A Java applet is an applet delivered to 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, and are written in programming languages that compile to Java bytecode, usually in Java, but also in other languages such as Jython,[8] JRuby,[9] or Eiffel (via SmartEiffel).[10]
Java applets run at speeds comparable to, but generally slower than, other compiled languages such as C++, but until approximately 2011 many times faster than JavaScript.[11] In addition they can use 3D hardware acceleration that is available from Java. This makes applets well suited for non trivial, computation intensive visualizations. When browsers have gained support for native hardware accelerated graphics in the form of Canvas and WebGL, as well as Just in Time compiled JavaScript, the speed difference has become less noticeable.
Since Java's bytecode is cross-platform or platform independent, Java applets can be executed by browsers for many platforms, including Microsoft Windows, Unix, Mac OS and Linux. It is also trivial to run a Java applet as an application with very little extra code. This has the advantage of running a Java applet in offline mode without the need for any Internet browser software and also directly from the integrated development environment (IDE).
Contents |
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 the user action an applet can change the provided graphic content. This makes applets well suitable for demonstration, visualization and teaching. There are online applet collections for studying various subjects, from physics[12] to heart physiology.[3] Applets are also used to create online game collections that allow players to compete against live opponents in real-time.
An applet can also be a text area only, providing, for instance, a cross platform command-line interface to some remote system.[13] 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 dedicated area, so they are less useful for improving the site appearance in general (while applets like news tickers[14] or WYSIWYG editors[15] are also known). Applets can also play media in formats that are not natively supported by the browser[16]
HTML pages may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on the parameters that were passed.
As applets have been available before CSS, they were also widely used for trivial effects like navigation buttons. This use is criticized and declining.[17]
Java applets are executed in a sandbox by most web browsers, preventing them from accessing local data like clipboard or file system. 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.
A Java applet extends the class java.applet.Applet, or in the case of a Swing applet, javax.swing.JApplet. The class must override methods from the applet class to set up a user interface inside itself (Applet is a descendant of Panel which is a descendant of Container. As applet inherits from container, it has largely the same user interface possibilities as an ordinary Java application, including regions with user specific visualization.
The first implementations involved downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation as slow loading components. However, since jars were introduced, an applet is usually delivered as a single file that has a size of the bigger image (hundreds of kilobytes to several megabytes).
The domain from where the applet executable has been downloaded is the only domain to which the usual (unsigned) applet is allowed to communicate. This domain can be different from the domain where the surrounding HTML document is hosted.
Java system libraries and runtimes are backwards compatible, allowing to write code that runs both on current and on future versions of the Java virtual machine.
Many Java developers, blogs and magazines are recommending that the Java Web Start technology be used in place of Applets.[18][19] Java Web Start also allows to launch unmodified applet code, only it then runs in a separate window (not inside the invoking browser).
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.
The applet can be displayed on the web page by making use of the deprecated applet HTML element,[20] or the recommended object element.[21] Embed element can be used[22] with Mozilla family browsers (embed is no longer deprecated in since HTML 5). This specifies the applet's source and location. 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 any reason.
However, despite object being officially a recommended tag, as of 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, the object tag currently requires JavaScript (that recognizes the browser and adjusts the tag), usage of additional browser-specific tags or delivering adapted output from the server side. Deprecating applet tag has been criticised.[23] Oracle now provides a maintained JavaScript code[24] to launch applets with cross platform workarounds.
The following example is made simple enough to illustrate the essential use of Java applets through its java.applet package. It also uses classes from the Java Abstract Window Toolkit (AWT) for producing actual output (in this case, the "Hello, world!" message).
import java.applet.Applet; import java.awt.*; // Applet code for the "Hello, world!" example. // This should be saved in a file named as "HelloWorld.java". public class HelloWorld extends Applet { // This method is mandatory, but can be empty (i.e., have no actual code). public void init() { } // This method is mandatory, but can be empty.(i.e.,have no actual code). public void stop() { } // Print a message on the screen (x=20, y=10). public void paint(Graphics g) { g.drawString("Hello, world!", 20,10); // Draws a circle on the screen (x=40, y=30). g.drawArc(40,30,20,20,0,360); } }
Additional simple applets are available at Wikiversity.[25]
For compiling, this code is saved on a plain-ASCII file with the same name as the class and .java extension, i.e. HelloWorld.java. The resulting HelloWorld.class applet should be placed on the web server and is invoked within an HTML page by using an <APPLET> or an <OBJECT> tag. For example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML> <HEAD> <TITLE>HelloWorld_example.html</TITLE> </HEAD> <BODY> <H1>A Java applet example</H1> <P>Here it is: <APPLET code="HelloWorld.class" WIDTH="200" HEIGHT="40"> This is where HelloWorld.class runs.</APPLET></P> </BODY> </HTML>
Displaying the HelloWorld_example.html page from a Web server, the result should look as this:
A Java applet example
Here it is: Hello, world!
