走近Guava(四): 并发

并发:

  • Guava集合处理是很强大的(这些在jdk8中都有些引入),但Guava发光的地方是并发

Monitor

  • Monitor实现同步
/**
 * 通过Monitor的Guard进行条件阻塞
 */


public class MonitorSample {
	private List<String> list = new ArrayList<String>();
	private static final int MAX_SIZE = 10;
	private Monitor monitor = new Monitor();
	
	private Monitor.Guard listBelowCapacity = new Monitor.Guard(monitor) {
		@Override
		public boolean isSatisfied() {
			return list.size() < MAX_SIZE;
		}
	};

	public void addToList(String item) throws InterruptedException {
		monitor.enterWhen(listBelowCapacity); //Guard(形如Condition),不满足则阻塞,而且我们并没有在Guard进行任何通知操作
		try {
			list.add(item);
		} finally {
			monitor.leave();
		}
	}
}
  • Monitor就像java本土的synchronized, ReentrantLock一样,每次只运行一个线程占用,且可重占用,每一次占用会对应一次退出占用。

Monitor最佳实践:

  • 就如上面,我们通过if条件来判断是否可进入Monitor代码块,并再try/finally中释放:
if (monitor.enterIf(guardCondition)) {
        try {
              doWork();
    } finally {
           monitor.leave();
       }
}

其他的Monitor访问方法:

Monitor.enter //进入Monitor块,将阻塞其他线程知道Monitor.leave
Monitor.tryEnter //尝试进入Monitor块,true表示可以进入, false表示不能,并且不会一直阻塞
Monitor.tryEnterIf //根据条件尝试进入Monitor块

这些方法都有对应的限时版本。

ListenableFuture类

  • jdk5之后有了Future这种异步执行的结构
ExecutorService executor = Executors.newCachedThreadPool();
   Future<Integer> future = executor.submit(new Callable<Integer>(){
                                public Integer call() throws Exception{
                                   return service.getCount();
} });
//Retrieve the value of computation
Integer count = future.get();
  • ListenableFuture对Future进行了扩展,允许注册一个回调函数,task执行完后自动调用。
  • 获取ListableFuture对象。

正如我们获取Future对象要通过ExecutorService.submit(Callable)来获取一样,我们可以这样创建ListenableFuture对象:

executorService = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(NUM_THREADS)); //包装Executors创建的线程池
ListenableFuture<String> listenableFuture = executorService.submit(new Callable<String>()...); //获取ListableFuture对象
listenableFuture.addListener(new Runnable() {
      @Override
      public void run() {
          methodToRunOnFutureTaskCompletion();
      }
}, executorService); //注册回调函数

FutureCallback类

  • FutureCallback定义了onSuccessonFailure方法,onSuccess方法会接收一个Future对象,这样我们就可以获取Future的结果。
  • 首先需要一个FutureCallback实现类。
/**
 * 定义一个FutureCallBack实现类
 */
public class FutureCallbackImpl implements FutureCallback<String> {
	private StringBuilder builder = new StringBuilder();

	@Override
	public void onSuccess(String result) {
		builder.append(result).append(" successfully");
	}

	@Override
	public void onFailure(Throwable t) {
		builder.append(t.toString());
	}

	public String getCallbackResult() {
		return builder.toString();
	}
}

使用实例:

ListeningExecutorService executorService = MoreExecutors.listeningDecorator(Executors.newCachedThreadPool());
ListenableFuture<String> futureTask = executorService.submit(new Callable<String>() { //创建ListenaleFuture对象
					@Override
					public String call() throws Exception {
						return "Task completed";
					}
				});
FutureCallbackImpl callback = new FutureCallbackImpl();
Futures.addCallback(futureTask, callback); //添加回调
callback.getCallbackResult(); //获取结果

如果CallBack是一个耗时操作,你应该选择另一个注册CallBack:

Futures.addCallback(futureTask,callback,executorService); //提供另一个线程池来执行性回调

SettableFuture类:

SettableFuture可以用来设置要返回得值:

 

SettableFuture<String> sf = SettableFuture.create();
//Set a value to return
sf.set("Success");
//Or set a failure Exception
sf.setException(someException);

AsyncFunction:

  • 该接口与函数式编程密切相关, 类似Function, 但apply方法会转换成一个ListenableFuture封装的范型对象。
public class AsyncFuntionSample implements AsyncFunction<Long, String> {
	private ConcurrentMap<Long, String> map = Maps.newConcurrentMap();
	private ListeningExecutorService listeningExecutorService;

	@Override
	public ListenableFuture<String> apply(final Long input) throws Exception {
		if (map.containsKey(input)) {
			SettableFuture<String> listenableFuture = SettableFuture.create(); //构建一个SettableFuture
			listenableFuture.set(map.get(input));
			return listenableFuture;
		} else {
			return listeningExecutorService.submit(new Callable<String>() {
				@Override
				public String call() throws Exception {
					String retrieved = //compute to get the data;
					map.putIfAbsent(input, retrieved);
					return retrieved;
				}
			});
		}
	}
}

FutureFallback类:

  • FutureFallback用于异常恢复的备份。
/**
 * 当Future任务失败后, 作为备份的Future
 */
public class FutureFallbackImpl implements FutureFallback<String> {
	@Override
	public ListenableFuture<String> create(Throwable t) throws Exception {
		if (t instanceof FileNotFoundException) {
			SettableFuture<String> settableFuture = SettableFuture.create();
			settableFuture.set("Not Found");
			return settableFuture;
		}
		throw new Exception(t);
	}
}

Futures类:

  • Futures类是有关Future实例的一个工具类。

异步转换:

ListenableFuture<Person> lf = Futures.transform(ListenableFuture<String> f,AsyncFunction<String,Person> af);

使用FutureFallbacks:

ListenableFuture<String> lf = Futures.withFallback(ListenableFuture<String> f,FutureFallback<String> fb);

RateLimiter:

  • RateLimiter限制访问每秒访问资源的线程数。有点类似信号量Semaphore。
RateLimiter limiter = RateLimiter.create(4.0); //每秒不超过4个任务被提交
limiter.acquire();  //请求RateLimiter, 超过permits会被阻塞
executor.submit(runnable); //提交任务

也有非阻塞式地尝试:

If(limiter.tryAcquire()){ //未请求到limiter则立即返回false
    doSomething();
}else{
    doSomethingElse();
}

来源:http://my.oschina.net/indestiny/blog/219368

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