Exploring Java Concurrency: Threads, Synchronization & More

When you write a Java program, it usually runs one step at a time. But what if you want it to do more than one thing at once — like loading data while also updating the screen? That’s where Java Concurrency can help. It lets your code run multiple tasks at the same time using threads.

Java Concurrency is super useful for building fast and smooth apps. In this blog, we’ll break down how threads work, how Java developers create and control them, and how to avoid common problems — all explained in one place. So, shall we begin?

How Do Threads Work in Java?

Thread is the smallest unit of execution in a program. From the above sentences, we can say that Java is multi-threaded as it can run multiple threads simultaneously, either independently or collectively interacting with one another.

Every Java program runs under at least one thread and is generally named the main thread. But when tasks pile up, like reading files, calling APIs, or handling user inputs, one thread can become a bottleneck.

This is where Java threads come in. They allow your application to carry out multiple actions at once, improving speed and responsiveness.

Processes vs Threads

• A process is an autonomous program with its own memory space and resources.
• A thread is a smaller unit within a process that shares the same memory and resources with other threads in the same process.
• Threads are lightweight and faster to create than processes. 
• Multiple threads can run tasks in parallel using CPU time in a shared process.
• The communication of threads is easy because they share the same memory space. However, processes need more complex inter-process communication.

Processes are heavy and isolated; threads are lightweight and work together within a single process. 

Creating Threads in Java

To run multiple tasks at once, you need to create threads in your program. Java offers more than one way to do this, depending on what you’re trying to achieve. Whether you’re keeping things simple or returning results from tasks, Java makes it flexible to get started with multi-threading.

There are three primary ways to create threads:

1. Extending the Thread class:

This is one of the most basic ways to create a thread. You create a new class that extends Thread and write your code using the run() method. Then, you create an object of your class and call .start() to run the thread.

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

Here, MyThread is your custom thread. When you call start(), Java creates a new thread and runs whatever is written inside run(). This is great when you just need a quick and simple thread. But it doesn’t allow you to extend any other class since Java doesn’t support multiple inheritance.

2. Implementing the Runnable interface:

This method is more flexible than extending Thread. Instead of creating a new thread class, you just write your task inside a class that implements Runnable. You then pass that class to a Thread object and start it.

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Runnable thread is running");
    }
}

This method lets your class extend something else if needed, since you’re not directly extending Thread. It’s also better when you’re working with multiple threads that share the same task.

3. Using Callable and Future (for returning results):

Runnable and Thread don’t return anything. But if you want your task to give back a result, use Callable. You define what to return, and use Future to get that result later.

Callable<Integer> task = () -> {
    return 123;
};

This creates a task that returns the number 123. To actually run it and get the result, you’ll need to use an ExecutorService, which we’ll talk about later in the blog.

This method is perfect when your thread needs to do some work and send a result back — like fetching data or doing some calculations.

Thread Lifecycle

A thread in Java goes through several stages from creation to completion:

• New: The thread is created but hasn’t started yet.
• Runnable: The thread is ready to run and waiting for CPU time.
• Running: The thread is actively executing.
• Blocked: The thread is waiting to access a resource locked by another thread.
• Waiting: The thread is paused indefinitely until another thread signals it.
• Timed Waiting: The thread waits for a specific period.
• Terminated: The thread has finished execution or was stopped.

These states help manage how threads behave and interact during program execution.

Threads are the foundation of Java’s concurrency model. Learning how to create and manage them will open the door to building applications that can handle more work without slowing down.

How Thread Synchronization Works in Java?

Synchronization is an essential part of managing concurrency in Java, especially when working with shared resources. When multiple threads access the same data, things can quickly go wrong if they interfere with each other.

This is called a race condition. One thread might be updating a value while another is trying to read it, leading to unexpected results.

Java provides synchronization tools to prevent this kind of conflict by controlling how and when threads can access shared resources.

Using Synchronized

The synchronized keyword in Java helps control access to shared resources by allowing only one thread to execute a block of code at a time.

Ways to Use Synchronized:

• Synchronized Method
  Locks the entire method, allowing only one thread to access it at a time.

public synchronized void updateValue() {
// code here
}

• Synchronized Block
  Locks only a specific part of the code using an object as a lock.

synchronized (lockObject) {
// critical section
}

Why Use It:

• Prevents race conditions
• Ensures data consistency
• Makes thread access predictable

Just remember – too much synchronization can slow things down, so use it only when needed.

Proper synchronization keeps your data safe and your application stable. It helps you avoid issues like race conditions and inconsistent output. While it’s a powerful tool, using it carefully ensures your program runs smoothly without unnecessary slowdowns.

Java Concurrency Utilities (java.util.concurrent)

Handling threads manually can get tricky, especially as your application grows. To make things easier, Java provides a set of built-in concurrency tools under the java.util.concurrent package.

These utilities help manage tasks, control thread behavior, and handle synchronization without writing complex code from scratch. They offer a cleaner and more efficient way to work with multi-threading.

To simplify concurrent programming, Java introduced the java.util.concurrent package. It includes:

• Executor Framework: Manages thread pools

ExecutorService executor = Executors.newFixedThreadPool(5);
executor.submit(() -> System.out.println("Task executed"));

• Callable and Future: For tasks that return values
• ScheduledExecutorService: For scheduled tasks
• ThreadPoolExecutor: Customizable thread pool management

To safely handle data sharing between threads, you can use a Java concurrent list like CopyOnWriteArrayList from java.util.concurrent.

The concurrency utilities in this package take a lot of the heavy lifting off your shoulders. From managing thread pools to safely sharing data between threads, these tools make it easier to build fast and reliable programs.

Locks and Advanced Synchronization

Sometimes, synchronization isn’t enough when you need more control over thread behavior. Java’s Lock interface and related tools offer flexible ways to manage access and avoid blocking issues in complex scenarios.

ReentrantLock

ReentrantLock is part of the java.util.concurrent.locks package and provides more control than the synchronized keyword. It allows a thread to lock a resource it already holds without getting blocked again–hence the name reentrant.

Key Features:

• Allows explicit lock and unlock control.
• Supports try-locking (non-blocking) and timed lock attempts.
• It can be interrupted while waiting for a lock.

Basic Usage:

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

ReentrantLock is useful in cases where you need advanced features like fairness, interruptible waits, or multiple condition variables.

Using advanced locks helps you fine-tune how threads interact, especially when performance and responsiveness matter. With careful use, you can avoid common issues like deadlocks while keeping your code efficient and easier to manage in high-concurrency scenarios.

Atomic Variables

Atomic variables are part of the java.util.concurrent.atomic package and provide a way to safely update variables across multiple threads without needing synchronization. These variables ensure that operations like incrementing, comparing, or setting values happen atomically, meaning they are done as a single, indivisible step.

Key Features:

• No need for locks, reducing overhead and improving performance.
• Provide atomic operations like get(), set(), incrementAndGet(), etc.
• Ensure thread safety for simple data types.

Example:

AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet(); // Atomically increment the counter

Atomic variables are especially useful in high-concurrency scenarios, where frequent updates to shared data need to be performed safely without slowing down the program with lock mechanisms.

For developers working with advanced concurrency integrations like java/com/instagram/common/util/concurrent/igrunnableid/ig_runnable_ids.py, understanding atomic operations is key.

The file path java/com/instagram/common/util/concurrent/igrunnableid/ig_runnable_ids.py refers to an internal Python script used by Instagram (Meta) in conjunction with their Java-based infrastructure. Despite the Java-style directory, this Python file plays a supporting role in the concurrency layer of their system. Specifically, it’s used to generate unique identifiers for Runnable tasks in Java, which are essential for tracing, debugging, and profiling concurrent operations.

In high-scale environments like Instagram, managing multi-threaded behavior efficiently requires visibility into task execution, and assigning consistent IDs helps track and monitor these tasks across logs and performance tools. This integration between Python and Java reflects a common engineering practice in large systems, using lightweight scripting to automate and enhance complex backend processes.

Fork/Join Framework

The Fork/Join Framework is part of Java’s concurrency utilities that allow parallel processing of tasks. It splits the task into smaller sub-tasks (forks), works on them in parallel, and combines the results (joins). This is particularly useful for those tasks that can be subdivided into independent chunks, such as processing huge amounts of data.

Key Features:

• Work Stealing: Available threads can “steal” tasks from busy threads to balance out the workload.
• RecursiveTask: This is useful for tasks that return results.
• RecursiveAction: This is useful for tasks that do not return anything.

Example:

ForkJoinPool pool = new ForkJoinPool();
RecursiveTask<Integer> task = new MyRecursiveTask();
Integer result = pool.invoke(task);

The Fork/Join Framework improves the performance of programs by leveraging multi-core processors, making it best for parallel processing of huge complex tasks. 

Common Concurrency Pitfalls

Concurrency in Java boosts performance. While it opens doors to many advantages, it also comes with some drawbacks. Below are common issues to look out for: 

• Race Conditions: This happens when two or more threads try to access the same piece of shared data synchronously. Because of this, the object fails to behave as expected. Always ensure that shared data is accessed safely.
• Deadlocks: It happens when there are multiple threads that are continually waiting for one another to release resources. Avoid this by acquiring locks in a consistent order and using timeouts.
• Thread Starvation: When certain threads are never able to access resources because others are always given priority. Use fair locks or adjust thread priorities to prevent this.
• Improper Use of Locks: Overusing locks or non-release of locks results in performance overruns or deadlock situations. Always release locks in a final block to ensure they are released even if an exception occurs.

By being aware of these pitfalls and taking care to design thread-safe code, you can avoid common concurrency issues and build more reliable applications.

Note: One of the common problems faced by the developers is the ConcurrentModificationException in Java, which occurs while the collection is modified while being iterated.

FAQs on Java Concurrency

Let’s Summarize

Java Concurrency is a powerful toolset that allows developers to build fast, efficient, and scalable applications. Whether you’re building microservices or high-performance apps, Java concurrency offers the structure and tools to succeed.

By using Java’s built-in features like ReentrantLock, atomic variables, and the Fork/Join framework, developers can tackle complex tasks with more control and fewer errors.But remember-concurrent programming can be tricky and complex at a certain point.

If you need help solving complex issues and maintaining the accuracy of your application, you can trust our Java development services!