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
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());
}
}
Explanation:
- Two threads (t1 and t2) access a shared counter variable concurrently.
- The inc() and get() methods are marked as synchronized.
- synchronized ensures that only one thread can execute these methods at a time.
- This prevents race conditions during concurrent updates.
- The use of synchronization ensures the counter is accurately and consistently updated by both threads.
- Final output reflects the correct cumulative result of all increments.
Synchronized Blocks in Java
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,
synchronized(sync_object)
{
// Access shared variables and other shared resources
}
Example: Below is an example of synchronization using Synchronized Blocks.
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());
}
}
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
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());
}
}
OutputWithdrawn: 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:
- Demonstrates process synchronization using a shared BankAccount object.
- Two threads operate concurrently: one for depositing, one for withdrawing.
- Both deposit() and withdraw() methods are marked as synchronized.
- synchronized ensures mutual exclusion only one thread can access the balance at a time.
- Prevents race conditions when threads access or modify the balance simultaneously.
- Ensures thread safety and maintains a consistent, accurate 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:
Example: Java Program to demonstrate thread synchronization for Ticket Booking System.
Java
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());
}
}
OutputBooked 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:
- Uses a shared TicketBooking class with a synchronized bookTicket() method.
- Ensures only one thread can book tickets at a time, preventing race conditions.
- Multiple threads try booking tickets in a loop.
- availableTickets is safely accessed and updated through synchronization.
- Prevents overbooking and ensures thread-safe ticket allocation.
- The program prints the remaining available tickets after all booking attempts.
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.
Java
.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");
}
}
}
OutputSending Hi
Hi Sent
Sending Bye
Bye Sent
Explanation:
- Sender is synchronized inside run() to ensure thread safety.
- Alternatively, making send() a synchronized method achieves the same.
- This removes the need to synchronize Sender in run().
- Both prevent concurrent access and ensure safe message sending.
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
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();
}
}
Explanation:
- The Test class defines a synchronized method test_func() to ensure thread safety.
- Two threads are created using anonymous classes, each calling test_func() with different input values.
- The synchronized keyword ensures only one thread accesses test_func() at a time.
- This prevents interleaved output and ensures consistent execution of the number sequence.
Volatile Keyword
The volatile keyword in Java ensures that all threads have a consistent view of a variable's value. It prevents caching of the variable's value by threads, ensuring that updates to the variable are immediately visible to other threads.
Working of Volatile Modifier:
- It applies only to variables.
- volatile guarantees visibility i.e. any write to a volatile variable is immediately visible to other threads.
- It does not guarantee atomicity, meaning operations like count++ (read-modify-write operations) can still result in inconsistent values
Example: Java program to demonstrate the volatile keyword.
Java
class Counter {
private volatile boolean running = true;
public void stop() {
running = false;
}
public void start() {
new Thread(() -> {
while (running) {
System.out.println("Running...");
try {
Thread.sleep(200);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
System.out.println("Stopped.");
}).start();
}
}
public class Geeks {
public static void main(String[] args) throws InterruptedException {
Counter counter = new Counter();
counter.start();
Thread.sleep(600); // Let it run briefly
counter.stop(); // Then stop the thread
}
}
OutputRunning...
Running...
Running...
Stopped.
Explanation:
- running is marked volatile to ensure changes are visible across threads.
- start() creates a thread that prints "Running..." while running is true.
- Thread.sleep(200) slows the loop to simulate work.
- main() waits briefly, then calls stop() to set running to false.
- The thread sees the change and exits the loop, printing "Stopped.
