Exploring Threads and Virtual Threads in Java: A Comprehensive Guide for Scalable Concurrency

In Java, both threads and virtual threads are mechanisms for achieving concurrency, but they differ significantly in managing resources, especially in terms of efficiency, scalability, and performance. Let’s dive into the details of each, including their internal workings and specific differences.

Traditional Java Threads

Java threads are managed by the operating system (OS), also known as platform threads. These threads map directly to the OS's native threads, meaning each Java thread corresponds to a single OS thread. Here’s an overview of how they work internally and their characteristics:

Characteristics of Traditional Threads

• Heavyweight Threads: Each Java thread is a wrapper around a native OS thread, and they are costly to create and manage.
• Blocking: Traditional threads consume an OS resource (like a kernel thread), even if they’re blocked (e.g., waiting for I/O).
• Limited Scalability: Because OS threads have significant memory overhead and require substantial CPU resources, the number of threads an application can manage is limited.
• Managed by OS Scheduler: The OS scheduler decides which threads to run and when. This can lead to context-switching overheads that become considerable with a high number of threads.

Internal Working of Traditional Threads

1. OS-Level Mapping: Java threads are mapped directly to kernel-level threads. Each thread has its own memory stack, maintained by the OS.
2. Scheduling and Context Switching: The OS scheduler is responsible for managing multiple threads, switching between them as needed. Context switching, which is essential for multitasking, is relatively expensive for OS threads.
3. Synchronization and Locks: Traditional threads synchronize using locks at the OS level, which adds more resource overhead.

Because of these characteristics, when you create thousands of threads in Java, it quickly becomes inefficient, with memory constraints and context-switching overheads causing performance issues. This limitation led to the exploration of lighter concurrency models, such as virtual threads.

Virtual Threads (Introduced in Project Loom)

Introduced as part of Project Loom in Java 19, virtual threads aim to make concurrency much more lightweight. They are also known as user-mode threads because they are managed by the JVM rather than the OS.

Characteristics of Virtual Threads

• Lightweight: Virtual threads are cheap to create and dispose of since they don’t have a 1:1 mapping to OS threads.
• Non-blocking by Design: If a virtual thread blocks, it yields control to the JVM rather than blocking an OS thread, which allows the system to manage more concurrent tasks without consuming as many resources.
• Highly Scalable: You can create millions of virtual threads in Java, making them highly scalable for I/O-bound applications.
• Managed by JVM Scheduler: Virtual threads are scheduled and managed by the JVM rather than the OS. The JVM uses a smaller number of OS threads (a pool) to run a large number of virtual threads.

Internal Working of Virtual Threads

1. User-Mode Scheduling: Virtual threads do not map directly to OS threads. Instead, the JVM manages a continuation-based scheduling model, in which virtual threads are backed by a carrier thread pool (typically a small number of OS threads).
2. Continuation and Yielding: When a virtual thread blocks (for example, waiting on I/O), it yields control back to the JVM. The JVM then parks this virtual thread and allows the OS thread to work on other virtual threads. This yields a cooperative multitasking model where the JVM, rather than the OS, manages scheduling and resource allocation.
3. Continuation-Based Execution: Internally, virtual threads use a construct called continuation, which can suspend and resume execution at any point. This mechanism allows virtual threads to pause their execution state and yield to another virtual thread without relying on OS-level context switching.
4. Stack Frames in Heap: Unlike OS threads, which maintain their own memory stack, virtual threads may store their stack frames in the Java heap. This makes it possible for virtual threads to be paused and resumed with minimal overhead.

Key Differences Between Traditional Threads and Virtual Threads

Example: Practical Use Case of Virtual Threads

Suppose you are building a web server that handles thousands of incoming requests. In traditional Java threading, each request would typically require its own thread, consuming a lot of resources. Using virtual threads, you can handle each request with a separate virtual thread, and even if requests involve blocking I/O, virtual threads can be suspended without consuming OS resources. This approach leads to massive scalability and optimal performance.

When to Use Traditional Threads vs. Virtual Threads

• Traditional Threads are still useful for CPU-bound applications where the number of tasks is relatively low, but each task is computationally intensive.
• Virtual Threads are ideal for I/O-bound tasks, especially when applications need to handle many concurrent operations, such as web servers, microservices, or message-driven systems.

In summary, virtual threads revolutionize concurrency in Java by introducing lightweight, scalable threading. They improve performance, especially for applications that need to handle large numbers of concurrent operations. The internal working of virtual threads with continuation-based scheduling and JVM management enables efficient handling of blocking operations, unlike traditional threads.vHere are some helpful reference links to learn more about threads, virtual threads, Project Loom, and concurrency in Java and Spring Boot:

Java Threads Documentation -https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/lang/Thread.html

Project Loom Overview: https://openjdk.org/projects/loom/

Virtual Threads in Java:

https://www.baeldung.com/java-virtual-threads

https://www.infoq.com/articles/virtual-threads-intro/

Concurrency and Virtual Threads in Spring Boot:

https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#scheduling)

https://www.baeldung.com/spring-async)

Asynchronous and Non-blocking Programming in Spring Boot:

https://www.baeldung.com/spring-boot-async)

These resources offer detailed explanations and code examples that build upon the blog content provided, helping you dive deeper into virtual threads, Java concurrency, and Spring Boot integration with asynchronous and concurrent processing.