2.1-单例实现方式

饿汉模式

线程安全，调用效率高，但是不能延时加载

public class SingletonObject1 {
    
    private static final SingletonObject1 instance = new SingletonObject1();

    private SingletonObject1() {}

    public static SingletonObject1 getInstance() {
        return instance;
    }
}

懒汉模式

线程不安全，会产生多个实例

public class SingletonObject2 {

    private static SingletonObject2 instance;

    private SingletonObject2() {}

    public static SingletonObject2 getInstance() {
        if (null == instance) {
            instance = new SingletonObject2();
        }
        return SingletonObject2.instance;
    }
}

getInstance()方法加锁（效率低）

public class SingletonObject3 {
    private static SingletonObject3 instance;

    private SingletonObject3() {}

    public synchronized static SingletonObject3 getInstance() {
        if (null == instance) {
            instance = new SingletonObject3();
        }
        return SingletonObject3.instance;
    }
}

双重检测锁模式

由于指令重排序问题，偶尔会出现异常，比如构造方法里面有大量的操作，jvm可能不会等到所有的操作都执行完，才返回实例指针

public class SingletonObject4 {

    private static SingletonObject4 instance;

    private SingletonObject4() { }

    public static SingletonObject4 getInstance() {
        if (null == instance) {
            synchronized (SingletonObject4.class) {
                if (null == instance) {
                    instance = new SingletonObject4();
                }
            }
        }
        return SingletonObject4.instance;
    }
}

5、双重检测锁模式2

通过volatile解决【4】的问题，严格要求happen before，也就是在执行【读】操作的时候必须看到完整的【写】操作

public class SingletonObject5 {

    private static volatile SingletonObject5 instance;

    private SingletonObject5() {}

    public static SingletonObject5 getInstance() {
        if (null == instance) {
            synchronized (SingletonObject4.class) {
                if (null == instance) {
                    instance = new SingletonObject5();
                }
            }
        }
        return SingletonObject5.instance;
    }
}

6、静态内部类

懒加载、线程安全、效率高

public class SingletonObject6 {

    private SingletonObject6() {}

    private static class InstanceHolder {
        private final static SingletonObject6 instance = new SingletonObject6();
    }

    public static SingletonObject6 getInstance() {
        return InstanceHolder.instance;
    }
}

7、枚举

枚举是线程安全的，而且只会被装载一次

public class SingletonObject7 {
    
    private SingletonObject7() {}

    private enum Singleton {
        INSTANCE;

        private final SingletonObject7 instance;

        Singleton() {
            instance = new SingletonObject7();
        }

        public SingletonObject7 getInstance() {
            return instance;
        }
    }

    public static SingletonObject7 getInstance() {
        return Singleton.INSTANCE.getInstance();
    }