event triggering and graphical layout. Failure to match behavior on each platform can cause subtle but difficult to resolve bugs that impact user interaction and the appearance of the GUI.

[edit] Example

The following is a Hello World program using Swing.

import javax.swing.JFrame;
import javax.swing.JLabel;
import java.awt.EventQueue; 

public final class HelloWorld {

    public static void main(String[] args) {
        // Swing has its own dispatch thread, which is 
        // distinct from the main JVM (launcher) thread.  
        // This means that even if the (launcher) thread exited 
        // (this) main method, the Swing GUI thread would
        // still be running, waiting to respond to user input, etc.
        // and if the user closes the window, then the program would
        // continue to run (due to the live gui thread). As of Java 1.4, the
        // GUI thread will automatically stop if all Components are hidden and
        // disposed.
        //
        // Execute all GUI code on the event dispatch thread, even initialization:
        EventQueue.invokeLater(new Runnable() { public void run() {

            // Create frame with title "Hello, World!"
            JFrame f = new JFrame("Hello, World!");

            // Previously we typically attached a window listener to our main JFrame
            // and on windowClosing() event notification callback, we would
            // explicitly call System.exit(stat) or some other such nastiness.
            // The following is the new (clean) way of defining default close behavior
            // for a JFrame.  The following will merely dispose the JFrame but it will not
            // stop the application unless *all* Windows are disposed.

            f.setDefaultCloseOperation (JFrame.DISPOSE_ON_CLOSE);
 
            // this stops the app on window close
            // (it is commented out.)
            //            f.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
 
            // JFrame, which is the 'window' component of Swing, has an interesting feature:
            // it has a layered content architecture which (conceptually) allows for placement
            // of a component of the JFrame in a designated layer.  The layers can be conceptually
            // viewed as a series of transparencies placed one above another in a overhead projector
            // Swing typically places user components (such as buttons, etc.) in what it calls its
            // 'content pane'.  This pane is layer that is below most of the other (transparent) layers.
            // What are the layers for?  Think of when you right-click on a control and it 'pops' a 
            // contextual menu 'over' the original control; or, when you press 'help' for a control or
            // hover the mouse for a 'tool tip' and the information is 'placed over the control'.  How is that
            // done?  By rendering the information in a layer 'above' the lower level 'content' pane.
            // So:
            // The standard way of adding a component to a 'multi-pane' container such as JFrame is by first
            // establishing a reference to its 'content pane' (which is, by default just a JPanel with
            // BorderLayout and adding the components the JFrame to the content pane and NOT directly 
            // to the JFrame.  (The latest Swing releasehowever alters the semantics of the JFrame.add(..)
            // to assume convenient addition to the content pane).
 
            f.getContentPane().add(new JLabel("Hello, World!"));
 
            // Remember the relative layouts of Swing?  
            // pack() tells the receiving container (here 'this' JFrame)
            // for it to optimally pack its components according to its layout.
            f.pack();

            f.setVisible(true);
        } });
    }
}

[edit] See also

[edit] References

[edit] External links

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