Concurrency in Java: Mastering Synchronized Blocks for Thread Safety

Concurrency is a critical aspect of modern software development, particularly in Java, where multiple threads can run simultaneously within an application. Whether you’re developing a high-performance web server, a financial application, or a multi-threaded desktop application, handling concurrency effectively is paramount. If you’ve ever encountered issues like race conditions or data inconsistency, you understand how vital proper synchronization is. In this article, we’ll dive deep into Java synchronized blocks, their importance for concurrency control, and practical examples of their usage.

What is a Java Synchronized Block?

A synchronized block in Java is a mechanism used to control access to critical sections of code, ensuring that only one thread executes the block at a time. This is particularly useful when multiple threads need to access shared resources, as it prevents issues like race conditions and ensures data consistency.

A synchronized block is marked with the synchronized keyword and can be applied either to entire methods or specific blocks of code within methods. This flexibility allows developers to fine-tune concurrency control for their applications.

Benefits of Using Synchronized Blocks

1. Prevention of Race Conditions By restricting simultaneous access to critical sections, synchronized blocks eliminate race conditions where multiple threads might attempt to modify shared data concurrently, leading to unpredictable results.
2. Data Consistency They ensure that only one thread can execute the synchronized code at a time, maintaining the integrity of shared resources.
3. Visibility Guarantees Changes made to shared variables within synchronized blocks are visible to other threads, thanks to the memory barrier enforced by the synchronized keyword.
4. Atomicity Operations within synchronized blocks are performed as a single, uninterruptible unit, preventing interference from other threads.

Synchronized Methods vs. Synchronized Blocks

Synchronized Methods

A synchronized method locks the entire method for a specific object, ensuring that only one thread can execute it at a time. For example:

In this case, all synchronized methods in the same object instance share the same lock (the instance itself), so only one thread can access any of these methods at a time.

Synchronized Blocks

Synchronized blocks provide finer-grained control, allowing you to lock only a specific portion of a method instead of the entire method. For example:

Here, only the code within the synchronized block is restricted to one thread at a time. This approach improves performance by allowing concurrent execution of non-critical sections of the method.

Monitor Objects: The Key to Synchronization

A monitor object is the lock associated with a synchronized block or method. It controls thread access and ensures exclusive execution within a synchronized section.

Common Synchronization Pattern

Using Custom Monitor Objects

Instead of synchronizing on this, you can use a custom monitor object to decouple synchronization logic from the instance:

This approach offers greater flexibility, particularly when synchronizing access to specific resources in complex applications.

Visibility Guarantees and Atomicity

One of the key challenges in multi-threaded programming is ensuring that changes made by one thread are visible to others. Java's synchronization mechanism provides:

1. Memory Barrier Enforcement When a thread enters a synchronized block, it refreshes variables from main memory. Upon exiting, it writes back any changes, making them visible to other threads.
2. Atomicity All operations within a synchronized block are executed as a single, indivisible unit, preventing partial updates by other threads.

Example: Synchronized Blocks in Action

Let’s see how synchronized blocks maintain data consistency in a multi-threaded environment:

Testing the Counter Implementation

This example demonstrates how synchronized blocks ensure accurate results even when multiple threads operate concurrently.

Limitations of Synchronized Blocks

1. Performance Overhead Synchronization involves locking and unlocking, which can reduce performance in highly concurrent applications.
2. No Fairness Guarantee The synchronized keyword does not ensure which thread will acquire the lock next, potentially causing thread starvation.
3. Scope Restriction Locks provided by synchronized blocks are limited to a single JVM and cannot be used for distributed systems.

Advanced Synchronization Techniques

In scenarios requiring more sophisticated concurrency control, consider alternatives from Java’s concurrency package:

• ReentrantLock: Provides explicit locking with features like fairness policies.
• ReentrantReadWriteLock: Allows multiple threads to read concurrently while ensuring exclusive access for writes.

