## Chapter 2. Creating and Destroying Objects(创建和销毁对象)
### Item 5: Prefer dependency injection to hardwiring resources(依赖注入优于硬连接资源)
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):
许多类依赖于一个或多个底层资源。例如,拼写检查程序依赖于字典。常见做法是,将这种类实现为静态实用工具类([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):
类似地,我们也经常看到它们的单例实现([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.
这两种方法都不令人满意,因为它们假设只使用一个字典。在实际应用中,每种语言都有自己的字典,特殊的字典用于特殊的词汇表。另外,最好使用一个特殊的字典进行测试。认为一本字典就足够了,是一厢情愿的想法。
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.**
你可以尝试让 SpellChecker 支持多个字典:首先取消 dictionary 字段的 final 修饰,并在现有的拼写检查器中添加更改 dictionary 的方法。但是在并发环境中这种做法是笨拙的、容易出错的和不可行的。**静态实用工具类和单例不适用于由底层资源参数化的类。**
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.
所需要的是支持类的多个实例的能力(在我们的示例中是 SpellChecker),每个实例都使用客户端需要的资源(在我们的示例中是 dictionary)。满足此要求的一个简单模式是在**创建新实例时将资源传递给构造函数。** 这是依赖注入的一种形式:字典是拼写检查器的依赖项,在创建它时被注入到拼写检查器中。
```
// 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).
依赖注入模式非常简单,许多程序员在不知道其名称的情况下使用了多年。虽然拼写检查器示例只有一个资源(字典),但是依赖注入可以处理任意数量的资源和任意依赖路径。它保持了不可变性([Item-17](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.md)),因此多个客户端可以共享依赖对象(假设客户端需要相同的底层资源)。依赖注入同样适用于构造函数、静态工厂([Item-1](/Chapter-2/Chapter-2-Item-1-Consider-static-factory-methods-instead-of-constructors.md))和构建器([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:
这种模式的一个有用变体是将资源工厂传递给构造函数。工厂是一个对象,可以反复调用它来创建类型的实例。这样的工厂体现了工厂方法模式 [Gamma95]。Java 8 中引入的 `Supplier<T>` 非常适合表示工厂。在输入中接受 `Supplier<T>` 的方法通常应该使用有界通配符类型([Item-31](/Chapter-5/Chapter-5-Item-31-Use-bounded-wildcards-to-increase-API-flexibility.md))来约束工厂的类型参数,以允许客户端传入创建指定类型的任何子类型的工厂。例如,这里有一个生产瓷砖方法,每块瓷砖都使用客户提供的工厂来制作马赛克:
```
Mosaic create(Supplier<? extends Tile> tileFactory) { ... }
```
Although dependency injection greatly improves flexibility and testability, it can clutter up(使杂乱) 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(适用于) use by these frameworks.
尽管依赖注入极大地提高了灵活性和可测试性,但它可能会使大型项目变得混乱,这些项目通常包含数千个依赖项。通过使用依赖注入框架(如 Dagger、Guice 或 Spring),几乎可以消除这种混乱。这些框架的使用超出了本书的范围,但是请注意,为手动依赖注入而设计的 API 很容易被这些框架所使用。
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.
总之,不要使用单例或静态实用工具类来实现依赖于一个或多个底层资源的类,这些资源的行为会影响类的行为,也不要让类直接创建这些资源。相反,将创建它们的资源或工厂传递给构造函数(或静态工厂或构建器)。这种操作称为依赖注入,它将大大增强类的灵活性、可复用性和可测试性。
---
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- Chapter 2. Creating and Destroying Objects(创建和销毁对象)
- Item 1: Consider static factory methods instead of constructors(考虑以静态工厂方法代替构造函数)
- Item 2: Consider a builder when faced with many constructor parameters(在面对多个构造函数参数时,请考虑构建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有构造函数或枚举类型实施单例属性)
- Item 4: Enforce noninstantiability with a private constructor(用私有构造函数实施不可实例化)
- Item 5: Prefer dependency injection to hardwiring resources(依赖注入优于硬连接资源)
- Item 6: Avoid creating unnecessary objects(避免创建不必要的对象)
- Item 7: Eliminate obsolete object references(排除过时的对象引用)
- Item 8: Avoid finalizers and cleaners(避免使用终结器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 优于 try-finally)
- Chapter 3. Methods Common to All Objects(对象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆盖 equals 方法时应遵守的约定)
- Item 11: Always override hashCode when you override equals(当覆盖 equals 方法时,总要覆盖 hashCode 方法)
- Item 12: Always override toString(始终覆盖 toString 方法)
- Item 13: Override clone judiciously(明智地覆盖 clone 方法)
- Item 14: Consider implementing Comparable(考虑实现 Comparable 接口)
- Chapter 4. Classes and Interfaces(类和接口)
- Item 15: Minimize the accessibility of classes and members(尽量减少类和成员的可访问性)
- Item 16: In public classes use accessor methods not public fields(在公共类中,使用访问器方法,而不是公共字段)
- Item 17: Minimize mutability(减少可变性)
- Item 18: Favor composition over inheritance(优先选择复合而不是继承)
- Item 19: Design and document for inheritance or else prohibit it(继承要设计良好并且具有文档,否则禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口优于抽象类)
- Item 21: Design interfaces for posterity(为后代设计接口)
- Item 22: Use interfaces only to define types(接口只用于定义类型)
- Item 23: Prefer class hierarchies to tagged classes(类层次结构优于带标签的类)
- Item 24: Favor static member classes over nonstatic(静态成员类优于非静态成员类)
