JVM16_类的概述、分类、ClassLoader源码分析、自定义类的加载器、双亲委派机制、沙箱安全机制（四）

 测试代码:
 ClassLoader.getSystemClassLoader().loadClass("com.xiaozhi.java.User");
 //resolve==true,加载class的同时需要进行解析操作
 protected Class<?> loadClass(String name, boolean resolve) 
        throws ClassNotFoundException
    {
    //同步操作,保证只能加载一次
        synchronized (getClassLoadingLock(name)) {
            // 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 {
          //获取当前类的父类加载器
                    if (parent != null) {
            //如果存在父类加载器,则调用父类加载器进行类的加载(双亲委派机制)
                        c = parent.loadClass(name, false);
                    } else {
            //parent==null 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
                }
        // 当前类的加载器的父类加载器未加载此类 or 当前类的加载器未加载此类
                if (c == null) {
                    // 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();
                }
            }
      //是否进行解析操作
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }