019 单例模式的5种写法

单例模式的定义：保证一个类仅有一个实例，并提供一个访问它的全局访问点！

1、懒汉

public class Singleton {
    private static Singleton instance;
    private Singleton (){}

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

2、饿汉

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

3、双重校验锁

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;
    }
}

4、枚举

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

5、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;  
    }  
}

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