Virtual threads are lightweight threads in Java that do not correspond to native threads in the operating system, allowing for reduced memory usage as their stack frames are stored in heap memory. The Java Virtual Machine (JVM) manages these threads by scheduling their execution on a limited number of platform threads and handling blocking operations efficiently. New executor services like ThreadPerTaskExecutor facilitate easy management of virtual threads by creating a new virtual thread for each submitted task.

1. Virtual Threads
Haim Michael
July 11th
, 2023
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2. © 2008 Haim Michael 20230307
What is Virtual Thread
 The virtual thread is a thread without a native thread running
on the operating system level.

3. © 2008 Haim Michael 20230307
Simple Demo
Thread t1 = Thread.ofVirtual().name("moshico1").start(() -> {
for(int i=0; i<100; i++) {
System.out.println("i="+i);
try {
Thread.sleep(100);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
});

4. © 2008 Haim Michael 20230307
The Stack
 The virtual thread stores the stack frames in the heap
memory. Thanks to that the memory footprint of virtual
threads is relatively small.

5. © 2008 Haim Michael 20230307
Executor Services
 We can use a new implementation of the
java.util.concurrent.ExecutorService tailored for
virtual threads. Its name is ThreadPerTaskExecutor.
 This executor creates a new virtual thread for every task we
submit.
var executor = Executors.newVirtualThreadPerTaskExecutor();

6. © 2008 Haim Michael 20230307
How Virtual Thread Work?
 The JVM maintains a pool of platform threads. Initially the
number of the platform threads equals the number of the CPU
cores.
 Whenever a virtual thread is created, the JVM schedules its
execution on one of the platform threads, and copies the
stack of the virtual thread from the heap to the stack of the
platform thread (AKA carrier thread).

7. © 2008 Haim Michael 20230307
How Virtual Thread Work?
 When a virtual thread is stuck on a blocking operation the
carrier thread is released and the stack chunk of the virtual
thread is copied back to the heap.
 When the blocked virtual thread finishes the blocking
operation, the scheduler schedules it again for execution on
the same carrier thread or another one.

8. © 2008 Haim Michael 20230307
Pinned Virtual Threads
 There are few cases where a blocking operation doesn’t
unmount the virtual thread from the carrier thread. As a result,
the carrier thread is blocked.
 In these cases we will describe the virtual thread as pinned to
the carrier thread.
 There are two cases in which a virtual thread is pinned to the
carrier thread: When it executes code inside a synchronized
block and when it calls a native method.

9. © 2008 Haim Michael 20230307
Questions & Answers
Thanks for Your Time!
Haim Michael
haim.michael@lifemichael.com
+972+3+3726013 ext:700
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