单例模式，懒汉式在多线程中的问题

Singleton2实现单例模式，懒汉式 

class Singleton2{
	private Singleton2(){}
	
	private static Singleton2 singleton2;
	
	
	public static Singleton2 getSingle2(){
		if(singleton2 == null){
			
			//这边模拟线程等待
			try {
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			singleton2 =  new Singleton2();
		}
		
		return singleton2;
	}
}

main方法

public static void main(String[] args) throws InterruptedException {
		
		//线程1
		new Thread(){
			@Override
			public void run() {
				Singleton2 s1 = Singleton2.getSingle2();
				System.out.println("线程1:"+s1);
			}
		}.start();
		
		//线程2
		new Thread(){
			@Override
			public void run() {
				Singleton2 s1 = Singleton2.getSingle2();
				System.out.println("线程2"+s1);
			}
		}.start();

	}

运行结果：获取的两个实力不一致，单例模式失效。

解决多线程下，懒汉式的线程安全问题

1、使用synchronized同步代码块

2、使用synchronized同步方法

class Singleton2{
	private Singleton2(){}
	
	private static Singleton2 singleton2;
	
	//解决多线程下懒汉式问题，使用synchronized同步方法
	public static synchronized Singleton2 getSingle2(){
		//解决多线程下的懒汉式问题，使用synchronized同步代码块
	//	synchronized (Singleton2.class) {
			if(singleton2 == null){
				
				//这边模拟线程等待
				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				singleton2 =  new Singleton2();
			}
	//	}
		
		return singleton2;
	}
}