Volatile vs Synchronized
Features | Synchronized | Volatile |
|---|
Applies to | It applies only to blocks or methods. | It applies to variables only. |
|---|
Purpose | It ensures mutual exclusion and visibility | It ensures visibility of changes to variables across threads |
|---|
Performance | Performance is relatively low compared to volatile because of the acquisition and release of the lock. | Performance is relatively high compared to synchronized Keyword. |
|---|
Similar Reads
Java Tutorial Java is a high-level, object-oriented programming language used to build web apps, mobile applications, and enterprise software systems. Known for its Write Once, Run Anywhere capability, which means code written in Java can run on any device that supports the Java Virtual Machine (JVM).Syntax and s
10 min read
Basics
Introduction to JavaJava is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It is platform-independent, which means we can write code once and run it anywhere using the Java Virtual Machine (JVM). Java is mostly used for building desktop applications, web applications, Android
4 min read
Java Programming BasicsJava is one of the most popular and widely used programming language and platform. A platform is an environment that helps to develop and run programs written in any programming language. Java is fast, reliable and secure. From desktop to web applications, scientific supercomputers to gaming console
4 min read
Java MethodsJava Methods are blocks of code that perform a specific task. A method allows us to reuse code, improving both efficiency and organization. All methods in Java must belong to a class. Methods are similar to functions and expose the behavior of objects.Example: Java program to demonstrate how to crea
7 min read
Access Modifiers in JavaIn Java, access modifiers are essential tools that define how the members of a class, like variables, methods, and even the class itself, can be accessed from other parts of our program. They are an important part of building secure and modular code when designing large applications. In this article
6 min read
Arrays in JavaIn Java, an array is an important linear data structure that allows us to store multiple values of the same type. Arrays in Java are objects, like all other objects in Java, arrays implicitly inherit from the java.lang.Object class. This allows you to invoke methods defined in Object (such as toStri
9 min read
Java StringsIn Java, a String is the type of object that can store a sequence of characters enclosed by double quotes and every character is stored in 16 bits, i.e., using UTF 16-bit encoding. A string acts the same as an array of characters. Java provides a robust and flexible API for handling strings, allowin
8 min read
Regular Expressions in JavaIn Java, Regular Expressions or Regex (in short) in Java is an API for defining String patterns that can be used for searching, manipulating, and editing a string in Java. Email validation and passwords are a few areas of strings where Regex is widely used to define the constraints. Regular Expressi
7 min read
OOPs & Interfaces
Classes and Objects in JavaIn Java, classes and objects are basic concepts of Object Oriented Programming (OOPs) that are used to represent real-world concepts and entities. A class is a template to create objects having similar properties and behavior, or in other words, we can say that a class is a blueprint for objects.An
10 min read
Java ConstructorsIn Java, constructors play an important role in object creation. A constructor is a special block of code that is called when an object is created. Its main job is to initialize the object, to set up its internal state, or to assign default values to its attributes. This process happens automaticall
10 min read
Java OOP(Object Oriented Programming) ConceptsBefore Object-Oriented Programming (OOPs), most programs used a procedural approach, where the focus was on writing step-by-step functions. This made it harder to manage and reuse code in large applications.To overcome these limitations, Object-Oriented Programming was introduced. Java is built arou
10 min read
Java PackagesPackages in Java are a mechanism that encapsulates a group of classes, sub-packages and interfaces. Packages are used for: Prevent naming conflicts by allowing classes with the same name to exist in different packages, like college.staff.cse.Employee and college.staff.ee.Employee.They make it easier
8 min read
Java InterfaceAn Interface in Java programming language is defined as an abstract type used to specify the behaviour of a class. An interface in Java is a blueprint of a behaviour. A Java interface contains static constants and abstract methods. Key Properties of Interface:The interface in Java is a mechanism to
11 min read
Collections
Exception Handling
Java Exception HandlingException handling in Java is an effective mechanism for managing runtime errors to ensure the application's regular flow is maintained. Some Common examples of exceptions include ClassNotFoundException, IOException, SQLException, RemoteException, etc. By handling these exceptions, Java enables deve
8 min read
Java Try Catch BlockA try-catch block in Java is a mechanism to handle exceptions. This make sure that the application continues to run even if an error occurs. The code inside the try block is executed, and if any exception occurs, it is then caught by the catch block.Example: Here, we are going to handle the Arithmet
4 min read
Java final, finally and finalizeIn Java, the keywords "final", "finally" and "finalize" have distinct roles. final enforces immutability and prevents changes to variables, methods, or classes. finally ensures a block of code runs after a try-catch, regardless of exceptions. finalize is a method used for cleanup before an object is
4 min read
