- Java applet
A Java applet is an
appletdelivered in the form of Java bytecode. Java applets can run in a Web browserusing a Java Virtual Machine(JVM), or in Sun's AppletViewer, a stand-alone tool for testing applets. Java applets were introduced in the first version of the Java language in 1995. Java applets are usually written in the Java programming language but they can also be written in other languages that compile to Java bytecode such as Jython.
Applets are used to provide interactive features to web applications that cannot be provided by
HTML. Since Java's bytecodeis platform independent, Java applets can be executed by browsers for many platforms, including Windows, Unix, Mac OSand Linux.There are open source tools like [http://sourceforge.net/projects/applet2app applet2app] which can be used to convert an applet to a stand alone Java application/windows executable/linux executable. This has the advantage of running a Java applet in offline mode without the need for internet browser software.
Java Servletis sometimes informally compared to be "like" a server-side applet, but it is different in its language, functions, and in each of the characteristics described here about applets.
Java applets are executed in a "sandbox" by most web browsers, preventing them from accessing local data. The code of the applet is downloaded from a
web serverand the browser either embeds the applet into a web page or opens a new window showing the applet's user interface. The applet can be displayed on the web page by making use of the deprecated
HTML element[http://www.w3.org/TR/html401/struct/objects.html#edef-APPLET] , or the recommended
objectelement [http://www.w3.org/TR/html401/struct/objects.html#edef-OBJECT] . This specifies the applet's source and the applet's location statistics.
A Java applet extends the class Javadoc:SE|package=java.applet|java/applet|Applet, or in the case of a Swing applet, Javadoc:SE|package=javax.swing|javax/swing|JApplet. The class must override methods from the applet class to set up a user interface inside itself (
Appletis a descendant of Javadoc:SE|java/awt|Panel which is a descendant of Javadoc:SE|java/awt|Container).
A Java applet can have any or all of the following advantages:
*it is simple to make it work on Linux, Windows and Mac OS i.e. to make it cross platform
*the same applet can work on "all" installed versions of Java at the same time, rather than just the latest
plug-inversion only. However, if an applet requires a later version of the JRE the client will be forced to wait during the large download.
*it is supported by most
cachein most web browsers, so will be quick to load when returning to a web page but may get stuck in the cache and have issues when new versions come out.
*it can have full access to the machine it is running on if the user agrees
*it can improve with use: after a first applet is run, the JVM is already running and starts quickly, benefitting regular users of Java but the JVM will need to restart each time the browser starts fresh.
*it can run at a speed that is comparable to (but generally slower than) other compiled languages such as
*it can move the work from the server to the client, making a web solution more scalable with the number of users/clients
A Java applet is open to any of the following disadvantages:
*it requires the Java
plug-in, which isn't available by default on all web browsers.
*an implementation of the Sun Java
plug-indoes not exist for AMD64/Intel64 processors.1
*it cannot start until the
Java Virtual Machineis running, and this may have significant startup time the first time it is used.
*if untrusted, it has severely limited access to the user's system - in particular having no direct access to the client's disk or clipboard.
*some organizations only allow software installed by the administrators. As a result, many users cannot view applets by default.
*applets may require a specific JRE.
*because of the platform independent nature of Java, it may be undesirably slow.
open sourceand freewareimplementations are available, such as the GCJplugin, and Blackdown Javaplugin. There is request for enhancement to [http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4802695 Support Java Plug-in on 64-bit AMD and Intel] .
Sun has made a considerable effort to ensure compatibility is maintained between Java versions as they evolve. For example,
Microsoft's Internet Explorer, the most popular web browser since the late 1990s, used to ship with the Microsoft Java Virtual Machineas the default. The MSJVM had some extra non-Java features added which, if used, would prevent MSJVM applets from running on Sun's Java (but not the other way round).Fact|date=February 2008 Sun sued for breach of trademark, as the point of Java was that there should be no proprietary extensions and that code should work everywhere. Development of MSJVM was frozen by a legal settlement, leaving many users with an extremely outdated Java virtual machine. Later, in October 2001, MS stopped including Java with Windows, and for some years it has been left to the computer manufacturers to ship Java independently of the OS.
Some browsers (notably Netscape) do not do a good job of handling height=100% on applets which makes it difficult to make an applet fill most of the browser window (
Alternative technologies exist (for example,
DHTML, Flash, and Microsoft Silverlight) that satisfy some of the scope of what is possible with an applet.
Another alternative to applets for client side Java is
Java Web Start, which runs outside the browser. In addition to the features available to applets, a simple permissions box can give Java Web Start programs read and/or write access to specified files stored on the client, and to the client's clipboard.
* [http://www.java.com/download/ Latest version of Sun Microsystems Java Virtual Machine] (includes browser plug-ins for running Java applets in most web browsers).
* [http://java.sun.com/docs/books/tutorial/applet/index.html Information about writing applets from Sun Microsystems]
* [http://java.sun.com/applets/jdk/1.4/index.html Demonstration applets from Sun Microsystems] (
JDK1.4 - include source code)
* [http://conceptualeclipse.googlepages.com/pressuresimulation Interactive Java Pressure Simulation Applet]
* [http://cmol.nbi.dk/models Examples of interactive Java Applets from physics, biology and social systems]
* [http://javaboutique.internet.com/javasource/ examples for Java Applets with sourcecode]
* [http://www.home.hs-karlsruhe.de/~frhe0012/Snake.html another example with sourcecode]
* [http://www.lalaclick.net applet for visual interaction with last.fm data]
* [http://faq.javaranch.com/java/AppletsFaq JavaRanch Applet FAQ]
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