How to Generate Java Deadlock Programmatically and How to Analyze Deadlock

Deadlock is a programming situation where two or more threads are blocked forever, this situation arises with at least two threads and two or more resources.

Here I have written a simple program that will cause deadlock scenario and then we will see how to analyze it.

Here is a simple Java Example:

package com.crunchify.tutorials;

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;

/**
 * @author Crunchify.com
 * 
 */

public class CrunchifyThreadDeadLock {

    public static void main(String[] args) throws InterruptedException {
        Object obj1 = new Object();
        Object obj2 = new Object();
        Object obj3 = new Object();

        Thread crunchifyThread1 = new Thread(new CrunchifySynchronizeThread(obj1, obj2), "crunchifyThread1");
        Thread crunchifyThread2 = new Thread(new CrunchifySynchronizeThread(obj2, obj3), "crunchifyThread2");
        Thread crunchifyThread3 = new Thread(new CrunchifySynchronizeThread(obj3, obj1), "crunchifyThread3");

        crunchifyThread1.start();
        Thread.sleep(3000);
        crunchifyThread2.start();
        Thread.sleep(3000);
        crunchifyThread3.start();
    }
}

class CrunchifySynchronizeThread implements Runnable {
    private Object obj1;
    private Object obj2;

    public CrunchifySynchronizeThread(Object obj1, Object obj2) {
        this.obj1 = obj1;
        this.obj2 = obj2;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        synchronized (obj1) {
            System.out.println(name + " acquired lock on Object1: " + obj1);
            try {
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            synchronized (obj2) {
                System.out.println(name + " acquired lock on Object2: " + obj2);
            }
            System.out.println(name + " released lock on Object2: " + obj2);
        }
        System.out.println(name + " released lock on Object1: " + obj1);
        System.out.println(name + " Finished Crunchify Deadlock Test.");
    }
}

Output:

crunchifyThread1 acquired lock on Object1: java.lang.Object@59e3bddb
crunchifyThread2 acquired lock on Object1: java.lang.Object@294e430c
crunchifyThread3 acquired lock on Object1: java.lang.Object@246972f1

While your app is hanging like in the example above, you can get a thread dump and see the deadlocked threads.

For example, on Mac, you can either do Ctrl-\ or simply use jstack and process id to get the thread dump which makes it very obvious where the deadlock is.

In this example, the thread dump looks like this:

"crunchifyThread3" 
   java.lang.Thread.State: RUNNABLE
        at sun.management.ThreadImpl.getThreadInfo1(Native Method)
        at sun.management.ThreadImpl.getThreadInfo(ThreadImpl.java:174)
        at com.crunchify.tutorials.CrunchifySynchronizeThread.getThreadDump(CrunchifyThreadDeadLock.java:64)
        at com.crunchify.tutorials.CrunchifySynchronizeThread.run(CrunchifyThreadDeadLock.java:50)
        at java.lang.Thread.run(Thread.java:722)

"crunchifyThread2" 
   java.lang.Thread.State: BLOCKED
        at com.crunchify.tutorials.CrunchifySynchronizeThread.run(CrunchifyThreadDeadLock.java:53)
        at java.lang.Thread.run(Thread.java:722)

"crunchifyThread1" 
   java.lang.Thread.State: BLOCKED
        at com.crunchify.tutorials.CrunchifySynchronizeThread.run(CrunchifyThreadDeadLock.java:53)
        at java.lang.Thread.run(Thread.java:722)

"Signal Dispatcher" 
   java.lang.Thread.State: RUNNABLE

"Finalizer" 
   java.lang.Thread.State: WAITING
        at java.lang.Object.wait(Native Method)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:189)

"Reference Handler" 
   java.lang.Thread.State: WAITING
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:503)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)

Noticed here BLOCKED and WAITING threads => Deadlock situation here.. In next example will provide simple Java Program to generate above thread dump programmatically.