Singleton类中的非静态成员

Question

I´m doing a singleton class as follows.

public class SingletonTest {

    private static SingletonTest instance;

    private Integer result;

    private SingletonTest() {   
    }

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

    public Integer calculateResult(int value1, int value2) {
        result = value1 + value2;
        return result;
    }
}

But a problem occurs when I call non-static members from multiple threads (with JMeter).

By example:

Thread 1: SingletonTest.getInstance().calculateResult(1,2) -> return 3

Thread 2: SingletonTest.getInstance().calculateResult(3,2) -> return 3

I think this happen because 2 threads are accessing in the same time at method and override de property called result.

Answer 1

You are right, the second thread is accesing the value of result set by the first thread.

If you are modifying a value that is visible to multiple threads, you should synchronize the operation.

public synchronized Integer calculateResult(int value1, int value2) {
    result = value1 + value2;
    return result;
}

This way the first thread to call the method gets a lock on the singleton, and the second thread can't access it until its finished.

You should probably review the official tutorials if you are going to use concurrency, especially this .

Answer 2

Your guess is correct and its happening because your shared result instance variable is not thread safe. Not sure why are you sharing it. Fix code is here:

public class SingletonTest {

    private static SingletonTest instance;

    private SingletonTest() {   
    }

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

    public Integer calculateResult(int value1, int value2) {
        Integer result = value1 + value2;
        return result;
    }
}

Answer 3

Try this

public class Singleton {

    private static Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
         return instance;
    }
}