// 檢查類是否已被裝載過
Class c = findLoadedClass(name);
if (c == null ) {
// 指定類未被裝載過
try {
if (parent != null ) {
// 如果父類加載器不為空， 則委派給父類加載
c = parent.loadClass(name, false );
} else {
// 如果父類加載器為空， 則委派給啟動(dòng)類加載加載
c = findBootstrapClass0(name);
}
} catch (ClassNotFoundException e) {
// 啟動(dòng)類加載器或父類加載器拋出異常后， 當(dāng)前類加載器將其
// 捕獲， 并通過findClass方法， 由自身加載
c = findClass(name);
}
}

public static Class forName(String className)
throws ClassNotFoundException {
return forName0(className, true , ClassLoader.getCallerClassLoader());
}
 
/** Called after security checks have been made. */
private static native Class forName0(String name, boolean initialize,
ClassLoader loader)
throws ClassNotFoundException;

// Now create the class loader to use to launch the application
try {
loader = AppClassLoader.getAppClassLoader(extcl);
} catch (IOException e) {
throw new InternalError(
"Could not create application class loader" );
}
 
// Also set the context class loader for the primordial thread.
Thread.currentThread().setContextClassLoader(loader);

import java.io.File;
import java.io.FileInputStream;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
 
/**
* 可以重新載入同名類的類加載器實(shí)現(xiàn)
*
 
* 放棄了雙親委派的加載鏈模式.
* 需要外部維護(hù)重載后的類的成員變量狀態(tài).
*
* @author ken.wu
* @mail ken.wug@gmail.com
* 2007-9-28 下午01:37:43
*/
public class HotSwapClassLoader extends URLClassLoader {
 
public HotSwapClassLoader(URL[] urls) {
super (urls);
}
 
public HotSwapClassLoader(URL[] urls, ClassLoader parent) {
super (urls, parent);
}
 
public Class load(String name)
throws ClassNotFoundException {
return load(name, false );
}
 
public Class load(String name, boolean resolve)
throws ClassNotFoundException {
if ( null != super .findLoadedClass(name))
return reload(name, resolve);
 
Class clazz = super .findClass(name);
 
if (resolve)
super .resolveClass(clazz);
 
return clazz;
}
 
public Class reload(String name, boolean resolve)
throws ClassNotFoundException {
return new HotSwapClassLoader( super .getURLs(), super .getParent()).load(
name, resolve);
}
}
 
public class A {
private B b;
 
public void setB(B b) {
this .b = b;
}
 
public B getB() {
return b;
}
}
 
public class B {}

public class TestHotSwap {
public static void main(String args[]) {
A a = new A();
B b = new B();
a.setB(b);
 
System.out.printf("A classLoader is %s n" , a.getClass().getClassLoader());
System.out.printf("B classLoader is %s n" , b.getClass().getClassLoader());
System.out.printf("A.b classLoader is %s n" ,   a.getB().getClass().getClassLoader());
 
HotSwapClassLoader c1 = new HotSwapClassLoader( new URL[]{ new URL( "file:\e:\test\")} , a.getClass().getClassLoader());
Class clazz = c1.load(" test.hotswap.A ");
Object aInstance = clazz.newInstance();
 
Method method1 = clazz.getMethod(" setB ", B.class);
method1.invoke(aInstance, b);
 
Method method2 = clazz.getMethod(" getB ", null);
Object bInstance = method2.invoke(aInstance, null);
 
System.out.printf(" reloaded A.b classLoader is %s n", bInstance.getClass().getClassLoader());
}
}