Mastering synchronized blocks and methods is fundamental for building robust multi-threaded Java applications. By providing a mechanism for concurrency control, synchronized blocks ensure thread safety, data integrity, and visibility of shared resources. While they come with trade-offs in terms of performance and complexity, their correct usage is indispensable for developing reliable applications in a multi-threaded world.

Here are some questions and their answers that might help you in grasping the concept

1. How does the synchronized keyword work in Java, and what are the different ways to apply it?

The synchronized keyword ensures that only one thread at a time can execute a synchronized block or method, preventing race conditions. It works by acquiring an intrinsic lock (monitor lock) on an object.

Ways to use synchronized:

Instance method: Locks on the current object (this)

Static method: Locks on the class object (Class.class)

2. What is the difference between synchronizing a static method and a non-static method?

• A non-static synchronized method locks on the instance (this), meaning only one thread can execute synchronized methods on a specific instance at a time.
• A static synchronized method locks on the class object (Class.class), meaning only one thread can execute any synchronized static method for that class, even if there are multiple instances.

🔹 Key Point: A thread holding the lock on an instance (this) can still allow other threads to access static synchronized methods because they lock on the class level.

3. What are intrinsic locks (monitor locks), and how does synchronized use them?

Intrinsic locks (or monitor locks) are the locks that every object in Java has. When a thread enters a synchronized method/block, it acquires the intrinsic lock of the object or class:

• Instance methods → Lock on this (current object)
• Static methods → Lock on the Class object
• Synchronized block → Lock on a custom object

A thread must release the lock when it exits the synchronized block/method. Other threads must wait until the lock is free.

4. What happens if a thread holding a lock on a synchronized method gets blocked or goes to sleep?

If a thread holding the lock:

• Gets blocked (e.g., waiting for I/O, network call) → It still holds the lock, preventing other threads from entering the synchronized block.
• Calls Thread.sleep(n) → It still holds the lock while sleeping.
• Calls wait() inside a synchronized block → It releases the lock and waits to be notified.

🔹 Key Point: wait() releases the lock, while sleep() and blocked I/O do not.

5. Can two threads execute two different synchronized methods of the same object at the same time? Why or why not?

No, they cannot. If both methods are synchronized and called on the same instance, only one thread can execute any synchronized method at a time because they share the same intrinsic lock (this).

However, if they are called on different instances, they can run simultaneously.

Key Point: If methodA() and methodB() are called on the same instance, one thread must finish before another can enter.

6. If a synchronized method calls another synchronized method on the same object, will it cause a deadlock? Why or why not?

No, it won’t cause a deadlock because Java uses reentrant locks. A thread that already holds a lock on an object can enter another synchronized method of the same object without blocking itself.

Key Point: Java’s synchronized mechanism allows nested locking on the same object by the same thread.

7. How does synchronized compare to ReentrantLock, and when would you use one over the other?

🔹 Use synchronized for simple mutual exclusion

🔹 Use ReentrantLock for more flexibility (timeouts, try-lock, fairness, multiple conditions)

8. What are the limitations of using synchronized, and how can they be mitigated?

Limitations of synchronized:

1. Thread Blocking: If a thread holding a lock blocks, no other thread can proceed.
2. No Timeout Mechanism: A thread can be stuck waiting indefinitely.
3. No Explicit Lock Control: Cannot check if a lock is available (tryLock() in ReentrantLock can).
4. Only One Condition Variable: Cannot wait for multiple conditions (Condition in ReentrantLock allows multiple).

Mitigation:

• Use ReentrantLock if timeout, fairness, or better control is needed.
• Use concurrent data structures like ConcurrentHashMap, CopyOnWriteArrayList.

9. Can you synchronize on an object that is not this or a class (Class.class)? Why might you do that?

Yes, you can synchronize on any arbitrary object. This is useful for finer control over locking.

🔹 Key Point: Synchronizing on a private object prevents external classes from interfering with locks.

10. What are some best practices when using synchronized to avoid performance issues and deadlocks?

✅ Use fine-grained locking (avoid synchronizing large blocks of code).

✅ Minimize the scope of synchronized blocks to reduce contention.

