Class not available after using ClassLoader

Question

If I load a class from file at runtime using a URLClassLoader:

ClassLoader classLoader = new URLClassLoader(new URL[] { classesUrl }, getClass().getClassLoader());
String name = fileName.replace("\\", ".").replace("/", ".").substring(0, s.lastIndexOf("."));

System.out.println("loading class " + name);
Class c = classLoader.loadClass(name);
System.out.println("loaded " + c.getCanonicalName()); // 1

This seems to work - output at 1 is loaded com.robert.test.NumberUtil.

If I then try to create an instance of that class using

Class.forName("com.robert.test.NumberUtil");

I get a ClassNotFoundException: com.robert.test.NumberTest. Is what I'm trying to do possible? Or do I have to use the class at 1 (i.e. use the object returned from classLoader.loadClass() and once it's out of scope reload it?).

Answer 1

I worked around it. I was trying to compile JUnit tests, then load the generated classes, and run any classes annotated with @RunWith(Suite.class), but when I tried to pass the classes to JUnitCore, the loaded classes seemed to get lost.

Anyway, because this is for a custom Ant task anyway, I used the Ant JUnitTask class, and used the canonical names of the classes to run and added the compiled test directory to the JUnitTask classpath.

Answer 2

Class.forName() does not create a new instance of your class, it just attempts to find the Class object for that class. To create a new instance, you may use newInstance() on some Class object:

Object something = c.newInstance(); // use proper type instead of Object!

This only works for instantiating classes with a default constructor; otherwise you have to use reflection to instantiate an object.

Also note that creating a new ClassLoader does not immediately affect classloading behaviour. For your example above, you had to to invoke

Class.forName("com.robert.test.NumberUtil", true, c); // pass explicit ClassLoader

To make your ClassLoader replace the current ClassLoader, you might want to say

Thread.currentThread().setContextClassLoader(c)