@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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