How to implement coroutines in Java?

What is a coroutine?

Coroutine is a lightweight thread in user mode, which can implement concurrent execution of multiple tasks in one thread. Compared with traditional threads, coroutines have less switching overhead, so they can achieve higher concurrency performance.

Coroutines can be seen as a cooperative multitasking mechanism. In a coroutine, tasks cooperate through a coroutine scheduler. When a task needs to wait for the I/O operation to complete, it will actively give up CPU time to other tasks, and resume execution until the I/O operation is completed.

How to implement coroutines in Java?

In Java, the implementation of coroutines usually requires the use of third-party libraries. The following are two commonly used Java coroutine libraries:

Quasar

Quasar is a JVM-based coroutine library that implements coroutines through bytecode enhancements. Quasar provides a class called Fiber to represent coroutines, which can be used to create, start, pause, resume, cancel, wait, and synchronize coroutines through the Fiber API.

The following is a sample code that uses Quasar to implement coroutines:

import co.paralleluniverse.fibers.Fiber;
import co.paralleluniverse.fibers.SuspendExecution;

public class FiberDemo {
    
    
    public static void main(String[] args) throws Exception {
    
    
        Fiber<Void> fiber1 = new Fiber<Void>() {
    
    
            @Override
            protected Void run() throws SuspendExecution, InterruptedException {
    
    
                System.out.println("Fiber 1 started");
                Fiber.sleep(1000);
                System.out.println("Fiber 1 resumed");
                return null;
            }
        };
        
        Fiber<Void> fiber2 = new Fiber<Void>() {
    
    
            @Override
            protected Void run() throws SuspendExecution, InterruptedException {
    
    
                System.out.println("Fiber 2 started");
                Fiber.sleep(500);
                System.out.println("Fiber 2 resumed");
                return null;
            }
        };
        
        fiber1.start();
        fiber2.start();
        
        fiber1.join();
        fiber2.join();
    }
}

In the above code, we create a coroutine by inheriting the Fiber class. In the run method, we can write the code that the coroutine needs to execute. With the Fiber.sleep method, we can suspend the coroutine for a period of time and then resume execution. At the end, we wait for the coroutine to finish executing with the fiber.join method.

Kotlin coroutines

Kotlin coroutine is a built-in coroutine library in Kotlin language, and its API design is very simple and easy to use. Kotlin coroutines mark suspendable functions with the suspend keyword, and use the CoroutineScope class to manage the life cycle of coroutines.

Here is a sample code for implementing coroutines using Kotlin coroutines:

import kotlinx.coroutines.*

fun main() = runBlocking {
    
    
    val job1 = launch {
    
    
        println("Coroutine 1 started")
        delay(1000)
        println("Coroutine 1 resumed")
    }
    
    val job2 = launch {
    
    
        println("Coroutine 2 started")
        delay(500)
        println("Coroutine 2 resumed")
    }
    
    job1.join()
    job2.join()
}

In the above code, we use the runBlocking function to create a CoroutineScope object, and then use the launch function to create the coroutine. In the coroutine, we use the delay function to make the coroutine pause for a period of time, and then resume execution. At the end, we use the join function to wait for the coroutine to finish executing.

Summarize

A coroutine is a lightweight thread that can achieve higher concurrency performance. In Java, we can use third-party libraries to implement coroutines. Quasar is a JVM-based coroutine library, which implements coroutines through bytecode enhancement; Kotlin coroutine is a built-in coroutine library in Kotlin language, and its API design is very simple and easy to use. No matter which coroutine library we use, we need to pay attention to the life cycle of coroutines to avoid problems such as resource leaks or deadlocks.