jvm双亲委派机制

JVM整体流程如上图所示.java类的加载是有加载器来进行的.JVM中有以下几种类加载器:
1.引导类加载器BootstrapClassLoader :负责加载支撑JVM运行的位于JRE的lib目录核心类库,比如rt.jar,charsets.jar等
2.扩展类加载器ExtClassLoader :负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录下的JAR包
3.应用程序类加载器:负责加载ClassPath路径下的类包
4.自定义加载器:负责加载用户自定义路径下的类包.

 public Launcher() {
        Launcher.ExtClassLoader var1;
        try {
            var1 = Launcher.ExtClassLoader.getExtClassLoader();
        } catch (IOException var10) {
            throw new InternalError("Could not create extension class loader", var10);
        }

        try {
            this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
        } catch (IOException var9) {
            throw new InternalError("Could not create application class loader", var9);
        }

        Thread.currentThread().setContextClassLoader(this.loader);
        String var2 = System.getProperty("java.security.manager");
        if (var2 != null) {
            SecurityManager var3 = null;
            if (!"".equals(var2) && !"default".equals(var2)) {
                try {
                    var3 = (SecurityManager)this.loader.loadClass(var2).newInstance();
                } catch (IllegalAccessException var5) {
                } catch (InstantiationException var6) {
                } catch (ClassNotFoundException var7) {
                } catch (ClassCastException var8) {
                }
            } else {
                var3 = new SecurityManager();
            }

            if (var3 == null) {
                throw new InternalError("Could not create SecurityManager: " + var2);
            }

            System.setSecurityManager(var3);
        }

    }

注意:ExtClassLoader 和AppClassLoader直接并没有继承关系只是AppClassLoader中的parent属性是ExtClassLoader


双亲委派源码实现机制:ClassLoader.loadClass();

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 {
                	//从第一张图中可以看出AppClassLoader.parent属性值ExtClassLoader
                   // ExtClassLoader的parent属性值为null
                    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;
        }
    }

该方法大体逻辑是:
1 首先，检查一下指定名称的类是否已经加载过，如果加载过了，就不需要再加载，直接 返回。
2. 如果此类没有加载过，那么，再判断一下是否有父加载器；如果有父加载器，则由父加 载器加载（即调用parent.loadClass(name, false);）.或者是调用bootstrap类加载器来加 载。
3. 如果父加载器及bootstrap类加载器都没有找到指定的类，那么调用当前类加载器的 findClass方法来完成类加载。