Java DeadLock

https://dzone.com/articles/how-detect-java-deadlocks
Detect deadlocks programmatically using ThreadMXBean class

Java 5 introduced ThreadMXBean - an interface that provides various monitoring methods for threads. I recommend you to check all of the methods as there are many useful operations for monitoring the performance of your application in case you are not using an external tool. 

public interface DeadlockHandler {
  void handleDeadlock(final ThreadInfo[] deadlockedThreads);
}

public class DeadlockDetector {

  private final DeadlockHandler deadlockHandler;
  private final long period;
  private final TimeUnit unit;
  private final ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
  private final ScheduledExecutorService scheduler = 
  Executors.newScheduledThreadPool(1);
  
  final Runnable deadlockCheck = new Runnable() {
    @Override
    public void run() {
      long[] deadlockedThreadIds = DeadlockDetector.this.mbean.findDeadlockedThreads();
    
      if (deadlockedThreadIds != null) {
        ThreadInfo[] threadInfos = 
        DeadlockDetector.this.mbean.getThreadInfo(deadlockedThreadIds);
      
        DeadlockDetector.this.deadlockHandler.handleDeadlock(threadInfos);
      }
    }
  };
  
  public DeadlockDetector(final DeadlockHandler deadlockHandler, 
    final long period, final TimeUnit unit) {
    this.deadlockHandler = deadlockHandler;
    this.period = period;
    this.unit = unit;
  }
  
  public void start() {
    this.scheduler.scheduleAtFixedRate(
    this.deadlockCheck, this.period, this.period, this.unit);
  }
}

Let’s test this in practice. First, we will create a handler to output deadlocked threads information to System.err. We could use this to send email in a real world scenario, for example:

public class DeadlockConsoleHandler implements DeadlockHandler {

  @Override
  public void handleDeadlock(final ThreadInfo[] deadlockedThreads) {
    if (deadlockedThreads != null) {
      System.err.println("Deadlock detected!");
      
      Map<Thread, StackTraceElement[]> stackTraceMap = Thread.getAllStackTraces();
      for (ThreadInfo threadInfo : deadlockedThreads) {
      
        if (threadInfo != null) {
      
          for (Thread thread : Thread.getAllStackTraces().keySet()) {
        
            if (thread.getId() == threadInfo.getThreadId()) {
              System.err.println(threadInfo.toString().trim());
                
              for (StackTraceElement ste : thread.getStackTrace()) {
                  System.err.println("\t" + ste.toString().trim());
              }
            }
          }
        }
      }
    }
  }
}

This iterates through all stack traces and prints stack trace for each thread info. This way we can know exactly on which line each thread is waiting, and for which lock. This approach has one downside - it can give false alarms if one of the threads is waiting with a timeout which can actually be seen as a temporary deadlock. Because of that, original thread could no longer exist when we handle our deadlock and findDeadlockedThreads will return null for such threads. To avoid possible NullPointerExceptions, we need to guard for such situations. Finally, lets force a simple deadlock and see our system in action:

DeadlockDetector deadlockDetector = new DeadlockDetector(new DeadlockConsoleHandler(), 5, TimeUnit.SECONDS);
deadlockDetector.start();

final Object lock1 = new Object();
final Object lock2 = new Object();

Thread thread1 = new Thread(new Runnable() {
  @Override
  public void run() {
    synchronized (lock1) {
      System.out.println("Thread1 acquired lock1");
      try {
        TimeUnit.MILLISECONDS.sleep(500);
      } catch (InterruptedException ignore) {
      }
      synchronized (lock2) {
        System.out.println("Thread1 acquired lock2");
      }
    }
  }

});
thread1.start();

Thread thread2 = new Thread(new Runnable() {
  @Override
  public void run() {
    synchronized (lock2) {
      System.out.println("Thread2 acquired lock2");
      synchronized (lock1) {
        System.out.println("Thread2 acquired lock1");
      }
    }
  }
});
thread2.start();

https://github.com/xieqilu/Qilu-leetcode/blob/master/PureStorage/LockAndDeadLock.cs
First, it's important to understand what a deadlock among threads is and the conditions that lead to one.
Many OS course textbooks will cite the four conditions necessary for a deadlock to occur:
1, A limited number of a particular resource. In the case of a monitor in C# (what you use when you employ the lock keyword),
this limited number is one, since a monitor is a mutual-exclusion lock (meaning only one thread can own a monitor at a time).
2, The ability to hold one resource and request another.
In C#, this is akin to locking on one object and then locking on another before releasing the first lock, for example:
lock(a)
   {
     lock(b)
         {
            ....
          }
   }
3, No preemption capability. In C#, this means that one thread can't force another thread to release a lock.
4, A circular wait condition. This means that there is a cycle of threads,
each of which is waiting for the next to release a resource before it can continue.