原文链接： Guide to Spring @Autowired
1. Overview 概述
Starting with Spring 2.5, the framework introduced annotations-driven Dependency Injection. The main annotation of this feature is @Autowired. It allows Spring to resolve and inject collaborating beans into our bean.
从Spring的2.5版本开始，引入了基于注解的依赖注入。这个新功能的主要注解就是@Autowired注解，它允许Spring框架解析它所标记的依赖，并把它注入到我们的bean对象中。
In this tutorial, we'll first take a look at how to enable autowiring and the various ways to autowire beans. Afterward, we'll talk about resolving bean conflicts using @Qualifier annotation, as well as potential exception scenarios.
这篇文章中，首先看一下如何启用自动注入，并查看有几种方式来注入依赖bean。稍后，将介绍如何使用@Qualifier注解来解决bean对象冲突，还有一些可能出现异常的场景。
2. Enabling @Autowired Annotations 启用@Autowired注解功能
The Spring framework enables automatic dependency injection. In other words, by declaring all the bean dependencies in a Spring configuration file, Spring container can autowire relationships between collaborating beans. This is called Spring bean autowiring.
Spring框架默认启动了自动依赖注入，换句话说，通过在配置文件中声明bean的依赖，Spring容器会自动关联声明了引用关系的bean对象，进行依赖注入，这就是Spring的自动注入。
To use Java-based configuration in our application, let's enable annotation-driven injection to load our Spring configuration:
要使用Java类进行配置，那么要启用基于注解的注入功能来加载Spring配置，示例如下：
@Configuration
@ComponentScan("com.baeldung.autowire.sample")
public class AppConfig {}Alternatively, the <context:annotation-config> annotation is mainly used to activate the dependency injection annotations in Spring XML files.
或者，在XML文件中可以使用<context:annotion-config>来标记启用注解注入功能，这个标签也是最常用的启动这一功能的标签。
Moreover, Spring Boot introduces the @SpringBootApplication annotation. This single annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan.
还有就是，SpringBoot 引入了 @SpringBootApplication 注解，它相当于同时启用了 @Configuration @EnableAutoConfiguration @ComponentScan 这三个注解。
Let's use this annotation in the main class of the application:
下面是@SpringBootApplication注解的使用示例：
@SpringBootApplication
class VehicleFactoryApplication {
public static void main(String[] args) {
SpringApplication.run(VehicleFactoryApplication.class, args);
}
}As a result, when we run this Spring Boot application, it will automatically scan the components in the current package and its sub-packages. Thus it will register them in Spring's Application Context, and allow us to inject beans using @Autowired.
按照上面代码进行编写，当我们启动它，会自动扫描当前包以及子包中的所有类，并在Spring程序上下文中进行注册，之后就可以作用于@Autowired注解标记的依赖，进行注入了。
3. Using @Autowired 使用@Autowired注解
After enabling annotation injection, we can use autowiring on properties, setters, and constructors.
启用了基于注解进行依赖注入的功能后，我们可以在字段、set方法及构造函数上进行使用
3.1. @Autowired on Properties 在字段上使用@Autowired注解
Let’s see how we can annotate a property using @Autowired. This eliminates the need for getters and setters.
来看一下如何在字段上使用@Autowired注解，这种方式可以省略字段所需的get/set方法
First, let's define a fooFormatter bean:
首先，定义一个bean对象，示例如下：
@Component("fooFormatter")
public class FooFormatter {
public String format() {
return "foo";
}
}Then, we'll inject this bean into the FooService bean using @Autowired on the field definition:
接着，把它注入到使用了@Autowired注解标记的字段上，当然，类型是相同的，示例如下：
@Component
public class FooService {
@Autowired
private FooFormatter fooFormatter;
}As a result, Spring injects fooFormatter when FooService is created.
结果就是，Spring框架正确处理了这个依赖注入，是在bean对象被创建时进行注入的。
3.2. @Autowired on Setters 在set方法上使用@Autowired注解
Now let's try adding @Autowired annotation on a setter method.
现在，试着在set方法上使用@Autowired注解
In the following example, the setter method is called with the instance of FooFormatter when FooServiceis created:
在下面的代码中，set方法在bean对象被创建的时个被调用，以此来注入所需的依赖
public class FooService {
private FooFormatter fooFormatter;
@Autowired
public void setFooFormatter(FooFormatter fooFormatter) {
this.fooFormatter = fooFormatter;
}
}
3.3. @Autowired on Constructors 在构造函数上使用@Autowired注解
Finally, let's use @Autowired on a constructor.
最后，在构造函数上使用@Autowired注解
We'll see that an instance of FooFormatter is injected by Spring as an argument to the FooServiceconstructor:
从下面代码中，可以看到，这种写法，即可把FooFormatter对象被当做参数注入到依赖它的bean对象中
public class FooService {
private FooFormatter fooFormatter;
@Autowired
public FooService(FooFormatter fooFormatter) {
this.fooFormatter = fooFormatter;
}
}
4. @Autowired and Optional Dependencies @Autowired注解与可选依赖
When a bean is being constructed, the @Autowired dependencies should be available. Otherwise, if Spring cannot resolve a bean for wiring, it will throw an exception.
在bean对象被构建时，所需的、被@Autowired注解标记的依赖bean应该已经被初始化完成，否则的话，Spring找不到依赖的对象时，就会抛出异常。
Consequently, it prevents the Spring container from launching successfully with an exception of the form:
因此，出现异常就会让Spring容器加载失败，会出现下面的错误：
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException:
No qualifying bean of type [com.autowire.sample.FooDAO] found for dependency:
expected at least 1 bean which qualifies as autowire candidate for this dependency.
