7.4.1 Setter-based dependency injection

Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

在调用一个无参构造器或无参的静态工厂方法区实例化你的bean后,基于Setter的依赖注入能够熟练的通过调用你的beans的setter方法.

The following example shows a class that can only be dependency-injected using pure setter injection. This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces, base classes or annotations.

下面的例子展示了一个只能够使用纯粹的setter注入的类.这是一个标准的Java类.他是普通的Java对象,它没有依赖于容器中特定的接口,基类或注解.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on the MovieFinder
    private MovieFinder movieFinder;

    // a setter method so that the Spring container can inject a MovieFinder
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // business logic that actually uses the injected MovieFinder is omitted...
}

The ApplicationContext supports constructor-based and setter-based DI for the beans it manages. It also supports setter-based DI after some dependencies have already been injected through the constructor approach. You configure the dependencies in the form of a BeanDefinition, which you use in conjunction with PropertyEditor instances to convert properties from one format to another. However, most Spring users do not work with these classes directly (i.e., programmatically) but rather with XML bean definitions, annotated components (i.e., classes annotated with @Component, @Controller, etc.), or @Bean methods in Java-based @Configuration classes. These sources are then converted internally into instances of BeanDefinition and used to load an entire Spring IoC container instance.

 ApplicationContext 能够它管理的beans支持基于构造器和基于setter的依赖注入.它也支持在一些依赖已经通过构造器的方法被注入后进行基于setter的依赖注入.你以 BeanDefinition的形式配置依赖,你结合使用 PropertyEditor 实例将属性从一种格式转换到另一种格式.然而,大多数Spring用户不会直接使用这些类(也就是说,以编程的方式)而去使用XML bean定义,注解组件(也就是说,在类上加 @Component, @Controller等等)或者使用基于java的 @Configuration 类的 @Bean 方法.这些源由 BeanDefinition 内部转化为实例和载入一个空的Spring IoC容器实例.

Constructor-based or setter-based DI? 

基于构造器或基于setter的依赖注入?

Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use constructors for mandatory dependencies and setter methods or configuration methods for optional dependencies. Note that use of the @Required annotation on a setter method can be used to make the property a required dependency.

自从你可以混合使用基于构造器和基于setter的依赖注入,它是一个很好的规则去使用构造器给强制的依赖,使用setter方法或配置方法给可选的依赖.注意使用@Required这个注解在一个setter方法上,可以让这个属性成为一个必要的依赖.

The Spring team generally advocates constructor injection as it enables one to implement application components as immutable objects and to ensure that required dependencies are not null. Furthermore constructor-injected components are always returned to client (calling) code in a fully initialized state. As a side note, a large number of constructor arguments is a bad code smell, implying that the class likely has too many responsibilities and should be refactored to better address proper separation of concerns.

Spring团队通常提倡使用构造器的注入方式,它能够实现一个应用组件作为一个不可变的对象,它能够确保必要的依赖不为空.此外,构造器注入组件总是返回一个完全初始化状态的客户端(调用)代码.另外要注意的是,大量的构造器参数并不是一段好代码,它暗示着这个类有太多的责任,他应该被重构,将他们的关系适当的分离开来.

Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency. One benefit of setter injection is that setter methods make objects of that class amenable to reconfiguration or re-injection later. Management through JMX MBeans is therefore a compelling use case for setter injection.

Setter注入应该主要用在可选参数的依赖上,它能够在类中适当的分配默认值.否则,非空检查必须被提供在这段代码使用了这个依赖的每一个地方.setter注入的一个好处就是通过setter方法制作了这个对象后还可以重新配置或重新注入.通过JMX MBeans管理也因此是一种强制的使用情况给setter注入.

Use the DI style that makes the most sense for a particular class. Sometimes, when dealing with third-party classes for which you do not have the source, the choice is made for you. For example, if a third-party class does not expose any setter methods, then constructor injection may be the only available form of DI.

给一个特定类使用有意义的依赖注入样式.有时,当处理自己没有源码的第三方类时,就给了你选择.例如,如果一个第三方类没有暴露任何setter方法,那么就只能够使用构造器注入的方式了.