Introduction to Deadlock in Java
In Java, Deadlock is a situation of multithreading where thread 1 waits for an object lock attained by thread 2 and thread 2 waits for an object lock that is attained by thread 1. Here, both thread 1 and thread 2 waits for each other to release the lock. Multithreaded programs in Java can cause deadlock as the keyword synchronized blocks the executing thread at the time of waiting for monitor or lock linked with the object mentioned. Let us see the working and examples of deadlock in the following sections.
How does Deadlock work?
As already discussed, the synchronized method can be used to lock a particular portion of the code. For each object in Java, a lock will be there and synchronization is a technique to lock a function or block of code to make sure that only 1 thread can access that function or block of code at a time.
When a specific thread needs to execute a synchronized function, it first attempts to obtain the lock. At the same time, if another thread has already obtained the lock then the first thread waits until thread 2 releases the lock. Even though Synchronization prevents issues on data inconsistency, there is an issue with synchronization.
Assume there are 2 threads ‘Thread 1’ and ‘Thread 2’. Thread 1 has attained the lock of Object 1 and Thread 2 has attained the lock of Object 2. Thread 1 which executes method 1 wants to attain the lock on Object 2. However, Thread 2 already attains the lock on Object 2.
In addition to that, Thread 2 also needs to attain the lock on Object 1, however, Thread 1 has a lock on Object 1. Here, both threads Thread 1 and Thread 2 can’t complete their execution and wait for the lock forever. This situation is called Deadlock.
Examples of Deadlock in Java
Here are the following examples mention below
Example #1
Java program to implement deadlock.
Code:
public class DeadLockExample {
//main method
public static void main(String[] args) throws InterruptedException {
//create three objects 1, 2 and 3
Object o1 = new Object();
Object o2 = new Object();
Object o3 = new Object();
//create three threads 1, 2 and 3
Thread th1 = new Thread(new SynchronizationThread(o1, o2), "thread 1");
Thread th2 = new Thread(new SynchronizationThread(o2, o3), "thread 2");
Thread th3 = new Thread(new SynchronizationThread(o3, o1), "thread 3");
//start thread 1
th1.start();
//thread sleeps for 5000 seconds
Thread.sleep(5000);
//start thread 2
th2.start();
//thread sleeps for 5000 seconds
Thread.sleep(5000);
//start thread 3
th3.start();
}
}
class SynchronizationThread implements Runnable
{
private Object o1;
private Object o2;
public SynchronizationThread(Object o1, Object o2){
this.o1=o1;
this.o2=o2;
}
//function run
@Override
public void run() {
//store the name of the thread
String nm = Thread.currentThread().getName();
System.out.println( nm + " attaining lock on "+ o1 ) ;
synchronized (o1)
{
System.out.println( nm + " attained lock on "+ o1 ) ;
work();
System.out.println( nm + " attaining lock on "+ o2 ) ;
synchronized (o2) {
System.out.println( nm + " attained lock on "+ o2 );
work();
}
System.out.println( nm + " released lock on "+ o2 ) ;
}
System.out.println( nm + " released lock on "+ o1 ) ;
System.out.println( nm + " completed execution.") ;
}
//function work
private void work() {
try {
//thread sleeps
Thread.sleep(30000);
}
//catch the exception
catch (InterruptedException exc)
{
exc.printStackTrace();
}
}
}
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Output:
Example #2
In this program, 3 threads running shares a resource and they run in a way that it attains lock on object 1 but when it tries to attain lock on object 2, it goes on wait state. In order to avoid deadlock, code can be rewritten as shown below.
Code:
public class DeadLockExample {
public static void main(String[] args) throws InterruptedException {
Object o1 = new Object();
Object o2 = new Object();
Object o3 = new Object();
Thread th1 = new Thread(new SynchronizationThread(o1, o2), "thread 1");
Thread th2 = new Thread(new SynchronizationThread(o2, o3), "thread 2");
Thread th3 = new Thread(new SynchronizationThread(o3, o1), "thread 3");
//start thread 1, 2 and 3
th1.start();
//thread sleeps for 5000 seconds
Thread.sleep(5000);
th2.start();
//thread sleeps for 5000 seconds
Thread.sleep(5000);
th3.start();
}
}
class SynchronizationThread implements Runnable{
private Object o1;
private Object o2;
public SynchronizationThread(Object o1, Object o2){
this.o1=o1;
this.o2=o2;
}
//function run
@Override
public void run() {
//store the name of the thread
String nm = Thread.currentThread().getName();
System.out.println( nm + " attaining lock on "+ o1 ) ;
synchronized (o1)
{
System.out.println( nm + " attained lock on "+ o1 ) ;
work();
}
System.out.println( nm + " released lock on "+ o1 ) ;
System.out.println( nm + " acquiring lock on " + o2 );
synchronized (o2) {
System.out.println( nm + " attained lock on "+ o2 );
work();
}
System.out.println( nm + " released lock on "+ o2 ) ;
System.out.println( nm + " released lock on "+ o1 ) ;
System.out.println( nm + " completed execution.") ;
}
//function work
private void work() {
try {
//thread sleeps
Thread.sleep(30000);
}
//catch the exception
catch (InterruptedException exc)
{
exc.printStackTrace();
}
}
}
Output:
How to avoid Deadlock in Java?
Following are the guidelines that helps in avoiding deadlock situations.
1. Avoid Locks that are nested
Nested locks are one of the common reasons for causing deadlocks. In order to avoid deadlock, do not lock another resource if it already holds one. If the user works with only 1 object, it is impossible to cause deadlock.
2. Lock Only What is Required
It is advised to lock the resources that is needed. That is, some users try to lock the resources even though they are not needed.
3. Avoid waiting indefinitely
Deadlock occurs if 2 threads wait for each other to complete indefinitely with the help of thread join. If the thread needs to wait for other threads to complete, it is good to use join with the maximum amount of time that has to wait for the thread to complete.
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