May 04

Java 21, the Next LTS Release, Delivers Virtual Threads, Record Patterns and Pattern Matching

It is suggested that there is no need to replace synchronized blocks and methods that are used infrequently (e.g., only performed at startup) or that guard in-memory operations. For example, if we scale a million virtual threads in the application, there will be a million ThreadLocal instances along with the data they refer to. Such a large number of instances can put enough burden on the physical memory and it should be avoided. Notice the blazing fast performance of virtual threads that brought down the execution time from 100 seconds to 1.5 seconds with no change in the Runnable code. They do not block the OS thread while they are waiting or sleeping. Another important note is that virtual threads are always daemon threads, meaning they’ll keep the containing JVM process alive until they complete.

java virtual threads

An example from the JEP definition itself shows the ability to get to the Point values, which are part of ColoredPoint, which is nested in a Rectangle. This makes it way more useful than before, when all the records needed to be deconstructed every time. By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Java Virtual Threads

Since then and still with the release of Java 19, a limitation was prevalent, leading to Platform Thread pinning, effectively reducing concurrency when using synchronized. The use of synchronized code blocks is not in java virtual threads of itself a problem; only when those blocks contain blocking code, generally speaking I/O operations. In fact, the same blocking code in synchronized blocks can lead to performance issues even without Virtual Threads.

java virtual threads

Such locks don’t pin the virtual thread, making the cooperative scheduling work again. The following sample program shows rate limiting with a simple semaphore that allows a small number of concurrent requests. When the maximum is exceeded, the acquire method blocks, but that is ok. The first category, asynchronous, initiate I/O operations which complete at some later time, possibly on a thread other than the thread that initiated the I/O operation. By definition, these APIs do not result in blocking system calls, and therefore require no special treatment when run in a virtual thread.

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When the system schedules Java threads, it will occupy a lot of resources to handle thread context switching. Because virtual threads are threads and have little new API surface of their own, there is relatively little to learn in order to use virtual threads. But there are actually quite a few things we need to unlearn in order to use them effectively. Multi-threaded applications that use security principals, transactions, and shared context will benefit from the scoped values. The example below shows the ScopeValue, which is created and used with the scope of runWhere, a runnable method.

  • However, we can still look at some concepts at the roots of virtual threads.
  • Notice how now the task is executed by two threads, the first one executing the code before the blocking call and the second one after that.
  • For decades, we have relied on the multithreading model described above to solve concurrent programming problems in Java.
  • This is very similar to the JavaScript example we gave earlier.
  • This is very easy to write, understand and debug, but what if the client requests something that performs a blocking call?
  • While the virtual thread is being executed by a platform thread – the virtual thread is said to be mounted
    to that thread.

Enter virtual threads, which solve this problem by mapping Java threads to carrier threads that manage (i.e., mount/unmount) thread operations to a carrier thread. It is an abstraction that gives more flexibility and control for developers. As we said at the beginning of this article, with virtual threads, it’s not the case anymore. Creating a virtual thread is very cheap, both in space and time. Also, they were designed with the idea of using a different virtual thread for each request.

Use design by contract to build Kubernetes Operators in Java

Note that I also set the compiler to use Java 19 and (as shown in Listing 2) added a line to the .mvn/jvm.config. I strongly suspect that Java threads have a performance advantage on Windows, but not on Linux. A long-form guide on how to make microservices communicate with gRPC in Scala. The above example shows how we wrote concurrent programs that were constrained until now. By the way, the code uses LockSupport.parkNanos instead of Thread.sleep so that we don’t have to catch the pesky InterruptedException.

Thanks to this PR, the loom-unit extension will be integrated into the @QuarkusTest to provide a simpler developer experience. We can use the Thread.Builder reference to create and start multiple threads. There are other ways to achieve what virtual thread do (such as NIO and the related Reactor pattern). This, however, entails using message loops and callbacks which warp your mind (that’s why so many people hate JavaScript).

No Time Slicing Between Virtual Threads

As we can see, the IO operation, the sleep() method, is after the infinite loop. We also defined an alwaysTrue() function, which returns true and allows us to write an infinite loop without using the while (true) construct that is not permitted by the compiler. To overcome the problems of callbacks, reactive programming, and async/await strategies were introduced. We used virtual.threads.playground, but we can use any name we want.

java virtual threads

In this way, Executor will be able to run 100 tasks at a time and other tasks will need to wait. As we have 10,000 tasks so the total time to finish the execution will be approximately 100 seconds. Indeed, there was some popular call to add async/await to Java, as C# and Kotlin have. On a modest desktop system with no special configuration options, running this program takes about 1.6 seconds in a cold start, and about 1.1 seconds after warmup. If we try running this program with a cached thread pool instead, depending on how much memory is available, it may well crash with OutOfMemoryError before all the tasks are submitted. And if we ran it with a fixed-sized thread pool with 1000 threads, it wont crash, but Littles Law accurately predicts it will take 100 seconds to complete.

Virtual threads in action

These things matter when you need to process millions of requests per day they can be a game changer and drastically increase your throughput in some situations. The code is the same, we just have to use Executors.newVirtualThreadPerTaskExecutor() which creates a new green thread every time a task is submitted to it. Virtual threads help in achieving the same high scalability and throughput as the asynchronous APIs with the same hardware configuration, without adding the syntax complexity. To increase the performance of the application, we will add more and more Java threads.

As an example, on my personal web site, I provide demo services for producing random items. If a large number of requests comes at an instant from the same IP address, the hosting company blacklists the IP address. There is no API for constructing a virtual thread with another thread group. Trying to call setPriority or setDaemon on a virtual thread has no effect.

Not the answer you’re looking for? Browse other questions tagged javamultithreading or ask your own question.

This works, we can now handle as many requests as we like if we pay enough to our cloud vendor, but with cloud technologies, one of the main driving factors is reducing the cost of operation. Sometimes we can’t afford the extra spending and we end up with a slow and barely usable system. At its core it performs a basic event loop that monitors all of the synchronous networking read, connect, and accept operations that are not immediately ready when invoked in a virtual thread. When the I/O operation becomes ready, the poller will be notified and subsequently unpark the appropriate parked virtual thread. This approach can also be used in production (so, not only in tests). You can also determine how long the carrier thread was blocked by correlating the pinned stack trace with other log events (like what happened just after in the same thread).

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