Synchronization in Java: The Secret to Building Robust Multi-threaded Applications

Synchronization in Java is a fundamental concept that ensures mutual exclusion when multiple threads access shared resources, preventing issues such as race conditions and data corruption. 

By using synchronization primitives, developers can ensure that only one thread accesses critical sections of code at a time. This guarantees consistent data integrity in concurrent environments.

In this blog, you’ll explore synchronization in Java, including locks, concurrency mechanisms, and performance optimization strategies for multi-threaded applications.

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Understanding Synchronization in Java

Synchronization in Java ensures that only one thread can access a shared resource at a time. This prevents race conditions and data inconsistencies when multiple threads are accessing the same resource. In simple terms, synchronization helps you avoid situations where threads conflict with each other, leading to unpredictable results.

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Think of synchronization like a traffic signal at a busy intersection. Without it, vehicles (or threads) would crash into each other, causing chaos. The traffic signal, just like the synchronized keyword in Java, controls the flow and keeps everything in order.

Code Example:

class Counter {
   private int count = 0;
   // Synchronized method to avoid thread interference
   public synchronized void increment() {
       count++;
   }
   public int getCount() {
       return count;
   }
}
public class SyncExample {
   public static void main(String[] args) {
       Counter counter = new Counter();
       // Creating two threads that increment the counter
       Thread t1 = new Thread(() -> {
           for (int i = 0; i < 1000; i++) {
               counter.increment();
           }
       });
       Thread t2 = new Thread(() -> {
           for (int i = 0; i < 1000; i++) {
               counter.increment();
           }
       });
       t1.start();
       t2.start();
       try {
           t1.join();
           t2.join();
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
       System.out.println("Final count: " + counter.getCount());
   }
}

Output:

Final count: 2000

Without synchronization in Java, threads may attempt to modify shared resources concurrently, leading to data corruption and unpredictable behavior. Here’s why synchronization is key:

• Thread Interference: Multiple threads trying to access shared data simultaneously can cause unexpected results. For instance, two threads withdrawing money from the same bank account might not reflect the correct balance.
• Memory Consistency Errors: When one thread updates a shared resource, that change might not be visible to others unless synchronization is used. This can lead to inconsistencies, especially in large multi-threaded systems.

Common Issues Without Synchronization:

• Race Conditions
• Deadlocks in Java
• Data Corruption

These issues can be avoided by using proper synchronization techniques to coordinate access to shared resources.

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Also Read: What Is Multithreading in Java? All You Need to Know in 2025

How Synchronization Works in Java?

Java offers two primary methods for achieving synchronization: synchronized methods and synchronized blocks. Let's dive deeper into both:

1. Synchronized Methods

By marking a method with the synchronized keyword, you lock the entire method. This means only one thread can execute the method at any given time.

Example:

public synchronized void increment() {
   count++;
}

Here, the method is locked, ensuring only one thread can update the count variable at a time.

2. Synchronized Blocks

For more control, you can use synchronized blocks within methods. This allows you to lock only a specific part of the code, reducing the overhead of locking the entire method.

Example:

public void increment() {
   synchronized (this) {
       count++;
   }
}

This ensures that only the increment operation is synchronized, while other operations in the method can proceed without locking.

Also Read: How to Make a Successful Career in Java in 2025

Types of Synchronization

Synchronization can be broadly classified into Process Synchronization and Thread Synchronization.

1. Process Synchronization ensures that different processes do not access shared resources concurrently, especially when working in operating systems or distributed systems.
2. Thread Synchronization controls how threads within a single process access shared resources.

In most Java applications, thread synchronization is what you deal with. This involves managing how threads within the same JVM interact with shared data. If synchronization is not applied, threads can cause inconsistent states due to concurrent data access.

Synchronization Mechanisms in Java

There are several tools in Java for managing thread synchronization. Here are the most common ones:

1. Intrinsic Locks

Each Java object has an intrinsic lock. When you use synchronized methods or blocks, the JVM automatically locks the object to ensure only one thread can access the synchronized code.

2. Explicit Locks

For more advanced synchronization, Java provides ReentrantLock, which allows more granular control over locks. This includes features like lock timeout and try-lock, which aren’t available with intrinsic locks.

