導航目錄
@Resources
Match by Name
Match by Type
Match by Qualifier
@Inject
@Autowired
本文簡述這三個Spring應用裡常用的注解差別。
官方文檔裡對@Resources的說明：
The @Resource annotation is part of the JSR-250 annotation collection and is packaged with Jakarta EE.
什麼是JSR-250呢？通路這個連結：
https://jcp.org/en/jsr/detail?id=250 裡面有很多PDF可以下載下傳：
文檔裡介紹，@Resources對Bean的注入按照如下的優先級進行：
我們來看看Match by name的例子。下面的代碼試圖通過Match by Name注入一個名稱為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的定義在如下代碼裡：
@Configuration
public class ApplicationContextTestResourceNameType {
@Bean(name="namedFile")
public File namedFile() {
File namedFile = new File("namedFile.txt");
return namedFile;
}
} 運作時，申明Bean依賴處的@Resource的Name屬性和Bean定義處@Bean的Name屬性值一緻，Match by Name測試通過。
将使用bean的消費者代碼裡@Resource注解的name屬性去掉，使其變成下面這樣：
@Resource
private File defaultFile; 測試仍然通過，是因為Match by Name的探測機制執行失敗後，進行下一輪Match by Type的探測，這一輪成功了。
定義兩個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;
}
} 測試代碼：
@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());
}
} 這一次執行失敗，遇到異常org.springframework.beans.factory.NoUniqueBeanDefinitionException.
原因是因為我們的測試代碼裡，沒有指定注入Bean的名稱，是以Spring的Match by Name探測失敗，進行Match by Type時，探測到兩個類型一樣的Bean，Spring架構不知道注入哪一個，是以就報異常了。
避免這個異常也很容易，使用@Qualifier.代碼如下：
@Resource
@Qualifier("defaultFile")
private File dependency1;
@Resource
@Qualifier("namedFile")
private File dependency2; 這個注解定義在JSR-330裡，文檔連結：
https://jcp.org/en/jsr/detail?id=330注入的優先級：
注意@Inject注入的最高優先級方式為Match by Type，而非@Resource的Match by Name.
任意定義一個待注入的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());
}
} public class AnotherArbitraryDependency extends ArbitraryDependency {
private final String label = "Another Arbitrary Dependency";
public String toString() {
return label;
}
} @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的第一次試圖通過Match by Type注入一樣失敗，遇到異常：NoUniqueBeanDefinitionException
利用@Qualifier避免這個異常：
@Inject
@Qualifier("defaultFile")
private ArbitraryDependency defaultDependency;
@Inject
@Qualifier("namedFile")
private ArbitraryDependency namedDependency; public class YetAnotherArbitraryDependency extends ArbitraryDependency {
private final String label = "Yet Another Arbitrary Dependency";
public String toString() {
return label;
}
} 消費者代碼：
@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代碼：
@Configuration
public class ApplicationContextTestInjectName {
@Bean
public ArbitraryDependency yetAnotherFieldInjectDependency() {
ArbitraryDependency yetAnotherFieldInjectDependency =
new YetAnotherArbitraryDependency();
return yetAnotherFieldInjectDependency;
}
}