Java 21 Virtual Threads: Benefits, Usage, and Performance Comparison

Java 21 introduces virtual threads, a lightweight alternative to OS threads that reduces memory usage and creation overhead, making it ideal for high‑concurrency and I/O‑intensive applications.

Key advantages:

Lightweight – managed by the JVM, allowing creation of hundreds of thousands of threads.

High concurrency support – especially for I/O‑bound workloads.

Automatic scheduling – the JVM adjusts scheduling without manual thread‑pool management.

Basic usage example:

import java.lang.Thread;
Thread virtualThread = Thread.ofVirtual().start(() -> {
    System.out.println("虚拟线程正在运行");
});
System.out.println("主线程正在运行");

Delayed start example:

import java.lang.Thread;
Thread virtualThread = Thread.ofVirtual()
    .name("虚拟线程")
    .unstarted(() -> System.out.println("虚拟线程运行中"));

virtualThread.start();
virtualThread.join(); // 等待虚拟线程完成

To enable virtual threads in a Spring Boot project, use Java 21 or later and add the --enable-preview flag in pom.xml :

org.apache.maven.plugins
maven-compiler-plugin
21
21
--enable-preview

Configure Tomcat to use a virtual‑thread executor:

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import java.util.concurrent.Executors;

@Configuration
public class VirtualThreadConfig {
    @Bean
    public TomcatProtocolHandlerCustomizer
protocolHandlerVirtualThreadExecutorCustomizer() {
        return protocolHandler -> protocolHandler.setExecutor(Executors.newVirtualThreadPerTaskExecutor());
    }
}

Performance experiment comparing 100 000 traditional threads vs. virtual threads shows execution time dropping from ~18.6 s to ~3.7 s (≈500 % speed‑up). HTTP request tests (1 600 requests, concurrency 400) also demonstrate lower latency and higher throughput for virtual threads.

Additional Java performance tips include using parallel streams, CompletableFuture for asynchronous I/O, reducing database round‑trips with caching, and employing object pools for resource reuse.

In summary, virtual threads simplify concurrency, dramatically improve performance in high‑load scenarios, and can be adopted with minimal Spring Boot configuration.