Question

I just compiled and run this program of java but the output is unpleasant.
I don’t know why the threads are in deadlock situation.
Can anyone help me to understand the output of the program.

class A {
synchronized void foo(B b) {
    String name=Thread.currentThread().getName();
    System.out.println(name+"entered A.foo");

    try {
        Thread.sleep(1000);
    } catch(Exception e) {}
    System.out.println(name+"trying to call B.last()");
    b.last();
}
synchronized void last() {
    System.out.println("inside A.last");
}
}

class B {
synchronized void bar(A a) {
    String name=Thread.currentThread().getName();
    System.out.println(name+"entered B.bar");
    try {
        Thread.sleep(1000);
    } catch(Exception e) {
        System.out.println("b interrupted");
    }
    System.out.println(name+"trying to call A.last()");
    a.last();
}
synchronized void last() {
    System.out.println("inside A.last");
}
}

class DeadLock implements Runnable {
A a=new A();
B b=new B();
DeadLock() {
    Thread.currentThread().setName("mainthread");
    Thread t=new Thread(this,"racingthread");
    t.start();
    a.foo(b);
    System.out.println("back in main thread");
}
public void run() {
    b.bar(a);
    System.out.println("back in other theread");
}
public static void main(String...d) {
    new DeadLock();
}
}

the output on my computer is

mainthreadentered A.foo

racingthreadentered B.bar

mainthreadtrying to call B.last()

racingthreadtrying to call A.last()

Answer 1

You have a classic deadlock.

• A.foo() locks a and calls b.last() which attempts to lock b
• B.bar() locks b1 and calls a.last() which attempts to lock a

Neither can proceed because each needs the lock of the other.

I don’t know why the threads are in deadlock situation.

Because that is what your program is designed to do.

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Given you don’t need any of the locks, the simplest solution is to remove all the synchronized keywords.