Chapter 23. Applets

One of the original promises of Java was that applications could be delivered over the network to your computer as needed. Instead of the old days of buying a shrink-wrapped box containing a word processor, installing it, and upgrading it every few years, it would now be possible to obtain and use software directly from the Internet, safely and on any platform. Today, we take networked distribution of software for granted. Mobile devices have driven the advent of app stores for installed software, and many (if not the majority) of the most highly used applications are now purely browser-based. Unfortunately for Java fans, these advances took different paths, arguably less elegant, than the Java platform offered and took longer to arrive than they likely could have.

This chapter is about the applet API, which was Java’s earliest mechanism for delivering applications to the web browser. Applets are not widely used today, but they are part of the vernacular and are still doing well in some niches. While you may not wish to write applets going forward, it is useful to understand their basic functionality. As we review applets, we will also touch on general areas of interest along the way, such as the Java security model.

An applet is a Java program that runs within the context of a web page. Like an image or hyperlink, it “owns” some rectangular area of the user’s screen. When the web browser loads a page that contains a Java applet, it knows how to load the classes of the applet and run them. This chapter describes how applets work and how they are incorporated into web pages. We’ll also talk about the Java Plug-in and related technologies such as Java Web Start briefly.

The Politics of Browser-Based Applications

First, a bit of history. The potential for applets to add dynamic content to web pages was one of the driving forces behind the spread of the Java programming language. Prior to Java’s introduction in 1994, there was really no standard way to do this; even the now-ubiquitous animated GIF images were not yet widely supported. Sun’s HotJava Java-based web browser was the first to support applets. It was Java’s original “killer application.” Later, in 1995, Netscape announced that it would support the Applet API in its browsers, and soon after that Microsoft jumped on the bandwagon. For a while, it seemed that Java would power the future of the Web, but there were many bumps in the road to come.

Many problems, both technical and political, plagued the early years of Java’s use in browsers and client-side applications. Performance issues were to be expected in such a young platform. But what really crippled Java early on was the nonportable and buggy AWT, Java’s original GUI toolkit. Many people overlook the fact that Java’s success as a portable language is in large part a result of just how much of the Java API is implemented in Java. You might be surprised to learn just how many Java internals involve no native code—everything from cryptography to DNS is done in Java—requiring no porting for new platforms. Similarly, the renaissance of Java GUI applications seen in later years was due almost entirely to the introduction of the pure Java Swing GUI toolkit. In contrast, the original AWT system was based on native code, which had to be ported to each new system, taking into account subtle and tricky platform dependencies. AWT was effectively a large, graphical C program that Java talked to through a set of interfaces and Java was, to some extent, unfairly painted as nonportable and buggy by association.

Java faced other, less technical obstacles as well. Netscape foisted the original AWT upon the world when it insisted that Java be released with “native look and feel” in lieu of a less capable, but truly portable, graphical toolkit. It later introduced a pure Java GUI toolkit called IFC (an ancestor of Swing), but it struggled to gain traction. Later, Microsoft effectively stuck us with AWT by freezing the level of the Applet API in its browsers at Java 1.1. Applets languished with poor GUIs while lawsuits between Sun and Microsoft dragged on in the 1990s. The result was that support for applets in web browsers remained a mess.

Sun made an attempt to insulate Java from the browser battles with the introduction of the Java Plug-in. The Plug-in allowed applets to run in an up-to-date Java VM, but by this point lacked the critical mass and near-universal adoption necessary to make such a plug-in useful. The weaker but more prevalent Adobe Flash plug-in flourished, as developers desperately wanted a way to deploy richer applications to the browser. In the 2000s, JavaScript and HTML matured greatly and made the browser itself more of a viable platform for deploying software, leading to the great irony that JavaScript, the slower and less capable namesake of Java, became the de facto programming language of the Web.

Find it a bit depressing? Well, take heart; JavaScript has evolved into a worthy platform for the client side, Flash is fading, and Java has dominated server-side development and made possible portable libraries that have fueled much of the Internet revolution. We cannot say what the future holds, but Java in some form will likely be a player in it for many decades to come.

