1. 非线程安全

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

2.线程安全（Synchronized）

效率低

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

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

3. 线程安全（双重检查🔒）

存在空指针问题

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

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

4. 线程安全（双重检查🔒+volatile）

public class Singleton{
    
    private Singleton(){}
    
    private static volatile Singleton instance;
    
     public static Singleton getInstance() {
        //第一次判断，如果instance的值不为null，不需要抢占锁，直接返回对象
        if(instance == null) {
            //给当前类的字节码对象加锁
            synchronized (Singleton.class){
                //第二次判断
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
    }
    return instance;
}

5. 线程安全（静态内部类方式）

//存在空指针异常的问题
public class Singleton{
    
    private Singleton(){}
    
    private static class SingletonHolder{
		private static final Singleton INSTANCE = new Singleton();
	}
	
	public static Singleton getInstance(){
		return SingletonHolder.INSTANCE;
	}
}