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FAQs: Applets

- What are the alternatives to using applets?

- Can I use a "native" two-tier driver for a browser applet?

- Why doesn't my browser applet connect to the database?

- Why does my applet work with Appletviewer, but not with a browser?

- What's causing ClassFormatErrors with my applet?

What are the alternatives to using applets?

BEA supports the use of server-side applications with HTTP servlets and Java Server Pages (JSPs) as part of the J2EE platform. We recommend that before you develop new applications, consider using either servlets or JSPs. A well-designed series of interactive Web pages using servlets and Java Server Pages (JSPs) generally yield a faster and more reliable Web site.If you are currently using applets, you may find that most can be converted to Java applications using Java Web Start and you can continue using WebLogic Server. For information, go to Sun's Java Web Start site.

For more information, see Using Applets With WebLogic Server.

Can I use a "native" two-tier driver for a browser applet?

No. Within an unsigned applet, you cannot load native libraries over the wire, access the local file system, or connect to any host except the host from which you loaded the applet. The applet security manager enforces these restrictions on applets as protection against applets being able to do unsavory things to unsuspecting users.

If you are trying to use jDriver for Oracle from an applet, then you are violating the first restriction. Your applet will fail when it attempts to load the native (non-Java layer) library that allows jDriver for Oracle to make calls into the non-Java Oracle client libraries. If you look at the exception that is generated, you will see that your applet fails in java.lang.System.loadLibrary, because the security manager determined that you were attempting to load a local library and halted the applet.

You can, however, use the WebLogic JTS or Pool driver for JDBC connectivity in applets. When you use one of these WebLogic multitier JDBC drivers, you need one copy of WebLogic jDriver for Oracle (or any other two-tier JDBC driver) for the connection between the WebLogic Server and the DBMS.

Why doesn't my browser applet connect to the database?

Problem: I'm using a WebLogic multitier driver in an applet as an interface to a DBMS. If I run the class using the Sun Appletviewer on my local machine, I have no problems. But when I try to run the applet in a Netscape browser, it will not connect.

If Appletviewer works and Netscape does not, it is an indication that you are violating a Netscape security restriction. In this case, the violation is that an applet cannot open a socket to a machine other than the one from which it loaded the applet. To solve this problem, you will have to serve your applet code from the same machine that hosts the DBMS.

In addition, the IP naming format you use in the applet CODEBASE and the constructor for the T3Client must match. That is, you can't use dot-notation in one place and a domain name in the other.

Why does my applet work with Appletviewer, but not with a browser?

Problem: I tried to run two of the applets in the examples directory of the distribution. I installed the WebLogic classes on my local machine (NT server) and on another machine (a Windows 95 client). I am not using any browsers, just trying to run the applets with Appletviewer. The applets work fine when I run Appletviewer from the NT server, but do not work at all from the Windows 95 client.

There are two possible problems: Either the CODEBASE tag is not properly set in the applet HTML file, or the class files are not properly loaded on the HTTP server.

The applet works on the NT server because you installed the WebLogic distribution on your NT server. Even if the applet cannot successfully load the necessary classes from the HTTP server, it does find them in your local CLASSPATH. But when you try to run it from the Windows 95 client, the applet must load the classes over the wire from the HTTP server, and if you haven't installed them correctly, it will fail.

What's causing ClassFormatErrors with my applet?

Problem: I downloaded your distribution and copied the classes to my HTTP server DocumentRoot. I created an applet that I ran successfully from my Netscape server. I placed it in the server directory /webz/ns-home/classes/applets/myapp.class and called it with the following:

>       <APPLET
>         CODEBASE=http://myserver.com/webz/ns-home/classes
>         CODE=applets.myapp.class>

Then I set my attributes in the Administration Console to listen on port 7001, and I started the WebLogic Server on the HTTP machine so I could use my applet with WebLogic JDBC, like this:

>       <APPLET
>              CODEBASE=t3://myserver.com:7001/webz/ns-home/classes
>              CODE=applets.myapp.class>

When I changed the CODEBASE tag to point to the WebLogic Server, I started getting ClassFormatErrors.

There are several problems with your setup. The most obvious have to do with your CODEBASE:

1. The CODEBASE tag in your applet should point to your HTTP server, not to WebLogic Server.

2. The directory path referenced in your CODEBASE tag is not an absolute directory path on the HTTP server; it is a configured path that originates from the HTTP Document Root. You are using the absolute path in your CODEBASE tag. If your class "myapp" is in the "applets" package, then the correct CODEBASE for your setup would be:

   >       <APPLET
   >               CODEBASE=http://myserver.com/classes
   >               CODE=applets.myapp.class>

In addition, if you are getting a ClassFormatError, it signals a problem with your HTTP server configuration. It could be that you haven't loaded the WebLogic or applet classes in the correct directory on the HTTP server, or you are specifying the CODEBASE or the CODE incorrectly in your APPLET tag.

Remember, too, that if you installed the WebLogic distribution on the machine from which you are running the applet, your applet will first look for the WebLogic classes in your local CLASSPATH, which may obscure the fact that you haven't properly installed the classes for serving from the HTTP server. To test your HTTP configuration properly, you need to temporarily rename the WebLogic classes in your local CLASSPATH or try your applet from another machine.

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