Applet Support and the Java Plug-in

As we mentioned earlier, the state of support for Java in web browsers has always been a mess. The most reliable way to run Java in a web browser has always been the Java Plug-in. The Plug-in is a free, user-installed component (implemented differently for each browser) that supports Java itself. Using the Plug-in makes an end-run around the poor support built into the browser by using a separate software package that can be installed and updated independently of the browser releases.

Recently, some platforms—notably Mac OS X—have dropped the built-in support for Java in the browser entirely and rely on users to install the Java Plug-in if they wish to run applets. This is not ideal, but does simplify things by eliminating some of the ambiguity. If you have installed Java 7 on OS X or Windows, then you should already have the Java Plug-in needed to run the applets discussed in this chapter. In some cases, the first time you attempt to view a page containing an applet, you may be prompted to enable Java in the browser. You may also want to consult the Java “control panel” in Windows or “preference pane” in Mac OS X for additional preferences related to Java after installing Java 7.

The JApplet Class

A JApplet is a Swing JPanel with a mission. It is a GUI container that has some extra structure to allow it to be used in the “alien” environment of a web browser. Applets also have a lifecycle that lets them act more like an application than a static component. Although applets tend to be relatively simple, there’s no inherent restriction on their complexity other than the issues of downloading and caching their content. Historically, applets have tended to be small “widgets.”

The javax.swing.JApplet class defines the core functionality of an applet. (java.awt.Applet is the older, AWT-based form.)

Structurally, an applet is a wrapper for your Java code. In contrast to a standalone graphical Java application, which starts up from a main() method and creates a GUI, an applet itself is a component that expects to be dropped into someone else’s GUI. Thus, an applet can’t run by itself; it runs in the context of a web browser or a special applet-viewer program (which we’ll talk about later). Instead of having your application create a JFrame to hold your GUI, you stuff your application inside a JApplet (which itself extends Container) and let the browser add your applet to the page.

Applets are placed on web pages with the <applet> HTML tag, which we’ll cover later in this chapter. At its simplest, you just specify the name of the applet class and a size in pixels for the applet:

    <applet code="AnalogClock" width="100" height="100"></applet>

Pragmatically, an applet is an intruder into someone else’s environment and therefore has to be treated with suspicion. The web browsers that run applets impose restrictions on what the applet is allowed to do. The restrictions are enforced by an applet security manager. The browser provides everything the applet needs through an applet context—the API the applet uses to interact with its environment.

A JApplet expects to be embedded in a page and used in a viewing environment that provides it with resources. In all other respects, however, applets are just ordinary Panel objects. As Figure 23-1 shows, an applet is a kind of Panel. Like any other Panel, a JApplet can contain user interface components and use all the basic drawing and event-handling capabilities of the Component class. You can draw on a JApplet by overriding its paint() method and respond to events in the JApplet’s display area by providing the appropriate event listeners. Applets also have additional structure that helps them interact with the browser environment.

Aside from the top-level structure and the security restrictions, there is no difference between an applet and an application. If your application can live within the limits imposed by a browser’s security manager, you can structure it to function as both an applet and a standalone application. Normally, you’ll use your applet class itself only as a thin wrapper to manage the lifecycle and appearance of your application—create the GUI, start, and stop. So, the bulk of your code should be easily adaptable to either a standalone or applet deployment.

The java.applet package
Figure 23-1. The java.applet package

Applet Lifecycle

The Applet class contains four methods that can be overridden to guide it through its lifecycle. The init(), start(), stop(), and destroy() methods are called by the appletviewer or web browser to direct the applet’s behavior. init() is called once, after the applet is created. The init() method is where you perform basic setup such as parsing parameters, building a user interface, and loading resources.

