單例模式（singleton pattern ）是一種在開發(fā)中經(jīng)常用到的設(shè)計模式。

模式定義

確保一個類僅有一個實例，并且提供一個訪問它的全局訪問點。

實現(xiàn)

1、單例模式-餓漢式

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();

    private Singleton(){
        System.out.println("new Singleton instance");
    }

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

2、單例模式-懶漢式(非線程安全)

public class Singleton {
    private static Singleton instance;
    private Singleton() {
        System.out.println("new Singleton instance");
    }

    //非線程安全
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

3、單例模式-懶漢式(線程安全)

public class Singleton {
    private volatile static Singleton instance;

    private Singleton(){
        System.out.println("new Singleton instance");
    }

    //double-check
    public static Singleton getInstance(){
        if(instance==null){
            synchronized (Singleton.class){
                if(instance==null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

4、單例模式-懶漢式

public class Singleton {

    private Singleton(){
        System.out.println("new Singleton instance");
    }

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

    private static class SingletonHolder{
        public static final Singleton INSTANCE = new Singleton();
    }
}

5、單例模式-枚舉

public enum Singleton {
    INSTANCE("singleton");

    private String value;
    Singleton(String value){
        this.value = value;
    }

    public void doSomeAction(){
        System.out.println("doSomeAction");
    }
}

源代碼

https://github.com/TiFG/design-patterns/tree/master/singleton