## Chapter 9. General Programming(通用程序設計)
### Item 66: Use native methods judiciously(明智地使用本地方法)
The Java Native Interface (JNI) allows Java programs to call native methods, which are methods written in native programming languages such as C or C++. Historically, native methods have had three main uses. They provide access to platform-specific facilities such as registries. They provide access to existing libraries of native code, including legacy libraries that provide access to legacy data. Finally, native methods are used to write performance-critical parts of applications in native languages for improved performance.
Java 本地接口(JNI)允許 Java 程序調用本地方法,這些方法是用 C 或 C++ 等本地編程語言編寫的。從歷史上看,本地方法主要有三種用途。它們提供對特定于平臺的設施(如注冊中心)的訪問。它們提供對現有本地代碼庫的訪問,包括提供對遺留數據訪問。最后,本地方法可以通過本地語言編寫應用程序中注重性能的部分,以提高性能。
It is legitimate to use native methods to access platform-specific facilities, but it is seldom necessary: as the Java platform matured, it provided access to many features previously found only in host platforms. For example, the process API, added in Java 9, provides access to OS processes. It is also legitimate to use native methods to use native libraries when no equivalent libraries are available in Java.
使用本地方法訪問特定于平臺的機制是合法的,但是很少有必要:隨著 Java 平臺的成熟,它提供了對許多以前只能在宿主平臺中上找到的特性。例如,Java 9 中添加的流 API 提供了對 OS 流程的訪問。在 Java 中沒有等效庫時,使用本地方法來使用本地庫也是合法的。
**It is rarely advisable to use native methods for improved performance.** In early releases (prior to Java 3), it was often necessary, but JVMs have gotten much faster since then. For most tasks, it is now possible to obtain comparable performance in Java. For example, when java.math was added in release 1.1, BigInteger relied on a then-fast multiprecision arithmetic library written in C. In Java 3, BigInteger was reimplemented in Java, and carefully tuned to the point where it ran faster than the original native implementation.
**為了提高性能,很少建議使用本地方法。** 在早期版本(Java 3 之前),這通常是必要的,但是從那時起 JVM 變得更快了。對于大多數任務,現在可以在 Java 中獲得類似的性能。例如,在版本 1.1 中添加了 `java.math`,BigInteger 是在一個用 C 編寫的快速多精度運算庫的基礎上實現的。在當時,為了獲得足夠的性能這樣做是必要的。在 Java 3 中,BigInteger 則完全用 Java 重寫了,并且進行了性能調優,新的版本比原來的版本更快。
A sad coda to this story is that BigInteger has changed little since then, with the exception of faster multiplication for large numbers in Java 8. In that time, work continued apace on native libraries, notably GNU Multiple Precision arithmetic library (GMP). Java programmers in need of truly high-performance multiprecision arithmetic are now justified in using GMP via native methods [Blum14].
這個故事的一個可悲的結尾是,除了在 Java 8 中對大數進行更快的乘法運算之外,BigInteger 此后幾乎沒有發生什么變化。在此期間,對本地庫的工作繼續快速進行,尤其是 GNU 多精度算術庫(GMP)。需要真正高性能多精度算法的 Java 程序員現在可以通過本地方法使用 GMP [Blum14]。
The use of native methods has serious disadvantages. Because native languages are not safe (Item 50), applications using native methods are no longer immune to memory corruption errors. Because native languages are more platform-dependent than Java, programs using native methods are less portable. They are also harder to debug. If you aren’t careful, native methods can decrease performance because the garbage collector can’t automate, or even track, native memory usage (Item 8), and there is a cost associated with going into and out of native code. Finally, native methods require “glue code” that is difficult to read and tedious to write.
使用本地方法有嚴重的缺點。由于本地語言不安全([Item-50](/Chapter-8/Chapter-8-Item-50-Make-defensive-copies-when-needed.md)),使用本地方法的應用程序不再能免受內存毀壞錯誤的影響。由于本地語言比 Java 更依賴于平臺,因此使用本地方法的程序的可移植性較差。它們也更難調試。如果不小心,本地方法可能會降低性能,因為垃圾收集器無法自動跟蹤本地內存使用情況([Item-8](/Chapter-2/Chapter-2-Item-8-Avoid-finalizers-and-cleaners.md)),而且進出本地代碼會產生相關的成本。最后,本地方法需要「粘合代碼」,這很難閱讀,而且編寫起來很乏味。
In summary, think twice before using native methods. It is rare that you need to use them for improved performance. If you must use native methods to access low-level resources or native libraries, use as little native code as possible and test it thoroughly. A single bug in the native code can corrupt your entire application.
總之,在使用本地方法之前要三思。一般很少需要使用它們來提高性能。如果必須使用本地方法來訪問底層資源或本地庫,請盡可能少地使用本地代碼,并對其進行徹底的測試。本地代碼中的一個錯誤就可以破壞整個應用程序。
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**[Back to contents of the chapter(返回章節目錄)](/Chapter-9/Chapter-9-Introduction.md)**
- **Previous Item(上一條目):[Item 65: Prefer interfaces to reflection(接口優于反射)](/Chapter-9/Chapter-9-Item-65-Prefer-interfaces-to-reflection.md)**
- **Next Item(下一條目):[Item 67: Optimize judiciously(明智地進行優化)](/Chapter-9/Chapter-9-Item-67-Optimize-judiciously.md)**
- 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)