困惑（线程锁）

写两种常用的单例模式实现，对比一下两种方式的实现有什么差异：

No1：

public class Test {private static Test instance = null;public static Test getInstance(){synchronized (Test.class) {        if (instance == null) {        instance = new Test();   }}return instance;}}

No2：

public class Test {private static Test instance = null;public synchronized static Test getInstance(){if (instance == null) {instance = new Test();}return instance;}}