## Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
### Item 5: Prefer dependency injection to hardwiring resources(依賴注入優于硬連接資源)
Many classes depend on one or more underlying resources. For example, a spell checker depends on a dictionary. It is not uncommon to see such classes implemented as static utility classes (Item 4):
許多類依賴于一個或多個底層資源。例如,拼寫檢查程序依賴于字典。常見做法是,將這種類實現為靜態實用工具類([Item-4](/Chapter-2/Chapter-2-Item-4-Enforce-noninstantiability-with-a-private-constructor.md)):
```
// Inappropriate use of static utility - inflexible & untestable!
public class SpellChecker {
private static final Lexicon dictionary = ...;
private SpellChecker() {} // Noninstantiable
public static boolean isValid(String word) { ... }
public static List<String> suggestions(String typo) { ... }
}
```
Similarly, it’s not uncommon to see them implemented as singletons (Item 3):
類似地,我們也經常看到它們的單例實現([Item-3](/Chapter-2/Chapter-2-Item-3-Enforce-the-singleton-property-with-a-private-constructor-or-an-enum-type.md)):
```
// Inappropriate use of singleton - inflexible & untestable!
public class SpellChecker {
private final Lexicon dictionary = ...;
private SpellChecker(...) {}
public static INSTANCE = new SpellChecker(...);
public boolean isValid(String word) { ... }
public List<String> suggestions(String typo) { ... }
}
```
Neither of these approaches is satisfactory, because they assume that there is only one dictionary worth using. In practice, each language has its own dictionary, and special dictionaries are used for special vocabularies. Also, it may be desirable to use a special dictionary for testing. It is wishful thinking to assume that a single dictionary will suffice for all time.
這兩種方法都不令人滿意,因為它們假設只使用一個字典。在實際應用中,每種語言都有自己的字典,特殊的字典用于特殊的詞匯表。另外,最好使用一個特殊的字典進行測試。認為一本字典就足夠了,是一廂情愿的想法。
You could try to have SpellChecker support multiple dictionaries by making the dictionary field nonfinal and adding a method to change the dictionary in an existing spell checker, but this would be awkward, error-prone,and unworkable in a concurrent setting. **Static utility classes and singletons are inappropriate for classes whose behavior is parameterized by an underlying resource.**
你可以嘗試讓 SpellChecker 支持多個字典:首先取消 dictionary 字段的 final 修飾,并在現有的拼寫檢查器中添加更改 dictionary 的方法。但是在并發環境中這種做法是笨拙的、容易出錯的和不可行的。**靜態實用工具類和單例不適用于由底層資源參數化的類。**
What is required is the ability to support multiple instances of the class (in our example, SpellChecker), each of which uses the resource desired by the client (in our example, the dictionary). A simple pattern that satisfies this requirement is to **pass the resource into the constructor when creating a new instance.** This is one form of dependency injection: the dictionary is a dependency of the spell checker and is injected into the spell checker when it is created.
所需要的是支持類的多個實例的能力(在我們的示例中是 SpellChecker),每個實例都使用客戶端需要的資源(在我們的示例中是 dictionary)。滿足此要求的一個簡單模式是在**創建新實例時將資源傳遞給構造函數。** 這是依賴注入的一種形式:字典是拼寫檢查器的依賴項,在創建它時被注入到拼寫檢查器中。
```
// Dependency injection provides flexibility and testability
public class SpellChecker {
private final Lexicon dictionary;
public SpellChecker(Lexicon dictionary) {
this.dictionary = Objects.requireNonNull(dictionary);
}
public boolean isValid(String word) { ... }
public List<String> suggestions(String typo) { ... }
}
```
The dependency injection pattern is so simple that many programmers use it for years without knowing it has a name. While our spell checker example had only a single resource (the dictionary), dependency injection works with an arbitrary number of resources and arbitrary dependency graphs. It preserves immutability (Item 17), so multiple clients can share dependent objects(assuming the clients desire the same underlying resources). Dependency injection is equally applicable to constructors, static factories (Item 1), and builders (Item 2).
依賴注入模式非常簡單,許多程序員在不知道其名稱的情況下使用了多年。雖然拼寫檢查器示例只有一個資源(字典),但是依賴注入可以處理任意數量的資源和任意依賴路徑。它保持了不可變性([Item-17](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.md)),因此多個客戶端可以共享依賴對象(假設客戶端需要相同的底層資源)。依賴注入同樣適用于構造函數、靜態工廠([Item-1](/Chapter-2/Chapter-2-Item-1-Consider-static-factory-methods-instead-of-constructors.md))和構建器([Item-2](/Chapter-2/Chapter-2-Item-2-Consider-a-builder-when-faced-with-many-constructor-parameters.md))。
A useful variant of the pattern is to pass a resource factory to the constructor.A factory is an object that can be called repeatedly to create instances of a type.Such factories embody the Factory Method pattern [Gamma95]. The `Supplier<T>` interface, introduced in Java 8, is perfect for representing factories. Methods that take a `Supplier<T>` on input should typically constrain the factory’s type parameter using a bounded wildcard type (Item 31) to allow the client to pass in a factory that creates any subtype of a specified type. For example, here is a method that makes a mosaic using a client-provided factory to produce each tile:
這種模式的一個有用變體是將資源工廠傳遞給構造函數。工廠是一個對象,可以反復調用它來創建類型的實例。這樣的工廠體現了工廠方法模式 [Gamma95]。Java 8 中引入的 `Supplier<T>` 非常適合表示工廠。在輸入中接受 `Supplier<T>` 的方法通常應該使用有界通配符類型([Item-31](/Chapter-5/Chapter-5-Item-31-Use-bounded-wildcards-to-increase-API-flexibility.md))來約束工廠的類型參數,以允許客戶端傳入創建指定類型的任何子類型的工廠。例如,這里有一個生產瓷磚方法,每塊瓷磚都使用客戶提供的工廠來制作馬賽克:
```
Mosaic create(Supplier<? extends Tile> tileFactory) { ... }
```
Although dependency injection greatly improves flexibility and testability, it can clutter up(使雜亂) large projects, which typically contain thousands of dependencies.This clutter can be all but eliminated by using a dependency injection framework, such as Dagger [Dagger], Guice [Guice], or Spring [Spring]. The use of these frameworks is beyond the scope of this book, but note that APIs designed for manual dependency injection are trivially adapted for(適用于) use by these frameworks.
盡管依賴注入極大地提高了靈活性和可測試性,但它可能會使大型項目變得混亂,這些項目通常包含數千個依賴項。通過使用依賴注入框架(如 Dagger、Guice 或 Spring),幾乎可以消除這種混亂。這些框架的使用超出了本書的范圍,但是請注意,為手動依賴注入而設計的 API 很容易被這些框架所使用。
In summary, do not use a singleton or static utility class to implement a class that depends on one or more underlying resources whose behavior affects that of the class, and do not have the class create these resources directly. Instead, pass the resources, or factories to create them, into the constructor (or static factory or builder). This practice, known as dependency injection, will greatly enhance the flexibility, reusability, and testability of a class.
總之,不要使用單例或靜態實用工具類來實現依賴于一個或多個底層資源的類,這些資源的行為會影響類的行為,也不要讓類直接創建這些資源。相反,將創建它們的資源或工廠傳遞給構造函數(或靜態工廠或構建器)。這種操作稱為依賴注入,它將大大增強類的靈活性、可復用性和可測試性。
---
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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)