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