Synchronization in Java
In multithreading, synchronization is important to make sure multiple threads safely work on shared resources. Without synchronization, data can become inconsistent or corrupted if multiple threads access and modify shared variables at the same time. In Java, it is a mechanism that ensures that only one thread can access a resource at any given time. This process helps prevent issues such as data inconsistency and race conditions when multiple threads interact with shared resources.
Example: Below is a Java Program to demonstrate synchronization.
// Java Program to demonstrate synchronization in Java
class Counter {
private int c = 0; // Shared variable
// Synchronized method to increment counter
public synchronized void inc() {
c++;
}
// Synchronized method to get counter value
public synchronized int get() {
return c;
}
}
public class Geeks {
public static void main(String[] args) {
Counter cnt = new Counter(); // Shared resource
// Thread 1 to increment counter
Thread t1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
cnt.inc();
}
});
// Thread 2 to increment counter
Thread t2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
cnt.inc();
}
});
// Start both threads
t1.start();
t2.start();
// Wait for threads to finish
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print final counter value
System.out.println("Counter: " + cnt.get());
}
}
Output
Counter: 2000
Explanation: Two threads, t1 and t2, increment the shared counter variable concurrently. The inc() and get() methods are synchronized, meaning only one thread can execute these methods at a time, preventing race conditions. The program ensures that the final value of the counter is consistent and correctly updated by both threads.
Need of Synchronization:
When multiple threads share resources, synchronization make sure that only one thread accesses the resource at a time. This prevents problem like data getting mixed up or broken because of multiple threads changing it together.
Synchronized Blocks in Java
Java provides a way to create threads and synchronise their tasks using synchronized blocks.
A synchronized block in Java is synchronized on some object. Synchronized blocks in Java are marked with the synchronized keyword. All synchronized blocks synchronize on the same object and can only have one thread executed inside them at a time. All other threads attempting to enter the synchronized block are blocked until the thread inside the synchronized block exits the block. If you want to master concurrency and understand how to avoid common pitfalls,
General Form of Synchronized Block
synchronized(sync_object)
{
// Access shared variables and other
// shared resources
}
This synchronization is implemented in Java with a concept called monitors or locks. Only one thread can own a monitor at a given time. When a thread acquires a lock, it is said to have entered the monitor. All other threads attempting to enter the locked monitor will be suspended until the first thread exits the monitor.
Example: Below is an example of synchronization using Synchronized Blocks.
// Java Program to demonstrate synchronization block in Java
class Counter {
private int c = 0; // Shared variable
// Method with synchronization block
public void inc() {
synchronized(this) { // Synchronize only this block
c++;
}
}
// Method to get counter value
public int get() {
return c;
}
}
public class Geeks {
public static void main(String[] args) {
Counter cnt = new Counter(); // Shared resource
// Thread 1 to increment counter
Thread t1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
cnt.inc();
}
});
// Thread 2 to increment counter
Thread t2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
cnt.inc();
}
});
// Start both threads
t1.start();
t2.start();
// Wait for threads to finish
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print final counter value
System.out.println("Counter: " + cnt.get());
}
}
Output
Counter: 2000
Types of Synchronization
There are two type of synchronizations in Java which are listed below:
- Process Synchronization
- Thread Synchronization
1. Process Synchronization in Java
Process Synchronization is a technique used to coordinate the execution of multiple processes. It ensures that the shared resources are safe and in order.
Example: Here is a popular example of Process Synchronization in Java
// Java Program to demonstrate Process Synchronization
class BankAccount {
private int balance
= 1000; // Shared resource (bank balance)
// Synchronized method for deposit operation
public synchronized void deposit(int amount)
{
balance += amount;
System.out.println("Deposited: " + amount
+ ", Balance: " + balance);
}
// Synchronized method for withdrawal operation
public synchronized void withdraw(int amount)
{
if (balance >= amount) {
balance -= amount;
System.out.println("Withdrawn: " + amount
+ ", Balance: " + balance);
}
else {
System.out.println(
"Insufficient balance to withdraw: "
+ amount);
}
}
public int getBalance() { return balance; }
}
// Main class
public class Geeks {
public static void main(String[] args)
{
BankAccount account
= new BankAccount(); // Shared resource
// Thread 1 to deposit money into the account
Thread t1 = new Thread(() -> {
for (int i = 0; i < 3; i++) {
account.deposit(200);
try {
Thread.sleep(50); // Simulate some delay
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
});
// Thread 2 to withdraw money from the account
Thread t2 = new Thread(() -> {
for (int i = 0; i < 3; i++) {
account.withdraw(100);
try {
Thread.sleep(
100); // Simulate some delay
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
});
// Start both threads
t1.start();
t2.start();
// Wait for threads to finish
try {
t1.join();
t2.join();
}
catch (InterruptedException e) {
e.printStackTrace();
}
// Print final balance
System.out.println("Final Balance: "
+ account.getBalance());
}
}
Output
Withdrawn: 100, Balance: 900 Deposited: 200, Balance: 1100 Deposited: 200, Balance: 1300 Withdrawn: 100, Balance: 1200 Deposited: 200, Balance: 1400 Withdrawn: 100, Balance: 1300 Final Balance: 1300
Explanation: It demonstrates process synchronization using a bank account with deposit and withdrawal operations. Two threads, one for depositing and one for withdrawing, perform operations on the shared account. The methods deposit() and withdraw() are synchronized to ensure thread safety, preventing race conditions when both threads access the balance simultaneously. This ensures accurate updates to the account balance.
