单例模式

单例模式，是一种常用的软件设计模式。在它的核心结构中只包含一个被称为单例的特殊类。通过单例模式可以保证系统中，应用该模式的类一个类只有一个实例。即一个类只有一个对象实例。

1. 懒汉 - 线程不安全

package com.my.test.singleton;

//懒汉 - 线程不安全
public class SingletonExample01 {

    private SingletonExample01(){

    }

    private static SingletonExample01 instance = null;

    public static SingletonExample01 getInstance(){
        if(instance == null){
            instance = new SingletonExample01();
        }
        return instance;
    }
}

2. 懒汉 - 线程安全

package com.my.test.singleton;

//懒汉 - 线程安全
public class SingletonExample02 {

    private SingletonExample02(){

    }

    private static SingletonExample02 instance = null;

    public static synchronized SingletonExample02 getInstance(){
        if(instance == null){
            instance = new SingletonExample02();
        }
        return instance;
    }
}

3. 双重校验锁

package com.my.test.singleton;

//懒汉 - 双重校验锁
public class SingletonExample03 {

    private SingletonExample03(){

    }

    private static volatile SingletonExample03 instance = null;

    public static SingletonExample03 getInstance(){
        if(instance == null){
            synchronized (SingletonExample03.class){
                if(instance == null){
                    instance = new SingletonExample03();
                }
            }
        }
        return instance;
    }
}

4. 饿汉模式

package com.my.test.singleton;

//饿汉模式
public class SingletonExample04 {

    private SingletonExample04(){

    }

    // 方法一
//    private static SingletonExample04 instance = new SingletonExample04();


    // 方法二
    private static SingletonExample04 instance = null;

    static {
        instance = new SingletonExample04();
    }

    public static SingletonExample04 getInstance(){
        return instance;
    }
}

5. 静态内部类

package com.my.test.singleton;

// 静态内部类
public class SingletonExample05 {

    private SingletonExample05(){

    }

    private static class InstanceSingletonExample05{
        private static SingletonExample05 instance = new SingletonExample05();
    }

    public static SingletonExample05 getInstance(){
        return InstanceSingletonExample05.instance;
    }
}

6. 枚举

package com.my.test.singleton;


// 枚举
public class SingletonExample06 {

    private SingletonExample06(){
    }

    public static SingletonExample06 getInstance(){
        return InstanceSingletonExample06.INSTANCE.getSingletonExample06();
    }

    private enum InstanceSingletonExample06{
        INSTANCE;

        private SingletonExample06 singletonExample06 = new SingletonExample06();

        public SingletonExample06 getSingletonExample06(){
            return singletonExample06;
        }
    }
}

7. 枚举2.0

package com.my.test.singleton;

public enum SingletonEnumExample{
    INSTANCE;

    public void test(){
        // do everything
    }
}