By convention, applets don’t provide an explicit constructor to do any setup. The reason for this is that the constructor is meant to be called by the applet’s environment, for simple creation of the applet. This might happen before the applet has access to certain resources, such as information about its environment. Therefore, an applet doesn’t normally do any work there; instead it should rely on the default constructor for the JApplet class and do its initialization in the init() method.

The start() method is called whenever the applet becomes visible; it shouldn’t be a surprise then that the stop() method is called whenever the applet becomes invisible. init() is called only once in the life of an applet, but start() and stop() can be called any number of times (although always in the logical sequence). The start() method is called when the applet is displayed, such as when it scrolls onto the screen; stop() will be called if the applet scrolls off the screen, or the viewer leaves the document. start() tells the applet it should be active. The applet may want to create threads, animate, or otherwise perform useful (or annoying) activity. stop() is called to let the applet know it should go dormant. Applets should cease CPU-intensive or wasteful activity when they are stopped and resume when (and if) they are restarted. However, there’s no requirement that an invisible applet stop computing; in some applications, it may be useful for the applet to continue running in the background. Just be considerate of your user, who doesn’t want an invisible applet dragging down system performance.

Finally, the destroy() method gives the applet a last chance to clean up before it’s removed—some time after the last call to stop(). For example, an applet might want to gracefully close down suspended communications channels at this time. Exactly when destroy() is called depends on the browser; Netscape calls destroy() just prior to deleting the applet from its cache. This means that although an applet can cling to life after being told to stop(), how long it can go on is unpredictable. If you want to maintain an applet as the user progresses through other pages of activities, you may have to put it in an HTML frame, so that it remains visible and won’t be told to stop() (see “Applet persistence and navigation”).

If you’ve read this entire book up until now, you’ve already seen a couple of applets that snuck in among other topics. In Chapter 9, we created a simple clock applet, and in Chapter 13, we used an applet to send packets of information from a web browser. Now let’s try a simple Swing-based example using JApplet. The following example, ShowApplet, shown in Figure 23-2, does nothing special, but you can use it to test the version of Java that’s running in your browser (and see if the Plug-in is installed) and to see when the applet is started and stopped. It’s a good reference.

    import javax.swing.*;
    import java.awt.event.*;

    public class ShowApplet extends JApplet {
       JTextArea text = new JTextArea();
       int startCount;

       public void init() {
          JButton button = new JButton("Press Me");
          button.addActionListener( new ActionListener() {
             public void actionPerformed( ActionEvent e ) {
                text.append("Button Pressed!\n");
             }
          } );
          getContentPane().add( "Center", new JScrollPane( text ) );
          JPanel panel = new JPanel();
          panel.add( button );
          getContentPane().add( "South", panel );
          text.append( "Java Version: "
             +System.getProperty("java.version")+"\n" );
          text.append( "Applet init()\n" );
       }
       public void start() {
          text.append( "Applet started: "+ startCount++ +"\n" );
       }
       public void stop() {
          text.append( "Applet stopped.\n" );
       }
    }
ShowApplet
Figure 23-2. ShowApplet

After compiling the applet, we have to create an HTML page in which to embed it. The following will do:

    <html><head><title>ShowApplet</title></head>
    <body>
    <applet code="ShowApplet" width="300" height="300">
       Your browser does not understand Java.</applet>
    </body>
    </html>

We’ll discuss the applet tag and other issues related to embedding applets in documents in detail later in this chapter. For now, just save this in a file called showapplet.html. Load the file with your favorite web browser and see what happens. (We’re assuming you have installed Java on your computer by this point; you may have to enable Java in your browser to see the applet if it is disabled by default.) If you have access to a web server, you can use it. Otherwise, you can open the file locally using either the browser’s Open File menu option or a URL such as:

    file:///Users/somedir/showapplet.html

The applet shows the version of Java running it and prints messages when its button is pressed. If you have installed the latest Java Plug-in you should see “Java version: 1.7” in the box, regardless of which browser you are using (including Microsoft Internet Explorer). The applet prints messages when its start() and stop() methods are called, along with a count. You can use this to experiment with different browsers and page-layout configurations to see when your applet is reloaded or restarted. If your browser fails to display the applet with the correct version of Java, don’t worry. Later in this chapter, we’ll talk about how to convert the HTML to force the browser to use the Java Plug-in explicitly.

