java的ClassLoader

java有两种类型的classload,一种是user-defined的，一种是jvm内置的bootstrap class loader，所有user-defined的class loader都是java.lang.ClassLoader的子类.

而jvm内置的class loader有3种，分别是 Bootstrap ClassLoader， Extension ClassLoader（即ExtClassLoader），System ClassLoader(即AppClassLoader).

而jvm加载时的双亲委派 就不说了，javaeye上有很多文章都有介绍..
可以分别看一下他们的构造器，其中Bootstrap ClassLoader是用c写的.
java.lang.ClassLoader

    protected ClassLoader(ClassLoader parent) {
	SecurityManager security = System.getSecurityManager();
	if (security != null) {
	    security.checkCreateClassLoader();
	}
//给父loader赋值.
	this.parent = parent;
	initialized = true;
    }
    protected ClassLoader() {
	SecurityManager security = System.getSecurityManager();
	if (security != null) {
	    security.checkCreateClassLoader();
	}
//这边将会把AppClassLoader付给父loader
	this.parent = getSystemClassLoader();
	initialized = true;
    }

这个构造器有带参数的，和不带构造器的2个。带参数的构造器传入的是这个 class loader的父loader,而不带参数的构造器则会把getSystemClassLoader()所返回的class loader当作他自己的父装载器.下面我们来看getSystemClassLoader()的代码

public static ClassLoader getSystemClassLoader() {
//所返回的class loader在这个方法里面被赋值
	initSystemClassLoader();
	if (scl == null) {
	    return null;
	}
	SecurityManager sm = System.getSecurityManager();
	if (sm != null) {
	    ClassLoader ccl = getCallerClassLoader();
	    if (ccl != null && ccl != scl && !scl.isAncestor(ccl)) {
		sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
	    }
	}
	return scl;
    }

    private static synchronized void initSystemClassLoader() {
	if (!sclSet) {
	    if (scl != null)
		throw new IllegalStateException("recursive invocation");
            sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
	    if (l != null) {
		Throwable oops = null;
//在这边被赋值
		scl = l.getClassLoader();
	         ...................................
.......................................
		}
	    }
	    sclSet = true;
	}
    }

这边的父class loader就是scl，也就是l.getClassLoader()所得到的,getClassLoader(),接下来看Launcher的源码：

private static Launcher launcher = new Launcher();

    public static Launcher getLauncher() {
	return launcher;
    }

    private ClassLoader loader;

    public Launcher() {
	// Create the extension class loader
	ClassLoader extcl;
	try {
//这里传入构造器的parent为空
	    extcl = ExtClassLoader.getExtClassLoader();
	} catch (IOException e) {
	    throw new InternalError(
		"Could not create extension class loader");
	}

	// Now create the class loader to use to launch the application
	try {
//这边可以看到默认的loader就是AppClassLoader,也就是说getSystemClassLoader返回的就是AppClassLoader
	    loader = AppClassLoader.getAppClassLoader(extcl);
	} catch (IOException e) {
	    throw new InternalError(
		"Could not create application class loader");
	}
//每一个当前线程一个classload,以防止多线程中的classload引起的混乱(这个是我自己理解的，呵呵)
	// Also set the context class loader for the primordial thread.
	Thread.currentThread().setContextClassLoader(loader);

	...................................
................................................
    }

    /*
     * Returns the class loader used to launch the main application.
     */
    public ClassLoader getClassLoader() {
	return loader;
    }

从中我们看到AppClassLoader的父loader是ExtClassLoader,而ExtClassLoader的父loader是什么呢？我们在来看ExtClassLoader的构造器:

public ExtClassLoader(File[] dirs) throws IOException {
	    super(getExtURLs(dirs), null, factory);
	    this.dirs = dirs;
	}

他的父loader为空，而他的顶级父类是java.lang.ClassLoader而当传入的parent为null时，我们使用 ExtClassLoader load一个类时，系统会调用Bootstrap ClassLoader.

protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
    {
	// First, check if the class has already been loaded
	Class c = findLoadedClass(name);
	if (c == null) {
	    try {
		if (parent != null) {
//先调用父loader来load.
		    c = parent.loadClass(name, false);
		} else {
//调用Bootstrap ClassLoader来load
		    c = findBootstrapClass0(name);
		}
	    } catch (ClassNotFoundException e) {
	        // If still not found, then invoke findClass in order
	        // to find the class.
	        c = findClass(name);
	    }
	}
	if (resolve) {
	    resolveClass(c);
	}
	return c;
    }

而这里findBootstrapClass0也就是调用Bootstrap ClassLoader这个最核心的class loader来load class.

最终我们可以看到getSystemClassLoader() 返回的class loader就是AppClassLoader.