java - 几个Runnable完成后的返回值

Question

目前我有如下代码:

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();
}

我读过有关可用于将进程拆分为多个线程的 Runnable 对象，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

你要找的不是Runnable，而是一个Callable .与 Runnable 不同，Callable 返回一个值。这通常与 ExecutorService 一起使用(一个线程池)。

在线程池中维护您的线程总是好的，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。每次提交都会返回一个 Future目的。 Future 对象允许您等待已提交给 ExecutorService 的 Callable 实例的返回值。

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());
}