2. Thread Synchronization in Java
Thread Synchronization is used to coordinate and ordering of the execution of the threads in a multi-threaded program. There are two types of thread synchronization are mentioned below:
- Mutual Exclusive
- Cooperation (Inter-thread communication in Java)
Example: Java Program to demonstrate thread synchronization for Ticket Booking System.
// Java Program to demonstrate thread synchronization for Ticket Booking System
class TicketBooking {
private int availableTickets = 10; // Shared resource (available tickets)
// Synchronized method for booking tickets
public synchronized void bookTicket(int tickets) {
if (availableTickets >= tickets) {
availableTickets -= tickets;
System.out.println("Booked " + tickets + " tickets, Remaining tickets: " + availableTickets);
} else {
System.out.println("Not enough tickets available to book " + tickets);
}
}
public int getAvailableTickets() {
return availableTickets;
}
}
public class Geeks {
public static void main(String[] args) {
TicketBooking booking = new TicketBooking(); // Shared resource
// Thread 1 to book tickets
Thread t1 = new Thread(() -> {
for (int i = 0; i < 2; i++) {
booking.bookTicket(2); // Trying to book 2 tickets each time
try {
Thread.sleep(50); // Simulate delay
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
// Thread 2 to book tickets
Thread t2 = new Thread(() -> {
for (int i = 0; i < 2; i++) {
booking.bookTicket(3); // Trying to book 3 tickets each time
try {
Thread.sleep(40); // Simulate delay
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
// Start both threads
t1.start();
t2.start();
// Wait for threads to finish
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print final remaining tickets
System.out.println("Final Available Tickets: " + booking.getAvailableTickets());
}
}
Output
Booked 2 tickets, Remaining tickets: 8 Booked 3 tickets, Remaining tickets: 5 Booked 3 tickets, Remaining tickets: 2 Booked 2 tickets, Remaining tickets: 0 Final Available Tickets: 0
Explanation: Here, the TicketBooking class contains a synchronized method bookTicket(), which ensures that only one thread can book tickets at a time, preventing race conditions and overbooking. Each thread attempts to book a set number of tickets in a loop, with thread synchronization ensuring that the availableTickets variable is safely accessed and updated. Finally, the program prints the remaining tickets.
Mutual Exclusion
Mutual Exclusion helps keep threads from interfering with one another while sharing data. There are three types of Mutual Exclusive mentioned below:
- Synchronized method.
- Synchronized block.
- Static synchronization.
Example: Below is the implementation of the Java Synchronization.
// A Java program to demonstrate working of synchronized.
import java.io.*;
// A Class used to send a message
class Sender {
public void send(String msg)
{
System.out.println("Sending " + msg); // Changed to print without new line
try {
Thread.sleep(100);
}
catch (Exception e) {
System.out.println("Thread interrupted.");
}
System.out.println(msg + "Sent"); // Improved output format
}
}
// Class for sending a message using Threads
class ThreadedSend extends Thread {
private String msg;
Sender sender;
// Receives a message object and a string message to be sent
ThreadedSend(String m, Sender obj)
{
msg = m;
sender = obj;
}
public void run()
{
// Only one thread can send a message at a time.
synchronized (sender)
{
// Synchronizing the send object
sender.send(msg);
}
}
}
// Driver class
class Geeks {
public static void main(String args[])
{
Sender send = new Sender();
ThreadedSend S1 = new ThreadedSend("Hi ", send);
ThreadedSend S2 = new ThreadedSend("Bye ", send);
// Start two threads of ThreadedSend type
S1.start();
S2.start();
// Wait for threads to end
try {
S1.join();
S2.join();
}
catch (Exception e) {
System.out.println("Interrupted");
}
}
}
Output
Sending Hi Hi Sent Sending Bye Bye Sent
Explanation: In the above example, we choose to synchronize the Sender object inside the run() method of the ThreadedSend class. Alternately, we could define the whole send() block as synchronized, producing the same result. Then we don't have to synchronize the Message object inside the run() method in the ThreadedSend class.
We do not always have to synchronize a whole method. Sometimes it is preferable to synchronize only part of a method. Java synchronized blocks inside methods make this possible.
Example: Below is the Java program shows the synchronized method using an anonymous class
// Java Pogram to synchronized method by
// using an anonymous class
import java.io.*;
class Test {
synchronized void test_func(int n)
{
// synchronized method
for (int i = 1; i <= 3; i++) {
System.out.println(n + i);
try {
Thread.sleep(100);
}
catch (Exception e) {
System.out.println(e);
}
}
}
}
// Driver Class
public class Geeks {
// Main function
public static void main(String args[])
{
// only one object
final Test O = new Test();
Thread a = new Thread() {
public void run() { O.test_func(15); }
};
Thread b = new Thread() {
public void run() { O.test_func(30); }
};
a.start();
b.start();
}
}
Output
16 17 18 31 32 33
Explanation: Here the Test class has a synchronized method test_func() that prints a sequence of numbers with a slight delay, ensuring thread safety when accessed by multiple threads. Two threads are created using anonymous classes, each calling the test_func() method with different values. The synchronized keyword ensures that only one thread can execute the method at a time.