The Applet Security Sandbox

Applets are quarantined within the browser by an applet SecurityManager. The SecurityManager is part of the web browser or appletviewer application. It is installed before the browser loads any applets and implements the basic restrictions that let the user run untrusted applets (loaded over the Internet) safely. Remember, there are no inherent security restrictions on a standalone Java application. It is the browser that limits what applets are allowed to do using a security policy.

Most browsers impose the following restrictions on untrusted applets:

  • Untrusted applets can’t read or write files on the local host.

  • Untrusted applets can open network connections (sockets) only to the server from which they originated.

  • Untrusted applets can’t start other processes on the local host.

  • Untrusted applets can’t have native methods.

The motivation for these restrictions should be fairly obvious: you clearly wouldn’t want a program coming from some random Internet site to access your files or run arbitrary programs. Although untrusted applets can’t directly read and write files on the client side or talk to arbitrary hosts on the network, applets can work with servers to store data and communicate. For example, an applet can use Java’s RMI facility to do processing on its server. An applet can communicate with other applets on the Net by proxy through its server.

Trusted applets

We’ve been using the term untrusted applet, so it should come as no surprise that it is also possible to have such a thing as a trusted applet. Applets become trusted through the use of digital signatures, by signing the JAR file containing your applet code. Because a signature identifies the applet’s origin unambiguously, the user can distinguish between trusted applets (i.e., applets that come from a site or person you trust not to do anything harmful) and run-of-the-mill untrusted applets. In browser environments that support signing, trusted applets can be granted permission to “go outside” of the applet security sandbox. Trusted applets can be allowed to do all of the things that standalone Java applications can do: read and write files, open network connections to arbitrary machines, and interact with the local operating system by starting processes. Trusted applets still can’t have native methods, but including native methods in an applet would destroy its portability anyway.

Because signed applets are now a fairly niche topic, we no longer cover them in this chapter. If you need more details on them, please visit the “extras” page for this book, where we post additional material not included in the book as well as the example source code.

Getting Applet Resources

An applet must communicate with its browser or applet viewer. For example, it may need configuration parameters from the HTML document in which it appears. An applet may also need to load images, audio clips, and other items. It may also want to ask the viewer about other applets on the same HTML page in order to communicate with them. To get resources from the environment, applets use the AppletStub and AppletContext interfaces, provided by the browser.

Applet parameters

An applet can get configuration parameters from <param> tags placed inside the <applet> tag in the HTML document, as we’ll describe later. You can retrieve these parameters using Applet’s getParameter() method. For example, the following code reads parameters called imageName and sheep from its HTML page:

    String imageName = getParameter( "imageName" );
    try  {
        int numberOfSheep = Integer.parseInt( getParameter( "sheep" ) );
    } catch ( NumberFormatException e ) { /* use default */ }

There is an API that allows an applet to provide information (help) about the parameters it accepts. The applet’s getParameterInfo() can return an array of string arrays, listing and describing the applet’s parameters. However, it’s unclear that anyone uses this API.

Applet resources

An applet can find out where it lives using the getDocumentBase() and getCodeBase() methods. getDocumentBase() returns the base URL of the document in which the applet appears; getCodeBase() returns the base URL of the Applet’s class files (these two are often the same). An applet can use these methods to construct relative URLs from which to load other resources from its server, such as images, sounds, and other data. The getImage() method takes a URL and asks for an image from the viewer environment. The image may be cached or loaded when later used. The getAudioClip() method, similarly, retrieves sound clips.

