How to Use Locks in Multi-Threaded Java Program?
Last Updated :
02 Feb, 2021
A lock may be a more flexible and complicated thread synchronization mechanism than the standard synchronized block. A lock may be a tool for controlling access to a shared resource by multiple threads. Commonly, a lock provides exclusive access to a shared resource: just one thread at a time can acquire the lock and everyone accesses to the shared resource requires that the lock be acquired first. However, some locks may allow concurrent access to a shared resource, like the read lock of a ReadWriteLock.
// Example of lock interface
Lock lock = new ReentrantLock();
lock.lock();
// critical section
lock.unlock();
Methods in the lock interface
There are certain methods in a lock interface. We are gonna look at those along with their modifiers:
| MODIFIERS | DESCRIPTION |
|---|
| void | lock() - It acquires the lock if it's available; if the lock isn't available a thread gets blocked until the lock is released |
| lockInterruptibly() - It is similar to lock() but it acquires the lock unless the thread is interrupted |
| unlock() - As the name suggests it simply releases the lock instance |
| condition | newCondition() - It simply returns the new condition instance |
| boolean | tryLock() - It attempts to accumulate the lock immediately, return true if locking succeeds |
| tryLock(long time, TimeUnit unit) - It is often almost like tryLock(), except it waits up the given timeout before abandoning trying to accumulate the Lock |
Implementation of locks
Let's see how can we implement some locks in Java:
1.readWriteLock()
ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
readWriteLock.readLock().lock();
// ....
......//
readWriteLock.readLock().unlock();
readWriteLock.writeLock().lock();
// only one writer can enter this section,
// and only if no threads are currently reading.
readWriteLock.writeLock().unlock();
Below is the implementation of readWriteLock() method:
Java
// Implementation of ReadWriteLock in Java
import java.io.*;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
class GFG<O> {
private final ReadWriteLock readWriteLock
= new ReentrantReadWriteLock();
private final Lock writeLock
= readWriteLock.writeLock();
private final Lock readLock = readWriteLock.readLock();
private final List<O> list = new ArrayList<>();
// setElement function sets
// i.e., write the element to the thread
public void setElement(O o)
{
// acquire the thread for writing
writeLock.lock();
try {
list.add(o);
System.out.println(
"Element by thread "
+ Thread.currentThread().getName()
+ " is added");
}
finally {
// To unlock the acquired write thread
writeLock.unlock();
}
}
// getElement function prints
// i.e., read the element from the thread
public O getElement(int i)
{
// acquire the thread for reading
readLock.lock();
try {
System.out.println(
"Elements by thread "
+ Thread.currentThread().getName()
+ " is printed");
return list.get(i);
}
finally {
// To unlock the acquired read thread
readLock.unlock();
}
}
public static void main(String[] args)
{
GFG<String> gfg = new GFG<>();
gfg.setElement("Hi");
gfg.setElement("Hey");
gfg.setElement("Hello");
System.out.println("Printing the last element : "
+ gfg.getElement(2));
}
}
OutputElement by thread main is added
Element by thread main is added
Element by thread main is added
Elements by thread main is printed
Printing the last element : Hello
2. reentrantLock()
public class lockImplement {
//...
ReentrantLock lock = new ReentrantLock();
int counter = 0;
public void testing() {
lock.lock();
try {
// Critical section here
count++;
} finally {
lock.unlock();
}
}
//...
}
Below is the implementation of reentrantLock() method:
Java
// Java code to illustrate Reentrant Locks
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReentrantLock;
class worker implements Runnable {
String name;
ReentrantLock re;
public worker(ReentrantLock rl, String n)
{
re = rl;
name = n;
}
public void run()
{
boolean done = false;
while (!done) {
// Getting Outer Lock
boolean ans = re.tryLock();
// Returns True if lock is free
if (ans) {
try {
Date d = new Date();
SimpleDateFormat ft
= new SimpleDateFormat("hh:mm:ss");
System.out.println(
"task name - " + name
+ " outer lock acquired at "
+ ft.format(d)
+ " Doing outer work");
Thread.sleep(1500);
// Getting Inner Lock
re.lock();
try {
d = new Date();
ft = new SimpleDateFormat(
"hh:mm:ss");
System.out.println(
"task name - " + name
+ " inner lock acquired at "
+ ft.format(d)
+ " Doing inner work");
System.out.println(
"Lock Hold Count - "
+ re.getHoldCount());
Thread.sleep(1500);
}
catch (InterruptedException e) {
e.printStackTrace();
}
finally {
// Inner lock release
System.out.println(
"task name - " + name
+ " releasing inner lock");
re.unlock();
}
System.out.println("Lock Hold Count - "
+ re.getHoldCount());
System.out.println("task name - " + name
+ " work done");
done = true;
}
catch (InterruptedException e) {
e.printStackTrace();
}
finally {
// Outer lock release
System.out.println(
"task name - " + name
+ " releasing outer lock");
re.unlock();
System.out.println("Lock Hold Count - "
+ re.getHoldCount());
}
}
else {
System.out.println("task name - " + name
+ " waiting for lock");
try {
Thread.sleep(1000);
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
public class test {
static final int MAX_T = 2;
public static void main(String[] args)
{
ReentrantLock rel = new ReentrantLock();
ExecutorService pool
= Executors.newFixedThreadPool(MAX_T);
Runnable w1 = new worker(rel, "Job1");
Runnable w2 = new worker(rel, "Job2");
Runnable w3 = new worker(rel, "Job3");
Runnable w4 = new worker(rel, "Job4");
pool.execute(w1);
pool.execute(w2);
pool.execute(w3);
pool.execute(w4);
pool.shutdown();
}
}
Output:
task name - Job2 waiting for lock
task name - Job1 outer lock acquired at 09:49:42 Doing outer work
task name - Job2 waiting for lock
task name - Job1 inner lock acquired at 09:49:44 Doing inner work
Lock Hold Count - 2
task name - Job2 waiting for lock
task name - Job2 waiting for lock
task name - Job1 releasing inner lock
Lock Hold Count - 1
task name - Job1 work done
task name - Job1 releasing outer lock
Lock Hold Count - 0
task name - Job3 outer lock acquired at 09:49:45 Doing outer work
task name - Job2 waiting for lock
task name - Job3 inner lock acquired at 09:49:47 Doing inner work
Lock Hold Count - 2
task name - Job2 waiting for lock
task name - Job2 waiting for lock
task name - Job3 releasing inner lock
Lock Hold Count - 1
task name - Job3 work done
task name - Job3 releasing outer lock
Lock Hold Count - 0
task name - Job4 outer lock acquired at 09:49:48 Doing outer work
task name - Job2 waiting for lock
task name - Job4 inner lock acquired at 09:49:50 Doing inner work
Lock Hold Count - 2
task name - Job2 waiting for lock
task name - Job2 waiting for lock
task name - Job4 releasing inner lock
Lock Hold Count - 1
task name - Job4 work done
task name - Job4 releasing outer lock
Lock Hold Count - 0
task name - Job2 outer lock acquired at 09:49:52 Doing outer work
task name - Job2 inner lock acquired at 09:49:53 Doing inner work
Lock Hold Count - 2
task name - Job2 releasing inner lock
Lock Hold Count - 1
task name - Job2 work done
task name - Job2 releasing outer lock
Lock Hold Count - 0
Note: The program might not work on an online IDE because of sleep call.