Java中的上下文加载器ContextClassLoader详解

public final class Class<T> implements java.io.Serializable,
                              GenericDeclaration,
                              Type,
                              AnnotatedElement {
    @CallerSensitive
    public static Class<?> forName(String className)
                throws ClassNotFoundException {
      	// 获取调用该方法的类
        Class<?> caller = Reflection.getCallerClass();
      	// ClassLoader.getClassLoader获取调用该方法的类的类加载器
        return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
    }

public final class ServiceLoader<S>
    implements Iterable<S>
{
    public static <S> ServiceLoader<S> load(Class<S> service) {
      	// 线程上下文类加载器,在Launcher类的构造器中被赋值为AppClassLoader,它可以读到ClassPath下的自定义类
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }     

public class Launcher {
    public Launcher() {
        ClassLoader extcl;
        try {
            // 首先创建扩展类加载器
            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 {
            // 再创建AppClassLoader并把extcl作为父加载器传递给AppClassLoader
            loader = AppClassLoader.getAppClassLoader(extcl);
        } catch (IOException e) {
            throw new InternalError(
                    "Could not create application class loader");
        }
        // 设置线程上下文类加载器,稍后分析
        Thread.currentThread().setContextClassLoader(loader);
        // 省略其他代码...
    }

public
class Thread implements Runnable {
    private ClassLoader contextClassLoader;
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        // 省略其他代码...
      	// 当前线程为父线程
        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            /* Determine if it's an applet or not */
            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }
            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }
        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();
        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }
        g.addUnstarted();
        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
      	// 子线程ContextClassLoader设置为父线程的ContextClassLoader
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        // 省略其他代码...
    }