To minimize download time, applets are usually delivered in a form of compressed zip archive (having jar extension). If all needed classes (only one in our case) are placed in compressed archive example.jar, the embedding code would look different:
<P>Here it is: <APPLET code="HelloWorld" WIDTH="200" HEIGHT="40" ARCHIVE="example.jar"> This is where HelloWorld.class runs.</APPLET></P>
Applet inclusion is described in detail in Sun's official page about the APPLET tag.[26]
A Java applet can have any or all of the following advantages:[27]
A Java applet may have any of the following disadvantages:
applet tag, the object tag needs workarounds to write a cross-browser HTML document.Sun has made a considerable effort 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[31] was filed after Microsoft modified its own Java Virtual Machine which shipped with Internet Explorer. Microsoft added about 50 methods and 50 fields[31] 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.[32]
Microsoft continued to ship its own unmodified Java virtual machine. Over years it has become 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.[33] In 2002 Sun filed an antitrust lawsuit, claiming that Microsoft's attempts at illegal monopolization have 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).[32] 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.[34][35]
Google has developed their own Android platform that uses Java features and concepts, yet is incompatible with standard libraries. This may be a violation of conditions under which Sun granted OpenJDK patents to use open source Java for all.[36] In 2010, Oracle sued Google[37] for using Java "in a wrong way", claiming that "Google's Android competes with Oracle America's Java" and that "Google has been aware of Sun’s patent portfolio ... since Google hired certain former Sun Java engineers". As of April 2011[update], the lawsuit is ongoing.
There are two applet types with very different security models: signed applets and unsigned applets.[38]
Limits on unsigned applets are understood as "draconian":[39] they have no access to the local filesystem and web access limited to the applet download site; there are also many other important restrictions. For instance, they cannot access all system properties, use their own class loader, call native code, execute external commands on a local system or redefine classes belonging to core packages included as part of a Java release. While they can run in a standalone frame, such frame contains a header, indicating that this is an untrusted applet. Successful initial call of the forbidden method does not automatically create a security hole as an access controller checks the entire stack of the calling code to be sure the call is not coming from an improper location.
As with any complex system, multiple security problems have been discovered and fixed since Java was first released. Some of these (like the Calendar serialization security bug)[40] persisted for many years with nobody being aware. However it seems that most (if not all) security holes are closed before they can be exploited on a larger scale.
Some studies mention applets crashing the browser or overusing CPU resources but these are classified as nuisances[41] and not as true security flaws. However, unsigned applets may be involved in combined attacks that exploit a combination of multiple severe configuration errors in other parts of the system.[42] An unsigned applet can also be more dangerous to run directly on the server where it is hosted because while code base allows it to talk with the server, running inside it can bypass the firewall. An applet may also try DoS attacks on the server where it is hosted but usually people who manage the web site also manage the applet, making this unreasonable. Communities may solve this problem via source code review or running applets on a dedicated domain.[43][44]
The unsigned applet can also try to download malware hosted on originating server. However it could only store such file into temporary folder (as its transient data) and has no means to complete the attack by executing it. There were attempts to use applets for spreading Phoenix and Siberia exploits this way, while these exploits do not use Java internally and were also distributed in several other ways.
As of 1999, no real security breaches involving unsigned applets have ever been publicly reported.[41][45] Using an up-to-date Web browser is usually enough to be safe against the known direct attacks from unsigned applets.
A signed applet[46] contains a signature that the browser should verify through a remotely running, independent certificate authority server. Producing this signature involves specialized tools and interaction with the authority server maintainers. Once the signature is verified, and the user of the current machine also approves, a signed applet can get more rights, becoming equivalent to an ordinary standalone program. The rationale is that the author of the applet is now known and will be responsible for any deliberate damage. This approach allows applets to be used for many tasks that are otherwise not possible by client-side scripting. However, this approach requires more responsibility from the user, deciding whom he or she trusts. The related concerns include a non-responsive authority server, wrong evaluation of the signer identity when issuing certificates, and known applet publishers still doing something that the user would not approve of. Hence signed applets that appeared from Java 1.1 may actually have more security concerns.[47]
Self-signed applets, which are applets signed by the developer themselves, may potentially pose a security risk; java plugins provide a warning when requesting authorization for a self-signed applet, as the function and safety of the applet is guaranteed only by the developer itself, and has not been independently confirmed. Such self-signed certificates are usually only used during development prior to release where third-party confirmation of security is unimportant, but most applet developers will seek third-party signing to ensure that users trust the applet's safety.
Java security problems are not fundamentally different from similar problems of any client-side scripting platform. In particular, all issues related to signed applets also apply to Microsoft ActiveX components.
Alternative technologies exist (for example, JavaScript, Curl, Flash, and Microsoft Silverlight) that satisfy some of the scope of what is possible with an applet. Of these, JavaScript is not always viewed as a competing replacement; JavaScript can coexist with applets in the same page, assist in launching applets (for instance, in a separate frame or providing platform workarounds) and later be called from the applet code.[48] JavaFX is an extension of the Java platform and may also be viewed as an alternative.
|
|||||||||||||||||||||||||
|
|||||||||||