假设我有这样的任务：
for(Object object: objects) {
Result result = compute(object);
list.add(result);
}
并行化每个 compute() 的最简单方法是什么（假设它们已经可并行化）？
我不需要与上述代码严格匹配的答案，只是一个一般性的答案。但如果您需要更多信息：我的任务是 IO 绑定的，这是针对 Spring Web 应用程序的，这些任务将在 HTTP 请求中执行。
【问题讨论】：
Result result = compute(object);吗？
标签： java multithreading parallel-processing
我建议您查看ExecutorService。
特别是这样的：
ExecutorService EXEC = Executors.newCachedThreadPool();
List<Callable<Result>> tasks = new ArrayList<Callable<Result>>();
for (final Object object: objects) {
Callable<Result> c = new Callable<Result>() {
@Override
public Result call() throws Exception {
return compute(object);
}
};
tasks.add(c);
}
List<Future<Result>> results = EXEC.invokeAll(tasks);
请注意，如果 objects 是一个大列表，则使用 newCachedThreadPool 可能会很糟糕。缓存的线程池可以为每个任务创建一个线程！您可能想要使用newFixedThreadPool(n)，其中 n 是合理的（例如您拥有的内核数量，假设 compute() 受 CPU 限制）。
以下是实际运行的完整代码：
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
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;
public class ExecutorServiceExample {
private static final Random PRNG = new Random();
private static class Result {
private final int wait;
public Result(int code) {
this.wait = code;
}
}
public static Result compute(Object obj) throws InterruptedException {
int wait = PRNG.nextInt(3000);
Thread.sleep(wait);
return new Result(wait);
}
public static void main(String[] args) throws InterruptedException,
ExecutionException {
List<Object> objects = new ArrayList<Object>();
for (int i = 0; i < 100; i++) {
objects.add(new Object());
}
List<Callable<Result>> tasks = new ArrayList<Callable<Result>>();
for (final Object object : objects) {
Callable<Result> c = new Callable<Result>() {
@Override
public Result call() throws Exception {
return compute(object);
}
};
tasks.add(c);
}
ExecutorService exec = Executors.newCachedThreadPool();
// some other exectuors you could try to see the different behaviours
// ExecutorService exec = Executors.newFixedThreadPool(3);
// ExecutorService exec = Executors.newSingleThreadExecutor();
try {
long start = System.currentTimeMillis();
List<Future<Result>> results = exec.invokeAll(tasks);
int sum = 0;
for (Future<Result> fr : results) {
sum += fr.get().wait;
System.out.println(String.format("Task waited %d ms",
fr.get().wait));
}
long elapsed = System.currentTimeMillis() - start;
System.out.println(String.format("Elapsed time: %d ms", elapsed));
System.out.println(String.format("... but compute tasks waited for total of %d ms; speed-up of %.2fx", sum, sum / (elapsed * 1d)));
} finally {
exec.shutdown();
}
}
}
【讨论】：
使用 Java8 及更高版本，您可以在集合上使用 parallelStream 来实现此目的：
List<T> objects = ...;
List<Result> result = objects.parallelStream().map(object -> {
return compute(object);
}).collect(Collectors.toList());
注意：结果列表的顺序可能与对象列表中的顺序不匹配。
此 stackoverflow 问题how-many-threads-are-spawned-in-parallelstream-in-java-8 中提供了如何设置正确线程数的详细信息
【讨论】：
可以简单地创建几个线程并得到结果。
Thread t = new Mythread(object);
if (t.done()) {
// get result
// add result
}
编辑：我认为其他解决方案更酷。
【讨论】：
如需更详细的答案，请阅读Java Concurrency in Practice 并使用java.util.concurrent。
【讨论】：
这是我在自己的项目中使用的东西：
public class ParallelTasks
{
private final Collection<Runnable> tasks = new ArrayList<Runnable>();
public ParallelTasks()
{
}
public void add(final Runnable task)
{
tasks.add(task);
}
public void go() throws InterruptedException
{
final ExecutorService threads = Executors.newFixedThreadPool(Runtime.getRuntime()
.availableProcessors());
try
{
final CountDownLatch latch = new CountDownLatch(tasks.size());
for (final Runnable task : tasks)
threads.execute(new Runnable() {
public void run()
{
try
{
task.run();
}
finally
{
latch.countDown();
}
}
});
latch.await();
}
finally
{
threads.shutdown();
}
}
}
// ...
