[英]Executing Dependent tasks in parallel in Java
我需要找到一種在java中並行執行任務(依賴和獨立)的方法。
我檢查了java.util.concurrent Future和Fork / Join,但看起來我們無法向Task添加依賴項。
任何人都可以指出我更正Java API。
在Scala中這很容易做到,我認為你最好使用Scala。 這是我從這里開始的一個例子http://danielwestheide.com/ (新手的Scala指南第16部分:從哪里開始)這個人有一個很棒的博客(我不是那個人)
讓我們一起喝咖啡吧。 要做的任務是:
或作為一棵樹:
Grind _
Coffe \
\
Heat ___\_Brew____
Water \_____Combine
/
Foam ____________/
Milk
在使用並發api的java中,這將是:
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
public class Barrista {
static class HeatWater implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("Heating Water");
Thread.sleep(1000);
return "hot water";
}
}
static class GrindBeans implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("Grinding Beans");
Thread.sleep(2000);
return "grinded beans";
}
}
static class Brew implements Callable<String> {
final Future<String> grindedBeans;
final Future<String> hotWater;
public Brew(Future<String> grindedBeans, Future<String> hotWater) {
this.grindedBeans = grindedBeans;
this.hotWater = hotWater;
}
@Override
public String call() throws Exception
{
System.out.println("brewing coffee with " + grindedBeans.get()
+ " and " + hotWater.get());
Thread.sleep(1000);
return "brewed coffee";
}
}
static class FrothMilk implements Callable<String> {
@Override
public String call() throws Exception {
Thread.sleep(1000);
return "some milk";
}
}
static class Combine implements Callable<String> {
public Combine(Future<String> frothedMilk, Future<String> brewedCoffee) {
super();
this.frothedMilk = frothedMilk;
this.brewedCoffee = brewedCoffee;
}
final Future<String> frothedMilk;
final Future<String> brewedCoffee;
@Override
public String call() throws Exception {
Thread.sleep(1000);
System.out.println("Combining " + frothedMilk.get() + " "
+ brewedCoffee.get());
return "Final Coffee";
}
}
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(2);
FutureTask<String> heatWaterFuture = new FutureTask<String>(new HeatWater());
FutureTask<String> grindBeans = new FutureTask<String>(new GrindBeans());
FutureTask<String> brewCoffee = new FutureTask<String>(new Brew(grindBeans, heatWaterFuture));
FutureTask<String> frothMilk = new FutureTask<String>(new FrothMilk());
FutureTask<String> combineCoffee = new FutureTask<String>(new Combine(frothMilk, brewCoffee));
executor.execute(heatWaterFuture);
executor.execute(grindBeans);
executor.execute(brewCoffee);
executor.execute(frothMilk);
executor.execute(combineCoffee);
try {
/**
* Warning this code is blocking !!!!!!!
*/
System.out.println(combineCoffee.get(20, TimeUnit.SECONDS));
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (TimeoutException e) {
System.out.println("20 SECONDS FOR A COFFEE !!!! I am !@#! leaving!!");
e.printStackTrace();
} finally{
executor.shutdown();
}
}
}
確保您添加超時以確保您的代碼不會永遠等待完成某些事情,這可以通過使用Future.get(long,TimeUnit)完成,然后相應地處理失敗。
它在scala中更好,但是,它就像在博客上一樣:准備一些咖啡的代碼看起來像這樣:
def prepareCappuccino(): Try[Cappuccino] = for {
ground <- Try(grind("arabica beans"))
water <- Try(heatWater(Water(25)))
espresso <- Try(brew(ground, water))
foam <- Try(frothMilk("milk"))
} yield combine(espresso, foam)
所有方法返回未來(鍵入的未來),例如grind將是這樣的:
def grind(beans: CoffeeBeans): Future[GroundCoffee] = Future {
// grinding function contents
}
對於所有實現,請查看博客,但這就是它的全部內容。 您也可以輕松集成Scala和Java。 我真的建議在Scala而不是Java中做這種事情。 Scala需要更少的代碼,更清晰和事件驅動。
具有依賴性的任務的通用編程模型是Dataflow 。 簡化模型,其中每個任務只有一個,但重復的依賴是Actor模型 。 Java有很多actor庫,但數據流很少。 另請參見: which-actor-model-library-framework-for-java , java-pattern-for-nested-callbacks
使用BlockingQueue。 將任務A的輸出放入隊列,任務B阻塞,直到隊列中有可用的東西。
這些文檔包含實現此目的的示例代碼: http : //docs.oracle.com/javase/6/docs/api/java/util/concurrent/BlockingQueue.html
你需要的是CountDownLatch 。
final CountDownLatch gate = new CountDownLatch(2);
// thread a
new Thread() {
public void run() {
// process
gate.countDown();
}
}.start();
// thread c
new Thread() {
public void run() {
// process
gate.countDown();
}
}.start();
new Thread() {
public void run() {
try {
gate.await();
// both thread a and thread c have completed
// process thread b
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}.start();
作為替代方案,根據您的方案,您也可以使用BlockingQueue來實現Producer-Consumer模式。 請參閱文檔頁面上的示例。
如果任務B依賴於任務A的輸出,我首先會質疑任務B是否真的是一個單獨的任務。 如果存在以下情況,則分離任務是有意義的:
假設它是一個單獨的任務,那么您可以允許任務A和B共享BlockingQueue
,以便任務A可以傳遞任務B數據。
使用此庫https://github.com/familysyan/TaskOrchestration 。 它為您管理任務依賴。
有一個專門用於此目的的java庫(免責聲明:我是這個庫的所有者),名為Dexecutor
以下是如何實現所需結果的方法,您可以在此處閱讀更多相關信息
@Test
public void testDependentTaskExecution() {
DefaultDependentTasksExecutor<String, String> executor = newTaskExecutor();
executor.addDependency("A", "B");
executor.addIndependent("C");
executor.execute(ExecutionBehavior.RETRY_ONCE_TERMINATING);
}
private DefaultDependentTasksExecutor<String, String> newTaskExecutor() {
return new DefaultDependentTasksExecutor<String, String>(newExecutor(), new SleepyTaskProvider());
}
private ExecutorService newExecutor() {
return Executors.newFixedThreadPool(ThreadPoolUtil.ioIntesivePoolSize());
}
private static class SleepyTaskProvider implements TaskProvider<String, String> {
public Task<String, String> provid(final String id) {
return new Task<String, String>() {
@Override
public String execute() {
try {
//Perform some task
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
String result = id + "processed";
return result;
}
@Override
public boolean shouldExecute(ExecutionResults<String, String> parentResults) {
ExecutionResult<String, String> firstParentResult = parentResults.getFirst();
//Do some logic with parent result
if ("B".equals(id) && firstParentResult.isSkipped()) {
return false;
}
return true;
}
};
}
}
Java定義了一個CompletableFuture類。
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html
這就是你要找的東西。 它有助於構建執行流程。
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