Lesson 9

The applet in this module was very simple. It did not do much more than the HTML form we used in previous modules. However, it was enough to demonstrate how an applet communicates with a servlet, and responds to what the servlet sends back. Now you know how to:

1. Add member variables to your applet to hold the information you will send to a servlet
2. Build a URL that represents a GET request
3. Open a connection to a servlet
4. Build a BufferedReader to read the servlet output
5. Display the servlet output in your applet

When a Java applet is embedded in a web page, a browser can download it and execute it automatically. If you think about it, that is a very dangerous thing to do. So, to protect the client, JDK 1.0 assumed all applets were untrusted and ran them under the watch of a SecurityManager that severely limited what they could do. For example, the security manager made sure applets could not write to the user's file system, read certain system properties, accept incoming socket connections, or establish outgoing socket connections to any host but the origin server.
This protected the client, but it limited the usefulness of applets. Consequently, JDK 1.1 introduced the concept of trusted applets, which are applets that can operate like normal applications with full access to the client machine. For an applet to be trusted, it has to be digitally signed by a person or company the client trusts (as marked in the client's browser). The signature authenticates the applet's origin and guarantees integrity during the transfer, so the client knows the applet code has not been surreptitiously changed. This allowed for more productive applets, but it was an all-or-nothing approach. To give the client more control, JDK 1.2 is introducing a fine-grained access control system. Under this new system, a digitally signed applet can be partially trusted, given certain abilities without being given free reign on the system. This promises to allow applets from unknown sources to be granted small privileges (such as writing to a single directory), without granting them the ability to wipe the client's hard drive.

The following quiz tests on how applets and servlets communicate.
Applets Servlets - Quiz