CompletableFuture - It may be useful to build chain of actions before execution.

下方這隻程式在Java8中應是相當常見:

CompletableFuture.supplyAsync(()->{
	dumpCurrentThreadName("supplyAsync");
	return new Response();
}).whenComplete((response, ex)->{
	dumpCurrentThreadName("whenComplete");
	// process response
});

它的問題是如果supplyAsync中的工作，在執行whenComplete前就結束了；當執行whenComplete時，有可能會使用client thread去執行工作而影響到工作接收的速度。以下是我的執行結果:

ForkJoinPool.commonPool-worker-3: supplyAsync
main: whenComplete

假如你的callback chain很長，遇到這個情況的機率就會越高。

面對這個問題，在Java9中，允許你將實際的執行工作放到callback chain之後:

CompletableFuture<Response> asyncJob = new CompletableFuture<>();
 
asyncJob.whenComplete((response, ex)->{
	dumpCurrentThreadName("whenComplete");
});
 
asyncJob.completeAsync(()->{
	dumpCurrentThreadName("supplyAsync");
	return new Response();
});

輸出結果如下:

ForkJoinPool.commonPool-worker-3: supplyAsync
ForkJoinPool.commonPool-worker-3: whenComplete

這樣可以避免client thread跑去執行原本預期要async的工作，而增加了client thread的回應速度。

至於Java8目前我使用下面方式來達到lazy launch:

CompletableFuture<Response> asyncJob = new CompletableFuture<>();
job.thenApply((ret)->{
	dumpCurrentThreadName("supplyAsync");
	return new Response();rrentThreadName("runAsync");
 
}).whenComplete((response, ex)->{
	dumpCurrentThreadName("whenComplete");
});
 
CompletableFuture.runAsync(()->{
	asyncJob.complete(null);
})	

我針對三種情況做測試，在whenComplete中會去計算submit時的context switch次數並delay 50ms，

• 直接透過runAsync launch程式。
• 透過JDK9 completeAsync lazy launch程式。
• JDK8的lazy launch程式。

由於我使用common的pool，因此我在執行測試之前，會先叫起所有的thread，以避免第一個執行的需要去產生thread。以下是我的測試結果:

launchImmediately-submit: 306
launchImmediately-done: 1855
count for context swtching of submit operation: 6
launchLazilyJDK9-submit: 2
launchLazilyJDK9-done: 1656
count for context swtching of submit operation: 0
launchLazily-submit: 2
launchLazily-done: 1712
count for context swtching of submit operation: 0

可以發現，如果在submit時發生了Problem中所敘述的情況，將會延長client thread與整體工作時間；JDK9與JDK8的做法其實差異不大，不過JDK9的寫法更為簡潔且少產生中繼物件。

我附上測試程式碼供大家參考:

package org.tonylin.test.lambda.parallel.async;
 
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.atomic.AtomicInteger;
 
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;
import org.tonylin.test.util.ThreadUtil;
 
public class TestLazyCompletableFuture {
 
	private int jobNum = 100;
	private long delayTime = 50;
	private List<CompletableFuture<Void>> jobs = new ArrayList<>();
	private AtomicInteger count = new AtomicInteger(0);
 
	@BeforeClass
	public static void classSetup(){
		ThreadUtil.disableLoggin();
		ThreadUtil.launchThreadsOfCommonPool();
	}
 
	@AfterClass
	public static void classTeardown(){
		ThreadUtil.enableLoggin();
	}
 
	private void countIfMainThread(){
		if(Thread.currentThread().getName().equals("main")){
			count.incrementAndGet();
		}
	}
 
	@Test
	public void launchImmediately(){
		long before = System.currentTimeMillis();
		for( int i = 0 ; i < jobNum ; i++ ) {
			CompletableFuture<Void> job = CompletableFuture.runAsync(()->{
				ThreadUtil.dumpCurrentThreadName("runAsync");
			}).whenComplete((ret,ex)->{
				countIfMainThread();
				ThreadUtil.dumpCurrentThreadName("whenComplete");
				ThreadUtil.sleep(delayTime);
			});		
			jobs.add(job);
		}
		dumpDuration("launchImmediately", before);
	}
 
	@Test
	public void launchLazily(){
		long before = System.currentTimeMillis();
		for( int i = 0 ; i < jobNum ; i++ ) {
			CompletableFuture<Void> job = new CompletableFuture<>(); 
			job.whenComplete((ret,ex)->{
				countIfMainThread();
				ThreadUtil.dumpCurrentThreadName("whenComplete");
				ThreadUtil.sleep(delayTime);
			});		
 
			jobs.add(CompletableFuture.runAsync(()->{
				ThreadUtil.dumpCurrentThreadName("runAsync");
				job.complete(null);
			}));
		}
		dumpDuration("launchLazily", before);
	}
 
	@Test
	public void launchLazilyJDK9() {
		long before = System.currentTimeMillis();
		for( int i = 0 ; i < jobNum ; i++ ) {
			CompletableFuture<Void> job = new CompletableFuture<>(); 
			job.whenComplete((ret,ex)->{
				countIfMainThread();
				ThreadUtil.dumpCurrentThreadName("whenComplete");
				ThreadUtil.sleep(delayTime);
			});		
 
			job.completeAsync(()->{
				ThreadUtil.dumpCurrentThreadName("runAsync");
				return null;
			});
 
			jobs.add(job);
		}
		dumpDuration("launchLazilyJDK9", before);
	}
 
	private void dumpDuration(String testCase, long before){
		long duration_submit = System.currentTimeMillis() - before;
		System.out.println(testCase+"-submit: " + duration_submit);
 
		CompletableFuture.allOf(jobs.toArray(new CompletableFuture[0])).join();
		long duration = System.currentTimeMillis() - before;
		System.out.println(testCase+"-done: " + duration);
		System.out.println("count for context swtching of submit operation: " + count);
	}
 
}

package org.tonylin.test.util;
 
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
 
public class ThreadUtil {
	private static boolean logginEnabled = true;
 
	public static void dumpCurrentThreadName(String suffix) {
		if( logginEnabled )
			System.out.println(Thread.currentThread().getName() + ": " + suffix);
	}
 
	public static void launchThreadsOfCommonPool(){
 
		List<ForkJoinTask<?>> tasks = new ArrayList<>();
		for( int i = 0 ; i < ForkJoinPool.getCommonPoolParallelism(); i++){
			ForkJoinTask<?> task = ForkJoinPool.commonPool().submit(()->{
				sleep(100);
			});	
			tasks.add(task);
		}
		tasks.forEach(task->task.join());
	}
 
	public static void sleep(long duration){
		try {
			Thread.sleep(duration);
		} catch (InterruptedException e) {
			Thread.currentThread().interrupt();
			throw new RuntimeException(e);
		}
	}
 
	public static void enableLoggin(){
		logginEnabled = true;
	}
 
	public static void disableLoggin(){
		logginEnabled = false;
	}
}

• Asynchronous API with CompletableFuture - Performance Tips and Tricks

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