Chained Exceptions in JavaChained Exceptions in Java allow associating one exception with another, i.e. one exception describes the cause of another exception. For example, consider a situation in which a method throws an ArithmeticException because of an attempt to divide by zero.But the root cause of the error was an I/O f
3 min read
Null Pointer Exception in JavaA NullPointerException in Java is a RuntimeException. It occurs when a program attempts to use an object reference that has the null value. In Java, "null" is a special value that can be assigned to object references to indicate the absence of a value.Reasons for Null Pointer ExceptionA NullPointerE
5 min read
Exception Handling with Method Overriding in JavaException handling with method overriding in Java refers to the rules and behavior that apply when a subclass overrides a method from its superclass and both methods involve exceptions. It ensures that the overridden method in the subclass does not declare broader or new checked exceptions than thos
4 min read
Java Advanced
Java Multithreading TutorialThreads are the backbone of multithreading. We are living in the real world which in itself is caught on the web surrounded by lots of applications. With the advancement in technologies, we cannot achieve the speed required to run them simultaneously unless we introduce the concept of multi-tasking
15+ min read
Synchronization in JavaIn 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
10 min read
File Handling in JavaIn Java, with the help of File Class, we can work with files. This File Class is inside the java.io package. The File class can be used to create an object of the class and then specifying the name of the file.Why File Handling is Required?File Handling is an integral part of any programming languag
6 min read
Java Method ReferencesIn Java, a method is a collection of statements that perform some specific task and return the result to the caller. A method reference is the shorthand syntax for a lambda expression that contains just one method call. In general, one does not have to pass arguments to method references.Why Use Met
9 min read
Java 8 Stream TutorialJava 8 introduces Stream, which is a new abstract layer, and some new additional packages in Java 8 called java.util.stream. A Stream is a sequence of components that can be processed sequentially. These packages include classes, interfaces, and enum to allow functional-style operations on the eleme
15+ min read
Java NetworkingWhen computing devices such as laptops, desktops, servers, smartphones, and tablets and an eternally-expanding arrangement of IoT gadgets such as cameras, door locks, doorbells, refrigerators, audio/visual systems, thermostats, and various sensors are sharing information and data with each other is
15+ min read
JDBC TutorialJDBC stands for Java Database Connectivity. JDBC is a Java API or tool used in Java applications to interact with the database. It is a specification from Sun Microsystems that provides APIs for Java applications to communicate with different databases. Interfaces and Classes for JDBC API comes unde
12 min read
Java Memory ManagementJava memory management is the process by which the Java Virtual Machine (JVM) automatically handles the allocation and deallocation of memory. It uses a garbage collector to reclaim memory by removing unused objects, eliminating the need for manual memory managementJVM Memory StructureJVM defines va
4 min read
Garbage Collection in JavaGarbage collection in Java is an automatic memory management process that helps Java programs run efficiently. Java programs compile to bytecode that can be run on a Java Virtual Machine (JVM). When Java programs run on the JVM, objects in the heap are created, which is a portion of memory dedicated
7 min read
Memory Leaks in JavaIn programming, a memory leak happens when a program keeps using memory but does not give it back when it's done. It simply means the program slowly uses more and more memory, which can make things slow and even stop working. Working of Memory Management in JavaJava has automatic garbage collection,
3 min read
Practice Java
Java Interview Questions and AnswersJava is one of the most popular programming languages in the world, known for its versatility, portability, and wide range of applications. Java is the most used language in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, and many more because of its features and per
15+ min read
Java Programs - Java Programming ExamplesIn this article, we will learn and prepare for Interviews using Java Programming Examples. From basic Java programs like the Fibonacci series, Prime numbers, Factorial numbers, and Palindrome numbers to advanced Java programs.Java is one of the most popular programming languages today because of its
8 min read
Java Exercises - Basic to Advanced Java Practice Programs with SolutionsLooking for Java exercises to test your Java skills, then explore our topic-wise Java practice exercises? Here you will get 25 plus practice problems that help to upscale your Java skills. As we know Java is one of the most popular languages because of its robust and secure nature. But, programmers
7 min read
Java Quiz | Level Up Your Java SkillsThe best way to scale up your coding skills is by practicing the exercise. And if you are a Java programmer looking to test your Java skills and knowledge? Then, this Java quiz is designed to challenge your understanding of Java programming concepts and assess your excellence in the language. In thi
1 min read
Top 50 Java Project Ideas For Beginners and Advanced [Update 2025]Java is one of the most popular and versatile programming languages, known for its reliability, security, and platform independence. Developed by James Gosling in 1982, Java is widely used across industries like big data, mobile development, finance, and e-commerce.Building Java projects is an excel
15+ min read