Deadlock in Java Multithreading

Deadlock occurs in Java when multiple threads block each other while waiting for locks held by one another. To prevent deadlocks, we can use the synchronized keyword to make methods or blocks thread-safe which means only one thread can have the lock of the synchronized method and use it, other threads have to wait till the lock releases other one acquires the lock. 

Example: Below is a simple example demonstrating a deadlock condition in Java.

// Utility class to pause thread execution
class Util { 
    static void sleep(long millis)
    {
        try {
            Thread.sleep(millis);
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
// this class is shared by both threads
class Shared {
  
    // first synchronized method
    synchronized void test1(Shared s2)
    {
        System.out.println(Thread.currentThread().getName()
                           + " enters test1 of " + this);
        Util.sleep(1000);
      
        // Trying to call test2 
        // on another object
        s2.test2();
        System.out.println(Thread.currentThread().getName()
                           + " exits test1 of " + this);
    }
  
    // Second synchronized method
    synchronized void test2()
    {
        System.out.println(Thread.currentThread().getName()
                           + " enters test2 of " + this);
        Util.sleep(1000);
      
        // taking object lock of s1 enters
        // into test1 method
        System.out.println(Thread.currentThread().getName()
                           + " exits test2 of " + this);
    }
}

class Thread1 extends Thread {
    private Shared s1;
    private Shared s2;
  
    // constructor to initialize fields
    public Thread1(Shared s1, Shared s2)
    {
        this.s1 = s1;
        this.s2 = s2;
    }
  
    // run method to start a thread
    @Override public void run() { s1.test1(s2); }
}

class Thread2 extends Thread {
    private Shared s1;
    private Shared s2;
  
    // constructor to initialize fields
    public Thread2(Shared s1, Shared s2)
    {
        this.s1 = s1;
        this.s2 = s2;
    }
  
    // run method to start a thread
    @Override public void run() { s2.test1(s1); }
}
public class Geeks {
  
  // In this class deadlock occurs
    public static void main(String[] args)
    {
        // creating one object
        Shared s1 = new Shared();
        Shared s2 = new Shared();
      
        // creating first thread and starting it
        Thread1 t1 = new Thread1(s1, s2);
        t1.setName("Thread1");
        t1.start();
      
        // creating second thread and starting it
        Thread2 t2 = new Thread2(s1, s2);
        t2.setName("Thread2");
        t2.start();
        Util.sleep(2000);
    }
}

Output:

Note: It is not recommended to run the program in an online IDE. We can run the above source code locally, but it gets stuck in a deadlock, preventing execution.

Explanation:

• Thread t1 starts by acquiring a lock on the s1 and enters the test1() method of s1.
• Thread t2 starts by acquiring a lock on the s2 and enters the test1() method of s2.
• In the test1() method both threads try to acquire locks on each other's objects but the locks are already held by the other thread causing both threads to wait indefinitely for the other to release the lock.
• Neither test1() nor test2() methods complete execution and the program remains stuck in the deadlock state.

Locks in Java

Below is the diagrammatic representation of how Locks work and prevent Deadlock conditions.

Detectecting Deadlocks

We can detect deadlocks in a running Java program using the following steps:

1. List the active Java processes:

jps -l

Response:

This will list the running Java processes and also mention that there is a deadlock if we want to generate a thread dump.

2. Identify the process ID (PID) of the target program and run:

jcmd <PID> Thread.print // replace PID with the process ID

Replace <PID> with the process ID from the list provided by jps -l. This command outputs the state of the threads, which you can then analyze for deadlocks.

After running the above two commands, we can see deadlock occurs:

As we can see it is mentioned that found 1 deadlock.

Preventing Deadlocks

We can avoid deadlock conditions by knowing its possibilities. It's a very complex process and not easy to catch. Still, if we try, we can avoid this. There are some methods by which we can avoid this condition. We can't completely remove its possibility but we can reduce it.

• Avoid Nested Locks: This is the main reason for deadlock. Mainly happens when we give locks to multiple threads. Avoid giving lock to multiple threads if we already have given to one.
• Avoid Unnecessary Locks: We should have lock only those members who are required. Having a lock on unnecessarily can lead to deadlock.
• Using thread join: Deadlock condition appears when one thread is waiting for the other to finish. If this condition occurs we can use Thread. Join the with the maximum time you think the execution will take.

Important Points: 

• If threads are waiting for each other to finish, then the condition is known as Deadlock.
• Deadlock condition is a complex condition which occurs only in case of multiple threads.
• Deadlock conditions can break our code at run time and can destroy business logic.
• We should avoid this condition as much as we can.