Java Applet Pdf

[Heron Mathis]

Whatis Applet?

An applet is a Java program that can be embedded into a web page. It runs inside the web browser and works at client side. An applet is embedded in an HTML page using the APPLET or OBJECT tag and hosted on a web server.

Applets are used to make the website more dynamic and entertaining.

Ans. Whenever we pass object reference in arguments then the object will be provided by its caller itself. In this case the caller of paint() method is browser, so it will provide an object. The same thing happens when we create a very basic program in normal Java programs. For Example:

Running the HelloWorld Applet :

After you enter the source code for HelloWorld.java, compile in the same way that you have been compiling java programs(using javac command). However, running HelloWorld with the java command will generate an error because it is not an application.

1. Using java enabled web browser : To execute an applet in a web browser we have to write a short HTML text file that contains a tag that loads the applet. We can use APPLET or OBJECT tag for this purpose. Using APPLET, here is the HTML file that executes HelloWorld :

The width and height statements specify the dimensions of the display area used by the applet. The APPLET tag contains several other options. After you create this html file, you can use it to execute the applet.

2. Using appletviewer : This is the easiest way to run an applet. To execute HelloWorld with an applet viewer, you may also execute the HTML file shown earlier. For example, if the preceding HTML file is saved with

RunHelloWorld.html, then the following command line will run HelloWorld :

3. appletviewer with java source file : If you include a comment at the head of your Java source code file that contains the APPLET tag then your code is documented with a prototype of the necessary HTML statements, and you can run your compiled applet merely by starting the applet viewer with your Java source code file. If you use this method, the HelloWorld source file looks like this :

Java applets are small applications written in the Java programming language, or another programming language that compiles to Java bytecode, and delivered to users in the form of Java bytecode.

At the time of their introduction, the intended use was for the user to launch the applet from a web page, and for the applet to then execute within a Java virtual machine (JVM) in a process separate from the web browser itself. A Java applet could appear in a frame of the web page, a new application window, a program from Sun called appletviewer,[6] or a stand-alone tool for testing applets.[clarification needed]

Java applets run at very fast speeds[citation needed] and until 2011, they were many times faster than JavaScript.[13] Unlike JavaScript, Java applets had access to 3D hardware acceleration, making them well-suited for non-trivial, computation-intensive visualizations. As browsers have gained support for hardware-accelerated graphics thanks to the canvas technology (or specifically WebGL in the case of 3D graphics),[14][15] as well as just-in-time compiled JavaScript,[16] the speed difference has become less noticeable.[citation needed]

Since Java bytecode is cross-platform (or platform independent), Java applets could be executed by clients for many platforms, including Microsoft Windows, FreeBSD, Unix, macOS and Linux. They could not be run on mobile devices, which do not support running standard Oracle JVM bytecode. Android devices can run code written in Java compiled for the Android Runtime.

The applets are used to provide interactive features to web applications that cannot be provided by HTML alone. They can capture mouse input and also have controls like buttons or check boxes. In response to user actions, an applet can change the provided graphic content. This makes applets well-suited for demonstration, visualization, and teaching. There are online applet collections for studying various subjects, from physics to heart physiology.

An applet can also be a text area only; providing, for instance, a cross-platform command-line interface to some remote system. If needed, an applet can leave the dedicated area and run as a separate window. However, applets have very little control over web page content outside the applet's dedicated area, so they are less useful for improving the site appearance in general, unlike other types of browser extensions (while applets like news tickers or WYSIWYG editors are also known). Applets can also play media in formats that are not natively supported by the browser.

As applets were available before HTML5, modern CSS and JavaScript interface DOM were standard, they were also widely used for trivial effects such as mouseover and navigation buttons. This approach, which posed major problems for accessibility and misused system resources, is no longer in use and was strongly discouraged even at the time.

Most browsers executed Java applets in a sandbox, preventing applets from accessing local data like the file system.[17] The code of the applet was downloaded from a web server, after which the browser either embedded the applet into a web page or opened a new window showing the applet's user interface.

The first implementations involved downloading an applet class by class. While classes are small files, there are often many of them, so applets got a reputation as slow-loading components. However, since .jar files were introduced, an applet is usually delivered as a single file that has a size similar to an image file (hundreds of kilobytes to several megabytes).

Many Java developers, blogs and magazines recommended that the Java Web Start technology be used in place of applets.[18] Java Web Start allowed the launching of unmodified applet code, which then ran in a separate window (not inside the invoking browser).

The applet would be displayed on the web page by making use of the deprecated applet HTML element,[19] or the recommended object element.[20] The embed element can be used[21] with Mozilla family browsers (embed was deprecated in HTML 4 but is included in HTML 5). This specifies the applet's source and location. Both object and embed tags can also download and install Java virtual machine (if required) or at least lead to the plugin page. applet and object tags also support loading of the serialized applets that start in some particular (rather than initial) state. Tags also specify the message that shows up in place of the applet if the browser cannot run it due to any reason.

However, despite object being officially a recommended tag in 2010, the support of the object tag was not yet consistent among browsers and Sun kept recommending the older applet tag for deploying in multibrowser environments,[22] as it remained the only tag consistently supported by the most popular browsers. To support multiple browsers, using the object tag to embed an applet would require JavaScript (that recognizes the browser and adjusts the tag), usage of additional browser-specific tags or delivering adapted output from the server side.

The Java browser plug-in relied on NPAPI, which nearly all web browser vendors have removed support for, or do not implement, due to its age and security issues. In January 2016, Oracle announced that Java runtime environments based on JDK 9 will discontinue the browser plug-in.[23]

Sun made considerable efforts to ensure compatibility is maintained between Java versions as they evolve, enforcing Java portability by law if required. Oracle seems to be continuing the same strategy.

The 1997 lawsuit,[26] was filed after Microsoft created a modified Java Virtual Machine of their own, which shipped with Internet Explorer. Microsoft added about 50 methods and 50 fields[26] into the classes within the java.awt, java.lang, and java.io packages. Other modifications included removal of RMI capability and replacement of Java Native Interface from JNI to RNI, a different standard. RMI was removed because it only easily supports Java to Java communications and competes with Microsoft DCOM technology. Applets that relied on these changes or just inadvertently used them worked only within Microsoft's Java system. 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. Microsoft agreed to pay Sun $20 million, and Sun agreed to grant Microsoft limited license to use Java without modifications only and for a limited time.[27]

Microsoft continued to ship its own unmodified Java virtual machine. Over the years it became extremely outdated yet still default for Internet Explorer. A later study revealed that applets of this time often contain their own classes that mirror Swing and other newer features in a limited way.[28] In 2002, Sun filed an antitrust lawsuit, claiming that Microsoft's attempts at illegal monopolization had harmed the Java platform. Sun demanded Microsoft distribute Sun's current, binary implementation of Java technology as part of Windows, distribute it as a recommended update for older Microsoft desktop operating systems and stop the distribution of Microsoft's Virtual Machine (as its licensing time, agreed in the prior lawsuit, had expired).[27] Microsoft paid $700 million for pending antitrust issues, another $900 million for patent issues and a $350 million royalty fee to use Sun's software in the future.[29][non-primary source needed]

There were two applet types with very different security models: signed applets and unsigned applets.[30] Starting with Java SE 7 Update 21 (April 2013) applets and Web-Start Apps are encouraged to be signed with a trusted certificate, and warning messages appear when running unsigned applets.[31] Further, starting with Java 7 Update 51 unsigned applets were blocked by default; they could be run by creating an exception in the Java Control Panel.[32]

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