✅ Avoid nested locks (A → B, B → A) to prevent deadlocks.

✅ Use concurrent utilities (ReentrantLock, Atomic, ConcurrentHashMap) for better performance.

✅ Always release locks in finally to avoid accidental lock retention.

11. If one thread is executing a synchronized method, can another thread concurrently execute a non-synchronized method on the same object?

Yes, a thread can run a non-synchronized method while another thread is running a synchronized method on the same object.

Here’s why:

• Synchronized methods: When a method is synchronized, it requires a lock (or monitor) on the object it belongs to (in case of instance methods) or on the class itself (in case of static methods). This lock ensures that only one thread at a time can execute that synchronized method on the given object or class.
• Non-synchronized methods: These methods don’t require a lock and can be executed by multiple threads concurrently, even if another thread is executing a synchronized method on the same object.

So, as long as the non-synchronized method does not require the same lock (which a synchronized method would hold), it can run without any conflict with the synchronized method.

12. Consider a scenario where multiple threads are accessing a shared resource. How do monitor objects work in Java, and what challenges can arise when multiple threads are sharing a monitor object for synchronization? Can you explain the potential risks of deadlock, race conditions, or thread starvation in this context? Answer?

In Java, monitor objects (or simply monitors) are used for thread synchronization. Each object in Java can be used as a monitor to control access to critical sections of code by multiple threads. When a thread enters a synchronized block or method, it acquires a lock on the monitor associated with the object. Other threads that attempt to enter the synchronized block or method must wait until the lock is released.

How monitor objects work in Java:

• When a thread enters a synchronized method or block, it must first acquire the monitor lock for the object (in the case of instance methods) or class (in the case of static methods).
• Once the thread has the lock, it can safely execute the synchronized code without interference from other threads trying to access the same method or block.
• When the thread exits the synchronized method or block, it releases the monitor lock, allowing other threads to acquire it and proceed with their execution.

However, sharing monitor objects introduces potential challenges, such as deadlock, race conditions, and thread starvation.

Challenges and potential risks:

1. Deadlock:
2. Prevention strategies:
3. Race Conditions:
4. Example: If two threads increment a shared counter without synchronization, the final value may be incorrect due to the threads interfering with each other. Prevention strategies:
5. Thread Starvation:
6. Example: A low-priority thread might never get CPU time to acquire a lock if higher-priority threads are continuously running and acquiring the lock first. Prevention strategies:

Using monitor objects to synchronize access to shared resources in a multithreaded Java program can help ensure data consistency and avoid race conditions. However, improper handling of synchronization can lead to serious issues like deadlock, race conditions, and thread starvation. To mitigate these risks, developers should use careful lock management, employ timeout strategies, and consider using higher-level concurrency utilities like ReentrantLock, Semaphore, or ExecutorService when appropriate.

13. What are the risks of using string constant objects, such as "lock", as synchronization locks in Java?

Using string constant objects (like "lock") as synchronization locks in Java is risky for several reasons:

1. String Interning: In Java, string literals are interned, meaning that all occurrences of the same string literal share the same reference. This can lead to unintended lock contention if different parts of the program or third-party libraries synchronize on the same string literal. For example, if one part of your program and a third-party library both synchronize on the string "lock", they will block each other, even if they are working on completely different resources, causing performance bottlenecks or even deadlocks.
2. Unintended Contention: Since string literals are interned, any synchronization on the same string literal could block unrelated code. This can result in significant performance degradation and reduce the concurrency of your application.
3. Hard to Maintain: Using string literals as locks makes it difficult to track where and why certain locks are being acquired. This can lead to subtle bugs that are hard to diagnose and fix, especially in large codebases.

Recommendation: It’s best to use dedicated lock objects (e.g., new Object()) or higher-level concurrency mechanisms (e.g., ReentrantLock), which allow you to better control synchronization and avoid unintended contention.

