by Mark Wutka
Although there are many differences between thecapabilities of applets and of applications in terms of security restrictions, there are very few environmental differences between the two. You can actually run an applet as a stand-alone application with just a few minor additions.
Applets automatically have a parent frame when they are loaded. When you run a stand-alone application, however, you have to create your own frame.
Applets also have the notion of document-base and code-base URLs, as well as an application context. This context represents the browser itself and supplies methods to load sound files, fetch images, and open other documents.
One of the biggest differences between an applet and an application is something that you might never think of. An applet has a network-aware class loader. When an applet needs to load a class, its class loader goes over the network to get it. The system class loader that is used by applications does not do this. If you want a stand-alone application to load classes over a network, you must write your own class loader to do it. The section "A Zipfile Class Loader" in Chapter 21, "Download Strategies," shows you how to write your own class loader.
Applets are, of course, intended to run inside Web browsers. In fact, you cannot run a stand-alone application in a Web browser, only an applet. You can, however, run an applet outside of a Web browser. You simply have to implement the same kind of framework that the browser provides automatically.
There are some benefits to being able to run an applet as a stand-alone
application. If you always write your user interfaces as applets,
you can run them inside of a browser or stand-alone without changing
your code. This means that you never have to go in and change
your code to run inside a browser, or to run stand-alone. Each
time you write a new user interface, it is automatically ready
to run wherever it is needed.
| Note |
The term application, when used in conjunction with Java, usually indicates a program running stand-alone. The term applet always refers to a program running within a browser. Unfortunately, this separation implies that the two are always separate things. A distributed application, in the traditional sense, is made up of many components. In the Java world, some of these components may be applets, and some may be stand-alone applications, but they all fit together to make a distributed application. |
You can run almost any applet as an application but some are easier than others. If your applet does not use the getDocumentBase, getCodeBase, or any of the AppletContext methods, you may be able to get away with creating a frame and launching the applet in the frame. In these cases, the applet is little more than a typical AWT container (remember, the Applet class is a subclass of java.awt.Panel).
Applets are first initialized by the init method, then started by the start method. Applications, on the other hand, are initialized and started with a static method called main. Fortunately, these methods can peacefully coexist in the same class.
By adding a main method that automatically creates a frame and adds the applet to the frame, you make your applet run either as an applet or as an application. Listing 13.1 shows a simple applet that will also run as an application.
Listing 13.1 Source Code for StandaloneApplet.java
import java.awt.*;
import java.applet.*;
// StandaloneApplet is an applet that runs either as
// an applet or a standalone application. To run
// standalone, it provides a main method that creates
// a frame, then creates an instance of the applet and
// adds it to the frame.
public class StandaloneApplet extends Applet
{
public void init()
{
add(new Button("Standalone Applet Button"));
}
public static void main(String args[])
{
// Create the frame this applet will run in
Frame appletFrame = new Frame("Some applet");
// The frame needs a layout manager, use the GridLayout to maximize
// the applet size to the frame.
appletFrame.setLayout(new GridLayout(1,0));
// Have to give the frame a size before it is visible
appletFrame.resize(300, 100);
// Make the frame appear on the screen. You should make the frame appear
// before you call the applet's init method. On some Java implementations,
// some of the graphics information is not available until there is a frame.
// If your applet uses certain graphics functions like getGraphics() in the
// init method, it may fail unless there is a frame already created and
// showing.
appletFrame.show();
// Create an instance of the applet
Applet myApplet = new StandaloneApplet();
// Add the applet to the frame
appletFrame.add(myApplet);
// Initialize and start the applet
myApplet.init();
myApplet.start();
}
}
Figure 13.1 shows StandaloneApplet running within a Web browser, while Figure 13.2 shows it running as a stand-alone application.
Figure 13.1 : Many applets act as simple AWT containers.
Figure 13.2 : Sometimes a simple frame is all you need to run an applet stand-alone.
An applet is itself just an AWT container with a few extra methods for interacting with its runtime environment. Most of the applet's other methods are really implemented by three interfaces-AppletStub, AppletContext, and AudioClip.
The AppletStub interface is used by a browser or other applet environment to give the Applet class access to its environment. The applet stub defines methods for determining the applet's document base and code base (the URLs from which the applet's parent document and .class file were loaded), the applet's parameters (usually specified by the <PARAM> HTML tag), and the applet's context.
The AppletContext interface
provides an applet with methods for loading images and audio clips,
as well as opening up new URLs in the browser, and finding out
what other applets are running in the current environment. Each
browser has its own AppletContext
class that knows how to perform specific tasks within the browser.