The following example uses getCodeBase() to construct a URL and load a properties configuration file, located in the same remote directory on the web server as the applet’s class file:

    Properties props = new Properties();
    try {
      URL url = new URL( getCodeBase(), "appletConfig.props" );
      props.load( url.openStream() );
    } catch ( IOException e ) { /* failed */ }

A better way to load resources is by calling the getResource() and getResourceAsStream() methods of the Class class, which search the applet’s JAR files (if any) as well as its codebase, which is an extension of the classpath for applets. The following code loads the same properties file in a more portable way:

    Properties props = new Properties();
    try {
      props.load( getClass().getResourceAsStream( "appletConfig.props") );
    } catch ( IOException e ) { /* failed */ }

An applet can ask its viewer to retrieve an image by calling the getImage() method. The location of the image to be retrieved is given as a URL, either absolute or fetched from an applet’s resources:

    public class MyApplet extends javax.swing.JApplet {
      public void init() {
        try {
          // absolute URL
          URL monaURL =
              new URL( "http://myserver/images/mona_lisa.gif");
          Image monaImage = getImage( monaURL );
          // applet resource URL
          URL daffyURL =
              getClass().getResource("cartoons/images/daffy.gif");
          Image daffyDuckImage = getImage( daffyURL );
        }
        catch ( MalformedURLException e ) {
            // unintelligable url
        }
      }
      // ...
    }

Again, using getResource() is preferred; it looks for the image in the applet’s JAR file (if there is one), before looking elsewhere in the applet’s classpath on the server. (We’ll talk more later about how classes are located for applets.)

Driving the browser

The status line is a blurb of text that usually appears somewhere in the web browser’s display, indicating a current activity. An applet can request that some text be placed in the status line with the showStatus() method. (The browser isn’t required to do anything in response to this call, but most browsers will oblige you.)

An applet can also ask the browser to show a new document. To do this, the applet makes a call to the showDocument(url) method of the AppletContext. You can get a reference to the AppletContext with the applet’s getAppletContext() method. Calling showDocument(url) replaces the currently showing document, which means that your currently running applet will be stopped.

Another version of showDocument() takes an additional String argument to tell the browser where to display the new URL:

    getAppletContext().showDocument( url, name );

The name argument can be the name of an existing labeled HTML frame; the document referenced by the URL is displayed in that frame. You can use this method to create an applet that “drives” the browser to new locations dynamically but keeps itself active on the screen in a separate frame. This is common for applets that act like navigation controls or menus. If the named frame doesn’t exist, the browser creates a new top-level window to hold it. Alternatively, name can have one of the following special values:

self

Show in the current frame

_parent

Show in the parent of our frame

_top

Show in outermost (top-level) frame

_blank

Show in a new top-level browser window

Both showStatus() and showDocument() requests may be ignored by a cold-hearted viewer or web browser. Nothing in browser-land is ever certain.

Inter-applet communication

Although it’s not very common, applets that are embedded in documents loaded from the same location on a website can use a simple mechanism to locate one another and coordinate their activities on a page. Once an applet has a reference to another applet, it can communicate with it just as with any other object, by invoking methods and sending events. The getApplet() method of the applet context looks for an applet by name:

    Applet clock = getAppletContext().getApplet("theClock");

Give an applet a name within your HTML document using the name attribute of the <applet> tag. Alternatively, you can use the getApplets() method to enumerate all the available applets in the pages.

The tricky thing with applet communications is that applets run inside the security sandbox. An untrusted applet can “see” and communicate only with objects that were loaded by the same class loader. Currently, the only reliable criterion for when applets share a class loader is when they share a common base URL. For example, all the applets contained in web pages loaded from the base URL of http://foo.bar.com/mypages/ should share a class loader and should be able to see each other. This includes documents such as mypages/foo.html and mypages/bar.html, but not mypages/morestuff/foo.html.

When applets do share a class loader, other techniques are possible, too. As with any other class, you can call static methods in applets by name. So you could use static methods in one of your applets as a “registry” to coordinate your activities.