public static void main(final String[] args) throws Exception
{
ParallelTasks tasks = new ParallelTasks();
final Runnable waitOneSecond = new Runnable() {
public void run()
{
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
}
}
};
tasks.add(waitOneSecond);
tasks.add(waitOneSecond);
tasks.add(waitOneSecond);
tasks.add(waitOneSecond);
final long start = System.currentTimeMillis();
tasks.go();
System.err.println(System.currentTimeMillis() - start);
}
在我的双核盒子上打印了超过 2000 个。
【讨论】：
您可以使用ThreadPoolExecutor。下面是示例代码：http://programmingexamples.wikidot.com/threadpoolexecutor（这里太长了，这里就不放了）
【讨论】：
Fork/Join 的并行数组是一种选择
【讨论】：
我要提到一个执行器类。下面是一些示例代码，您可以放置在 executor 类中。
private static ExecutorService threadLauncher = Executors.newFixedThreadPool(4);
private List<Callable<Object>> callableList = new ArrayList<Callable<Object>>();
public void addCallable(Callable<Object> callable) {
this.callableList.add(callable);
}
public void clearCallables(){
this.callableList.clear();
}
public void executeThreads(){
try {
threadLauncher.invokeAll(this.callableList);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public Object[] getResult() {
List<Future<Object>> resultList = null;
Object[] resultArray = null;
try {
resultList = threadLauncher.invokeAll(this.callableList);
resultArray = new Object[resultList.size()];
for (int i = 0; i < resultList.size(); i++) {
resultArray[i] = resultList.get(i).get();
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return resultArray;
}
然后要使用它，您需要调用执行器类来填充和执行它。
executor.addCallable( some implementation of callable) // do this once for each task
Object[] results = executor.getResult();
【讨论】：
一种巧妙的方法是利用 ExecutorCompletionService。
假设您有以下代码（如您的示例）：
public static void main(String[] args) {
List<Character> letters = IntStream.range(65, 91).mapToObj(i -> (char) i).collect(Collectors.toList());
List<List<Character>> list = new ArrayList<>();
for (char letter : letters) {
List<Character> result = computeLettersBefore(letter);
list.add(result);
}
System.out.println(list);
}
private static List<Character> computeLettersBefore(char letter) {
return IntStream.range(65, 1 + letter).mapToObj(i -> (char) i).collect(Collectors.toList());
}
现在要并行执行任务，您需要做的就是创建由线程池支持的 ExecutorCompletionService。然后提交任务并阅读结果。由于 ExecutorCompletionService 在后台使用 LinkedBlockingQueue，结果一旦可用就可以获取（如果您运行代码，您会注意到结果的顺序是随机的）：
public static void main(String[] args) throws InterruptedException, ExecutionException {
final ExecutorService threadPool = Executors.newFixedThreadPool(3);
final ExecutorCompletionService<List<Character>> completionService = new ExecutorCompletionService<>(threadPool);
final List<Character> letters = IntStream.range(65, 91).mapToObj(i -> (char) i).collect(Collectors.toList());
List<List<Character>> list = new ArrayList<>();
for (char letter : letters) {
completionService.submit(() -> computeLettersBefore(letter));
}
// NOTE: instead over iterating over letters again number of submitted tasks can be used as a base for loop
for (char letter : letters) {
final List<Character> result = completionService.take().get();
list.add(result);
}
threadPool.shutdownNow(); // NOTE: for safety place it inside finally block
System.out.println(list);
}
private static List<Character> computeLettersBefore(char letter) {
return IntStream.range(65, 1 + letter).mapToObj(i -> (char) i).collect(Collectors.toList());
}
【讨论】：