Java virtual thread is a new feature available from JDK 19. It has potential to improve the application’s availability, throughput and code quality on top of reducing memory consumption. If you are interested in learning more about Java virtual thread, here is a quick introduction to it.

Video: To see the visual walk-through of this post, click below:

In this post, let’s learn the pitfalls one should avoid when switching from Java platform threads to virtual threads:

1. Avoid synchronized blocks/methods

2. Avoid Thread pools to limit resource access

3. Reduce ThreadLocal usage

Let’s review these pitfalls in detail.

1. Avoid synchronized blocks/methods

When a method is synchronized in Java, only one thread would be allowed to enter that method at a time. Let’s consider the below example:
1: public synchronized void getData() {
3:  makeDBCall();
4: }
In the above code snippet, ‘getData()’ method is ‘synchronized’. Let’s say thread-1 attempts to enter the ‘getData()’ method first. While this thread-1 is executing the getData() method, thread-2 attempts to execute this method. Since ‘thread-1’ is currently executing the ‘getData()’ method, ‘thread-2’ will not be allowed to execute. It will be put in the BLOCKED state. If you are using a virtual thread in this scenario, when the thread is moved to BLOCKED state, ideally it should relinquish its control of the underlying OS thread and move back to the heap memory. However due to the limitation of the current virtual thread implementation, when a virtual thread gets BLOCKED because of synchronized method (or block), it will not relinquish its control over the underlying OS thread. Thus, you wouldn’t gain the benefits of switching to virtual threads

In this circumstance, you should consider replacing synchronized methods/blocks with ‘ReentrantLock‘. Above example code synchronized getData() method can be rewritten like this using ‘ReentrantLock’:
1: private ReentrantLock myLock = new ReentrantLock();
3: public void getData() {
5:   myLock.lock(); // acquire lock
6:   try {
8:      makeDBCall();
9:   } finally {
11:      myLock.unlock(); // release lock
12:   }
13: }
When you replace the synchronized method with ReentrantLock, the virtual thread will relinquish the control of the underlying OS thread and you can enjoy the benefits of virtual threads.

Note: Virtual thread not releasing the underlying operating system thread when working on synchronized method, is the current limitation in JDK 19. It could be addressed in the future release of Java.

2. Avoid Thread pools to limit resource access

Sometimes, in our programming constructs, we might have used thread pool to limit the access to certain resources. Say suppose we want to make only 10 concurrent calls to a backend system, it might have been programmed using thread pool as shown below:
1: private ExecutorService BACKEND_THREAD_POOL = Executors.newFixedThreadPool(10);
3: public <T> Future<T> queryBackend(Callable<T> query) {
5:   return BACKEND_THREAD_POOL.submit(query);
6: }
In line #1, you can notice a ‘BACKEND_THREAD_POOL’ is created with 10 threads. This thread pool is used in the ‘queryBackend()’ method to make backend calls. This thread pool will ensure that not more than 10 concurrent calls will be made to the backend system.

At the time of writing this post (Jan’ 2023) there is no API available in JDK to create an Executor (i.e. thread pool) with a fixed number of virtual threads. Here is the list of all the APIs to create virtual threads. When you are using Executor, you can create only an unlimited number of virtual threads. To address this problem, you can consider replacing Executor with Semaphore. In the above example, ‘queryBackend()’ method can be rewritten using ‘Semaphore’ as shown below:
1:   private static Semaphore BACKEND_SEMAPHORE = new Semaphore(10);
3:   public static <T> T queryBackend(Callable<T> query) throws Exception {
5:      BACKEND_SEMAPHORE.acquire(); // allow only 10 concurrent calls
6:      try {
8:         return;
9:      } finally {
11:         BACKEND_SEMAPHORE.release();
12:      }
13: }
If you are not familiar with Semaphore, you may read this ‘Java sempahore – easy introduction‘ post. If you notice in line #1, we are creating a ‘BACKEND_SEMAPHORE’ with 10 permits. This semaphore will allow only 10 concurrent calls to the backend system. This is a good alternative to the Executor.

3. Reduce ThreadLocal Usage
Few applications tend to use ThreadLocal variables. If you aren’t familiar with ThreadLocal variables, you may read this ‘Java ThreadLocal – easy introduction’ post. But in a nutshell, Java ThreadLocal variables are created and stored as variables within the scope of a particular thread alone and it cannot be accessed by other threads. If your application creates millions of virtual threads and each virtual thread has its own ThreadLocal variable, then it can quickly consume java heap memory space. Thus, you want to be cautious of the size of data that is stored as ThreadLocal variables.

You might wonder why ThreadLocal variables are not problematic in platform threads. The difference is that, in platform threads we don’t create millions of threads, whereas in virtual threads we do. Millions of threads, each with their own copy of ThreadLocal variables can quickly fill up memory. They say small drops of water make an ocean. It’s very true here.

In general, Java ThreadLocal variables are tricky to manage & maintain. It can also cause nasty production problems. Thus, limiting the scope of use of ThreadLocal variables, can benefit your application especially when using virtual threads.