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Java applet
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A Java applet is an applet delivered to the users in the form of Java bytecode. Java applets can run in a Web browser using 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.
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Encyclopedia
A Java applet is an applet delivered to the users in the form of Java bytecode. Java applets can run in a Web browser using 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 bytecode is platform independent, Java applets can be executed by browsers for many platforms, including Windows, Unix, Mac OS and Linux. There are open source tools like 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.
Many influential Java developers, blogs and magazines are recommending that the Java Web Start technology be used in place of Applets .
A Java Servlet is 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.
Technical information
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 server and 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 applet HTML element , or the recommended object element . This specifies the applet's source and the applet's location statistics.
A Java applet extends the class , or in the case of a Swing applet, . The class must override methods from the applet class to set up a user interface inside itself (Applet is a descendant of which is a descendant of ).
Advantages
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-in version 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 web browsers
- it will cache in 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 C++, but many times faster than JavaScript
- it can move the work from the server to the client, making a web solution more scalable with the number of users/clients
- developers can develop and debug an applet direct simply by creating a main routine (either in the applet's class or in a separate class) and call init and start on the applet, thus allowing for development in their favorite J2SE development environment. All one has to do after that is re-test the applet in the appletviewer program or a web browser to ensure it conforms to security restrictions.
Disadvantages
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.
- Prior to version 6u12, Sun did not provide a 64-bit version of its Java plug-in, forcing users to use the 32-bit plugin with a 32-bit browser.1
- It cannot start until the Java Virtual Machine is 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 (although some would argue that this is a security benefit instead of a disadvantage, as ad-hoc unrestricted access to a client's disk would be incredibly dangerous).
- 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.2
1. However, open source and freeware implementations are available, such as the GCJ plugin, and Blackdown Java plugin.
2. Applet may specify java version as of 6u10.
Compatibility issues
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 Machine as 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). 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 (JavaScript can, with difficulty, be used for this). Having the applet create its own main window is not a good solution either, as this leaves the browser window as a largely useless extra window and leads to a large chance of the applet being terminated unintentionally by the user closing the parent browser window.
Alternatives
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.
See also
External links
- (includes browser plug-ins for running Java applets in most web browsers).
- Applet section of Wikibook's Java book
- (JDK 1.4 - include source code)
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