How to create Deadlocks using Java thread

DeadLock

Deadlock describes a situation where two or more threads are blocked forever, waiting for each other
Here is one simple example to create dead lock and remove deadlock using Java threads

package com.vinod.test;

import java.util.ArrayList;
import java.util.List;

/**
 * In this program two threads are trying to read students and teachers details.
 *
 *@authorvinod
 *
 */
public class SimpleDeadLock {
   public static void main(String[] args) {
      final List<String> students = new ArrayList<String>();
      students.add("Raju");
      students.add("Radha");

      final List<String> teachers = new ArrayList<String>();
      teachers.add("Gopal");
      teachers.add("Ragav");

      Thread t1 = new Thread() {
         public void run() {
            synchronized (students) {
               System.out.println("Now thread 1 locked student arraylist");
               System.out.println("students details from thread1" + students);

               try {
                  Thread.sleep(110);
               } catch (InterruptedException e) {
               }

               synchronized (teachers) {
                  System.out.println("Now thread 1 locked teacher arraylist");
                  System.out.println("teacher details from thread1" + teachers);
               }
            }
         }
      };

      Thread t2 = new Thread() {
         public void run() {
            synchronized (teachers) {
               System.out.println("Now thread 2 locked teacher arraylist");
               System.out.println("teacher details from thread 2" + teachers);
               try {
                  Thread.sleep(110);
               } catch (InterruptedException e) {
               }

               synchronized (students) {
                  System.out.println("Now thread 2 locked student arraylist");
                  System.out.println("students details from thread2" + students);
               }
            }
         }
      };

      t1.start();
      t2.start();
   }
}

 

Output

Now thread 2 locked teacher arraylist
Now thread 1 locked student arraylist
students details from thread1[Raju, Radha]
teacher details from thread 2[Gopal, Ragav]

How to remove this deadlock?

This above example will never end and each thread is waiting for the resources mutually. Here i am changing the order of synchronization block and remove the dead lock

package com.vinod.test;

import java.util.ArrayList;
import java.util.List;

/**
 * In this program two threads are trying to read students and teachers details.
 *
 *@authorvinod
 *
 */
public class SimpleDeadLock {
   public static void main(String[] args) {

      final List<String> students = new ArrayList<String>();
      students.add("Raju");
      students.add("Radha");

      final List<String> teachers = new ArrayList<String>();
      teachers.add("Gopal");
      teachers.add("Ragav");

      Thread t1 = new Thread() {
         public void run() {
            synchronized (students) {
               System.out.println("Now thread 1 locked student arraylist");
               System.out.println("students details from thread1" + students);

               try {
                  Thread.sleep(110);
               } catch (InterruptedException e) {
               }

               synchronized (teachers) {
                  System.out.println("Now thread 1 locked teacher arraylist");
                  System.out.println("teacher details from thread1" + teachers);
               }
            }
         }
      };

      Thread t2 = new Thread() {
         public void run() {
            synchronized (students) {
               System.out.println("Now thread 2 locked student arraylist");
               System.out.println("students details from thread2" + students);
            }
            synchronized (teachers) {
               System.out.println("Now thread 2 locked teacher arraylist");
               System.out.println("teacher details from thread 2" + teachers);
               try {
                  Thread.sleep(110);
               } catch (InterruptedException e) {
               }

            }
         }
      };

      t1.start();
      t2.start();
   }
}

Output

Now thread 1 locked student arraylist
students details from thread1[Raju, Radha]
Now thread 1 locked teacher arraylist
teacher details from thread1[Gopal, Ragav]
Now thread 2 locked student arraylist
students details from thread2[Raju, Radha]
Now thread 2 locked teacher arraylist
teacher details from thread 2[Gopal, Ragav]