14. What are the potential issues with using String.class as a monitor object for synchronization in Java?

 Using String.class as a monitor object (i.e., synchronizing on the String class object) is generally not recommended due to the following issues:

1. Global Lock Contention: String.class is a shared object across the entire JVM. This means that any thread that synchronizes on String.class could block other threads synchronizing on the same class, even if they are working on unrelated resources. This creates global contention for the lock and can severely impact performance. For instance, one part of the program might be synchronizing on String.class for string processing, while another part (possibly a third-party library) synchronizes on String.class for different functionality. These threads would block each other unnecessarily.
2. Lack of Control Over Locking: Since String.class is a class-level object that is shared globally, you're not in full control of the lock. It could be accessed and locked by other parts of the system, which could lead to synchronization issues that are difficult to manage.
3. Maintainability and Debugging Challenges: Synchronizing on String.class makes it harder to track the flow of execution because the lock is shared and globally accessible. It’s difficult to pinpoint where the contention is occurring, and this can make your code harder to maintain and debug.

Recommendation: Instead of using String.class, it’s better to use a dedicated lock object (e.g., new Object()) or a more flexible locking mechanism such as ReentrantLock. These allow better control and reduce the risk of unintentional contention, leading to clearer, more maintainable code.

15. How do synchronized blocks in Java provide visibility guarantees for shared variables between threads?

Synchronized blocks in Java provide visibility guarantees through the happens-before relationship. When a thread exits a synchronized block, any changes made to shared variables within that block are guaranteed to be visible to any other thread that later enters a synchronized block on the same object. This ensures that updates to shared data are properly propagated between threads.

16. Can you explain what the "happens-before" relationship is in the context of synchronized blocks in Java?

The "happens-before" relationship refers to the ordering of actions in multithreaded programs. In the context of synchronized blocks, it means that when a thread exits a synchronized block and releases the lock, all changes made to shared variables are visible to any thread that acquires the same lock later. This guarantees that modifications made inside the synchronized block are reflected across other threads.

17. If two threads are executing methods that modify and read a shared variable, how can synchronization ensure the changes are visible to both threads?

If both threads synchronize on the same object, any changes made to the shared variable inside the synchronized methods will be visible to the other thread when it acquires the lock to enter a synchronized block or method. This is because synchronization ensures that changes to shared variables are flushed to main memory when a thread exits the synchronized block, and the next thread entering the synchronized block will see those updates.

18. Why is synchronization important in concurrent programming, and what might happen if it's not used when accessing shared resources?

Synchronization is important in concurrent programming because it prevents race conditions, which can occur when multiple threads access shared resources simultaneously and modify them in unpredictable ways. Without synchronization, threads may see stale or inconsistent data, leading to incorrect behavior or data corruption. Synchronization ensures that shared resources are accessed in a controlled manner and that updates are visible across threads.

19. What could go wrong if synchronization is not used when two threads are accessing and modifying a shared variable?

Without synchronization, threads may operate on outdated or inconsistent data, leading to race conditions. One thread might read a value while another is in the middle of updating it, leading to inconsistent or incorrect results. Additionally, changes made by one thread might not be visible to others due to issues like CPU caching or compiler optimizations. This could cause unpredictable behavior and make debugging difficult.

20. What are some alternatives to synchronized blocks for managing thread synchronization in Java? When might you prefer them over synchronized blocks?

Some alternatives to synchronized blocks are ReentrantLock, ReadWriteLock, and Atomic classes like AtomicInteger. These higher-level concurrency utilities offer more flexibility and control compared to synchronized blocks, such as the ability to try acquiring a lock without blocking, support for fair locking, and the ability to lock specific parts of data. You might prefer them when you need more advanced locking mechanisms or greater control over concurrency patterns (e.g., fine-grained locking, lock timeouts).

21. What impact does synchronization have on the performance of a multithreaded program?

Synchronization can introduce performance overhead because acquiring and releasing locks takes time. The more threads that compete for the same lock, the more contention can occur, potentially causing context switching, thread blocking, and increased latency. Using synchronized blocks wisely—by locking only the critical section—can help reduce this overhead. However, for highly concurrent programs, you might consider using advanced concurrency utilities (e.g., ReentrantLock) that offer more efficient lock management.