单例模式

懒加载，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;
	}
}

懒加载，线程安全，线程方法排序

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

synchronized表示多线程排序执行方法。

懒加载，线程安全，线程属性传递

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

volatile表示变量修改最后的值。

类加载，线程安全

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

静态内部类加载，线程安全

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

枚举

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

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