关于volatile的具体作用

Question

为什么下面这段代码是线程不安全的。 public class Singleton { private static Singleton singleton; private Singleton (){} public static Singleton getSingleton() { if (singleton == null) { ① synchronized (Singleton.class) { ② if (singleton == null) { ③ singleton = new Singleton(); ④ } } } return singleton; } } 而下面这段代码singleton被volatile修饰后就是线程安全的 public class Singleton { private volatile static Singleton singleton; private Singleton (){} public static Singleton getSingleton() { if (singleton == null) { ① synchronized (Singleton.class) { ② if (singleton == null) { ③ singleton = new Singleton(); ④ } } } return singleton; } }

Answer 1

volatile 保证了变量修改在不同线程的可见性