同步单例模式写法

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

}

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

}

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

}