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?).

can you provide more clarifications for a) Are you loading a file url, network url? b) what version of Java are you using c) At what later point do you call `Class.forName()`: a different method, a different thread? — c.s., Jul 15 '13 at 22:55

score 3 · Answer 1 · answered Jul 15 '13 at 21:52

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)

score 0 · Accepted Answer · answered Jul 24 '13 at 17:56

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.

Class not available after using ClassLoader

2 Answers2