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Judul : How To Purpose Locks Inwards Multi-Threaded Coffee Program
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How To Purpose 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 ofttimes have asking to explicate well-nigh how to usage Locks inward Java, therefore I idea to write a unproblematic Java program, which is multi-threaded in addition to uses rather novel Lock interface. Remember Lock is your tool to guard shared resources which tin endure anything e.g. database, File system, a Prime number Generator or a Message processor. Before using Locks inward Java program, it’s also ameliorate to acquire to a greater extent than or less basics. Lock is an interface from java.util.concurrent package. It was introduced inward JDK 1.5 loose every bit an choice of synchronized keyword. If you lot have got never written whatever multi-threading program, therefore I propose starting fourth dimension start amongst synchronized keyword because it’s easier to usage them. Once you lot are familiar amongst working of multi-threading computer programme e.g. How threads portion data, how inter thread communication works, you lot tin start amongst Lock facility. As I told you lot Lock is an interface, therefore 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 in addition to ReentrantReadWriteLock, afterwards provides ii to a greater extent than inner implementation known every bit ReentrantReadWriteLock.ReadLock in addition to ReentrantReadWriteLock.WriteLock. For our unproblematic multi-threaded Java program's role 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 encounter that Lock is used to protect a resource, therefore that alone ane thread tin access it at a time. Why nosotros do that? to brand certain our application bear properly.

For illustration nosotros tin usage Lock to protect a counter, whose sole role 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, therefore 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 in addition to unlock() method to loose lock.

Always recall to loose lock inward in conclusion block, because every object has alone ane lock in addition to if a thread doesn't loose it therefore no ane tin acquire it, which may resultant inward your computer 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 every bit presently every bit 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 in conclusion block, to brand certain it run fifty-fifty if endeavour block throws exception or not.

In adjacent department nosotros volition encounter illustration of our multi-threaded Java program, which uses Lock to protect shared Counter.

Java Lock in addition to ReentrantLock Example

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

import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;  /**  *  * Java Program to demo 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 in addition to 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 endure 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 agree 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 seat a long loop within Runnable's run() method to telephone telephone getCount() numerous time, if you lot encounter a duplicate way in that place is a work amongst your code, but without whatever duplicate way it’s working fine.

Common Mistakes made yesteryear beginners piece using Locks inward Java

Here are to a greater extent than or less of the mutual mistakes I have got observed yesteryear looking at Java beginners lock related code :

1) Instead of sharing lock they supply dissimilar locks to each thread. This ofttimes happens to them unknowingly because they unremarkably seat the lock in addition to guarded block within Runnable, in addition to they overstep ii different instances of Runnable to ii dissimilar threads e.g. where SimpleLock is a Runnable, every bit 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 in addition to secondThread are using dissimilar lock in addition to they tin run protected code simultaneously.

2) Second error Java beginners do is forget to telephone telephone unlock() method, only similar inward a higher house example. without calling unlock() method, Thread volition non loose its lock in addition to to a greater extent than or less other thread waiting for that lock volition never acquire that. Nothing volition move on inward this testify program, but in ane lawsuit you lot write this form of code inward existent application, you lot volition encounter nasty issues similar deadlock, starvation in addition to information corruption.  By the way Lock interface also offers several advantages over synchronized keyword, banking concern tally here to acquire more.

That's all well-nigh how to usage Locks inward multi-threaded Java computer programme for synchronization. Let me know if you lot have got whatever hard agreement Locks inward Java or anything related to multi-threading, Will endure glad to aid you. For farther reading, you lot tin explore Java documentation of Lock interface in addition to it's diverse implementation classes

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

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