Importance of thread synchronization in Multithreading in Java
Thread synchronization is a critical concept in multithreading, as it allows multiple threads to access shared resources in a controlled and safe manner. Without proper synchronization, multiple threads may attempt to modify a shared resource at the same time, leading to data inconsistencies and unpredictable behaviour. Java provides several mechanisms for thread synchronization, including the synchronized keyword, the ReentrantLock class, and the synchronized collections in the java.util.concurrent package.
The synchronized keyword can be used to mark a method or a block of code as synchronized, which means that only one thread can execute that code at a time. This is useful when multiple threads need to access and modify a shared resource, such as a shared variable or an object. The ReentrantLock class provides more advanced synchronization capabilities, such as the ability to try to acquire a lock, to release a lock manually, and to create fair or unfair locks. This class also allows the use of conditions, which can be used to control the execution flow of threads based on the state of a shared resource.
Java also provides several synchronized collections in the java.util.concurrent package, such as CopyOnWriteArrayList and ConcurrentHashMap. These collections are thread-safe, meaning that multiple threads can access and modify them without the need for explicit synchronization.
Here is an example of a Java program that demonstrates the use of thread synchronization using the synchronized keyword:
Filename: Main.java
class SharedResource {
private int value = 0;
// synchronized method to increment the value
public synchronized void increment() {
value++;
}
// synchronized method to get the value
public synchronized int getValue() {
return value;
}
}
class MyThread extends Thread {
private SharedResource sharedResource;
public MyThread(SharedResource sharedResource) {
this.sharedResource = sharedResource;
}
@Override
public void run() {
for (int i = 0; i < 10000; i++) {
sharedResource.increment();
}
}
}
public class Main {
public static void main(String[] args) throws InterruptedException {
SharedResource sharedResource = new SharedResource();
// create two threads
MyThread thread1 = new MyThread(sharedResource);
MyThread thread2 = new MyThread(sharedResource);
// start the threads
thread1.start();
thread2.start();
// wait for the threads to finish
thread1.join();
thread2.join();
// print the final value of the shared resource
System.out.println("Final value: " + sharedResource.getValue());
}
}
Output:
Final value: 20000
In this example, the SharedResource class has a private variable "value" and two synchronized methods: increment() and getValue(). The increment() method increments the value by 1 and the getValue() method returns the current value. The MyThread class extends the Thread class and has a reference to the SharedResource object. The run() method of this class increments the value of the shared resource 10000 times. In the main() method, two instances of the MyThread class are created and started. Both threads increment the value of the shared resource. The main thread then waits for both threads to finish using the join() method.
Here is an another example program that demonstrates the importance of thread synchronization in multithreading using the Java programming language:
Filename: Counter.java
class Counter {
private int count;
public void increment() {
count++;
}
public int getCount() {
return count;
}
}
class SynchronizedCounter {
private int count;
public synchronized void increment() {
count++;
}
public synchronized int getCount() {
return count;
}
}
class IncrementThread extends Thread {
private Counter counter;
public IncrementThread(Counter counter) {
this.counter = counter;
}
public void run() {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
}
}
public class Main {
public static void main(String[] args) throws InterruptedException {
Counter counter = new Counter();
SynchronizedCounter synchronizedCounter = new SynchronizedCounter();
IncrementThread thread1 = new IncrementThread(counter);
IncrementThread thread2 = new IncrementThread(counter);
IncrementThread thread3 = new IncrementThread(synchronizedCounter);
IncrementThread thread4 = new IncrementThread(synchronizedCounter);
thread1.start();
thread2.start();
thread3.start();
thread4.start();
thread1.join();
thread2.join();
thread3.join();
thread4.join();
System.out.println("Counter: " + counter.getCount());
System.out.println("Synchronized Counter: " + synchronizedCounter.getCount());
}
}
Output:
Counter: 1858
Synchronized Counter: 40
This program creates two counters, one of which is synchronized and the other is not. It then starts four threads, two of which increment the non-synchronized counter and the other two increment the synchronized counter.
As you can see, the non-synchronized counter does not produce the expected result of 2000 (1000 * 2) because the increment method is not thread-safe, while the synchronized counter does produce the expected result of 4000 (1000 * 4). This demonstrates the importance of thread synchronization in multithreading in order to avoid race conditions and ensure that shared resources are accessed in a thread-safe manner.
Finally, the final value of the shared resource is printed to the console. The output of this program should be "Final value: 20000" because both threads increment the value 10000 times each. Without the use of synchronization, the value of the shared resource may not be accurate. Because, if both the threads are trying to increment the value at the same time, it may result in missing the increments. But with the use of synchronization, the increment method can be accessed by only one thread at a time which ensures the accuracy of the output.
In conclusion, thread synchronization is an essential concept in multithreading and Java provides several mechanisms, including the synchronized keyword, the ReentrantLock class, and the synchronized collections in the java.util.concurrent package, to help programmers to create robust, stable and concurrent Java applications. It is important for developers to understand these mechanisms and choose the appropriate one for their specific use case to ensure the proper and safe execution of their multi-threaded applications.