JEP 491 - Synchronize Virtual Threads without Pinning.

Giriş

Açıklaması şöyle

Improve the scalability of Java code that uses synchronized methods and statements by arranging for virtual threads that block in such constructs to release their underlying platform threads for use by other virtual threads. This will eliminate nearly all cases of virtual threads being pinned to platform threads, which severely restricts the number of virtual threads available to handle an application's workload.

synchronized block kullanıyorsak Virtual Thread pinned oluyor. Bu JEP ile artık olmuyor

IntelliJ Idea Comment İçin Code Style

Comment At Firt Column

Cevap burada

• Settings | Editor | Code Style | Java | Code Generation | Comment Code | Line comment at first column -> Uncheck
• Settings | Editor | Code Style | Java | Code Generation | Comment Code | Block comment at first column -> Uncheck
• Settings | Editor | Code Style | Java | Wrapping and Braces | Keep when reformatting | Comment at first column -> Uncheck

Ayrıca Method Call Argument uzunsa ayrı satıra gelsin diye

• Settings | Editor | Code Style | Java | Wrapping and Braces | Method Call Arguments | -> Wrap if long

Ayrıca Chained method calls her biri ayrı satıra gelsin diye

• Settings | Editor | Code Style | Java | Wrapping and Braces | Chanied Method Calls | -> Wrap always

javadoc komutu

Örnek

Şöyle yaparız. docs/index.html dosyasını oluşturur

javadoc --enable-preview --source 22 -d docs Calculator.java

JMeter CLI

CLI vs Non-CLI

Açıklaması şöyle

JMeter has two modes: CLI and Non-CLI. .... CLI mode doesn't have any user interface. But Non-CLI mode has a user interface where you can point-and-click and interact with the JMeter.

Non-CLI mode should be used for recording, scripting, and smoke testing. This mode utilizes more resources (CPU and memory). CLI mode must be used for load testing, stress testing, and other forms of performance testing as well as for CI/CD pipelines. This mode doesn't eat up more resources as there is no user interface to load and render.

Örnek

Şöyle yaparız

jmeter -n -t test_plan.jmx -l test_results.jtl

Java record İle Yapılamayacak Şeyler

1. Records Cannot Extend Another Class

2. Records Cannot Be Extended

Örnek

Şu kod derlenmez

public class PremiumRecord extends InsuranceRecord {…}

3. Records Cannot Have Additional Instance Fields

Örnek

Şu kod derlenmez

public record InsuranceRecord(String type, float premium) {
  private String policyNumber;  // This won't compile
}

4. Private Canonical Constructors Are Not Allowed

Örnek

Şu kod derlenmez

public record InsuranceRecord(String type, float premium) {
  private InsuranceRecord(String type, float premium) {
    this.type = type;
    this.premium = premium;
  }
  public static InsuranceRecord newInstance(String type, float premium) {
    return new InsuranceRecord(type, premium);
  }
}

5.  Records Cannot Have Setters

Örnek

Şu kod derlenmez

public record InsuranceRecord(String type, float premium) {
  public void setType(String type) {
    this.type = type;  // Won't compile
  }
}

@Serial Anotasyonu

Giriş

Bu anotasyon sadece kodda var yani byte code'a girmiyor. Açıklaması şöyleşöyle

This annotation exists purely to engage better compile-time type checking. It is analogous in this way to the @Override annotation, which exists purely to capture design intent, so that humans and tools have more information to work with. 

Virtual Threads - Thread Pinning

Thread Pinning 

Virtual Threads için JDK'daki bir çok kod baştan yazılmış. Böylece thread unloading işlemi yapılabiliyor.  Ancak bir kaç işlemde istisna var. Buna Thread Pinning deniliyor

1. Object.wait()

2. synchronized blok içinde

3. native metod çalıştırırken

4. UDP Sockets

5. Files & DNS

Açıklaması şöyle

Pinning describes the condition where a virtual thread, which maps the platform thread, is stuck to the carrier thread. It means that the virtual thread can’t be unmounted from the carrier threads because the state of it can’t be stored in the heap memory. The pinned thread prevents others from utilizing the same platform thread.

1. synchronized Thread Pinning

Açıklaması şöyle

Only one thread can enter it at a time. The other threads attempting to enter will be blocked until the current (running) thread exits. They would need an uninterrupted access to shared resources to prevent race conditions as well as an accurate state of the data.

Örnek

Şu kod carrier thread'i bloke eder. 

var e = Executors.newVirtualThreadPerTaskExecutor();
for (int i = 0; i < 1000; i++) {
    e.submit(() -> { new Test().test(); });
}
e.shutdown();
e.awaitTermination(1, TimeUnit.DAYS);

class Test {
  synchronized void test() {
    sleep(4000);
  }
}

synchronized yerine  ReentrantLock kullanılır. Şöyle yaparız

var e = Executors.newVirtualThreadPerTaskExecutor();
for (int i = 0; i < 1000; i++) {
    e.submit(() -> { new Test().test(); });
}
e.shutdown();
e.awaitTermination(1, TimeUnit.DAYS);

class Test {
  private ReentrantLock lock = new ReentrantLock();
  void test() {
    lock.tryLock();
    try {
      sleep(4000);
    } finally {
      lock.unlock();
    }
  }
}

MySQL-J JDBC sürücüsündeki bir açıklama şöyle

Synchronized blocks in the Connector/J code were replaced with ReentrantLocks. This allows carrier threads to unmount virtual threads when they are waiting on IO operations, making Connector/J virtual-thread friendly. Thanks to Bart De Neuter for contributing to this patch. ”

2. native method Thread Pinning

Açıklaması şöyle

This method lets you use code from other languages into your JAVA project, like C or C++. Native methods are invoked within the JAVA virtual machine but executed outside its control.

3. Sockets Thread Pinning

TCP socketlerini okurken Virtual Threads bloke olmuyor, ama UDP socketlerinde oluyor. Açıklaması şöyle

However, not all Java constructs have been retrofitted that way. Such operations include synchronized methods and code blocks. Using them causes the virtual thread to become pinned to the carrier thread. When a thread is pinned, blocking operations will block the underlying carrier thread-precisely as it would happen in pre-Loom times.

Eğer carrier therad'in bloke olma durumu varsa, virtual thread sayısı artırılır. En fazla kaç thread olabileceği  jdk.virtualThreadScheduler.maxPoolSize ile atanabilir

pinned thread Tracing

Açıklaması şöyle

Use the following JVM parameters as options to trace the pinned thread

-Djdk.tracePinnedThreads=full Prints a complete stack trace when a thread jams while pinned, highlighting native frames and frames holding monitors.

-Djdk.tracePinnedThreads=short Limits the output to just the problematic frames.