@TOC

本文简述这三个Spring应用里常用的注解区别。

@Resources

官方文档里对@Resources的说明：

The @Resource annotation is part of the JSR-250 annotation collection and is packaged with Jakarta EE.

什么是JSR-250呢？访问这个链接：https://jcp.org/en/jsr/detail?id=250

里面有很多PDF可以下载：

打开一看，其实就是Java支持的注解的文档。这些文档是最权威的：

文档里介绍，@Resources对Bean的注入按照如下的优先级进行：

1. Match by Name
2. Match by Type
3. Match by Qualifier

Match by Name

我们来看看Match by name的例子。下面的代码试图通过Match by Name注入一个名称为namedFile的Bean：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
  loader=AnnotationConfigContextLoader.class,
  classes=ApplicationContextTestResourceNameType.class)
public class FieldResourceInjectionIntegrationTest {
 
    @Resource(name="namedFile")
    private File defaultFile;
 
    @Test
    public void givenResourceAnnotation_WhenOnField_ThenDependencyValid(){
        assertNotNull(defaultFile);
        assertEquals("namedFile.txt", defaultFile.getName());
    }
}

Bean的定义在如下代码里：

@Configuration
public class ApplicationContextTestResourceNameType {
 
    @Bean(name="namedFile")
    public File namedFile() {
        File namedFile = new File("namedFile.txt");
        return namedFile;
    }
}

运行时，申明Bean依赖处的@Resource的Name属性和Bean定义处@Bean的Name属性值一致，Match by Name测试通过。

Match by Type

将使用bean的消费者代码里@Resource注解的name属性去掉，使其变成下面这样：

@Resource
private File defaultFile;

测试仍然通过，是因为Match by Name的探测机制执行失败后，进行下一轮Match by Type的探测，这一轮成功了。

Match by Qualifier

定义两个Bean：

@Configuration
public class ApplicationContextTestResourceQualifier {
 
    @Bean(name="defaultFile")
    public File defaultFile() {
        File defaultFile = new File("defaultFile.txt");
        return defaultFile;
    }
 
    @Bean(name="namedFile")
    public File namedFile() {
        File namedFile = new File("namedFile.txt");
        return namedFile;
    }
}

测试代码：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
  loader=AnnotationConfigContextLoader.class,
  classes=ApplicationContextTestResourceQualifier.class)
public class QualifierResourceInjectionIntegrationTest {
 
    @Resource
    private File dependency1;
     
    @Resource
    private File dependency2;
 
    @Test
    public void givenResourceAnnotation_WhenField_ThenDependency1Valid(){
        assertNotNull(dependency1);
        assertEquals("defaultFile.txt", dependency1.getName());
    }
 
    @Test
    public void givenResourceQualifier_WhenField_ThenDependency2Valid(){
        assertNotNull(dependency2);
        assertEquals("namedFile.txt", dependency2.getName());
    }
}

这一次执行失败，遇到异常org.springframework.beans.factory.NoUniqueBeanDefinitionException.

原因是因为我们的测试代码里，没有指定注入Bean的名称，因此Spring的Match by Name探测失败，进行Match by Type时，探测到两个类型一样的Bean，Spring框架不知道注入哪一个，所以就报异常了。

避免这个异常也很容易，使用@Qualifier.代码如下：

@Resource
@Qualifier("defaultFile")
private File dependency1;
 
@Resource
@Qualifier("namedFile")
private File dependency2;

@Inject

这个注解定义在JSR-330里，文档链接：

https://jcp.org/en/jsr/detail?id=330

注入的优先级：

1. Match by Type
2. Match by Qualifier
3. Match by Name

Match by Type

注意@Inject注入的最高优先级方式为Match by Type，而非@Resource的Match by Name.

任意定义一个待注入的Component：

@Component
public class ArbitraryDependency {
 
    private final String label = "Arbitrary Dependency";
 
    public String toString() {
        return label;
    }
}

使用@Inject注入：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
  loader=AnnotationConfigContextLoader.class,
  classes=ApplicationContextTestInjectType.class)
public class FieldInjectIntegrationTest {
 
    @Inject
    private ArbitraryDependency fieldInjectDependency;
 
    @Test
    public void givenInjectAnnotation_WhenOnField_ThenValidDependency(){
        assertNotNull(fieldInjectDependency);
        assertEquals("Arbitrary Dependency",
          fieldInjectDependency.toString());
    }
}

Match by Qualifier

定义一个新的待注入组件：

public class AnotherArbitraryDependency extends ArbitraryDependency {
 
    private final String label = "Another Arbitrary Dependency";
 
    public String toString() {
        return label;
    }
}

测试代码：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
  loader=AnnotationConfigContextLoader.class,
  classes=ApplicationContextTestInjectQualifier.class)
public class FieldQualifierInjectIntegrationTest {
 
    @Inject
    private ArbitraryDependency defaultDependency;
 
    @Inject
    private ArbitraryDependency namedDependency;
 
    @Test
    public void givenInjectQualifier_WhenOnField_ThenDefaultFileValid(){
        assertNotNull(defaultDependency);
        assertEquals("Arbitrary Dependency",
          defaultDependency.toString());
    }
 
    @Test
    public void givenInjectQualifier_WhenOnField_ThenNamedFileValid(){
        assertNotNull(defaultDependency);
        assertEquals("Another Arbitrary Dependency",
          namedDependency.toString());
    }
}

和之前@Resource的第一次试图通过Match by Type注入一样失败，遇到异常：NoUniqueBeanDefinitionException

利用@Qualifier避免这个异常：

@Inject
@Qualifier("defaultFile")
private ArbitraryDependency defaultDependency;
 
@Inject
@Qualifier("namedFile")
private ArbitraryDependency namedDependency;

Match by Name

public class YetAnotherArbitraryDependency extends ArbitraryDependency {
 
    private final String label = "Yet Another Arbitrary Dependency";
 
    public String toString() {
        return label;
    }
}

消费者代码：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
  loader=AnnotationConfigContextLoader.class,
  classes=ApplicationContextTestInjectName.class)
public class FieldByNameInjectIntegrationTest {
 
    @Inject
    @Named("yetAnotherFieldInjectDependency")
    private ArbitraryDependency yetAnotherFieldInjectDependency;
 
    @Test
    public void givenInjectQualifier_WhenSetOnField_ThenDependencyValid(){
        assertNotNull(yetAnotherFieldInjectDependency);
        assertEquals("Yet Another Arbitrary Dependency",
          yetAnotherFieldInjectDependency.toString());
    }
}

application context代码：

@Configuration
public class ApplicationContextTestInjectName {
 
    @Bean
    public ArbitraryDependency yetAnotherFieldInjectDependency() {
        ArbitraryDependency yetAnotherFieldInjectDependency =
          new YetAnotherArbitraryDependency();
        return yetAnotherFieldInjectDependency;
    }
}

测试通过

@Autowired

这个注解和@Inject的用法一致，唯一区别就是@Autowired 属于Spring框架提供的注解。例子略。

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