Tech Tutorials: August 2018

In this post, I will discuss what happens when another new thread comes to synchronized block and what happens to its state.

Brief Introduction:

Lets start with understanding what is synchronized block. In Java, synchronized block is the set of code under curly brackets which can only be accessed by a thread holding lock of the object on which it is defined. For example

public class TestSynchronizedBlock {
 public void method1(){
  synchronized(this) {
   System.out.println("Inside synchronized block");
   // Do your stuff here
   System.out.println("Exiting synchronized block");
  }
 }
}

Consider the code above, in class TestSynchronizedBlock, inside method1 the synchronized block can be accessed by the thread who is holding lock for current object of TestSynchronizedBlock (as it is holding lock for this i.e. object of TestSynchronizedBlock class. Every Object (instance of class) in Java has one monitor lock (it is the part of Object class, thus is part of any class object) and the thread which first try to enter into synchronized block gets the monitor lock.

Conclusion:

As I said above, the thread which first try to enter into synchronized block has the exclusive monitor lock and it keeps the locks till the thread is executing inside the block. So if Thread A is executing inside synchronized block, what happens when Thread B try to execute. Thread B will go to wait state (waiting for lock) and it will release any other monitor lock it holds, if any. After Thread A will complete executing the synchronized block, it will release the lock and Thread B will get notified and will hold the monitor lock and then start executing (means it will come out of wait state and goes to Execution state). This holds true incase of multiple threads trying to acquire lock. It is worth to mention that If there is more than one thread waiting for lock and in wait state, after current thread completes the execution, only one thread out of waiting n thread will get notified and there is no such sequence as which one with get notified. You can also refer Spin lock for deep dive on locking mechanism.

That's all for synchronized block in context of multiple threads and its status. If you have any query / doubt / suggestion, please drop a comment.

In this article we will see how to implement Spin Lock in Java.

Introduction:

Lets first understand what is Spin Lock. Spin Lock is a type of lock where the threads instead of getting blocked keeps looking for locks. In Spin Lock, the thread does not go into wait state. The thread remains in Runnable / Running state and continuously trying to get lock (say in while loop) and this is the reason it is named as Spin Lock.

Usecase / Advantage:

So the next question that pop ups in mind is that where to use the Spin Lock over Wait Lock. Here are the following use-cases:

Avoiding Deadlock :

Spin Lock is another way to avoid Deadlock. Since the thread trying for lock does not go into wait state instead spinning for lock, using Spin Lock avoids deadlock.

When there is very small functionality between lock and unlock :

Suppose I want to just write small functionality bounded by locks. If you use mechanism where thread goes to wait state (eg. Reentrant Lock ) and then back to Runnable state (after acquiring lock), this means you are making call cpu expensive for a thread going into wait and runnable state. Perhaps you could choose Spin Lock which is a kind of busy waiting for a very short period of time before acquiring lock.

Disadvantage:

Do-nothing loop : As Spin Locks constantly checking if locks are available, it keeps wasting CPU time which could be allocated to other productive thread. Thats why longer duration of spin lock is not recommended.

Code Example:

Here is the Code Example in Java for Spin Lock Implementation:

SpinLock.java

package com.blogspot.tech693;

import java.util.concurrent.locks.ReentrantLock;

public class SpinLock extends ReentrantLock{

public SpinLock() {
  super();
 }


 public void lock() {
  while(!super.tryLock()) {
    // Do Nothing
  }

 }

 public void unlock() {
  super.unlock();
 }

}

Thats all for Spin Lock Example in Java. If you have questions / doubts, please write it on comment sections Thanks.

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