懒汉模式

@ThreadSafe
public class SingletonExample1 {
    //私有的构造函数
    private SingletonExample1() {

    }
    //单列对象
    private static SingletonExample1 instance = null;

    //静态的工厂方法
    public static  synchronized SingletonExample1 getInstance() {
        if (instance == null) {
            instance = new SingletonExample1();
        }
        return instance;
    }
}

饿汉模式

@ThreadSafe
public class SingletonExample2 {
    //私有的构造函数
    private SingletonExample2() {

    }
    //单列对象
    private static SingletonExample2 instance = new SingletonExample2();

    //静态的工厂方法
    public static SingletonExample2 getInstance() {
        return instance;
    }
}

双重同步锁单例模式

@ThreadSafe
public class SingletonExample4 {
    //私有的构造函数
    private SingletonExample4() {
    }
    //单列对象
    private static SingletonExample4 instance = null;

    //静态的工厂方法
    public static  SingletonExample4 getInstance() {
        if (instance == null) {
            synchronized (SingletonExample4.class){
                if(instance==null){
                    instance = new SingletonExample4();
                }
            }
        }
        return instance;
    }
}

volatile+双重同步锁单例模式

@ThreadSafe
public class SingletonExample5 {

    //私有的构造函数
    private SingletonExample5() {

    }
    //volatile+双重同步锁单例模式》  关键字可以防止cup指令重排
    //单列对象
    private volatile static SingletonExample5 instance = null;

    //静态的工厂方法
    public static SingletonExample5 getInstance() {
        if (instance == null) {
            synchronized (SingletonExample5.class){
                if(instance==null){
                    instance = new SingletonExample5();
                }
            }
        }
        return instance;
    }
}

静态的工厂方法

@ThreadSafe
public class SingletonExample6 {
    //私有的构造函数
    private SingletonExample6() {

    }
    //单列对象
    private static SingletonExample6 instance =null;

    static {
        instance = new SingletonExample6();
    }

    //静态的工厂方法
    public static SingletonExample6 getInstance() {
        return instance;
    }

    public static void main(String[] args) {
        System.out.println(getInstance().hashCode());
        System.out.println(getInstance().hashCode());
    }
}

枚举

@ThreadSafe
public class SingletonExample7 {

    private SingletonExample7() {
    }

    public static SingletonExample7 getInstance() {
        return Singleton.INSTANCE.getInstance();
    }

    private enum Singleton {

        INSTANCE;

        private SingletonExample7 singleton;

        //JVM保证这个方法只被调用一次
        Singleton() {
            singleton = new SingletonExample7();
        }

        public SingletonExample7 getInstance() {
            return singleton;
        }
    }
}