Using URLClassLoader

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By MageLang Institute

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Help is available for each task, or you can go straight to the solution source code.

Task 1

In the URLTester.java skeleton, create an array holding the URL of the file (directory name) specified from the command line, as args[0].

URL urlList[] = {new File(args[0]).toURL()};

Task 2

Create an instance of URLClassLoader, using this array in its constructor.

ClassLoader loader = new URLClassLoader(urlList);

Task 3

Next, have the test program load a class whose name will be provided from the command line, also, as args[1].

Class c = loader.loadClass (args[1]);

Task 4

After loading the class, create an instance of it. You can use reflection and the Constructor object, or just the newInstance method if the class has a no argument constructor.

Object tester = c.newInstance();

Task 5

The Tester.java file contains a test class to use for the class loader. The only thing it does is print messages when the class file is loaded and instances are created. Save and compile the file.

Shift-click on the Tester.java link to save. Then compile with the following command:

javac Tester.java

Task 6

If you have a "." in your CLASSPATH, move the Tester.class file to another directory.

Something like the following will do. Depending upon your platform, the specific command may vary.

mkdir test
move Tester.class test

Task 7

Run the URLTester program. The first argument to the program is the directory you just moved the Tester.class file to. The second argument is the class you wish to load, Tester.

java URLTester test Tester

Task 8

To demonstrate the flexibility of the URLClassLoader, place the Tester.class file in a JAR file. Then, tell the URLTester program to load the file from the JAR file, instead of a directory.

jar cf test.jar -C test Tester.class
java URLTester test.jar Tester

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