Dynamic Class Loading

RMI system uses object serialization to transmit data from one machine to another machine. When an object is passed from one program to another program, either as a parameter or return value of a remote method, then the receiving program must resolve that object. When parameters and return values are unmarshalled on the receiving end, they become live objects. The classes for parameters and return values are required for all types of objects. If you pass standard Java objects (e.g., String, Vector etc.), the classes will be loaded locally because these classes are available in every JVM. If you declare a remote method with a certain return type that is not known to the client (like custom classes and interfaces), then these classes must also be resolved at the time of unmarshaling. One way to make these classes available is to copy these files from one machine to another. However, if the server adds new classes for return types, then the client must be updated. In a small local area network (LAN), it is possible to update client application, every time the server is updated, but if it were a Wide Area Network (WAN), then it would be a problem to keep updating the client every time the server is updated. For that reason, RMI provides the facility of dynamic class loading. The RMI system uses an existing web server to dynamically load classes (see Figure 6.1). Whenever a program loads classes from another network location, there is a security issue. For dynamic loading of classes, a security manager must be installed in the client code. You may install a security manager in the server code, if the server is itself a client.

Figure 6.1

This chapter shows two examples of dynamic class loading.

The source code needed for the first example is stored in the examples\dynamic directory. The "dynamic" directory contains the following files:

• TDInterface.java
• TDServerImpl.java
• TDApplet.java
• TDClient.html
• Compile.bat
• Rmicompile.bat
• RunServer.bat
• RunApplet.bat

To run this example you need the following applications:

Netscape Navigator or appletviewer
Java Web Server