第3章：注释配置

JavaEE开发的颠覆者SpringBoot实战

@Required

第一个注释是 @Required，其用法如下：

public class SimpleMovieLister {
        private MovieFinder movieFinder;
        @Required
        public void setMovieFinder(MovieFinder movieFinder) {
                this.movieFinder = movieFinder;
        }
        // ...
}

@Required的含义是让容器在初始化的就检查Bean的属性需要被注入，防止在运行时抛出NullPointerException异常。

@Autowired

先看下定义：

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
    /**
     * Declares whether the annotated dependency is required.
     * <p>Defaults to {@code true}.
     */
    boolean required() default true;
}

保留到运行时，然后可以放在基本所有的类型上。运行时IoC容器会根据注释自动注入依赖。

@Primary

此处如果我们使用@Autowired注释，可能带来的问题是会有多个候选者，这个时候我们可以通过@Primary指定到底使用哪个。

public class MovieRecommender {
        @Autowired
        private MovieCatalog movieCatalog;
        // ...
}

xml配置：

<bean class="example.SimpleMovieCatalog" primary="true">
                <!-- inject any dependencies required by this bean -->
        </bean>
        <bean class="example.SimpleMovieCatalog">
                <!-- inject any dependencies required by this bean -->
        </bean>
        <bean id="movieRecommender" class="example.MovieRecommender"/>
</beans>

@Qualifier

当我们需要对@Autowired进行更精细的控制的时候，就可以使用@Qualifier了，相比较于@Primary能够对Bean进行更精细的控制，看例子：

public class MovieRecommender {
        private MovieCatalog movieCatalog;
        private CustomerPreferenceDao customerPreferenceDao;
        @Autowired
        public void prepare(@Qualifier("main")MovieCatalog movieCatalog,
                        CustomerPreferenceDao customerPreferenceDao) {
                this.movieCatalog = movieCatalog;
                this.customerPreferenceDao = customerPreferenceDao;
        }
        // ...
}

@Qualifier能够修饰方法和参数，上面的例子是修饰的参数，再来看xml中的配置。

<bean class="example.SimpleMovieCatalog">
                <qualifier value="main"/>
                <!-- inject any dependencies required by this bean -->
        </bean>
        <bean class="example.SimpleMovieCatalog">
                <qualifier value="action"/>
                <!-- inject any dependencies required by this bean -->
        </bean>
        <bean id="movieRecommender" class="example.MovieRecommender"/>

@Qualifier除了修饰函数和参数外，还能修饰注释本省，看例子：

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {
        String genre();
        Format format();
}

然后代码中的注释：

public class MovieRecommender {
        @Autowired
        @MovieQualifier(format=Format.VHS, genre="Action")
        private MovieCatalog actionVhsCatalog;
        @Autowired
        @MovieQualifier(format=Format.VHS, genre="Comedy")
        private MovieCatalog comedyVhsCatalog;
        @Autowired
        @MovieQualifier(format=Format.DVD, genre="Action")
        private MovieCatalog actionDvdCatalog;
        @Autowired
        @MovieQualifier(format=Format.BLURAY, genre="Comedy")
        private MovieCatalog comedyBluRayCatalog;
        // ...
}

@Resource

这个是JSR-250的标准，里面可以声明Bean的名字，如果不声明，则默认取属性名或者方法名，看例子：

public class SimpleMovieLister {
        private MovieFinder movieFinder;
        //@Resource(name="movieFinder")
        @Resource
        public void setMovieFinder(MovieFinder movieFinder) {
                this.movieFinder = movieFinder;
        }
}