Deadlock happens when threads (or whatever your platform calls its execution units) acquire resources, where each resource can only be held by one thread at a time, and holds on to those resources in a such a way that the holds cannot be preempted, and there exists some "circular" relationship between the threads such that each thread in the deadlock is waiting to acquire some resource held by another thread.
So, an easy way to avoid deadlock is to give some total ordering to resources and impose a rule that resources are only ever acquired by threads in order. Conversely, a deadlock can be intentionally created by running threads that acquire resources, but do not acquire them in order. For example:
Two threads, two locks. The first thread runs a loop that attempts to acquire the locks in a certain order, the second thread runs a loop that attempts to acquire the locks in the opposite order. Each thread releases both locks after successfully acquiring the locks.
public class HighlyLikelyDeadlock {
static class Locker implements Runnable {
private Object first, second;
Locker(Object first, Object second) {
this.first = first;
this.second = second;
}
@Override
public void run() {
while (true) {
synchronized (first) {
synchronized (second) {
System.out.println(Thread.currentThread().getName());
}
}
}
}
}
public static void main(final String... args) {
Object lock1 = new Object(), lock2 = new Object();
new Thread(new Locker(lock1, lock2), "Thread 1").start();
new Thread(new Locker(lock2, lock1), "Thread 2").start();
}
}
Now, there have been a few comments in this question that point out the difference between the likelihood and the certainty of deadlock. In some sense, the distinction is an academic issue. From a practical standpoint, I'd certainly like to see a running system that doesn't deadlock with the code I've written above :)
However, interview questions can be academic at times, and this SO question does have the word "surely" in the title, so what follows is a program that certainly deadlocks. Two Locker objects are created, each is given two locks and a CountDownLatch used to synchronize between the threads. Each Locker locks the first lock then counts down the latch once. When both threads have acquired a lock and counted down the latch, they proceed past the latch barrier and attempt to acquire a second lock, but in each case the other thread already holds the desired lock. This situation results in a certain deadlock.
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class CertainDeadlock {
static class Locker implements Runnable {
private CountDownLatch latch;
private Lock first, second;
Locker(CountDownLatch latch, Lock first, Lock second) {
this.latch = latch;
this.first = first;
this.second = second;
}
@Override
public void run() {
String threadName = Thread.currentThread().getName();
try {
first.lock();
latch.countDown();
System.out.println(threadName + ": locked first lock");
latch.await();
System.out.println(threadName + ": attempting to lock second lock");
second.lock();
System.out.println(threadName + ": never reached");
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
}
public static void main(final String... args) {
CountDownLatch latch = new CountDownLatch(2);
Lock lock1 = new ReentrantLock(), lock2 = new ReentrantLock();
new Thread(new Locker(latch, lock1, lock2), "Thread 1").start();
new Thread(new Locker(latch, lock2, lock1), "Thread 2").start();
}
}
