Thursday, December 6

Applets in Java

Where Do Applets and Classes Come from?
When a web browser sees an applet tag and decides to download and play the applet,
it starts a long chain of events. Let's say your browser sees the following applet tag:

<APPLET codebase=""
code="Animation.class" width="200" height="100" >

1.      The web browser sets aside a rectangular area on the page 200 pixels wide and
100 pixels high. In most web browsers, this area has a fixed size and cannot be
modified once created. The appletviewer in the JDK is a notable exception.
2.      The browser opens a connection to the server specified in the codebase parameter, using port 80 unless another port is specified in the codebase URL. If there's no codebase parameter, then the browser connects to the same server that served the HTML page.
3.      The browser requests the .class file from the web server as it requests any other file. If a codebase is present, it is prefixed to the requested filename. Otherwise, the document base (the directory that contains the HTML page) is used. For example:
GET /javafaq/classes/Animation.class HTTP 1.0

4.      The server responds by sending a MIME header followed by a blank line (\r\n) followed by the binary data in the .class file. A properly configured server sends .class files with MIME type application/octet-stream. For example:
HTTP 1.0 200 OK
Date: Mon, 10 Jun 1999 17:11:43 GMT
Server: Apache/1.2.8
Content-type: application/octet-stream
Content-length: 2782
Last-modified: Fri, 08 Sep 1998 21:53:55 GMT
5.      The web browser receives the data and stores it in a byte array. The byte code verifier goes over the byte codes that have been received to make sure they don't do anything forbidden, such as converting an integer into a pointer.
6.      If the byte code verifier is satisfied with the bytes that were downloaded, then the raw data is converted into a Java class using the defineClass( ) and loadClass( ) methods of the current ClassLoader object.
7.      The web browser instantiates the Animation class using its noargs constructor.
8.      The web browser invokes the init( ) method of Animation.
9.       The web browser invokes the start( ) method of Animation.

10.  If the Animation class references another class, the Java interpreter first searches for
the new class in the user's CLASSPATH. If the class is found in the user's CLASSPATH,
then it is created from the .class file on the user's hard drive. Otherwise the web
browser goes back to the site from which this class came and downloads the .class file for the new class. The same procedure is followed for the new class and any other
class that is downloaded from the Net. If the new class cannot be found, a
ClassNotFoundException is thrown

Who Can an Applet Talk to and What Can It Say?

1.      Applets cannot access arbitrary addresses in memory. Unlike the other restrictions in the list, which are enforced by the browser's SecurityManager instance, this restriction is a property of the Java language itself and the byte code verifier.
2.      Applets cannot access the local filesystem in any way. They cannot read from or write to the local filesystem nor can they find out any information about files. Therefore, they cannot find out whether a file exists or what its modification date may be.
3.      Applets cannot launch other programs on the client. In other words, they
cannot call System.exec( ) or Runtime.exec( ).
4.      Applets cannot load native libraries or define native method calls.
5.      Applets are not allowed to use System.getProperty( ) in a way that reveals
information about the user or the user's machine, such as a username or home
directory. They may use System.getProperty( ) to find out what version of
Java is in use.
6.       Applets may not define any system properties.
7.      In Java 1.1 and later, applets may not create or manipulate any Thread or
ThreadGroup that is not in the applet's own ThreadGroup. They may do this
in Java 1.0.
8.       Applets cannot define or use a new instance of ClassLoader, SecurityManager, ContentHandlerFactory, SocketImplFactory, or URLStreamHandlerFactory. They must use the ones already in place.

9.      An applet can only open network connections to the host from which the applet itself was downloaded.
10.  An applet cannot listen on ports below 1,024. (Internet Explorer 5.0 doesn't
allow applets to listen on any ports.)
11.  Even if an applet can listen on a port, it can accept incoming connections only
from the host from which the applet itself was downloaded.

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