Example:

ReentrantLock lock = new ReentrantLock();
lock.lock(); // Acquire the lock
try {
   // Critical section
} finally {
   lock.unlock(); // Release the lock
}

3. Read-Write Locks

A ReentrantReadWriteLock allows multiple threads to read a resource at the same time, but only one thread can write to it at any given moment. This is particularly useful when read operations far outnumber write operations.

Example:

ReadWriteLock rwLock = new ReentrantReadWriteLock();
rwLock.readLock().lock();  // Acquire read lock
try {
    // Read operation
} finally {
    rwLock.readLock().unlock();  // Release read lock
}

4. Atomic Variables

If you're dealing with simple thread-safe operations, Atomic Variables like AtomicInteger provide a lock-free mechanism for thread safety. These are often more efficient than traditional synchronization.

Example:

AtomicInteger count = new AtomicInteger(0);
count.incrementAndGet();  // Thread-safe increment

These synchronization mechanisms give you flexibility depending on your specific application needs.

Also Read: How to Code, Compile, and Run Java Projects in 2025

With a clear understanding of synchronization in Java, let’s move to the best practices and advanced concepts before using it.

Best Practices and Advanced Concepts

To optimize performance and avoid common pitfalls like deadlocks, starvation, and livelocks, it’s important to follow best practices and advanced strategies. Minimizing lock contention, using high-level concurrency utilities, and managing synchronization efficiently can significantly enhance your Java applications. 

The following points will delve deeper into these concepts, offering actionable insights to improve your thread management and overall performance.

Common Pitfalls in Synchronization

Without proper knowledge of synchronization in Java, developers often encounter these challenges:

1. Deadlocks

Deadlocks happen when two or more threads are waiting for each other to release resources they hold. It creates a cycle where no thread can proceed.

Example: Imagine two threads needing two resources: Resource A and Resource B. Thread 1 locks Resource A and waits for Resource B, while Thread 2 locks Resource B and waits for Resource A. Neither thread can proceed, causing a deadlock.

How to Avoid It:

• Always acquire locks in a fixed order.
• Use ReentrantLock with tryLock() for timeout-based locking to break the cycle.

2. Starvation

Starvation occurs when threads are blocked indefinitely, waiting for resources that are always allocated to other threads.

Example: If a low-priority thread is always preempted by high-priority threads, it may never get the chance to execute, causing starvation.

How to Avoid It:

• Use fair locks, which allow threads to acquire resources in the order they requested.
• Consider using Java's Semaphore or CountDownLatch for controlling thread access.

3. Livelocks

A livelock happens when two or more threads continuously change their state in response to each other but still don’t make any progress.

Example: Two threads trying to acquire two resources keep giving up their locks to avoid deadlocks, but they end up in a loop of releasing and requesting resources without actually doing any useful work.

How to Avoid It:

• Implement backoff mechanisms, where threads wait for some time before trying again.
• Ensure thread priority is managed properly.

By recognizing these pitfalls, you can avoid thread interference, ensuring your application runs smoothly and efficiently.

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Best Practices for Effective Synchronization

Effective synchronization in Java is about reducing contention, avoiding deadlocks, and ensuring the proper ordering of threads. Here are some practices you should follow:

1. Minimize Synchronized Blocks

It’s best to keep synchronized blocks as small as possible. The smaller the synchronized section, the less time threads will be blocked, improving performance.

Example: Instead of synchronizing the entire method, only synchronize the critical section that modifies shared resources.

public void increment() {
    // Synchronize only the critical section
    synchronized (this) {
        count++;
    }
}

2. Avoid Nested Locks

Nested locks can easily lead to deadlocks. If Thread A holds Lock 1 and waits for Lock 2, while Thread B holds Lock 2 and waits for Lock 1, neither thread can proceed.

How to Avoid It:

• Always acquire locks in a consistent order to prevent circular dependencies.
• Avoid nested synchronization whenever possible.

3. Use High-Level Concurrency Utilities

The java.util.concurrent package offers advanced synchronization utilities like ExecutorService, CountDownLatch, Semaphore, and CyclicBarrier. These tools provide better control and more efficient thread management than manual synchronization using the synchronized keyword.

Example: Using CountDownLatch to synchronize tasks across multiple threads.

