枚舉類實現單例模式

實現單例模式時候，要思考四個問題：

1. 是否線程安全
2. 多線程安全了，效率是否能接受
3. 是否可以延遲加載
4. 是否支持序列化

請看下面一張圖：

枚舉實現單例模式如下：簡潔優雅

public enum Singleton 0 {
    INSTANCE;
    int anInt;

    public int getAnInt() {
        return anInt;
    }

    public void setAnInt( int anInt ) {
        this.anInt = anInt;
    }

    public  void dosomethings(  ) {
        System.out.println("枚舉類實現單例");
    }
}

惡漢式單例模式如下：

public class Singleton 1{
    private static Singleton instance = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
}

飽漢式單例模式

public class Singleton2 {
 
	private static Singleton2 singleton;
	
	private Singleton2(){
		
	}
	
	public static synchronized Singleton2 getInstance(){
		if(singleton == null) 
			singleton = new Singleton2();
		return singleton;
	}

雙重鎖模式(DCL):

public class Singleton3 {
	private static Singleton3 singleton;
	
	private Singleton3(){
		
	}
	
	public static Singleton3 getInstance(){
		if(singleton == null){
			synchronized(Singleton3.class){
				if(singleton == null){
					singleton = new Singleton3();
				}
			}
		}
		return singleton;
	}

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