Applets

A Java applet must be run from within another program, called a host application. At this point, most host applications are Web browsers. The interaction between an applet and its host application is rather involved.

From the viewpoint of an applet, the interaction involves defining a subclass of the java.applet.Applet class. The Applet class defines a number of methods that control the applet. A subclass of Applet overrides one or more of the methods:

• init()
• The init() method is called to initialize the applet. Most initialization of an applet is done here instead of in a constructor because the constructor may be called before the hosting program is ready to provide all of the services needed for initialization.
• start()
• The start() method is called in a separate thread to tell the applet to start doing whatever it is supposed to do.
• paint()
• The paint() method is called at unpredictable times to draw the applet onto the screen.
• stop()
• The stop() method is called to tell the applet to stop doing whatever it does.
• destroy()
• The destroy() method is called to tell the applet to release any resources that it holds.

From the viewpoint of the host application, the interaction typically follows a standard sequence of events. The host application usually does the following:

1. Installs a SecurityManager object to implement a security policy.
2. Creates a ClassLoader object to load the applet.
3. Loads the applet and calls its default constructor.
4. Passes an AppletStub object to the applet's setStub() method.
5. Calls the applet's init() method in a separate thread.
6. Marks the applet as active.
7. Starts a new thread to run the applet's start() method.
8. Calls the applet's show() method, which makes the applet visible and causes the applet's paint() method to be called for the first time.
9. Calls the applet's paint() method whenever the applet needs to be refreshed.
10. Calls the applet's start() and stop() methods when the host wants the applet to start or stop. These methods are typically called when the applet is exposed or hidden.
11. Calls the applet's hide() method followed by its destroy() method when the host wants to shut down the applet.

Embedding an Applet in a Web Page

Web pages are written in a language called HTML. This explanation of how to embed an applet in a Web page assumes that you have some knowledge of basic HTML. An applet is embedded in a Web page using an <applet> tag. A minimal <applet> tag looks as follows:

<applet code=Clock height=300 width=350>
</applet> 

The code attribute of this sample <applet> tag specifies that the applet to be run is a class named Clock. The width and height attributes specify that the applet should be given a screen area that is 300 pixels high and 350 pixels wide.

The following list shows all of the attributes that can be specified in an <applet> tag. The attributes should be specified in the order in which they are listed. The code, height, and width attributes are required in an <applet> tag; the other attributes are optional:

• codebase
• The codebase attribute should specify a URL that identifies the directory used to find the .class files needed for the applet. Files for classes that belong to the default package should be in this directory. Files for classes that belong to named packages should be in subdirectories of this directory, where the relative path is specified by individual identifiers in the package name. If codebase is not specified, the <applet> tag uses the directory that contains the HTML file as a default.
• code
• The code attribute specifies the name of the class that implements the applet. If the applet is part of a named package, you must specify the fully qualified class name. So, if the name of the class is DataPlot and it is part of a package called COM.geomaker.graph, the value of the code attribute should be:

  code=COM.geomaker.graph.DataPlot.class 
  

  The browser locates the compiled code for the class by appending .class to the filename and searching the directory specified by the base URL for the document.

• object
• The object attribute specifies the name of a file that contains a serialized representation of an applet. If this attribute is specified, the applet is created by deserialization, rather than by calling its default constructor. The serialization is assumed to have occurred after the applet's init() method has been invoked, so the start() method is called instead of the init() method. Any attributes specified when the applet was serialized are not restored; the applet sees the attributes specified for this invocation.

  The object attribute is new as of Java 1.1. An <applet> tag must include either the code attribute or the object attribute, but it cannot include both.

• archive
• The archive attribute specifies a list of one or more archives that contain classes or other resources for an applet. Archives can be JAR or ZIP files. If this attribute is specified, the resources in the archives are loaded before the applet is run. If multiple archives are listed, they should be separated by commas. The archive attribute is new for Java 1.1.
• alt
• The alt attribute specifies the text that should be displayed by Web browsers that understand the <applet> tag but cannot run Java applets. If the text contains space characters, it should be enclosed in quotation marks.
• name
• The name attribute specifies a name for a particular instance of an applet. An applet can get a reference to another applet on the same Web page using the getApplet() method.
• width
• The width attribute specifies the width of the applet in pixels.
• height
• The height attribute specifies the height of the applet in pixels.
• align
• The align attribute specifies the positioning of the applet. The possible values are: left, right, top, texttop, middle, absmiddle, baseline, bottom, or absbottom.
• vspace
• The vspace attribute specifies the amount of vertical space above and below the applet in pixels.
• hspace
• The hspace attribute specifies the amount of horizontal space to the left and right of the applet in pixels.

Applet-specific parameters can be provided to an applet using <param> tags inside the <applet> tag. A <param> tag must specify name and value attributes. For example:

<param name=speed value=65> 

If a Web browser does not support the <applet> tag, it ignores the tag and simply displays any HTML content provided inside the tag. However, if the browser understands the <applet> tag, this HTML content is ignored. This means that you can provide HTML content inside an <applet> tag to inform users of non-Java-enabled browsers about what they are missing.

Here is an example that combines all of these elements:

<applet code=Compass height=400 width=300>
<param name=direction value=north>
<param name=speed value=65>
<p>
<i>If you can see this message, your Web browser is not Java enabled. There is a Java applet on this Web page that you are not seeing.</i>
<p>
</applet> 

If a non-Java-enabled browser is used to view this HTML file, the following text is displayed:

If you can see this message, your Web browser is not Java-enabled. There is a Java applet on this Web page that you are not seeing.