DZone
Thanks for visiting DZone today,
Edit Profile
  • Manage Email Subscriptions
  • How to Post to DZone
  • Article Submission Guidelines
Sign Out View Profile
  • Post an Article
  • Manage My Drafts
Over 2 million developers have joined DZone.
Log In / Join
Refcards Trend Reports Events Over 2 million developers have joined DZone. Join Today! Thanks for visiting DZone today,
Edit Profile Manage Email Subscriptions Moderation Admin Console How to Post to DZone Article Submission Guidelines
View Profile
Sign Out
Refcards
Trend Reports
Events
Zones
Culture and Methodologies Agile Career Development Methodologies Team Management
Data Engineering AI/ML Big Data Data Databases IoT
Software Design and Architecture Cloud Architecture Containers Integration Microservices Performance Security
Coding Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks
Culture and Methodologies
Agile Career Development Methodologies Team Management
Data Engineering
AI/ML Big Data Data Databases IoT
Software Design and Architecture
Cloud Architecture Containers Integration Microservices Performance Security
Coding
Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance
Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks

ReentrantLock Cheat Sheet

This quick and easy guide will show you how to make ReentrantLocks work for you, including examples of some advanced tricks.

Thomas Krieger user avatar by
Thomas Krieger
·
Oct. 05, 16 · Tutorial
Like (15)
Save
Tweet
Share
14.04K Views

Join the DZone community and get the full member experience.

Join For Free

The ReentrantLock is a replacement for the easier-to-use synchronized statement when you need one of the following advanced techniques: lockInterrupibly, tryLock, lock coupling, multiple conditions, or fair locks.

In the cheat sheet below, I summarized each technique. And in the rest of this blog post, I give a detailed description of those techniques.

ReentrantLock Cheat Sheet

Similar to synchronized statements, this lock is reentrant, which means the same thread can acquire the lock multiple times.

Lock Interruptible

private final ReentrantLock lock = new ReentrantLock(); 

public void m() throws InterruptedException  { 
    lock.lockInterruptibly(); 
        try { 
        // ... method body 
    } 
    finally { 
        lock.unlock() 
    } 
} 

This throws an InterruptedException when the thread.interrupt method was called by another thread. This allows you to interrupt the processing even while trying to acquire a lock — something that is not possible with the synchronized statement.

Try Lock

private final ReentrantLock lock = new ReentrantLock(); 

public boolean m() throws InterruptedException  { 
    if(! lock.tryLock(2 , TimeUnit.MILLISECONDS ) )  { 
        return false; 
    } 
    try 
    { 
        // ... method body 
    } 
    finally { 
        lock.unlock() 
    } 
    return true; 
} 

Similar to aqcuireInterrubtable, this throws a thread interrupted exception when the thread is interrupted by another thread. Additionally, it allows you to give a time span for how long to try to acquire the lock.

This is particularly useful when you have a task that is only valid for a specific amount of time. You can also use it to avoid deadlocks. When you need to acquire two locks simultaneously acquire the locks with tryLock. If you cannot acquire one of the locks at the given interval, release both locks, wait a little bit, and try again.

Lock Coupling

The reentrant lock allows you to unlock a lock immediately after you have successfully acquired another lock, leading to a technique called lock coupling. This can, for example, be used to lock a linkedlist. You can see an implementation here.

Multiple Conditions

Create:

Condition notFull = lock.newCondition();

Wait for Condition to become true:

lock.lock()
try { 
    while( condition not fullfilled) { 
        notFull.await(); 
    } 
}
finally { 
    lock.unlock() 
} 

Signal that Condition has become true:

lock.lock() 
try { 
    notFull.signal(); 
} 
finally { 
    lock.unlock() 
} 

Be cautious that you do not mix the methods from Condition, ‘await’, 'signal', with the wait and notify methods from java.lang.Object.

That's useful when you need to wait for different conditions. ArrayBlockingQueue, for example, uses two conditions one to wait that the queue becomes not full the other to wait till the queue becomes not empty.

Fair Locks

private final ReentrantLock lock = new ReentrantLock(true);

Threads get the lock in the order they requested it. This has a high performance penalty (see the following benchmark for details).

Benchmark

synchronized vs. ReentrantLock

The figure shows the throughput of synchronized vs. reentrant lock in a fair and unfair mode for different thread counts. 

The benchmark was run on JDK 8 on an Intel i5 4-core CPU using jmh. The source of the benchmark can be downloaded here.

Conclusion

A reentrant lock is one of the advanced techniques used to write multithreaded Java code. Whatever technique you use, you should test the multithreaded part of your application. Read here more about testing multithreaded programs.

Lock (computer science)

Published at DZone with permission of Thomas Krieger, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

Popular on DZone

  • How Do the Docker Client and Docker Servers Work?
  • Unleashing the Power of JavaScript Modules: A Beginner’s Guide
  • Upgrade Guide To Spring Data Elasticsearch 5.0
  • Using JSON Web Encryption (JWE)

Comments

Partner Resources

X

ABOUT US

  • About DZone
  • Send feedback
  • Careers
  • Sitemap

ADVERTISE

  • Advertise with DZone

CONTRIBUTE ON DZONE

  • Article Submission Guidelines
  • Become a Contributor
  • Visit the Writers' Zone

LEGAL

  • Terms of Service
  • Privacy Policy

CONTACT US

  • 600 Park Offices Drive
  • Suite 300
  • Durham, NC 27709
  • support@dzone.com
  • +1 (919) 678-0300

Let's be friends: