Asynchronous Programming in Java

Asynchronous Programming in Java

Parallel vs. Concurrent vs. Asynchronous

• Parallel: Multiple tasks executed at the same time. For example, I can walk and talk at the same time.
• Concurrent: I can work on multiple tasks, but I can do only one task at a moment in time. For example, I can talk or drink water at a moment.
• Asynchronous: It just means non-blocking. For example, I can brew a coffee, and in the meantime, I could do something else.
  • responsive
  • preemptive
  • performance

For Async programming JavaScript used callback. Callbacks created a problems of its own like callback hell. Promise came to solve the problem.

Promise

State of a promise.

• Pending when the promise is executed.
• Resolved when the promise is completed.
• Rejected when promise failed.

Completable Future

• CompletableFuture in Java is Promises in JavaScript
• runAsync - triggers the promise and forgets.
• supplyAsync - get the promise response.
• thenApply - maps response of promise
• thenAccept - consume the response of promise
• get - blocks the thread for promise to resolve or reject
• thenCompose - if the response is another promise
• thenCombine - to combine multiple promise

import lombok.SneakyThrows;
import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

@Slf4j
public class Sample {

	@SneakyThrows
	public static int compute(int n) {
		Thread.sleep(1000);
		return n * 2;
	}

	public static CompletableFuture<Integer> create(int n) {
		return CompletableFuture.supplyAsync(() -> compute(n));
	}

	public static void main(String[] args) throws ExecutionException, InterruptedException {
		log.info("future: {}", create(5)); // Not Complete
		create(4)
				.thenApply(data -> data + 1)
				.thenAccept(r -> log.info("future: {}", r))
				.get();

		var cf1 = create(2);
		var cf2 = create(3);

        // thenCombine
		cf1.thenCombine(cf2, Integer::sum)
				.thenAccept(v -> log.info("sum: {}", v));

        // thenCompose
		create(2)
//				.thenApply(Sample::create)
				.thenCompose(Sample::create)
				.thenAccept(r -> log.info("future: {}", r));
	}
}