Applet persistence and navigation

One of the biggest shortcomings of the Applet API is the lack of a real context for coordinating their activities during navigation across a multipage document or web application. The Applet API simply wasn’t designed for this. Although an applet’s life cycle is well defined in terms of its API, it is not well defined in terms of management by the browser or scope of visibility. As we described in the previous section, applets loaded from the same codebase can rendezvous at runtime using their name attributes. But there are no guarantees about how long an applet will live—or whether it will be stopped as opposed to being destroyed—once it is out of view. If you experiment with our ShowApplet in various browsers and in the Java Plug-in (which we’ll discuss later), you’ll see that, in some cases, the applet is stopped and restarted when the user leaves the page, but more often the applet is reinitialized from scratch. This makes designing multipage applications that work in all browsers difficult.

One solution has been to use static methods as a shared “registry,” as mentioned earlier. However, the details governing how classes loaded by applets are managed are even less well-defined than the management of the applets themselves. In Java 1.4, a pair of methods was added to the AppletContext to support short-term applet persistence: setStream() and getStream(). With these methods, an applet can ask the context to save a stream of byte data by a key value and return it later. The notion of providing the state to the context as a stream is a little odd, but easy enough to accommodate. Here is an example:

    getAppletContext.setStream("myStream",
        new ByteArrayInputStream( "This is some test data...".getBytes() ) );

Later, the stream data can be retrieved:

    InputStream in = getAppletContext.getStream( "myStream" );

Currently, the data is retained only as long as the browser is running. If you need more complex state and navigation capabilities, you might consider using a signed applet to write to a file or taking advantage of the Java Web Start API to install your application locally.

The <applet> Tag

Applets are embedded in HTML documents with the <applet> tag. The <applet> tag resembles the HTML <img> image tag. It contains attributes that identify the applet to be displayed and, optionally, give the web browser hints about how it should be shown.[48]

The standard image tag sizing and alignment attributes, such as height and width, can be used inside the applet tag. However, unlike images, applets have both an opening <applet> and a closing </applet> tag. Sandwiched between these can be any number of <param> tags that contain configuration data to be passed to the applet:

    <applet attributeattribute ... >
        <param parameter >
        <param parameter >
        ...
    </applet>

Attributes

Attributes are name/value pairs that are interpreted by a web browser or applet viewer. Attributes of the <applet> tag specify general features that apply to any applet, such as size and alignment. The definition of the <applet> tag lists a fixed set of recognized attributes; specifying an incorrect or nonexistent attribute should be considered an HTML error.

Three attributes are required in the <applet> tag. Two of these attributes, width and height, specify the space that the applet occupies on the screen. The third required attribute must be either code or object; you must supply one of these attributes, and you can’t specify both. The code attribute specifies the class file from which the applet is loaded; the object attribute specifies a serialized representation of an applet. Most often, you’ll use the code attribute.

The following is an HTML fragment for a simple clock applet that takes no parameters and requires no special HTML layout:

    <applet code="AnalogClock" width="100" height="100"></applet>

The HTML file that contains this <applet> tag must be stored in the same directory as the AnalogClock.class class file. The applet tag is not sensitive to spacing in the HTML, so the previous tag could be also be formatted a little more readably like so:

    <applet
        code="AnalogClock"
        width="100"
        height="100">
    </applet>

Parameters

Parameters are analogous to command-line arguments; they provide a way to pass information to an applet. Each <param> tag contains a name and a value that are passed as string values to the applet:

    <param name = "parameter_name" value = "parameter_value">

Parameters provide a means of embedding application-specific data and configuration information within an HTML document. Our AnalogClock applet, for example, might accept a parameter that selects between local and universal time:

    <applet code="AnalogClock" width="100" height="100">
        <param name="zone" value="GMT">
    </applet>

Presumably, this AnalogClock applet is designed to look for a parameter named zone with a possible value of GMT.

Parameter names and values should be quoted and can contain spaces and other whitespace characters. The parameters a given applet expects are, of course, determined by the developer of that applet. There is no standard set of parameter names or values; it’s up to the applet to interpret the parameter name/value pairs that are passed to it. Any number of parameters can be specified, and the applet may choose to use or ignore them as it sees fit.