Dependency annotations:
{@org.springframework.beans.factory.annotation.Autowired(required=true)}To fix this, we need to declare a bean of the required type:
为也避免上面的异常，可以在@Autowired注解中声明依赖需求的级别：
public class FooService {
@Autowired(required = false)
private FooDAO dataAccessor;
}5. Autowire Disambiguation 消除自动注入过程中的歧义现象
By default, Spring resolves @Autowired entries by type. If more than one bean of the same type is available in the container, the framework will throw a fatal exception.
默认情况下，Spring框架根据@Autowired标记对象的类型来完成注入。但若是容器中出现了同一类型的多个bean实例，就会出现异常，毕竟它也不知道应该使用哪一个，毕竟没有选择策略。
To resolve this conflict, we need to tell Spring explicitly which bean we want to inject.
为也解决上述问题，就需要明确告知Spring框架，哪一个bean对象都是所需的依赖。
5.1. Autowiring by @Qualifier 使用@Qualifier注解配合自动注入
For instance, let's see how we can use the @Qualifier annotation to indicate the required bean.
通过举例，来阐明如何使用@Qualifier注解来指明所需的bean依赖。
First, we'll define 2 beans of type Formatter:
首先，定义Formatter类型的两个实例对象
@Component("fooFormatter")
public class FooFormatter implements Formatter {
public String format() {
return "foo";
}
}@Component("barFormatter")
public class BarFormatter implements Formatter {
public String format() {
return "bar";
}
}Now let's try to inject a Formatter bean into the FooService class:
接着，通过@Autowired注解使框架自动进行依赖注入
public class FooService {
@Autowired
private Formatter formatter;
}In our example, there are two concrete implementations of Formatter available for the Spring container. As a result, Spring will throw a NoUniqueBeanDefinitionException exception when constructing the FooService:
这上面的示例中，在Spring容器中会有两个Formatter的实现类的实例，所以，Spring会在构造FooService时抛出一个NoUniqueBeanDEfinitionException异常
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException:
No qualifying bean of type [com.autowire.sample.Formatter] is defined:
expected single matching bean but found 2: barFormatter,fooFormatter
We can avoid this by narrowing the implementation using a @Qualifier annotation:
可以使用@Qualifier注解来限制使用的实例对象来避免这个问题，示例如下
public class FooService {
@Autowired
@Qualifier("fooFormatter")
private Formatter formatter;
}When there are multiple beans of the same type, it's a good idea to use @Qualifier to avoid ambiguity.
当出现一个类型的多个bean对象时，使用@Qualifier注解是一个好主意来解决这种依赖不明确问题。
Please note that the value of the @Qualifier annotation matches with the name declared in the @Component annotation of our FooFormatter implementation.
需要注意的是，@Qualifier注解中的bean名称，是组件在使用@Component注解标记时的名称。
5.2. Autowiring by Custom Qualifier 实现自定义的Qualifier限定注入
Spring also allows us to create our own custom @Qualifier annotation. To do so, we should provide the @Qualifier annotation with the definition:
Spring框架允许开发者创建自己的@Qualifier注解，实现方式就是，在自己的注解上使用@Qualifier注解标记，示例如下：
@Qualifier
@Target({
ElementType.FIELD, ElementType.METHOD, ElementType.TYPE, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface FormatterType {
String value();
}Then we can use the FormatterType within various implementations to specify a custom value:
之后就可以使用我们的自定义注解@FormatterType来指定实例名称：
@FormatterType("Foo")
@Component
public class FooFormatter implements Formatter {
public String format() {
return "foo";
}
}@FormatterType("Bar")
@Component
public class BarFormatter implements Formatter {
public String format() {
return "bar";
}
}Finally, our custom Qualifier annotation is ready to use for autowiring:
最后，可以使用用于自动依赖注入
@Component
public class FooService {
@Autowired
@FormatterType("Foo")
private Formatter formatter;
}
The value specified in the @Target meta-annotation restricts where to apply the qualifier, which in our example is fields, methods, types, and parameters.
在自定义注解的文件中，@Target注解用于限制自定义注解的使用范围，在上面示例中，可以用于字段、方法、java类及参数。
5.3. Autowiring by Name 使用变量名进行查找依赖并注入
Spring uses the bean's name as a default qualifier value. It will inspect the container and look for a bean with the exact name as the property to autowire it.
默认情况下，Spring框架会使用bean对象的名称做为实例对象名，并会检查引用了这个名称的bean进行依赖注入。
Hence, in our example, Spring matches the fooFormatter property name to the FooFormatterimplementation. Therefore, it injects that specific implementation when constructing FooService:
因此，在下面的例子中，Spring框架会找寻类型为FooFormatter类型的fooFormatter实例对象。因此，在构造FooService时，会在容器中查找fooFormatter实例并进行注入，找不到就抛异常。
public class FooService {
@Autowired
private Formatter fooFormatter;
}6. Conclusion 最后
In this article, we discussed autowiring and the different ways to use it. We also examined ways to solve two common autowiring exceptions caused by either a missing bean or an ambiguous bean injection.
这篇文章，认识了一下自动注入，还知道了使用它的几种方式。并且，晓得如何解决两种常见的自动依赖注入异常，一是缺少对应bean对象，另一个是不明确的bean对象引用。
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