When a browser loads an applet, it calls setStub
in the Applet object, which
sets the applet's stub (as you might guess). This stub, in turn,
has a method called getAppletContext,
which returns the applet's AppletContext
object. If you want to implement your own AppletContext
object, you must also create your own AppletStub
object. Otherwise, there would be no way to associate your AppletContext
object with an applet-there's no setAppletContext
method in the Applet class.
| Note |
As an applet programmer, you never access the AppletContext and AppletStub interfaces directly. The Applet class presents all the methods available in the AppletContext and AppletStub interfaces. The methods in the Applet class simply call the corresponding methods in the AppletContext and AppletStub interfaces. This technique is called delegation. |
Figure 13.3 shows the relationship between the Applet, the AppletStub, and the AppletContext.
If you look at the Applet class, you'll notice that it has methods for playing audio clips. Like most of the other methods provided by the Applet class (with the exception of the AWT container methods), the audio methods simply call methods in the AudioClip interface.
When you create an applet manually and add it to a frame, the applet is missing some key information that it may rely on. Many applets make use of the getDocumentBase and getCodeBase methods, as well as some methods in the AppletContext class. If you want to run an applet stand-alone that uses these methods, you have to create an applet stub and an applet context for the applet.
The applet stub interface defines a set of methods that an applet uses to get information about where it was loaded from. The getCodeBase, getDocumentBase, and getParameter methods in the applet class actually go to the applet's stub to get these pieces of information.
In addition, the applet stub is responsible for finding the AppletContext object for the applet. The AppletContext object has methods for fetching images and sound clips, opening up URLs in the browser, and displaying status messages in the browser's status window area (on Netscape, this is at the bottom of the browser). The AppletContext also provides methods to locate other applets running within the browser.
Listing 13.2 shows the RunAppletContext, which implements an AppletContext that can be used to run almost any applet in a stand-alone application.
Listing 13.2 Source Code for RunAppletContext.java
import java.applet.*;
import java.util.*;
import java.awt.*;
import java.net.*;
// This class provides a generic applet context for standalone
// applications. It is implemented as a singleton object, which
// means that there is only one instance of this class within the
// runtime environment. It stores all the loaded applets in a
// hash table so it can provide working getApplet and getApplets methods.
public class RunAppletContext extends Object implements AppletContext,
AudioClip
{
// The pointer to the lone instance of this class
protected static RunAppletContext context;
// The table of all the known applets in the runtime environment.
protected Hashtable applets;
protected RunAppletContext()
{
applets = new Hashtable();
}
// Returns the lone instance of the RunAppletContext. If there isn't
// an instance, it creates a new one.
public synchronized static RunAppletContext instance()
{
if (context == null) {
context = new RunAppletContext();
}
return context;
}
// Adds an applet to the table of known applets
public void addApplet(Applet applet, String name)
{
applets.put(name, applet);
}
// Locates an applet in the table
public Applet getApplet(String name)
{
return (Applet) applets.get(name);
}
// Returns an enumeration of all the known applets
public Enumeration getApplets()
{
return applets.elements();
}
// Tries to load an audio clip using Sun's AppletAudioClip
// which is distributed with the JDK. This class may not be
// available in all Java implementations since it is not a
// documented part of the JDK.
public AudioClip getAudioClip(URL url)
{
try {
return new sun.applet.AppletAudioClip(url);
} catch (Exception e) {
return this;
}
}
// Uses the AWT Toolkit class to fetch an image from a URL
public Image getImage(URL url)
{
return Toolkit.getDefaultToolkit().getImage(url);
}
// Since we aren't running in a browser and there aren't really
// any classes to render HTML in Java, we have to wimp out with
// the showDocument method and just print a message that the
// applet wanted to load a URL.
public void showDocument(URL url)
{
System.out.println("Wanted to show document on: "+url);
}
public void showDocument(URL url, String target)
{
System.out.println("Wanted to show document on: "+url+
" in frame "+target);
}
// Just print to System.out for showStatus.
public void showStatus(String status)
{
System.out.println(status);
}
// If we can't create an instance of sun.applet.AppletAudioClip, we
// return a pointer to this same object, which happens to also implement
// the AudioClip interface, but it doesn't do anything with them.
// The following three methods are the methods for AudioClip:
public void play() {};
public void loop() {};
public void stop() {};
}
To use a custom applet context, you need a custom applet stub since the stub is the class that returns the applet context. The stub contains the very useful getDocumentBase, getCodebase, and getParameter methods.
Listing 13.3 shows a handy RunAppletStub that allows you to customize the code and document bases as well as the applet parameters by using the system properties. It also returns an instance of RunAppletContext for the applet's context.
Listing 13.3 Source Code for RunAppletStub.java
import java.applet.*;
import java.net.*;
import java.awt.*;
// This class provides an applet stub for applets running
// as standalone applications. You can set the document base
// by setting the "docbase" system property. Likewise, you can set
// the code base through the "codebase" property.
