java多线程程序设计二、线程返回值，优先级以及暂停的问题

1、待返回值的任务的建立需要实现的是Callable接口，实现其call方法，而且这个任务只能通过ExecutorService 的submit方法进行调用，且返回值是一个Future对象，下面看一个例子：

package multiThread;

import java.util.concurrent.Callable;
//带返回值的任务

public class TaskWithResult implements Callable<String> {
private  int id;
public TaskWithResult(int id){
this.id=id;
}

public String call(){
return "TaskWithResult "+id;
}

}

//test

ExecutorService exec=Executors.newCachedThreadPool();
ArrayList<Future<String>> results=new ArrayList<Future<String>>();
for(int i=0;i<10;++i){
results.add(exec.submit(new TaskWithResult(i)));
}
for (Future<String> future : results) {
try {
System.out.println(future.get());
} catch (InterruptedException ie) {
System.out.println(ie.getMessage());
}finally{
exec.shutdown();
}
}

结果如下：



2、我们可以调用sleep方法让线程休眠一会例如：

public void run(){
while(countDown-->0){
System.out.println(status());
//Thread.yield();
try {
TimeUnit.MILLISECONDS.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

注意：在此处发生的异常不能够通过跨越线程传达到main方法中，所以我们需要在这里进行必要的捕获和处理。
3、线程的优先级
优先级的设置一般会在run函数里面进行
例如：

package multiThread;

public class SimplePriorities implements Runnable {
private int countDown=5;
private volatile double d;//no optimization
private int priority;
public SimplePriorities(int priority){
this.priority=priority;
}
public String toString(){
return Thread.currentThread()+": "+countDown;
}

public void run(){
Thread.currentThread().setPriority(priority);

while(true){
for(int i=1;i<10000;++i){
d+=(Math.PI+Math.E)/(double)i;
if(i%1000==0)
Thread.yield();//suggest other threads to run
}
System.out.println(this);
if(--countDown==0)return;
}
}

}
ExecutorService executorService=Executors.newCachedThreadPool();
for(int i=0;i<5;++i){
executorService.execute(new SimplePriorities(Thread.MIN_PRIORITY));
}
executorService.execute(new SimplePriorities(Thread.MAX_PRIORITY));
executorService.shutdown();