## Chapter 11. Concurrency(并發)
### Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器)
When many threads are runnable, the thread scheduler determines which ones get to run and for how long. Any reasonable operating system will try to make this determination fairly, but the policy can vary. Therefore, well-written programs shouldn’t depend on the details of this policy. **Any program that relies on the thread scheduler for correctness or performance is likely to be nonportable.**
當許多線程可以運行時,線程調度器決定哪些線程可以運行以及運行多長時間。任何合理的操作系統都會嘗試公平地做出這個決定,但是策略可能會有所不同。因此,編寫良好的程序不應該依賴于此策略的細節。**任何依賴線程調度器來保證正確性或性能的程序都可能是不可移植的。**
The best way to write a robust, responsive, portable program is to ensure that the average number of runnable threads is not significantly greater than the number of processors. This leaves the thread scheduler with little choice: it simply runs the runnable threads till they’re no longer runnable. The program’s behavior doesn’t vary too much, even under radically different thread-scheduling policies. Note that the number of runnable threads isn’t the same as the total number of threads, which can be much higher. Threads that are waiting are not runnable.
編寫健壯、響應快、可移植程序的最佳方法是確保可運行線程的平均數量不顯著大于處理器的數量。這使得線程調度器幾乎沒有選擇:它只運行可運行線程,直到它們不再可運行為止。即使在完全不同的線程調度策略下,程序的行為也沒有太大的變化。注意,可運行線程的數量與線程總數不相同,后者可能更高。正在等待的線程不可運行。
The main technique for keeping the number of runnable threads low is to have each thread do some useful work, and then wait for more. **Threads should not run if they aren’t doing useful work.** In terms of the Executor Framework (Item 80), this means sizing thread pools appropriately [Goetz06, 8.2] and keeping tasks short, but not too short, or dispatching overhead will harm performance.
保持可運行線程數量低的主要技術是讓每個線程做一些有用的工作,然后等待更多的工作。**如果線程沒有做有用的工作,它們就不應該運行。** 對于 Executor 框架([Item-80](/Chapter-11/Chapter-11-Item-80-Prefer-executors,-tasks,-and-streams-to-threads.md)),這意味著適當調整線程池的大小 [Goetz06, 8.2],并保持任務短小(但不要太短),否則分派開銷依然會損害性能。
Threads should not busy-wait, repeatedly checking a shared object waiting for its state to change. Besides making the program vulnerable to the vagaries of the thread scheduler, busy-waiting greatly increases the load on the processor, reducing the amount of useful work that others can accomplish. As an extreme example of what not to do, consider this perverse reimplementation of CountDownLatch:
線程不應該處于循環檢查共享對象狀態變化。除了使程序容易受到線程調度器變化無常的影響之外,循環檢查狀態變化還大大增加了處理器的負載,還影響其他線程獲取處理器進行工作。作為反面的極端例子,考慮一下 CountDownLatch 的不正確的重構實現:
```
// Awful CountDownLatch implementation - busy-waits incessantly!
public class SlowCountDownLatch {
private int count;
public SlowCountDownLatch(int count) {
if (count < 0)
throw new IllegalArgumentException(count + " < 0");
this.count = count;
}
public void await() {
while (true) {
synchronized(this) {
if (count == 0)
return;
}
}
}
public synchronized void countDown() {
if (count != 0)
count--;
}
}
```
On my machine, SlowCountDownLatch is about ten times slower than Java’s CountDownLatch when 1,000 threads wait on a latch. While this example may seem a bit far-fetched, it’s not uncommon to see systems with one or more threads that are unnecessarily runnable. Performance and portability are likely to suffer.
在我的機器上,當 1000 個線程等待一個鎖存器時,SlowCountDownLatch 的速度大約是 Java 的 CountDownLatch 的 10 倍。雖然這個例子看起來有點牽強,但是具有一個或多個不必要運行的線程的系統并不少見。性能和可移植性可能會受到影響。
When faced with a program that barely works because some threads aren’t getting enough CPU time relative to others, **resist the temptation to “fix” the program by putting in calls to Thread.yield.** You may succeed in getting the program to work after a fashion, but it will not be portable. The same yield invocations that improve performance on one JVM implementation might make it worse on a second and have no effect on a third. **Thread.yield has no testable semantics.** A better course of action is to restructure the application to reduce the number of concurrently runnable threads.
當面對一個幾乎不能工作的程序時,而原因是由于某些線程相對于其他線程沒有獲得足夠的 CPU 時間,那么 **通過調用 `Thread.yield` 來「修復」程序** 你也許能勉強讓程序運行起來,但它是不可移植的。在一個 JVM 實現上提高性能的相同的 yield 調用,在一些JVM 實現上可能會使性能變差,而在其他 JVM 實現上可能沒有任何影響。**`Thread.yield` 沒有可測試的語義。** 更好的做法是重構應用程序,以減少并發運行線程的數量。
A related technique, to which similar caveats apply, is adjusting thread priorities. **Thread priorities are among the least portable features of Java.** It is not unreasonable to tune the responsiveness of an application by tweaking a few thread priorities, but it is rarely necessary and is not portable. It is unreasonable to attempt to solve a serious liveness problem by adjusting thread priorities. The problem is likely to return until you find and fix the underlying cause.
一個相關的技術是調整線程優先級,類似的警告也適用于此技術,即,線程優先級是 Java 中最不可移植的特性之一。通過調整線程優先級來調優應用程序的響應性并非不合理,但很少情況下是必要的,而且不可移植。試圖通過調整線程優先級來解決嚴重的活性問題是不合理的。在找到并修復潛在原因之前,問題很可能會再次出現。
In summary, do not depend on the thread scheduler for the correctness of your program. The resulting program will be neither robust nor portable. As a corollary, do not rely on Thread.yield or thread priorities. These facilities are merely hints to the scheduler. Thread priorities may be used sparingly to improve the quality of service of an already working program, but they should never be used to “fix” a program that barely works.
總之,不要依賴線程調度器來判斷程序的正確性。生成的程序既不健壯也不可移植。因此,不要依賴 `Thread.yield` 或線程優先級。這些工具只是對調度器的提示。線程優先級可以少量地用于提高已經工作的程序的服務質量,但絕不應該用于「修復」幾乎不能工作的程序。
---
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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)