Java单例模式

懒汉式单例

1.适用于单线程环境（不推荐）

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

2.适用于多线程环境，但效率不高（不推荐）

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

3.双重检验锁（推荐）

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

4.静态内部类（Effective Java 上推荐）

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

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

饿汉式单例

static final

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private static final Singleton INSTANCE=new Singleton();

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

枚举单例

enum

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public enum EasySingleton{
    INSTANCE;
}