Attributes

Attributes are name/value pairs that are interpreted by a Web browser or applet viewer. (Many HTML tags besides <applet> have attributes.) Attributes of the <applet> tag specify general features that apply to all applets, 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, code, width, and height, are always required in the <applet> tag. code specifies the name of the applet to be loaded; width and height determine its initial size. Other attributes are optional.

The following is an HTML fragment for a hypothetical, 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 needs to be stored in the same directory as the AnalogClock.class class file. The applet tag is not sensitive to spacing, so the above is therefore equivalent to:

<applet 
    code=AnalogClock 
    width=100 
    height=100> 
<\applet> 

You can use whatever form seems appropriate.

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 strings 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.[3] Our AnalogClock applet, for example, might accept a parameter that selects between local and universal time:

[3] If you are wondering why the applet's parameters are specified in yet another type of tag, here's the reason. In the original alpha release of Java, applet parameters were included inside of a single <app> tag along with formatting attributes. However, this format was not SGML-compliant, so the <param> tag was added.

<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 can be quoted to contain spaces and other special characters. We could therefore be more verbose and use a parameter value like the following:

<param name=zone value="Greenwich Mean Time"> 

The parameters a given applet expects are determined by the developer of that applet. There is no fixed 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. The applet might also consider parameters to be either optional or required and act accordingly.

Hablo Applet?

Web browsers ignore tags they don't understand; if the Web browser doesn't interpret the <applet> or <param> tags, they should disappear and any HTML between the <applet>and </applet> tags should appear normally.

By convention, Java-enabled Web browsers do the opposite and ignore any extra HTML between the <applet> and </applet> tags. This means we can place some alternate HTML inside the <applet> tag, which is then 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 can now spell out the full-blown <applet> tag:[4]

[4] The HTML working group of the IETF is investigating standardization of embedded objects in HTML. A draft document can be found at ftp://ds.internic.net/internet-drafts/draft-ietf-html-cda-00.txt. They would prefer a slightly less application-centric name such as <embed>. However, their discussion, for the most part, parallels the <applet> tag.

<applet 
    code = class name 
    width = pixels wide 
    height = pixels high 
    [ codebase = location URL ] 
    [ name = instance name ] 
    [ alt = alternate text ] 
    [ align = alignment style ] 
    [ vspace = vertical pad pixels ] 
    [ hspace = horizontal pad pixels ] 
> 
[ <param name = parameter name value = parameter value> ] 
[ <param name = parameter name value = parameter value> ] 
... 
[ HTML for non Java aware browsers ] 
<\applet> 

The width, height, align, vspace, and hspace attributes have the same meanings as those of the <img> tag and determine the preferred size, alignment, and padding respectively.

The alt attribute specifies alternate text that is displayed by browsers that understand the <applet> tag and its attributes, but can't actually run applets. This attribute can also describe the applet, since in this case any alternate HTML between <applet> and <\applet> is ignored.

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 could 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 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 a 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; I'll discuss packages in a moment. By default, the Java run-time 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 can be used to specify 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

Packages are groups of Java classes; see Chapter 5, Objects in Java for more information. A package name is a little like an Internet hostname, in that they both use a hierarchical, dot-separated naming convention. A Java class file can be identified by its full name by prefixing the class name with the package name. We might therefore have a package called time for time-related Java classes, and perhaps a subordinate package called time.clock to hold classes related to one or more clock applications.

In addition to providing a naming scheme, packages can be used to locate classes stored at a particular location. Before a class file is retrieved from a server, its package-name component is translated by the client into a relative path name under the base URL of the document. The components of a class package name are simply turned into the components of a path name, just like with classes on your local system.

Let's suppose that our AnalogClock has been placed into the time.clock package and now has the fully qualified name of 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 the new base URL:

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

One possible package-naming convention proposes that Internet host and domain names be incorporated as part of package names to form a unique identifier for classes produced by a given organization. If a company with the domain name foobar.com produced our AnalogClock class, they might distribute it in a package called com.foobar.time.clock. The fully qualified name of the AnalogClock class would then be com.foo.time.clock.AnalogClock. This would presumably be a unique name stored on an arbitrary server. A future version of the Java class loader might use this to automatically search for classes on remote hosts.

Perhaps soon we'll run Sun's latest and greatest Web browser directly from its source with:

% java com.sun.java.hotjava.HotJava

Viewing Applets

Sun's JDK 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 AnalogClock at the URL shown above, use the following command:

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

The appletviewer retrieves all applets in the specified document and displays each one in a separate window. appletviewer is not a Web browser; it doesn't attempt to display HTML. It's primarily a convenience for testing and debugging applets. If the document doesn't contain <applet> tags, appletviewer does nothing.