fighting with the tricky ClassLoader

elgs

The conceptions of ClassLoader are the most tricky ones in the javaworld...
一想到Reflection, ClassLoader就像做恶梦, 但他们确实非常有趣, 每一个问题都是一个挑战. 和这些东西鏖战几个月后, 经过无数测试, 今天又弄明白一个概念, 所以写下来, 希望能对大家有所帮助, 同时也加深我的记忆.
假设有两个ClassLoader, parentLoader and childLoader, parentLoader is the parent classloader of the childLoader. Now, according to java's defalt delegation class loading mechanism, when the child class loader wants to load a class, say com.foo.Bar, it first asks his father, hey, dad, do you have the class com.foo.Bar previously loaded or in your classpath, then, recursively, the father does the same thing. If the father answered yes, then it returned the class to his son, if not, he told the son, no, I don't have it previously loaded or in my classpath, finally, the son search his own classpath, if it still can not find the class in his own classpath, then, a ClassNotFoundException thrown. This is the story in the java class loading world.
Now the child class loader hopes to get a class com.foo.Bar. but the class is found in the parent class loader's classpath.
ClassLoader parentLoader = new URLClassLoader(someURLs, null);
ClassLoader childLoader = new URLClassLoader(someOtherUrls, parentLoader);
Class c = childLoader.loadClass("com.foo.Bar");
Method m = c.getMethod("main", new Class[] { args.getClass() });
m.invoke(null, new Object[] { args });
Now, everything is happening in the new world(classpath). And, suddenly, a ClassNotFoundException is thrown when loading a class within the child class loader occured. Then, I move the class desired in to the father's classpth, it returns calm. So we see, everything is now happening in the father's world!! We load a class from the child class loader, but the class is actually found in the father's class loader, and we invoke a method on the class loaded. Then next time within the method, it load a new class, it search from the parent class loader, not the child. Really tricky!


本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/4960/showart_39476.html