Java Deadlock is a situation that can occur when two or more threads are blocked forever, waiting for each other to release the resource they need to proceed. In other words, each thread is waiting for a resource that is held by another thread, causing both threads to be stuck in an infinite loop of waiting.
A common cause of deadlock is when multiple threads acquire locks on different resources, but then they try to acquire locks on the same resources that are held by other threads, resulting in a circular dependency. This situation can occur when threads acquire locks on resources in different orders, causing each thread to wait indefinitely for the other thread to release the lock.
Here's an example of Java deadlock:
public class DeadlockExample {
private final Object lock1 = new Object();
private final Object lock2 = new Object();
public void method1() {
synchronized (lock1) {
System.out.println("method1 acquired lock1");
synchronized (lock2) {
System.out.println("method1 acquired lock2");
}
}
}
public void method2() {
synchronized (lock2) {
System.out.println("method2 acquired lock2");
synchronized (lock1) {
System.out.println("method2 acquired lock1");
}
}
}
}
In this example, we have a DeadlockExample class that has two methods, method1() and method2(). Each method acquires two locks, lock1 and lock2, but in different order. If two threads call these methods simultaneously, and one thread acquires lock1 and the other thread acquires lock2, then both threads will wait indefinitely for each other to release the lock they need to proceed, causing a deadlock.
To avoid deadlocks in Java, it's important to ensure that threads always acquire locks in the same order. This can be achieved by using a global ordering of resources, or by using a timeout mechanism that detects and recovers from deadlock situations. It's also important to minimize the use of synchronized blocks and use alternative synchronization mechanisms, such as java.util.concurrent classes, whenever possible.