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                ## Chapter 11. Concurrency(并發) ### Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流優于直接使用線程) The first edition of this book contained code for a simple work queue [Bloch01, Item 49]. This class allowed clients to enqueue work for asynchronous processing by a background thread. When the work queue was no longer needed, the client could invoke a method to ask the background thread to terminate itself gracefully after completing any work that was already on the queue. The implementation was little more than a toy, but even so, it required a full page of subtle, delicate code, of the sort that is prone to safety and liveness failures if you don’t get it just right. Luckily, there is no reason to write this sort of code anymore. 本書的第一版包含一個簡單工作隊列的代碼 [Bloch01, Item 49]。這個類允許客戶端通過后臺線程為異步處理排隊。當不再需要工作隊列時,客戶端可以調用一個方法,要求后臺線程在完成隊列上的任何工作后優雅地終止自己。這個實現只不過是一個玩具,但即便如此,它也需要一整頁的代碼,如果你做得不對,就很容易出現安全和活性失敗。幸運的是,沒有理由再編寫這種代碼了。 By the time the second edition of this book came out, java.util.concurrent had been added to Java. This package contains an Executor Framework, which is a flexible interface-based task execution facility. Creating a work queue that is better in every way than the one in the first edition of this book requires but a single line of code: 當這本書的第二版出版時,`java.util.concurrent` 已經添加到 Java 中。這個包有一個 Executor 框架,它是一個靈活的基于接口的任務執行工具。創建一個工作隊列,它在任何方面都比在這本書的第一版更好,只需要一行代碼: ``` ExecutorService exec = Executors.newSingleThreadExecutor(); Here is how to submit a runnable for execution: exec.execute(runnable); And here is how to tell the executor to terminate gracefully (if you fail to do this,it is likely that your VM will not exit): exec.shutdown(); ``` You can do many more things with an executor service. For example, you can wait for a particular task to complete (with the get method, as shown in Item 79, page 319), you can wait for any or all of a collection of tasks to complete (using the invokeAny or invokeAll methods), you can wait for the executor service to terminate (using the awaitTermination method), you can retrieve the results of tasks one by one as they complete (using an ExecutorCompletionService), you can schedule tasks to run at a particular time or to run periodically (using a ScheduledThreadPoolExecutor), and so on. 你可以使用 executor 服務做更多的事情。例如,你可以等待一個特定任務完成(使用 get 方法,參見 [Item-79](/Chapter-11/Chapter-11-Item-79-Avoid-excessive-synchronization.md),319 頁),你可以等待任務集合中任何或全部任務完成(使用 invokeAny 或 invokeAll 方法),你可以等待 executor 服務終止(使用 awaitTermination 方法),你可以一個接一個檢索任務,獲取他們完成的結果(使用一個 ExecutorCompletionService),還可以安排任務在特定時間運行或定期運行(使用 ScheduledThreadPoolExecutor),等等。 If you want more than one thread to process requests from the queue, simply call a different static factory that creates a different kind of executor service called a thread pool. You can create a thread pool with a fixed or variable number of threads. The java.util.concurrent.Executors class contains static factories that provide most of the executors you’ll ever need. If, however, you want something out of the ordinary, you can use the ThreadPoolExecutor class directly. This class lets you configure nearly every aspect of a thread pool’s operation. 如果希望多個線程處理來自隊列的請求,只需調用一個不同的靜態工廠,該工廠創建一種稱為線程池的不同類型的 executor 服務。你可以使用固定或可變數量的線程創建線程池。`java.util.concurrent.Executors` 類包含靜態工廠,它們提供你需要的大多數 executor。但是,如果你想要一些不同尋常的東西,你可以直接使用 ThreadPoolExecutor 類。這個類允許你配置線程池操作的幾乎每個方面。 