public Class<?> loadClass(String name) throws ClassNotFoundException ：加载名称为name的类，返回结果为java.lang.Class类的实例，如果找不到类，则返回ClassNotFoundException异常，该方法中的逻辑就是双亲委派模式的实现
//测试代码ClassLoader.getSystemClassLoader().loadClass("com.chapter11.User")涉及以下方法调用
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) { //同步代码，保证多个线程只能有一个运行
// 首先，在缓存中判断是否已经加载同名的类
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
//获取当前类加载器的父类加载器，则调用父类加载器进行类的加载
if (parent != null) {
c = parent.loadClass(name, false);
} else { //扩展类加载器父类加载器是引导类加载器
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) { //当前类加载器的父类加载器未加载此类或当前类加载器未加载此类
// 调用当前ClassLoader的findClass()
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c); //是否允许解析
}
return c;
}
}
//在URLClassLoader中重写了findClass方法，代码如下
protected Class<?> findClass(final String name)
throws ClassNotFoundException
{
final Class<?> result;
try {
result = AccessController.doPrivileged(
new PrivilegedExceptionAction<Class<?>>() {
public Class<?> run() throws ClassNotFoundException {
String path = name.replace('.', '/').concat(".class");
Resource res = ucp.getResource(path, false);
if (res != null) {
try {
return defineClass(name, res);
} catch (IOException e) {
throw new ClassNotFoundException(name, e);
}
} else {
return null;
}
}
}, acc);
} catch (java.security.PrivilegedActionException pae) {
throw (ClassNotFoundException) pae.getException();
}
if (result == null) {
throw new ClassNotFoundException(name);
}
return result;
}
protected final Class<?> defineClass(String name, byte[] b, int off, int len) throws ClassFormatError：根据给定的字节数组b转换为Class实例，off和len参数表示实际Class信息在byte数组中的位置和长度，其中byte数组b是ClassLoader从外部获取的，这是受保护的方法，只有在自定义ClassLoader子类中可以使用
