Classloader的双亲委派模型

代码体现loadClass方法

 protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    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);
            }
            return c;
        }
    }

类的加载流程

• findLoadedClass(name)检查该类是否加载过,加载过则直接返回该类的class对象
• 然后判断是否有父类的ClassLoader加载器,如果有则调用父类加载器加载该类,没有则调用bootstrapClassLoader去加载该类
• 如何还有没有找到,则调用自身的findClass方法,所以当我们自定义ClassLoader是需覆盖此方法

 /**
     * Returns a class loaded by the bootstrap class loader;
     * or return null if not found.
     */
    private Class<?> findBootstrapClassOrNull(String name)
    {
        if (!checkName(name)) return null;

        return findBootstrapClass(name);
    }

该机制有效的保证了java核心类的正确加载,保护了java虚拟机的安全