面試常問問題——多線程,如何給run方法傳參，如何實現線程的返回值

如何給run方法傳參？

（1）構造函數傳參
（2） 成員變量傳參
（3） 回調函數傳參

如何實現處理線程的返回值？

（1）主線程等待法

public class CycleWait implements Runnable{
    private  String value;
    @Override
    public void run() {
        try{
            Thread.currentThread().sleep(5000);
        }catch (InterruptedException e) {
            e.printStackTrace();
        }
        value="we have data now";
    }

    public static void main(String[] args) throws InterruptedException {
        CycleWait cycleWait=new CycleWait();
        //給方法附上多線程的屬性。
        Thread thread =new Thread(cycleWait);
        thread.start();
        //cycleWait.value主線程執行到這裏的時候有可能子線程還沒有獲取到值，所以需要在值爲空的時候，讓主線程等待。
        int i=0;
        while (cycleWait.value==null){
            //主線程等待法
		        Thread.currentThread().sleep(100);
		         i++;
           		 System.out.println(i);
        }
        System.out.println("value="+cycleWait.value);
    }
}

運行發現需要循環50次即主線程等待50個100毫秒才能取到值，而如果最後一次主線程取到值了，但是主線程還在等待，會有資源佔用等其他問題。

（2）使用Thread類的join()方法，阻塞當前線程以等待子線程處理完畢

 public static void main(String[] args) throws InterruptedException {
        CycleWait cycleWait=new CycleWait();
        Thread thread =new Thread(cycleWait);
        thread.start();
        int i=0;
        //join阻塞法
        thread.join();
        System.out.println("value="+cycleWait.value);
    }

通過callable接口實現：通過FutureTask 或者線程池獲取

需要jdk是1.5以上。

public class MyCallable implements Callable {
    @Override
    public Object call() throws Exception {
        String value ="test";
        System.out.println("work ready to work");
        Thread.currentThread().sleep(5000);
        System.out.println("task done");
        return value;
    }
    }

調用時

   public static void main(String[] args) throws ExecutionException, InterruptedException {
        FutureTask<String>  futureTask= new FutureTask<String>(new MyCallable());
        new Thread(futureTask).start();
        if(!futureTask.isDone()){
            System.out.println("FutureTask is not done ,please wait!");
        }
        System.out.println("value = "+futureTask.get());
    }

線程池：

public class ThreadPoolDemo {
    public static void main(String[] args) {
       //線程池提交 mycallable的任務。
        ExecutorService executorService = Executors.newCachedThreadPool();
        //向線程池提交
        Future future = executorService.submit(new MyCallable());
        Future future1 =executorService.submit(new MyCallable());
        if(!future.isDone()){
            System.out.println("task has not finished,please wait");
        }
        try {
            System.out.println("value ="+future.get());
            System.out.println("value ="+future1.get());


        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }finally {
            //結束線程池
            executorService.shutdown();
        }
    }
}