七种写法的单列模式

目录

• 七种写法的单列模式
  • 版本1：饿汉式
  • 版本2：多线程可能导致多个实例
  • 版本3：效率有待优化
  • 版本4：Double Check可能出现空指针异常
  • 版本5：Double Check
  • 版本6：通过静态内部类构造
  • 版本7：通过枚举构造

七种写法的单列模式

版本1：饿汉式

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

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

版本2：多线程可能导致多个实例

/**
    多线程可能导致多个实例
 */
public class SingletonObject2 {

    private static SingletonObject2 instance;

    private SingletonObject2(){}

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

}

版本3：效率有待优化

/**
  将整个方法锁住，效率有待优化
 */
public class SingletonObject3 {

    private static SingletonObject3 instance;

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

}

版本4：Double Check可能出现空指针异常

/**
 * Double Check
 * 可能会导致空指针异常：
 */
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 instance;
    }
}

版本5：Double Check

public class SingletonObject5 {

    //volatile 保证读的过程，写必须完成，
    // 也就new SingletonObject5的时候里面的内容全部构造完成
    private volatile static SingletonObject5 instance;

    private SingletonObject5(){}

    public static SingletonObject5 getInstance() {
        if (null == instance){
            synchronized (SingletonObject5.class){
                if (null == instance) { //这里一定需要再次判断
                    instance = new SingletonObject5();
                }
            }
        }
        return instance;
    }

}

版本6：通过静态内部类构造

public class SingletonObject6 {
    
    private SingletonObject6() {}

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

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

版本7：通过枚举构造

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();
    }

}