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Java Tip 39: The trick to using a basic Java 1.1 network and file class loader

Having trouble with that class loader? Here's how to use one in Java 1.1 that works either over the 'Net or from a local file

By Jack Harich, JavaWorld.com, 09/01/97

For his "Java In Depth" column in the October '96 issue of JavaWorld, Chuck McManis discussed in "The basics of Java class loaders." Based on that column, this Java Tip demonstrates how to successfully use the Java 1.1 class loader and goes a lot further.

Java class loader requirements

Let's first examine our requirements for a class loader. We'd like one that:

• can load classes from any legitimate URL pointing to a class file
  
  
• can also load classes from any local class file; this is useful for
  
  

  testing or simulating an Internet server on a client.

• can optionally translate periods in the class name to a preferred character, such as "_"; this is useful for placing classes from different packages/directories into the same directory
  
  
• follows the rules that all class loaders must obey
  
  

High-level design

The most flexible approach to the above requirements is these three classes:

     public abstract class MultiClassLoader extends ClassLoader
     public class URLClassLoader extends MultiClassLoader
     public class FileClassLoader extends MultiClassLoader

This allows the type of class loader to be determined at runtime and the rest of the application is unaffected.

Unit test

Hmmmm, fulfilling the above requirements doesn't sound too hard. Let's start with a "Use Case," namely a code example of how we'll use MultiClassLoader. This is best done by designing our test class, called TestLoader. Now, let's write the TestLoader unit test class. This shows how to set the class loader type at runtime, and then uses it in a variety of ways to illustrate how to use a class loader effectively. Here is the source code for TestLoader.java.

Let's examine the heart of the test code:

    // Init the preferred class loader
    MultiClassLoader loader = null;
    if (args.length == 0) {
        String root = "http://www.mindspring.com/happyjac/";
        loader = new URLClassLoader(root);
    } else {
        loader = new FileClassLoader("store\\");
    }
    loader.setClassNameReplacementChar('_');
    // A series of tests:
    Class testClass = null;
    try {
        testClass = loader.loadClass("Hello");
    } catch(Exception ex) {
        print("Load failed");
        ex.printStackTrace();
        return;
    }
    print("Loaded class " + testClass.getName() );
    try {
        Runnable hello = (Runnable)testClass.newInstance();
        hello.run();
    } catch(Exception ex) {
        print("Failed to instantiate");
        ex.printStackTrace();
    }

Good tests are understandable and short. The test first creates an instance of class loader called "loader". Then it uses an optional feature to translate periods to "_".

Next we decide whether to load from the Internet or from a local file, depending on whether any command-line argument was entered. Note that you will need to set the root or filePrefix to whatever source URL you're using as a base. We have subclassed MultiClassLoader, providing a clean way to provide different specific class sources.