## 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]。
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- Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
- Item 1: Consider static factory methods instead of constructors(考慮以靜態工廠方法代替構造函數)
- Item 2: Consider a builder when faced with many constructor parameters(在面對多個構造函數參數時,請考慮構建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有構造函數或枚舉類型實施單例屬性)
- Item 4: Enforce noninstantiability with a private constructor(用私有構造函數實施不可實例化)
- Item 5: Prefer dependency injection to hardwiring resources(依賴注入優于硬連接資源)
- Item 6: Avoid creating unnecessary objects(避免創建不必要的對象)
- Item 7: Eliminate obsolete object references(排除過時的對象引用)
- Item 8: Avoid finalizers and cleaners(避免使用終結器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 優于 try-finally)
- Chapter 3. Methods Common to All Objects(對象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆蓋 equals 方法時應遵守的約定)
- Item 11: Always override hashCode when you override equals(當覆蓋 equals 方法時,總要覆蓋 hashCode 方法)
- Item 12: Always override toString(始終覆蓋 toString 方法)
- Item 13: Override clone judiciously(明智地覆蓋 clone 方法)
- Item 14: Consider implementing Comparable(考慮實現 Comparable 接口)
- Chapter 4. Classes and Interfaces(類和接口)
- Item 15: Minimize the accessibility of classes and members(盡量減少類和成員的可訪問性)
- Item 16: In public classes use accessor methods not public fields(在公共類中,使用訪問器方法,而不是公共字段)
- Item 17: Minimize mutability(減少可變性)
- Item 18: Favor composition over inheritance(優先選擇復合而不是繼承)
- Item 19: Design and document for inheritance or else prohibit it(繼承要設計良好并且具有文檔,否則禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口優于抽象類)
- Item 21: Design interfaces for posterity(為后代設計接口)
- Item 22: Use interfaces only to define types(接口只用于定義類型)
- Item 23: Prefer class hierarchies to tagged classes(類層次結構優于帶標簽的類)
- Item 24: Favor static member classes over nonstatic(靜態成員類優于非靜態成員類)
- Item 25: Limit source files to a single top level class(源文件僅限有單個頂層類)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始類型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 優于數組)
- Item 29: Favor generic types(優先使用泛型)
- Item 30: Favor generic methods(優先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的靈活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可變參數)
- Item 33: Consider typesafe heterogeneous containers(考慮類型安全的異構容器)
- Chapter 6. Enums and Annotations(枚舉和注解)
- Item 34: Use enums instead of int constants(用枚舉類型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用實例字段替代序數)
- Item 36: Use EnumSet instead of bit fields(用 EnumSet 替代位字段)
- Item 37: Use EnumMap instead of ordinal indexing(使用 EnumMap 替換序數索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模擬可擴展枚舉)
- Item 39: Prefer annotations to naming patterns(注解優于命名模式)
- Item 40: Consistently use the Override annotation(堅持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用標記接口定義類型)
- Chapter 7. Lambdas and Streams(λ 表達式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表達式優于匿名類)
- Item 43: Prefer method references to lambdas(方法引用優于 λ 表達式)
- Item 44: Favor the use of standard functional interfaces(優先使用標準函數式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用無副作用的函數)
- Item 47: Prefer Collection to Stream as a return type(優先選擇 Collection 而不是流作為返回類型)
- Item 48: Use caution when making streams parallel(謹慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(檢查參數的有效性)
- Item 50: Make defensive copies when needed(在需要時制作防御性副本)
- Item 51: Design method signatures carefully(仔細設計方法簽名)
- Item 52: Use overloading judiciously(明智地使用重載)
- Item 53: Use varargs judiciously(明智地使用可變參數)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或數組,而不是 null)
- Item 55: Return optionals judiciously(明智地的返回 Optional)
- Item 56: Write doc comments for all exposed API elements(為所有公開的 API 元素編寫文檔注釋)
- Chapter 9. General Programming(通用程序設計)
- Item 57: Minimize the scope of local variables(將局部變量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循環優于傳統的 for 循環)
- Item 59: Know and use the libraries(了解并使用庫)
- Item 60: Avoid float and double if exact answers are required(若需要精確答案就應避免使用 float 和 double 類型)
- Item 61: Prefer primitive types to boxed primitives(基本數據類型優于包裝類)
- Item 62: Avoid strings where other types are more appropriate(其他類型更合適時應避免使用字符串)
- Item 63: Beware the performance of string concatenation(當心字符串連接引起的性能問題)
- Item 64: Refer to objects by their interfaces(通過接口引用對象)
- Item 65: Prefer interfaces to reflection(接口優于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地進行優化)
- Item 68: Adhere to generally accepted naming conventions(遵守被廣泛認可的命名約定)
- Chapter 10. Exceptions(異常)
- Item 69: Use exceptions only for exceptional conditions(僅在確有異常條件下使用異常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(對可恢復情況使用 checked 異常,對編程錯誤使用運行時異常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 異常)
- Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)
- Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)
- Item 74: Document all exceptions thrown by each method(為每個方法記錄會拋出的所有異常)
- Item 75: Include failure capture information in detail messages(異常詳細消息中應包含捕獲失敗的信息)
- Item 76: Strive for failure atomicity(盡力保證故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略異常)
- Chapter 11. Concurrency(并發)
- Item 78: Synchronize access to shared mutable data(對共享可變數據的同步訪問)
- Item 79: Avoid excessive synchronization(避免過度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流優于直接使用線程)
- Item 81: Prefer concurrency utilities to wait and notify(并發實用工具優于 wait 和 notify)
- Item 82: Document thread safety(文檔應包含線程安全屬性)
- Item 83: Use lazy initialization judiciously(明智地使用延遲初始化)
- Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(優先選擇 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常謹慎地實現 Serializable)
- Item 87: Consider using a custom serialized form(考慮使用自定義序列化形式)
- Item 88: Write readObject methods defensively(防御性地編寫 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(對于實例控制,枚舉類型優于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考慮以序列化代理代替序列化實例)