Choosing the executor service for a particular application can be tricky. For a small program, or a lightly loaded server, Executors.newCachedThreadPool is generally a good choice because it demands no configuration and generally “does the right thing.” But a cached thread pool is not a good choice for a heavily loaded production server! In a cached thread pool, submitted tasks are not queued but immediately handed off to a thread for execution. If no threads are available, a new one is created. If a server is so heavily loaded that all of its CPUs are fully utilized and more tasks arrive, more threads will be created, which will only make matters worse. Therefore, in a heavily loaded production server, you are much better off using Executors.newFixedThreadPool, which gives you a pool with a fixed number of threads, or using the ThreadPoolExecutor class directly, for maximum control. 為特定的應用程序選擇 executor 服務可能比較棘手。對于小程序或負載較輕的服務器,`Executors.newCachedThreadPool` 通常是一個不錯的選擇,因為它不需要配置,而且通常「做正確的事情」。但是對于負載沉重的生產服務器來說,緩存的線程池不是一個好的選擇!在緩存的線程池中,提交的任務不會排隊,而是立即傳遞給線程執行。如果沒有可用的線程,則創建一個新的線程。如果服務器負載過重,所有 CPU 都被充分利用,并且有更多的任務到達,就會創建更多的線程,這只會使情況變得更糟。因此,在負載沉重的生產服務器中,最好使用 `Executors.newFixedThreadPool`,它為你提供一個線程數量固定的池,或者直接使用 ThreadPoolExecutor 類來實現最大限度的控制。 Not only should you refrain from writing your own work queues, but you should generally refrain from working directly with threads. When you work directly with threads, a Thread serves as both a unit of work and the mechanism for executing it. In the executor framework, the unit of work and the execution mechanism are separate. The key abstraction is the unit of work, which is the task. There are two kinds of tasks: Runnable and its close cousin, Callable (which is like Runnable, except that it returns a value and can throw arbitrary exceptions). The general mechanism for executing tasks is the executor service. If you think in terms of tasks and let an executor service execute them for you, you gain the flexibility to select an appropriate execution policy to meet your needs and to change the policy if your needs change. In essence, the Executor Framework does for execution what the Collections Framework did for aggregation. 你不僅應該避免編寫自己的工作隊列,而且通常還應該避免直接使用線程。當你直接使用線程時,線程既是工作單元,又是執行它的機制。在 executor 框架中,工作單元和執行機制是分開的。關鍵的抽象是工作單元,即任務。有兩種任務:Runnable 和它的近親 Callable(與 Runnable 類似,只是它返回一個值并可以拋出任意異常)。執行任務的一般機制是 executor 服務。如果你從任務的角度考慮問題,并讓 executor 服務為你執行這些任務,那么你就可以靈活地選擇合適的執行策略來滿足你的需求,并在你的需求發生變化時更改策略。本質上,Executor 框架執行的功能與 Collections 框架聚合的功能相同。 In Java 7, the Executor Framework was extended to support fork-join tasks, which are run by a special kind of executor service known as a fork-join pool. A fork-join task, represented by a ForkJoinTask instance, may be split up into smaller subtasks, and the threads comprising a ForkJoinPool not only process these tasks but “steal” tasks from one another to ensure that all threads remain busy, resulting in higher CPU utilization, higher throughput, and lower latency. Writing and tuning fork-join tasks is tricky. Parallel streams (Item 48) are written atop fork join pools and allow you to take advantage of their performance benefits with little effort, assuming they are appropriate for the task at hand. 在 Java 7 中,Executor 框架被擴展為支持 fork-join 任務,這些任務由一種特殊的 Executor 服務(稱為 fork-join 池)運行。由 ForkJoinTask 實例表示的 fork-join 任務可以劃分為更小的子任務,由 ForkJoinPool 組成的線程不僅處理這些任務,而且還從其他線程「竊取」任務,以確保所有線程都處于繁忙狀態,從而提高 CPU 利用率、更高的吞吐量和更低的延遲。編寫和調優 fork-join 任務非常棘手。并行流([Item-48](/Chapter-7/Chapter-7-Item-48-Use-caution-when-making-streams-parallel.md)) 是在 fork 連接池之上編寫的,假設它們適合當前的任務,那么你可以輕松地利用它們的性能優勢。 A complete treatment of the Executor Framework is beyond the scope of this book, but the interested reader is directed to Java Concurrency in Practice [Goetz06]. 對 Executor 框架的完整處理超出了本書的范圍,但是感興趣的讀者可以在實踐中可以參閱《Java Concurrency in Practice》 [Goetz06]。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-11/Chapter-11-Introduction.md)** - **Previous Item(上一條目):[Item 79: Avoid excessive synchronization(避免過度同步)](/Chapter-11/Chapter-11-Item-79-Avoid-excessive-synchronization.md)** - **Next Item(下一條目):[Item 81: Prefer concurrency utilities to wait and notify(并發實用工具優于 wait 和 notify)](/Chapter-11/Chapter-11-Item-81-Prefer-concurrency-utilities-to-wait-and-notify.md)**
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