CountDownLatch latch = new CountDownLatch(3);
ExecutorService executor = Executors.newFixedThreadPool(3);

for (int i = 0; i < 3; i++) {
    executor.submit(() -> {
        System.out.println("Task executed.");
        latch.countDown();
    });
}

latch.await();  // Main thread waits for all tasks to complete
System.out.println("All tasks finished.");

4. Prefer Atomic Variables
For simple operations, prefer using atomic variables like AtomicInteger, AtomicReference, etc. These classes handle synchronization internally, providing thread-safe operations without the need for manual locking.

Example:

AtomicInteger count = new AtomicInteger(0);
count.incrementAndGet();  // Thread-safe increment

These best practices ensure that your synchronization in Java is both efficient and effective, helping you avoid common synchronization issues like deadlocks, starvation, and livelocks.

Also Read: Java Full Stack Developer Roadmap For 2025

Advanced Synchronization Techniques

Once you're comfortable with basic synchronization, you can move to more advanced techniques that help fine-tune performance and optimize thread safety.

1. Double-Checked Locking

Double-checked locking improves performance by checking the lock condition before and after acquiring the lock. This ensures that the thread only acquires the lock when necessary.

Example (for Singleton Pattern):

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

• Why use it? This technique prevents the overhead of acquiring the lock each time when the resource is already initialized.

2. Volatile Keyword

The volatile keyword ensures visibility of changes to variables across threads. When a variable is marked as volatile, any write to that variable by one thread is immediately visible to other threads.

Example:

private volatile boolean flag = false;

public void toggleFlag() {
    flag = !flag;  // Changes made by one thread are visible to others immediately
}

• Why use it? This helps in scenarios where multiple threads need to update a flag or shared variable without synchronization.

3. ThreadLocal Variables

In some cases, you may want each thread to have its own instance of a variable. This is where ThreadLocal comes in, providing thread-local storage. Each thread has its own separate copy of the variable, which avoids synchronization altogether.

Example:

ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(() -> 1);

public void increment() {
    threadLocal.set(threadLocal.get() + 1);
}

• Why use it? ThreadLocal is ideal for managing per-thread data like database connections or user sessions, where synchronization is not required.

Also Read: Top 10 Free Java Courses with Certificates for In-Demand Java Jobs

Performance Considerations

Synchronization is essential, but it comes with its own set of performance challenges. You must understand the overhead it introduces and the ways to minimize contention.

1. Synchronization Overhead

While synchronization guarantees thread safety, it also introduces performance bottlenecks. The JVM needs to manage locks and ensure only one thread accesses synchronized sections, which can slow down execution, especially in high-traffic applications.

2. Lock Contention
When multiple threads are competing for the same lock, it causes lock contention, reducing performance. If many threads need access to a single resource, they end up waiting for the lock, which can degrade performance.

Alternative Solutions:

• Use StampedLock, which allows for multiple readers and a single writer, reducing lock contention.

StampedLock lock = new StampedLock();
long stamp = lock.readLock();
try {
    // Read operation
} finally {
    lock.unlockRead(stamp);
}

3. Scalability

As the number of threads increases, so does the complexity of synchronization. High contention can reduce the scalability of your application. When scaling to multi-core systems, the cost of synchronization grows due to increased context switching and lock management.

• Tip: Use concurrency tools like ExecutorService to manage the number of threads and reduce unnecessary contention.

Also Read: 50 Java Projects With Source Code in 2025

Having explored performance considerations before implementing synchronization in java, let’s see how upGrad can help in learning these Java concepts.

Be Proficient in Synchronization in Java with upGrad!

Effective synchronization is essential for thread-safe Java applications. To avoid issues like race conditions and deadlocks, minimize synchronized blocks, and use tools like ExecutorService and CountDownLatch. For simple operations, prefer atomic variables like AtomicInteger to ensure thread safety without added overhead.

To deepen your expertise and apply these concepts effectively, upGrad’s courses offer hands-on learning with practical projects. With upGrad’s structured learning path, you’ll have the tools and support you need to become a proficient Java developer.

In addition to the specialized courses mentioned above, here are some additional free courses to help you get started.

• Learn Basic Python Programming
• Java Object-oriented Programming
• JavaScript Basics from Scratch
• Advanced JavaScript for All
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Still struggling to find the right Java software development course to match your goals in 2025? Reach out to upGrad for personalized counseling and valuable insights, or visit your nearest upGrad offline center for more details.

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