双检锁

public class DoubleCheckLockingSingletonDemo {

    private DoubleCheckLockingSingletonDemo() {
    }

    private static volatile DoubleCheckLockingSingletonDemo instance;

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

}

枚举

public enum EnumSingletonDemo {

    INSTANCE;

    EnumSingletonDemo() {
    }

    public void whateverMethod() {
    }
}

静态内部类

public class InternalStaticClassSingletonDemo {

    private InternalStaticClassSingletonDemo() {
    }

    static class InternalStaticClass {
        public static volatile InternalStaticClassSingletonDemo instance = new InternalStaticClassSingletonDemo();
    }

    public static InternalStaticClassSingletonDemo getInstance() {
        return InternalStaticClass.instance;
    }

}

同步方法

public class SynchronizedFuncSingletonDemo {

    private SynchronizedFuncSingletonDemo() {
    }

    private static SynchronizedFuncSingletonDemo instance;

    public synchronized SynchronizedFuncSingletonDemo getInstance1() {
        if (null == instance) {
            instance = new SynchronizedFuncSingletonDemo();
        }
        return instance;
    }

}

问题

• 为什么需要 volatile ?
  • volatile可以禁止指令重排序
• 为什么需要禁止指令重排序?
  • 因为new并不是一个原子操作, new可以简单分成三个操作:
    • 分配一块内存
    • 将内存的地址赋值给变脸
    • 在内存上初始化对象
  • 所以不加volatile可能会抛出空指针