¿Habla Applet?

Web browsers are supposed to ignore tags they don’t understand; if the web browser doesn’t know about the <applet> or <param> tags, we would expect them to disappear, and any HTML between the <applet> and </applet> tags to appear normally. By convention, Java-enabled web browsers ignore any extra HTML between the <applet> and </applet> tags. Combined, this means we can place some alternative HTML inside the <applet> tag, which is displayed only by web browsers that can’t run the applet.

For our AnalogClock example, we could display a small text explanation and an image of the clock applet as a teaser:

    <applet code="AnalogClock" width="100" height="100">
        <param name="zone" value="GMT">
        <strong>If you see this, you don't have a Java-enabled Web
        browser. Here's a picture of what you are missing.</strong>
        <img src="clockface.gif">
    </applet>

The Complete <applet> Tag

We’ll now spell out the complete syntax for the <applet> tag:

    <applet
        code = class_name

or:

        object = serialized_applet_name

        width = pixels_high
        height = pixels_wide

        [ codebase = location_URL ]
        [ archive = comma_separated_list_of_archive_files ]
        [ name = applet_instance_name ]
        [ alt = alternate_text ]
        [ align = style ]
        [ vspace = vertical pad pixels ]
        [ hspace = horizontal pad pixels ]
    >
        [ <param name = parameter_name value = parameter_value> ]
        [ <param ...  ]

        [ HTML code for non-Java-aware browsers ]
    </applet>

Either the code attribute or the object attribute must be present to specify the applet to run. The code attribute specifies the applet’s class file; you’ll see this most frequently. The object attribute specifies a serialized representation of an applet. When you use the object attribute to load an applet, the applet’s init() method is not called. However, the serialized applet’s start() method is called.

The width, height, align, vspace, and hspace attributes determine the preferred size, alignment, and padding, respectively. The width and height attributes are required.

The codebase attribute specifies the base URL to be searched for the applet’s class files. If this attribute isn’t present, the browser looks in the same location as the HTML file. The archive attribute specifies a list of JAR or ZIP files in which the applet’s class files are located. To put two or more files in the list, separate the filenames with commas; for example, the following attribute tells the browser to load and search three archives for the applet’s classes:

    archive="Part1.jar,Part2.jar,Utilities.jar"

The archive files listed by the archive tag are loaded from the codebase URL. When searching for classes, a browser checks the archives before searching any other locations on the server.

The alt attribute specifies alternate text that is displayed by browsers that understand the <applet> tag and its attributes but have Java disabled or don’t run applets. This attribute can also describe the applet because in this case, any alternate HTML between <applet> and </applet> is, by convention, ignored by Java-enabled browsers.

The name attribute specifies an instance name for the executing applet. This is a name specified as a unique label for each copy of an applet on a particular HTML page. For example, if we include our clock twice on the same page (using two applet tags), we should give each instance a unique name to differentiate them:

    <applet code="AnalogClock" name="bigClock" width="300" height="300">
    </applet>
    <applet code="AnalogClock" name="smallClock" width="50" height="50">
    </applet>

Applets can use instance names to recognize and communicate with other applets on the same page. We could, for instance, create a “clock setter” applet that knows how to set the time on an AnalogClock applet and pass it the instance name of a particular target clock on this page as a parameter. This might look something like:

    <applet code="ClockSetter">
        <param name="clockToSet" value="bigClock">
    </applet>

Loading Class Files

The code attribute of the <applet> tag should specify the name of an applet. This is either a simple class name or a package path and class name. For now, let’s look at simple class names; we’ll discuss packages in a moment. By default, the Java runtime system looks for the class file in the same location as the HTML document that contains it. This location is known as the base URL for the document.

