Question

I have some trouble understanding the concept of deadlock situation in this program.
I am getting the output as :
Entered amethod
Entered bmethod
and then the deadlock situation occurs.
Now since my amethod is a synchronized method, shouldn’t it execute first completely i.e. by calling bsum method and then start the new thread. ?
Please explain…

public class Deadlock 
{
    public static void main(String[] args) 
    {

        A a= new A();
        B b= new B();
        new MainClass1(a,b);
        new MainClass2(a,b);
    }

}
class MainClass1 extends Thread
{
    A a;
    B b;
    MainClass1(A a,B b)
    {
        super();
        this.a=a;
        this.b=b;
        start();
    }
    public void run()
    {
        a.amethod(b);
    }
}
class MainClass2 extends Thread
{
    A a;
    B b;
    MainClass2(A a,B b)
    {
        super();
        this.a=a;
        this.b=b;
        start();
    }
    public void run()
    {
        b.bmethod(a);
    }

}
class A
{
    public synchronized void amethod(B b)
    {
        System.out.println("Entered amethod");
        try{
            Thread.sleep(500);
        }catch(Exception e){}
        b.bsum(2,3);
    }
    public synchronized void asum(int a,int b)
    {
        System.out.println("Sum in A is");
        System.out.println(a+b);
    }
}
class B
{
    public synchronized void bmethod(A a)
    {
        System.out.println("Entered bmethod");
        try{
            Thread.sleep(500);
        }catch(Exception e){}
        a.asum(3, 5);
    }
    public synchronized void bsum(int a, int b)
    {
        System.out.println("Sum in B is");
        System.out.println(a+b);
    }
}

Answer 1

actually you have started both the threads… lets call the threads 1 and 2

so what happens is when thread 1 acquired the lock over object A and calls the method amethod at the same time
thread 2 has acquired the lock over object B and called the bmethod.
now A wants to call the b’s sum method that is locked because B has already the lock over the object B.
and B wants to call the sum method of A where A has already got the object of A and its not releasing the lock until its done with the calling of sum method.

just remove the synchronized keyword from the sum methods and it will work( i mean not go into deadlock condition)