// You can provide applet parameters by setting system properties
// with the applet's name followed by the parameter. For example:
// <PARAM name="stooge" value="moe">
// for an applet named MyApplet, could be set in this stub
// with by setting the system property "MyAppletstooge" to "moe".
// You can also just set the "stooge" property, but it will try
// using the appletname in front first. This allows you to run
// multiple applets at once that have the same parameter names.
public class RunAppletStub extends Object implements AppletStub
{
Frame appletFrame;
Applet applet;
String appletName;
String startDir;
public RunAppletStub()
{
}
// startDir is the local directory where this applet is started, or
// another directory if you prefer. If you don't specify a code base
// or a document base, the startDir is used for those. The directory
// separators must be '/' and not '\' or the URL class gets confused.
public RunAppletStub(Frame appletFrame, Applet applet, String name,
String startDir)
{
this.appletFrame = appletFrame;
this.applet = applet;
this.appletName = name;
this.startDir = startDir;
RunAppletContext.instance().addApplet(applet, name);
}
public void setParams(Frame appletFrame, Applet applet, String name,
String startDir)
{
this.appletFrame = appletFrame;
this.applet = applet;
this.appletName = name;
this.startDir = startDir;
RunAppletContext.instance().addApplet(applet, name);
}
public boolean isActive() { return true; }
// Return the document base URL. Try getting the docbase system parameter.
// If that isn't available, use the startDir directory.
public URL getDocumentBase()
{
String docbase = System.getProperty("docbase");
try {
if (docbase == null) {
return new URL("file://"+startDir);
} else {
return new URL(docbase);
}
} catch (MalformedURLException e) {
return null;
}
}
// Return the code base URL. Try getting the codebase system parameter.
// If that isn't available, use the startDir directory.
public URL getCodeBase()
{
String codebase = System.getProperty("codebase");
try {
if (codebase == null) {
return new URL("file://"+startDir);
} else {
return new URL(codebase);
}
} catch (MalformedURLException e) {
return null;
}
}
// fetch a parameter for the applet from the system properties. First
// try the applet name followed by the param name. If that's null,
// try just the param name.
public String getParameter(String paramName)
{
String prop = System.getProperty(appletName+paramName);
if (prop != null) return prop;
return System.getProperty(paramName);
}
public AppletContext getAppletContext()
{
return RunAppletContext.instance();
}
// appletResize is the only reason we need a reference to the applet's
// frame. If the applet wants to resize, we resize the frame, then
// the applet.
public void appletResize(int width, int height)
{
appletFrame.resize(width+10, height+20);
applet.resize(width, height);
}
}
All you have to do in your applet to use the RunAppletStub is create the stub and call the setStub applet method. Listing 13.4 shows the stand-alone applet updated to use the RunAppletStub class.
Listing 13.4 Source Code for Standalone2.java
import java.awt.*;
import java.applet.*;
// StandaloneApplet is an applet that runs either as
// an applet or a standalone application. To run
// standalone, it provides a main method that creates
// a frame, then creates an instance of the applet and
// adds it to the frame.
public class Standalone2 extends Applet
{
public void init()
{
add(new Button("Standalone Applet Button"));
}
public static void main(String args[])
{
// Create the frame this applet will run in
Frame appletFrame = new Frame("Some applet");
// The frame needs a layout manager, use the GridLayout to maximize
// the applet size to the frame.
appletFrame.setLayout(new GridLayout(1,0));
// Have to give the frame a size before it is visible
appletFrame.resize(300, 100);
// Make the frame appear on the screen. You should make the frame appear
// before you call the applet's init method. On some Java implementations,
// some of the graphics information is not available until there is a frame.
// If your applet uses certain graphics functions like getGraphics() in the
// init method, it may fail unless there is a frame already created and
// showing.
appletFrame.show();
// Create an instance of the applet
Applet myApplet = new Standalone2();
// Add the applet to the frame
appletFrame.add(myApplet);
// Now try to get an applet stub for this class.
RunAppletStub stub = new RunAppletStub(appletFrame,
myApplet, "standalone-applet", "http://localhost/");
myApplet.setStub(stub);
// Initialize and start the applet
myApplet.init();
myApplet.start();
}
}
You have to write this special main method only a few times before you start wondering whether you couldn't create a loader that automatically did all that stuff for you. The RunApplet class, included on the CD with this book, can load multiple applets.
When you start an applet, you can specify the applet's width, height, name, and starting directory. For example, the following command line starts the applet Applet1 with a size of 400¥300, a name of myapplet, and a starting directory of /home/mark:
java RunApplet Applet1,width=400,height=300,name=myapplet,startDir=/home/mark
Make sure there are no spaces in the parameters for a single applet; otherwise, they will be confused with parameters for another applet. You can run multiple applets by putting them all on the same command line. The following command runs applets named Applet1 and Applet2:
java RunApplet Applet1 Applet2
Notice that the width, height, name, and startDir parameters are optional.
