Multi-threaded Singleton Design Mode

I have no other hands, but I am familiar with it! Multi-threaded Singleton Design Mode

As we all know, in singleton mode, constructor is private and calls constructor internally through static methods to return instance objects of this class. The common code is as follows:

?

12345678910

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

　　

In the case of a single thread, there is indeed only one instance, but in the case of multiple threads, unexpected situations will occur.

Create a testsingleton class as follows:

?

public class TestSingleton implements Runnable {     private Singleton s = null;         public Singleton getS() {        return s;    }     public void setS(Singleton s) {        this.s = s;    }     public static void main(String[] args) {        TestSingleton ts1 = new TestSingleton();        TestSingleton ts2 = new TestSingleton();        Thread t1 = new Thread(ts1);        Thread t2 = new Thread(ts2);        t1.start();        t2.start();                 Singleton s1 = ts1.getS();        Singleton s2 = ts2.getS();                 System.out.println(s1 == s2);    }     @Override    public void run() {                 s = Singleton.getInstance();    }}

　　

The running result returns false.

In a multi-threaded environment, you need to consider the synchronization problem and modify the code in the singleton mode as follows:

?

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

　　

Re-run testsingleton and return the result "true" to ensure that the singleton mode is correct even in a multi-threaded environment.