Consider an HTML document, clock.html, that contains our clock applet example:

    <applet code="AnalogClock" width="100" height="100"></applet>

Let’s say we retrieve the document at the following URL:

    http://www.time.ch/documents/clock.html

Java tries to retrieve the applet class file from the same base location:

    http://www.time.ch/documents/AnalogClock.class

The codebase attribute of the <applet> tag specifies an alternative base URL for the class file search. Let’s say our HTML document now specifies codebase, as in the following example:

    <applet
        codebase="http://www.joes.ch/stuff/"
        code="AnalogClock"
        width="100"
        height="100">
    </applet>

Java now looks for the applet class file at:

    http://www.joes.ch/stuff/AnalogClock.class

Packages

For “loose” applet class files that are not packaged into archives, Java uses the standard package name to directory path mapping to locate files on the server. The only difference is that the requests are not local file lookups, but instead are requests to the web server at the applet’s codebase URL. Before a class file is retrieved from a server, its package name component is translated by the client into a relative pathname under the applet’s codebase.

Let’s suppose that our AnalogClock has been placed into a package called time.clock (a subordinate package for clock-related classes, within a package for time-related classes). The fully qualified name of our class is time.clock.AnalogClock. Our simple <applet> tag would now look like:

    <applet code="time.clock.AnalogClock" width="100" height="100"></applet>

Let’s say the clock.html document is once again retrieved from:

    http://www.time.ch/documents/clock.html

Java now looks for the class file in the following location:

    http://www.time.ch/documents/time/clock/AnalogClock.class

The same is true when specifying an alternative codebase:

    <applet
        codebase="http://www.joes.ch/stuff/"
        code="time.clock.AnalogClock"
        width="100"
        height="100">
    </applet>

Java now tries to find the class in the corresponding path under this base URL:

    http://www.joes.ch/stuff/time/clock/AnalogClock.class

appletviewer

The Java SDK comes with an applet viewer program, aptly called appletviewer. To use appletviewer, specify the URL of the document on the command line. For example, to view our (still only theoretical) AnalogClock at the URL shown earlier, use the following command:

    % appletviewer http://www.time.ch/documents/clock.html

appletviewer retrieves all applets in the specified document and displays each one in a separate window. appletviewer isn’t a web browser; it doesn’t attempt to display HTML. It was primarily useful before the Java Plug-in as a way to test an applet in a specific version of the Java runtime.

Java Web Start

The Java Web Start API is an alternative to using applets. Java Web Start uses the Java Network Launching Protocol (JNLP) to transparently download and install Java applications locally. The only thing the user has to do is to click on the install link on a web page. The installed applications can then be launched just like any installed application, by clicking on an icon on the desktop or through the Start menu, but they continue to be managed by the Java security policy unless otherwise authorized by the user. Web Start applications also automatically check for upgrades and update themselves over the Net. Java Web Start is a form of zero administration client installation, which implies that the client doesn’t have to do any work to install or maintain the application. JNLP applications may be signed (allowing the user to grant them fine-grained privileges), or unsigned. But even unsigned JNLP applications can take advantage of standard APIs that prompt the user for permission to perform basic operations, such as opening files and printing.

Packaging your application to use JNLP is relatively easy, but we won’t get into it here. The process mainly involves creating a JNLP deployment file that lists your JARs and specifies any special permission they require. You must then include an appropriate link in your web page that uses Web Start. The first time a user tries to install a JNLP application, he will have to install the Java Plug-in. Thereafter, the Java Web Start component manages all JNLP installs. See Oracle’s website for more information.

Conclusion

In this chapter, we covered some of the events that led to the current, fractured world of Java in the web browser and set the scene as it is. The Java Plug-in is currently your only real option for running Java in the browser. New technologies such as Java Web Start provide alternative directions for client-side deployment that may also be appealing. Finally, there are a variety of third-party products that produce clickable installers for Java applications that you may wish to consider.



[48] If you aren’t familiar with HTML or other markup languages, you may want to refer to HTML & XHTML: The Definitive Guide by Chuck Musciano and Bill Kennedy (O’Reilly) for a complete reference on HTML and structured web documents.