The RunApplet class is arranged slightly differently from the preceding Standalone2 class. Most of the work that is done in the main method in Standalone2 is now in a method called StartApplet. Listing 13.5 shows the startApplet method for the RunApplet class.
Listing 13.5 startApplet Method from RunApplet.java
// Creates the frame, sets the stub, starts the applet
public static void startApplet(Applet applet, int width, int height,
String name, String startDir)
{
// Create the applet's frame
Frame appletFrame = new Frame(name);
// Allow room for the frame's borders
appletFrame.resize(width+10, height+20);
// Use a grid layout to maximize the applet's size
appletFrame.setLayout(new GridLayout(1, 0));
// Add the applet to the frame
appletFrame.add(applet);
// Show the frame, which makes sure all the graphics info is loaded
// for the applet to use.
appletFrame.show();
// Create and set the stub
AppletStub stub = new RunAppletStub(appletFrame, applet, name,
startDir);
applet.setStub(stub);
// initialize the applet
applet.init();
// Make sure the frame shows the applet
appletFrame.validate();
// Start up the applet
applet.start();
The bulk of the RunApplet class is taken up by the main method, which spends all its time parsing command-line arguments. For each command-line argument, the main method creates a StringTokenizer object that uses a comma as the separator. For each token, the method checks to see whether it contains any of the allowable parameters, and, if so, parses the parameter. The applet's startDir parameter is used by the applet stub to return the document base and code base URLs. The URL class requires all directories to use the forward slash (/), as opposed to the backward slash (\) used by Windows and OS/2. The main method has to scan through the startDir parameter and replace any backward slashes with forward slashes. It uses a StringBuffer object to do this. The StringBuffer class allows you to build and edit strings more efficiently than using the String class, because you can directly change the characters in a StringBuffer object.
The main method simply turns the startDir
parameter into a StringBuffer
object, scans through the buffer replacing \s with /s, and then
converts the StringBuffer
object back into a String.
Listing 13.6 shows the source code for the main
method.
| Tip |
The technique of using a StringBuffer object to manipulate a String object is used very often by Java, but you don't always know it. Whenever you combine an integer and a string, like "Count: "+5, the Java compiler actually generates calls to the StringBuffer class to create the new string. |
Listing 13.6 main Method from RunApplet.java
public static void main(String[] args)
{
if (args.length < 1) {
System.err.println("Please supply the applet name.");
}
// For each arg, parse out the applet class name and other params
for (int i=0; i < args.length; i++) {
StringTokenizer tokenizer = new StringTokenizer(
String className = null;
// default to 300x200 frame
int width = 300;
int height = 200;
String name = null;
String startDir = null;
while (tokenizer.hasMoreTokens()) {
String token = tokenizer.nextToken();
// Look for width= parameter, if found, get the integer. If there's
// an error parsing the int, just ignore it
if (token.startsWith("width=")) {
try {
width = Integer.valueOf(
token.substring(6)).
intValue();
} catch (Exception ignore) {
}
// Look for the height parameter, ignore if there's an error
} else if (token.startsWith("height=")) {
try {
height = Integer.valueOf(
token.substring(7)).
intValue();
} catch (Exception ignore) {
}
// Look for the optional applet name
} else if (token.startsWith("name=")) {
name = token.substring(5);
// Normally, you just give the applet's class name in the parameter
// list, but if you like, you can be more specific and say
// applet=xxx.
} else if (token.startsWith("applet=")) {
className = token.substring(7);
// Set the home directory for the applet. If not set, will
// use the current directory (from System property "user.dir")
} else if (token.startsWith("startdir=")) {
startDir = token.substring(9);
} else {
if (className == null) {
className = token;
} else {
System.err.println(
"Invalid parameter - "+
token);
}
}
}
if (className == null) {
System.err.println(
"No class name specified in: "+
args[i]);
}
if (name == null) name = className;
// If no startDir set, use the "user.dir" property
if (startDir == null) {
startDir = System.getProperty("user.dir")+"//";
}
// This little piece of bogosity changes any \'s in the start dir
// to /'s, since the URL classes require /'s.
StringBuffer buff = new StringBuffer(startDir);
for (int j=0; j < buff.length(); j++) {
if (buff.charAt(j) == '\\') {
buff.setCharAt(j, '/');
}
}
// Convert the string buffer back to a string.
startDir = new String(buff);
// Load the applet's class
try {
Class appletClass = Class.forName(className);
Applet runme = (Applet) appletClass.
newInstance();
// Start the applet
startApplet(runme, width, height, name,
startDir);
} catch (Exception e) {
// If there's an error, just say which applet had the problem,
// but don't quit.
System.err.println("Error starting applet - "+
args[i]);
System.err.println(e);
}
}
}