一、问题背景
Quite often, you need a class to perform tasks like data processing, listening to events, or checking another class' activities during the application’s lifetime. To achieve this, you probably use threads with a set of locks and notifications. Java Thread API is well documented, but you need a great deal of code and experience to make your thread work properly and efficiently. You can avoid writing such classes from scratch every time you need them and build a more robust application by applying the framework we'll discuss in this article.
在应用程序中我们经常需要一个类去完成像数据处理、监听事件或检查另一个类的活动等任务。为了达到这个目标，我们可能使用带有一套锁和消息通知的线程。JAVA 线程API已经很好的文档化，但为了使线程能够正确而高效地运行，程序员仍然需要丰富的编程经验并编写大量的代码。通过应用本篇文章中讨论的框架，程序员能够避免忍受煎熬写大量的代码，快速创建健壮的应用程序。
二、长时间运行任务的程序框架
Framework for long-running tasks
The primary thing about a long-lived task is that it should somehow be kept running during the application lifetime. The right way to accomplish this is to provide a thread of execution for a particular task. You create a task as a thread or as an implementation of the java.lang.Runnable interface. If you implement Runnable, you can gain better object-oriented design and avoid the single-inheritance problems. You can also more efficiently manipulate with Runnable instances, for example, using a thread pool that usually needs a Runnable instance, not a thread, to run.
关于长时间运行的任务的主要事情是如何在应用程序的生命期使它一直保持运行。实现的恰当方法是提供一个线程来执行这个特定的任务。我们可以通过继承Thread类或实现java.lang.Runnable接口来达到该目标。如果采用实现Runnable接口的方式，就可以能够获得更好的面向对象的设计，同时可以避免JAVA中的单继承问题。另外，我们也能更有效的处理Runnable实例（例如使用线程池通常需要一个Runnable实例而不是线程来运行）。
The essence of the framework is the abstract class Worker ( Listing A), which implements the Runnable interface and provides the helper methods for efficient task handling. Some of the methods are fully implemented, like the run() method, but some are abstract and have to be filled by you. If you want to create a long-running class, you need only to extend the Worker class and implement several abstract methods. Let’s look at these methods in more detail.
框架的基础是一个叫Worker的抽象类，它实现了Runnable接口，并提供了有效处理任务的好方法。这些方法有些已经被实现，如run()方法，但有些是抽象方法，开发人员必须自己来实现。如果要创建一个长时间运行的类，你只需要继承Worker类并实现几个抽象方法。让我们看看这些方法的细节。
The run() method of the Worker class is designed to continuously execute the work() method until it is stopped. The work() method can be responsible for data processing, reaction to some event, file reading or writing, SQL execution, etc. It can throw an exception, so it is a good practice to propagate it and let the run() method handle it.
Worker 类的run()方法被设计成只要不停止运行就持续的执行work()方法。work()方法可以负责数据处理、事件响应、文件读写、，执行SQL命令等操作。这样work()方法能够抛出异常，并将异常传给run()，然后由run()方法来处理这些异常。
The run() method has two levels of try-catch clause: outside and inside the while-loop. The first try-catch clause is meant to catch all nonprogrammed exceptions and guarantee that the run() method never exits. The second clause will catch any kind of exceptions belonging to business logic and behave accordingly. If some waiting operation takes place in the work() method (e.g., waiting on an InputStream or a Socket), it is advisable to propagate an InterruptedException. The thing to keep in mind is that the work() method does not need to have any while-loop to keep it going as long as an application runs. The Worker does this for you.
run()方法有内外两层try-catch语句：一层处于while-loop循环外，一层在while-loop循环内。前一个try-catch用于捕获非编程异常以确保run()方法不退出。后一个try-catch语句捕获关于业务逻辑和相应行为的各种异常。如果在work()方法中发生了一些等待操作（例如等待一个输入流或一个Socket），抛出一个InterruptedException的方法是可取的。要记住的是只要应用程序在运行，work()方法不需要任何while-loop循环去维持它运行，这一切由Worker代办了。
When the run() method starts, it calls the prepareWorker() which is designed to prepare all resources needed for a long-running task (Listing A). In this method call, you can, for example, establish a database connection or open a file that will be used further. It is especially good to place here some blocking operations like opening a socket, because they will be done in a separate thread and thus will not block the main thread of execution.
run()开始时，调用prepareWorker()方法来准备长时间运行任务需要的所有资源（参考程序清单A）。例如 ，在这个方法中可以打开一个将要用到的数据库连接或文件。尤其对于那些像建立一个socket这样的阻塞操作放在这儿是很好的。因为若让它们在一个独立的线程中运行，则不会阻塞主线程的执行。
The opposite of the previous method is the releaseWorker() which is called when the run() method is about to exit (Listing A). Here, you can put the code to dispose of system resources used by this task or to perform other cleanup. This method is similar to java.lang.Object.finalize(), but it is explicitly called before a thread terminates.
与前面方法相反的是releaseWorker()，它在run()方法准备退出时被调用（参考程序清单A）。在该方法中你可以编写那些释放系统资源或执行其它清除动作的代码。该方法类似于java.lang.Object.finalize()，但它在线程中止时被显式的调用。
三、框架中的错误处理机制
Handling errors in the framework
Another important method is the handleError(), which takes a java.lang.Throwable as a parameter. This method is called each time an error situation occurs within the run() method. It is up to you how to implement error handling. One way is to log errors and control task termination by calling halt() method (Listing A).
另一个重要的方法是handleError()，它带有一个java.lang.Throwable的输入参数。在run()方法每次发生错误时调用这个方法。这依赖于你怎么实现错误处理。方法之一是写错误日志并通过调用halt()方法中止任务（参考程序清单A）。
The isCondition() method is used to tell whether execution of the work() method can be started, thus allowing granular control over a task. It is useful in event-triggered frameworks when execution of the work() method is pending until some condition?for example, a buffer is not empty?is fulfilled. In Worker’s implementation, the condition is checked upon a lock notification and periodically with a time interval you specify in the setTimeout() method (Listing A). If you don’t need any waiting blocks in a task, just make the isCondition() method always return true.
isCondition()方法用于判断work()方法是否能够被执行。因此允许细粒度地控制任务。这在事件触发的框架中非常有用。当work()方法的执行条件未满足时，work方法将被挂起，直到条件完全满足（例如，缓存区非空）。在Worker的实现中这个条件将按在方法setTimeout()中指定的时间周期地检查一个锁通知。如果在任务中不需要任何等待阻塞，仅仅只要使isCondition()方法总是返回真值。
四、任务终止时机
When to terminate
You'll also need the isRunning(), broadcast(), and halt() methods. Querying isRunning(), you can check whether a task is still running and make a decision whether to terminate it. The broadcast() method just notifies the lock object and makes a task proceed if it has been waiting on this lock. The halt() method stops a task, so the run() method will exit as soon as the next isRunning() status is checked. Because this method notifies only one lock that may block this task’s thread, it is advisable to use t
