Deadlock in Java

Java provides a concept of multithreading that allows a process to execute more than one thread at the same time. Multithreading provides a wide range of benefits such as maximum CPU utilization, saving time, decreasing maintenance cost, etc. However, it has some disadvantages as well and among them, the most common limitation is that multiple threads go into a waiting state and are blocked forever and this situation is called a deadlock in java.

In this write-up, we are going to learn the below-listed aspects of deadlocks in java:

What is a Deadlock?
When and Where a Deadlock can occur
Examples of Deadlock
How to Prevent a Deadlock in Java

So let’s start!

What is a Deadlock?

Deadlock describes a situation where two more threads are trying to access the same resource simultaneously and as a result, no one gets the resource and is eventually blocked forever.

When and Where a Deadlock can occur

In java, the deadlock scenario can occur only in the multithreaded environment where more than one thread executes at the same time. Moreover, a synchronized keyword can cause a deadlock in java.

From the above figure, we can observe that thread1 is waiting for thread2 to release the resources and vice versa.

Examples of Deadlock

Let’s consider the below-given example for a profound understanding of how a deadlock occurs in java:

public class DeadlockExample {
public static void main(String[] args) {
final String res1 = "First Resource";
final String res2 = "Second Resource";
Thread thread1 = new Thread() {
public void run() {
synchronized (res1) {
System.out.println("Thread 1: locked the res1");

try {
Thread.sleep(2000);
} catch (Exception excep) {
}

synchronized (res2) {
System.out.println("Thread 1: locked the res2");
}
}
}
};
Thread thread2 = new Thread() {
public void run() {
synchronized (res2) {
System.out.println("Thread 2: locked the res2");

try {
Thread.sleep(2000);
} catch (Exception excep) {
}

synchronized (res1) {
System.out.println("Thread 2: locked the res1");
}
}
}
};
thread1.start();
thread2.start();
}
}

In this example, we have two threads thread1 and thread2, both the threads start and call the run() method.

Following will be the output for the above snippet:

From the above snippet, it is clear that both the threads locked each other.

How to Prevent a Deadlock in Java

Below is the list of preventions that can be adopted to avoid a deadlock in java:

Avoid the use of nested and unnecessary locks.
Acquire the locks in the same order.
Use Thread Joins to avoid the Deadlock i.e., release a lock after a timeout period.

Conclusion

Deadlock describes a situation where two more threads are trying to access the same resource simultaneously and as a result, no one gets the resource and is eventually blocked forever. In java, a deadlock occurs only in the multithreaded environment where more than one thread executes at the same time. Moreover, a synchronized keyword can cause a deadlock in java. In java, a deadlock can’t be resolved completely however, it can be avoided by adopting some necessary precautions/rules e.g. avoiding the use of unnecessary locks, nested locks, etc.

This write-up presents a detailed overview of what are deadlocks in java, how deadlocks occur, and how to avoid the deadlocks in java.