[Java] Volatile

1) Volatile


  • Used as an indicator to Java Compiler and Thread to not cache value of this variable and always read it from the main memory.
  • In multi-threaded environment, threads might cache variables locally. Volatile (or synchronization) ensures the variable is read from main memory.
  • Makes variable atomic by implementation. 
  • Only possible with variables; cannot be used with method or class (illegal operation)
  • Guarantees visibility and ordering; write to any volatile variable happens before any read.
  • Prevents compiler or JVM form reordering code or moving away them away form synchronization barrier. 

2) Example


private boolean bExit;




 while(!bExit) {



    checkUserPosition();



    updateUserPosition();



 }

  • The focus here is the variable, bExit. 
  • One Thread (Game Thread) can cache the value of bExit instead of getting it form main memory everytime. If in between, any other thread (Event handler thread) changes the value; it would be not be visible to the GameThread.
  • Making bExit volatile in Java ensures this will not happen.


3) When to use Volatile?
3.1) Nonatomic 64-bit operations

In Java 64-bit, long and double values are treated as two 32-bit values. This means, a 64-bit write operation is basically performed as two separate 32-bit operations.
When you want to read/write long and double variable atomically, you should use volatile.

3.2) Less overhead version of synchronization

Ensures atomicity of variable without using synchronization (which has more overhead); explained more later.

3.3) Informs Compiler

Inform compiler a variable is subjected to be accessed by multiple threads, which prevents compiler from reordering or making undesired optimizations.

3.4) Double Checked Locking (Singleton Pattern)

4) Important points on Volatile

  • Read/Writes are atomic for most reference variable and most primitive variables (except long and double) even without use of volatile keyword.
  • Volatile variable don't block, since we are only doing simple read/write.
  • Java volatile variable can be null.
  • Volatile =/= atomic. For example, volatile++ is not atomic; to make operation atomic, you will need to ensure exclusive access using synchronized method/block in Java.
  • If variable is not shared between threads, volatile is not necessary.

5) Synchronized vs Volatile

  • Volatile is field modifier while synchronized modifies code blocks and methods.
  • Synchronized block, but volatile doesn't; thus, more performance impacting.
  • Synchronized have more overhead than volatile, because it synchronizes the value of all variables between thread memory and main memory. Whereas volatile synchronizes only one variable.

Resource


Comments

Post a Comment

Popular posts from this blog

[Redis] Redis Cluster vs Redis Sentinel

1) In-Depth Guides to Redis Cluster and Sentinel For more in-depth explanation for Redis Cluster or Redis Sentinel, please read my other posts: Sentinel:  https://notafraidofwong.blogspot.com/2019/05/redis-redis-sentinel.html Cluster:  https://notafraidofwong.blogspot.com/2019/05/redis-redis-cluster.html 2) Deciding between Redis Sentinel and Redis Cluster Similarities : Both solutions provide high availability for your system. 2.1) Redis Cluster 2.1.1) Pros Shards data across multiple nodes Has replication support Has built-in failover of master 2.1.2) Cons Not every library supports it May not be as robust (yet) as Standalone Redis or Sentinel Setup and maintenance is more complicated 2.2) Redis Sentinel 2.2.1) Pros Automatically selects new master in case of failure Easy to setup, (seems) easy to maintain 2.2.2) Cons No data sharding; master might be overutilized. Another distributed system to maintain. 3) So how do I decide? In
Read more

[Unit Testing] Test Doubles (Stubs, Mocks....etc)

Image
In unit testing, isolation is key. The goal of unit testing is to test individual components, and not an entire system . The class/object/function you are testing is System Under Test (SUT) , and the other components of the system are Collaborators or Dependencies. Test Double  is a generic term for any kind of 'pretend' object used in place of a real object for testing purposes.  There are several types of Testing Doubles. We will start with: Fake Stub Mock Command Queries Mocks vs Stubs And then: Dummy Spies Spies vs Mocks I) Basic Testing Doubles a) Fake Fake are objects that have working implementations, but not same as production one. Usually they take some shortcut and have simplified version of production code. Note that Fake is a generic term - that can point to anything; usually mock and stubs. An example of this is an in-memory implementation of a database. This fake implementation will not access the actual database, but will u
Read more

[Node.js] Pending HTTP requests lead to unresponsive nodeJS

This is a recap conclusion to the potential problems of a NodeJS program I worked on previously. The program simulates thousands of clients navigating a website by sending thousands of http requests to a given API . The idea is to load test a particular API to make sure they can handle high traffic. Because we are simulating clients, it's important to keep track of the state of the simulated client as well. The problem with this program is it is buggy and becomes unresponsive from time to time (the only solution is to restart it). The problem was fixed by re-writing the entire program in Java, but it was regretful that I never had enough time to find the exact reason the nodeJS version of it failed. The following are potential causes I suspect: 1. Exceptions The NodeJS program was not written very neatly and stacked callbacks on top of callbacks on top of callbacks. This made it very difficult to keep track of exceptions and unexpected http responses are prone to happen. So
Read more