java并发编程(五)CompletionService

以你之姓@ 2023-07-19 04:55 1阅读 0赞

CompletionService可以在多个任务中依时间先后获取最先执行完成的任务结果。废话不说，直接上代码。

import java.util.concurrent.Callable;
    
    public class BlockCall extends Thread implements Callable<String>{
    	private int blocktime;
    	public BlockCall(String name,int blocktime) {
    		super.setName(name);
    		this.blocktime=blocktime;
    	}
    	public String call() throws Exception {
    		Thread.sleep(blocktime);
    		return super.getName();
    	}
    
    }

import java.text.SimpleDateFormat;
    import java.util.concurrent.CompletionService;
    import java.util.concurrent.ExecutorCompletionService;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.Future;
    
    public class TestService {
    	ExecutorService newCachedThreadPool = Executors.newCachedThreadPool();
    	final String strformat = "ThreadName:%s %s";
    	CompletionService service = null;
    	public static void main(String[] args) throws Exception {
    		//new TestService().notBlock();
    		new TestService().block();	
    	}
    	private void block() throws Exception{
    		initial();
    		SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    		int resultCount=0;
    		while(resultCount<4) {
    			resultCount++;
    			Object object = service.take().get();
    			System.out.println(object+" "+ dateFormat.format(System.currentTimeMillis()));
    		}
    		newCachedThreadPool.shutdown();
    	}
    	private void notBlock() throws Exception{
    		initial();
    		SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    		int resultCount=0;
    		while(resultCount<4) {
    			Future poll = service.poll();
    			Object object = "null ";
    			if(poll!=null) {
    				object=poll.get();
    				resultCount++;
    			}
    			Thread.sleep(200);
    			System.out.println(object+" "+ dateFormat.format(System.currentTimeMillis()));
    		}
    		newCachedThreadPool.shutdown();
    	}
    	private void initial() {
    		String[] names = new String[] {"A","B","C","D"};
    		Integer[] times = new Integer[] {4000,3000,2000,1000};
    		BlockCall[] calls = new BlockCall[4];
    		for(int i=0;i<calls.length;i++) {
    			calls[i] = new BlockCall(names[i],times[i]);
    		}
    		service = new ExecutorCompletionService<String>(newCachedThreadPool);
    		for(int i=0;i<calls.length;i++) {
    			service.submit(calls[i]);
    		}
    	}
    }

下面是take方式得到的执行结果。

D 2020-03-29 18:19:03.866
    C 2020-03-29 18:19:04.866
    B 2020-03-29 18:19:05.866
    A 2020-03-29 18:19:06.866

下面是poll方式得到的执行结果。

null  2020-03-29 18:24:45.033
    null  2020-03-29 18:24:45.235
    null  2020-03-29 18:24:45.435
    null  2020-03-29 18:24:45.635
    null  2020-03-29 18:24:45.835
    D 2020-03-29 18:24:46.035
    null  2020-03-29 18:24:46.235
    null  2020-03-29 18:24:46.435
    null  2020-03-29 18:24:46.635
    null  2020-03-29 18:24:46.835
    C 2020-03-29 18:24:47.035
    null  2020-03-29 18:24:47.235
    null  2020-03-29 18:24:47.435
    null  2020-03-29 18:24:47.635
    null  2020-03-29 18:24:47.835
    B 2020-03-29 18:24:48.035
    null  2020-03-29 18:24:48.235
    null  2020-03-29 18:24:48.435
    null  2020-03-29 18:24:48.635
    null  2020-03-29 18:24:48.835
    A 2020-03-29 18:24:49.035

结论：take和poll都可以获取到Future得到的值。但在使用时需要根据计算的任务迭代得到值。这个非常重要。否则程序直接跳过，只能得到最先获取的值。突然想到，如果有一个任务，可以由多种不同的方法计算出结果。这个方法倒是一个较好的选择。即只要程序得到第一个结果，后面就可以放弃，不用管它。程序继续向下运行。