Java 单例模式的7种写法

1.  懒汉(线程不安全)

// 懒汉,线程不安全
public class Singleton {
    private static Singleton instance;
    private Singleton (){}
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
} 

2.  懒汉(线程安全)

// 懒汉,线程安全
public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}  

3.  饿汉

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

4.  饿汉(变种)

// 饿汉,变种
public class Singleton {  
    private Singleton instance = null;  
    static {  
        instance = new Singleton();  
    }  
    private Singleton (){}  
    public static Singleton getInstance() {  
        return this.instance;  
    }  
}  

5.  静态内部类

// 静态内部类
public class Singleton {  
    private static class SingletonHolder {  
        private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
        return SingletonHolder.INSTANCE;  
    }  
}  

6.  枚举

// 枚举
public enum Singleton {  
    INSTANCE;  
    public void whateverMethod() {  
    }  
} 

7.  双重效验锁

// 双重效验锁
public class Singleton {  
    private volatile static Singleton singleton;  
    private Singleton (){}  
    public static Singleton getSingleton() {  
        if (singleton == null) {  
            synchronized (Singleton.class) {  
            if (singleton == null) {  
                singleton = new Singleton();  
            }  
        }  
    }  
    return singleton;  
    }  
}  

 

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