双亲委派模型

一、类加载器简介：
JVM启动时，会形成由三个类加载器组成的初始类加载器层次结构：

• Bootstrap ClassLoader：根类加载器，也称为引导或原始类加载器。它负责加载Java的核心类。它比较特殊，不是ClassLoader的子类，而是由JVM自身实现的。
• Extension ClassLoader：扩展类加载器。它负责加载JRE的扩展目录（JAVA_HOME/jre/lib/ext 或者由 java.ext.dirs 系统属性指定的目录）中JAR的类包。通过这种方式，我们就可以为java扩展核心类以外的新功能，只要我们把自己开发的类打包成JAR文件，然后放入JAVA_HOME/jre/lib/ext 路径即可。
• System ClassLoader：系统（也称为应用）类加载器，。它负责在JVM启动时，加载来自命令java中的-classpath选项或java.class.path系统属性，或CLASSPATH环境变量所指定的JAR包和类路径。程序可以通过ClassLoader的静态方法getSystemClassLoader()获取该类加载器。如果没有特别指定，则用户自定义的类加载器都以该类加载器作为它的父加载器。
  二、加载机制之双亲委派模型源码分析：

/**
     * Loads the class with the specified binary name.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * 

     *
     *   
 Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  
     *
     *   
 Invoke the {@link #loadClass(String) loadClass} method
     *   on the parent class loader.  If the parent is null the class
     *   loader built-in to the virtual machine is used, instead.  
     *
     *   
 Invoke the {@link #findClass(String)} method to find the
     *   class.  
     *
     * 
     *
     * 
 If the class was found using the above steps, and the
     * resolve flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting Class object.
     *
     * 
 Subclasses of ClassLoader are encouraged to override {@link
     * #findClass(String)}, rather than this method.  
     *
     * 
 Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock getClassLoadingLock} method
     * during the entire class loading process.
     *
     * @param  name
     *         The binary name of the class
     *
     * @param  resolve
     *         If true then resolve the class
     *
     * @return  The resulting Class object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    protected Class loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            // 首先，检查该类是否已经加载了
            Class c = findLoadedClass(name);
            if (c == null) {
                // 表明目标类还没被加载
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        // 如果当前类加载器不是根类加载器，则委派父类去加载目标类
                        c = parent.loadClass(name, false);
                    } else {
                        // 如果当前类加载器已经是根类加载器，则由根类加载器去加载目标类
                        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.
                    // 如果所有父类加载器都没加载成功，则调用本身类加载器的findClass进行
                    // 类加载（自定义类加载器要实现该方法）
                    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;
        }
    }