- Item 25: Limit source files to a single top level class(源文件仅限有单个顶层类)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始类型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 优于数组)
- Item 29: Favor generic types(优先使用泛型)
- Item 30: Favor generic methods(优先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的灵活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可变参数)
- Item 33: Consider typesafe heterogeneous containers(考虑类型安全的异构容器)
- Chapter 6. Enums and Annotations(枚举和注解)
- Item 34: Use enums instead of int constants(用枚举类型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用实例字段替代序数)
- 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 替换序数索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模拟可扩展枚举)
- Item 39: Prefer annotations to naming patterns(注解优于命名模式)
- Item 40: Consistently use the Override annotation(坚持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用标记接口定义类型)
- Chapter 7. Lambdas and Streams(λ 表达式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表达式优于匿名类)
- Item 43: Prefer method references to lambdas(方法引用优于 λ 表达式)
- Item 44: Favor the use of standard functional interfaces(优先使用标准函数式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用无副作用的函数)
- Item 47: Prefer Collection to Stream as a return type(优先选择 Collection 而不是流作为返回类型)
- Item 48: Use caution when making streams parallel(谨慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(检查参数的有效性)
- Item 50: Make defensive copies when needed(在需要时制作防御性副本)
- Item 51: Design method signatures carefully(仔细设计方法签名)
- Item 52: Use overloading judiciously(明智地使用重载)
- Item 53: Use varargs judiciously(明智地使用可变参数)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或数组,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(为所有公开的 API 元素编写文档注释)
- Chapter 9. General Programming(通用程序设计)
- Item 57: Minimize the scope of local variables(将局部变量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循环优于传统的 for 循环)
- Item 59: Know and use the libraries(了解并使用库)
- Item 60: Avoid float and double if exact answers are required(若需要精确答案就应避免使用 float 和 double 类型)
- Item 61: Prefer primitive types to boxed primitives(基本数据类型优于包装类)
- Item 62: Avoid strings where other types are more appropriate(其他类型更合适时应避免使用字符串)
- Item 63: Beware the performance of string concatenation(当心字符串连接引起的性能问题)
- Item 64: Refer to objects by their interfaces(通过接口引用对象)
- Item 65: Prefer interfaces to reflection(接口优于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地进行优化)
- Item 68: Adhere to generally accepted naming conventions(遵守被广泛认可的命名约定)
- Chapter 10. Exceptions(异常)
- Item 69: Use exceptions only for exceptional conditions(仅在确有异常条件下使用异常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(对可恢复情况使用 checked 异常,对编程错误使用运行时异常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 异常)
- Item 72: Favor the use of standard exceptions(鼓励复用标准异常)
- Item 73: Throw exceptions appropriate to the abstraction(抛出能用抽象解释的异常)
- Item 74: Document all exceptions thrown by each method(为每个方法记录会抛出的所有异常)
- Item 75: Include failure capture information in detail messages(异常详细消息中应包含捕获失败的信息)
- Item 76: Strive for failure atomicity(尽力保证故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略异常)
- Chapter 11. Concurrency(并发)
- Item 78: Synchronize access to shared mutable data(对共享可变数据的同步访问)
- Item 79: Avoid excessive synchronization(避免过度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流优于直接使用线程)
- Item 81: Prefer concurrency utilities to wait and notify(并发实用工具优于 wait 和 notify)
- Item 82: Document thread safety(文档应包含线程安全属性)
- Item 83: Use lazy initialization judiciously(明智地使用延迟初始化)
- Item 84: Don’t depend on the thread scheduler(不要依赖线程调度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(优先选择 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常谨慎地实现 Serializable)
- Item 87: Consider using a custom serialized form(考虑使用自定义序列化形式)
- Item 88: Write readObject methods defensively(防御性地编写 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(对于实例控制,枚举类型优于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考虑以序列化代理代替序列化实例)