几个Runnable完成后的返回值

Question

目前我的代码如下：

public String getValue()
{
   StringBuilder builder = new StringBuilder();
   // 1st step
   //some logic to retreive code from database returning value
   //doing something with the returned value add it to collection
   builder.append(someString1);
   // 2nd step
   //some logic to retreive code from database returning value
   //doing something with the returned value add it to collection
   builder.append(someString2);
   // 3rd step
   //some logic to retreive code from database returning value
   //doing something with the returned value add it to collection
   builder.append(someString3);

   return builder.toString();
}

我已经阅读了有关可用于将进程拆分为多个线程的 Runnable 对象的信息，这会将我的代码更改为如下内容：

public String getValue()
{
     Thread firstTread = new Thread(new Process1());
     firstTread.start();
     Thread secondTread = new Thread(new Process1());
     secondTread.start();
     Thread thirdTread = new Thread(new Process1());
     thirdTread.start();

     // here i got confuse how to determine whether all thread allready finished 
     // before returning builder.toString();
}
//this is internal class
class Process1 implements Runnable
{
    public void run()
    {
      //do something and add to StringBuilder
    }
}

class Process2 implements Runnable
{
    public void run()
    {
      //do something and add to StringBuilder
    }
}

class Process3 implements Runnable
{
    public void run()
    {
      //do something and add to StringBuilder
    }
}

如何实现将进程拆分为多个线程的目标？

Answer 1

您使用 Callable，这几乎是一个 Runnable 返回值。而且您使用 ExecutorService 而不是自己创建 Thread 对象。

public String getValue()
{
    ExecutorService threaded = Executors.newFixedThreadPool(NUM_THREADS);
    List<CompletableFuture> results = threaded.invokeAll(
        Arrays.asList(this::process1, this::process2, this::process3);
    results.stream()
        .map(Future::get)  // collects the result from each thread
        .collect(Collectors.joining());
}
private String process1() { return "1"; }
private String process2() { return "2"; }
private String process3() { return "3"; }

编辑：请注意，输出不会是 "in different order" 因为 invokeAll returns "A list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list".

Answer 2

还有其他方法可以并行化您的工作，但要回答有关如何等待线程完成的直接问题，您可以使用

firstTread.join();
secondTread.join();
thirdTread.join();

Answer 3

Callable 更适合这样的任务，因为它是 returns 结果。

ExecutorService 有助于封装线程管理。

  public static void main(String[] args) throws Exception {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    List<Future<String>> futures = new ArrayList<>();
    futures.add(executorService.submit(new Process1()));
    futures.add(executorService.submit(new Process2()));
    futures.add(executorService.submit(new Process3()));

    StringBuilder sb = new StringBuilder();
    for (Future<String> future : futures) {
      // handle exceptions
      sb.append(future.get());
    }
  }

Answer 4

你要找的不是Runnable，而是一个Callable. Unlike Runnable, a Callable returns a value. This is usually used together with ExecutorService（线程池）。

在线程池中维护线程总是好的，而不是像那样手动生成它们。这可以防止创建不必要且昂贵的线程。这个想法是，不是调用 Thread.start()，而是将 Callable 的实例提交给具有预定义线程数的 ExecutorService 实例。每次提交 return 一个 Future 对象。 Future 对象允许您等待已提交给 ExecutorService 的 Callable 实例的 return 值。

这是您原始代码的修订版：

class Process1 implements Callable<String> {
    @Override
    public String call() throws Exception {
        return "Some string from this callable";
    }
}
// Insert implementation for Process2 and Process2 Callables
   ...

public static void main(String[] args) throws Exception {
   ExecutorService executor = Executors.newFixedThreadPool(3);

   Future<String> process1Future = executor.submit(new Process1());
   Future<String> process2Future = executor.submit(new Process2());
   Future<String> process3Future = executor.submit(new Process3());

   // It will wait here 
   String processedStringByProcess1 = process1Future.get();
   String processedStringByProcess2 = process2Future.get();
   String processedStringByProcess3 = process3Future.get();

   StringBuilder builder = new StringBuilder();
   builder.append(processedStringByProcess1);
   builder.append(processedStringByProcess2);
   builder.append(processedStringByProcess3);

   System.out.println(builder.toString());
}

几个Runnable完成后的返回值

Returning value after several Runnable finish

java

multithreading

runnable