## Chapter 10. Exceptions(異常)
### Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)
It is disconcerting when a method throws an exception that has no apparent connection to the task that it performs. This often happens when a method propagates an exception thrown by a lower-level abstraction. Not only is it disconcerting, but it pollutes the API of the higher layer with implementation details. If the implementation of the higher layer changes in a later release, the exceptions it throws will change too, potentially breaking existing client programs.
當一個方法拋出一個與它所執行的任務沒有明顯關聯的異常時,這是令人不安的。這種情況經常發生在由方法傳播自低層抽象拋出的異常。它不僅令人不安,而且讓實現細節污染了上層的 API。如果高層實現在以后的版本中發生變化,那么它拋出的異常也會發生變化,可能會破壞現有的客戶端程序。
To avoid this problem, **higher layers should catch lower-level exceptions and, in their place, throw exceptions that can be explained in terms of the higher-level abstraction.** This idiom is known as exception translation:
為了避免這個問題,**高層應該捕獲低層異常,并確保拋出的異常可以用高層抽象解釋。** 這個習慣用法稱為異常轉換:
```
// Exception Translation
try {
... // Use lower-level abstraction to do our bidding
} catch (LowerLevelException e) {
throw new HigherLevelException(...);
}
```
Here is an example of exception translation taken from the AbstractSequentialList class, which is a skeletal implementation (Item 20) of the List interface. In this example, exception translation is mandated by the specification of the get method in the `List<E>` interface:
下面是來自 AbstractSequentialList 類的異常轉換示例,該類是 List 接口的一個框架實現([Item-20](/Chapter-4/Chapter-4-Item-20-Prefer-interfaces-to-abstract-classes.md))。在本例中,異常轉換是由 `List<E>` 接口中的 get 方法規范強制執行的:
```
/**
* Returns the element at the specified position in this list.
* @throws IndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()}).
*/
public E get(int index) {
ListIterator<E> i = listIterator(index);
try {
return i.next();
}
catch (NoSuchElementException e) {
throw new IndexOutOfBoundsException("Index: " + index);
}
}
```
A special form of exception translation called exception chaining is called for in cases where the lower-level exception might be helpful to someone debugging the problem that caused the higher-level exception. The lower-level exception (the cause) is passed to the higher-level exception, which provides an accessor method (Throwable’s getCause method) to retrieve the lower-level exception:
如果低層異常可能有助于調試高層異常的問題,則需要一種稱為鏈式異常的特殊異常轉換形式。低層異常(作為原因)傳遞給高層異常,高層異常提供一個訪問器方法(Throwable 的 getCause 方法)來檢索低層異常:
```
// Exception Chaining
try {
... // Use lower-level abstraction to do our bidding
}
catch (LowerLevelException cause) {
throw new HigherLevelException(cause);
}
```
The higher-level exception’s constructor passes the cause to a chaining-aware superclass constructor, so it is ultimately passed to one of Throwable’s chaining-aware constructors, such as Throwable(Throwable):
高層異常的構造函數將原因傳遞給能夠接收鏈式異常的超類構造函數,因此它最終被傳遞給 Throwable 的一個接收鏈式異常的構造函數,比如 `Throwable(Throwable)`:
```
// Exception with chaining-aware constructor
class HigherLevelException extends Exception {
HigherLevelException(Throwable cause) {
super(cause);
}
}
```
Most standard exceptions have chaining-aware constructors. For exceptions that don’t, you can set the cause using Throwable’s initCause method. Not only does exception chaining let you access the cause programmatically (with getCause), but it integrates the cause’s stack trace into that of the higher-level exception.
大多數標準異常都有接收鏈式異常的構造函數。對于不支持鏈式異常的異常,可以使用 Throwable 的 initCause 方法設置原因。異常鏈接不僅允許你以編程方式訪問原因(使用 getCause),而且還將原因的堆棧跟蹤集成到更高層異常的堆棧跟蹤中。
**While exception translation is superior to mindless propagation of exceptions from lower layers, it should not be overused.** Where possible, the best way to deal with exceptions from lower layers is to avoid them, by ensuring that lower-level methods succeed. Sometimes you can do this by checking the validity of the higher-level method’s parameters before passing them on to lower layers.
雖然異常轉換優于底層異常的盲目傳播,但它不應該被過度使用。在可能的情況下,處理低層異常的最佳方法是確保低層方法避免異常。有時,你可以在將高層方法的參數傳遞到低層之前檢查它們的有效性。
If it is impossible to prevent exceptions from lower layers, the next best thing is to have the higher layer silently work around these exceptions, insulating the caller of the higher-level method from lower-level problems. Under these circumstances, it may be appropriate to log the exception using some appropriate logging facility such as java.util.logging. This allows programmers to investigate the problem, while insulating client code and the users from it.
如果不可能從低層防止異常,那么下一個最好的方法就是讓高層靜默處理這些異常,使較高層方法的調用者免受低層問題的影響。在這種情況下,可以使用一些適當的日志工具(如 `java.util.logging`)來記錄異常。這允許程序員研究問題,同時將客戶端代碼和用戶與之隔離。
In summary, if it isn’t feasible to prevent or to handle exceptions from lower layers, use exception translation, unless the lower-level method happens to guarantee that all of its exceptions are appropriate to the higher level. Chaining provides the best of both worlds: it allows you to throw an appropriate higherlevel exception, while capturing the underlying cause for failure analysis (Item 75).
總之,如果無法防止或處理來自低層的異常,則使用異常轉換,但要保證低層方法的所有異常都適用于較高層。鏈式異常提供了兼顧兩方面的最佳服務:允許拋出適當的高層異常,同時捕獲并分析失敗的潛在原因([Item-75](/Chapter-10/Chapter-10-Item-75-Include-failure-capture-information-in-detail-messages.md))。
---
**[Back to contents of the chapter(返回章節目錄)](/Chapter-10/Chapter-10-Introduction.md)**
- **Previous Item(上一條目):[Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)](/Chapter-10/Chapter-10-Item-72-Favor-the-use-of-standard-exceptions.md)**
- **Next Item(下一條目):[Item 74: Document all exceptions thrown by each method(為每個方法記錄會拋出的所有異常)](/Chapter-10/Chapter-10-Item-74-Document-all-exceptions-thrown-by-each-method.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)