How To Role Locks Inwards Multi-Threaded Coffee Program

Many Java programmers confused themselves similar hell piece writing multi-threaded Java programs e.g. where to synchronized? Which Lock to use? What Lock to usage etc. I oft have asking to explicate most how to usage Locks inward Java, thus I idea to write a uncomplicated Java program, which is multi-threaded as well as uses rather novel Lock interface. Remember Lock is your tool to guard shared resources which tin live anything e.g. database, File system, a Prime number Generator or a Message processor. Before using Locks inward Java program, it’s also amend to larn roughly basics. Lock is an interface from java.util.concurrent package. It was introduced inward JDK 1.5 unloose equally an option of synchronized keyword. If y'all have got never written whatsoever multi-threading program, as well as thus I advise start start amongst synchronized keyword because it’s easier to usage them. Once y'all are familiar amongst working of multi-threading programme e.g. How threads part data, how inter thread communication works, y'all tin start amongst Lock facility. As I told y'all Lock is an interface, thus nosotros cannot usage it directly, instead nosotros demand to usage its implementation class. Thankfully Java comes amongst ii implementation of java.util.concurrent.locks.Lock interface, ReentrantLock as well as ReentrantReadWriteLock, after provides ii to a greater extent than inner implementation known equally ReentrantReadWriteLock.ReadLock as well as ReentrantReadWriteLock.WriteLock. For our uncomplicated multi-threaded Java program's usage ReentrantLock is enough.
Here is the idiom to usage Locks inward Java :

Lock l = ...; l.lock();  try {     // access the resources protected yesteryear this lock  } finally {    l.unlock(); }
 

You tin run into that Lock is used to protect a resource, thus that solely i thread tin access it at a time. Why nosotros do that? to brand certain our application deport properly.

For illustration nosotros tin usage Lock to protect a counter, whose sole usage is to render a count incremented yesteryear one, when anyone calls its getCount() method. If nosotros don't protect them yesteryear parallel access of thread, as well as thus it’s possible that ii thread receives same count, which is against the program's policies.

Now, coming dorsum to semantics, nosotros have got used lock() method to acquire lock as well as unlock() method to unloose lock.

Always recollect to unloose lock inward lastly block, because every object has solely i lock as well as if a thread doesn't unloose it as well as thus no i tin acquire it, which may upshot inward your programme hung or threads going into deadlock.

That's why I said that synchronized keyword is simpler than lock, because Java itself brand certain that lock acquired yesteryear thread yesteryear entering into synchronized block or method is released equally before long equally it came out of the block or method. This happens fifty-fifty if thread came out yesteryear throwing exception, this is also nosotros have got unlock code inward lastly block, to brand certain it run fifty-fifty if attempt block throws exception or not.

In side yesteryear side department nosotros volition run into illustration of our multi-threaded Java program, which uses Lock to protect shared Counter.

Java Lock as well as ReentrantLock Example

Here is a sample Java program, which uses both Lock as well as ReentrantLock to protect a shared resource. In our illustration it’s an object, a counter's object. Invariant of Counter cast is to render a count incremented yesteryear 1 each fourth dimension person calls getCount() method. Here for testing 3 threads volition telephone band getCount() method simultaneously but guard provided yesteryear Lock volition preclude shared counter. As an exercise y'all tin also implement same cast using synchronized keyword. Here is consummate code :

import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;  /**  *  * Java Program to exhibit how to usage Locks inward multi-threading   * e.g. ReentrantLock, ReentrantReadWriteLock etc.  *  * @author Javin Paul  */ public class LockDemo {      public static void main(String args[]) {          // Let's do a counter as well as shared it betwixt 3 threads         // Since Counter needs a lock to protect its getCount() method         // nosotros are giving it a ReentrantLock.         final Counter myCounter = new Counter(new ReentrantLock());          // Task to live executed yesteryear each thread         Runnable r = new Runnable() {             @Override             public void run() {                 System.out.printf("Count at thread %s is %d %n",                         Thread.currentThread().getName(), myCounter.getCount());             }         };          // Creating 3 threads         Thread t1 = new Thread(r, "T1");         Thread t2 = new Thread(r, "T2");         Thread t3 = new Thread(r, "T3");          //starting all threads         t1.start();         t2.start();         t3.start();     } }

class Counter {     private Lock lock; // Lock to protect our counter     private int count; // Integer to concur count      public Counter(Lock myLock) {         this.lock = myLock;     }      public final int getCount() {         lock.lock();         try {             count++;             return count;         } finally {             lock.unlock();         }              } } Output: Count at thread T1 is 1 Count at thread T2 is 2 Count at thread T3 is 3

You tin fifty-fifty position a long loop within Runnable's run() method to telephone band getCount() numerous time, if y'all run into a duplicate way in that place is a work amongst your code, but without whatsoever duplicate way it’s working fine.

Common Mistakes made yesteryear beginners piece using Locks inward Java

Here are roughly of the mutual mistakes I have got observed yesteryear looking at Java beginners lock related code :

1) Instead of sharing lock they render dissimilar locks to each thread. This oft happens to them unknowingly because they normally position the lock as well as guarded block within Runnable, as well as they top ii different instances of Runnable to ii dissimilar threads e.g. where SimpleLock is a Runnable, equally shown below :

Thread firstThread = new Thread(new SimpleLock()); Thread secondThread = new Thread(new SimpleLock());  class SimpleLock implements Runnable {         private Lock myLock = new ReentrantLock();         public void printOutput() {             System.out.println("Hello!");         }         public void run() {             if (myLock.tryLock()) {                 myLock.lock();                 printOutput();             }else                 System.out.println("The lock is non accessible.");         }     }

Since hither myLock is instance variable, each instance of SimpleLock has their ain myLock instance, which way firstThread as well as secondThread are using dissimilar lock as well as they tin run protected code simultaneously.

2) Second error Java beginners do is forget to telephone band unlock() method, only similar inward a higher house example. without calling unlock() method, Thread volition non unloose its lock as well as roughly other thread waiting for that lock volition never acquire that. Nothing volition move on inward this assay out program, but in i lawsuit y'all write this sort of code inward existent application, y'all volition run into nasty issues similar deadlock, starvation as well as information corruption.  By the way Lock interface also offers several advantages over synchronized keyword, cheque here to larn more.

That's all most how to usage Locks inward multi-threaded Java programme for synchronization. Let me know if y'all have got whatsoever hard agreement Locks inward Java or anything related to multi-threading, Will live glad to assist you. For farther reading, y'all tin explore Java documentation of Lock interface as well as it's diverse implementation classes

Further Learning
Multithreading as well as Parallel Computing inward Java
Java Concurrency inward Practice - The Book
Applying Concurrency as well as Multi-threading to Common Java Patterns
Java Concurrency inward Practice Course yesteryear Heinz Kabutz

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