I just released the Starter Class of "Learn Spring Security":
1. Overview
This Spring Framework article will demonstrate the use of annotations related to dependency injection, namely the @Resource, @Inject, and @Autowired annotations. These annotations provide classes with a declarative way to resolve dependencies. For example:
@Autowired ArbitraryClass arbObject;
as opposed to instantiating them directly (the imperative way), for example:
ArbitraryClass arbObject = new ArbitraryClass();
Two of the three annotations belong to the Java extension package: javax.annotation.Resource and javax.inject.Inject. The @Autowired annotation belongs to the org.springframework.beans.factory.annotation package.
Each of these annotations can resolve dependencies either by field injection or by setter injection. A simplified, but practical, example will be used to demonstrate the distinction between the three annotations, based on the execution paths taken by each annotation.
The examples will focus on how to use the three injection annotations during integration testing. The dependency required by the test can either be an arbitrary file or an arbitrary class.
2. The @Resource Annotation
The @Resource annotation is part of the JSR-250 annotation collection and is packaged with Java EE. This annotation has the following execution paths, listed by precedence:
- Match by Name
- Match by Type
- Match by Qualifier
These execution paths are applicable to both setter and field injection.
2.1. Field Injection
Resolving dependencies by field injection is achieved by annotating an instance variable with the @Resource annotation.
2.1.1. Match by Name
The integration test used to demonstrate match-by-name field injection is listed as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestResourceNameType.class)
public class FieldResourceInjectionTest {
@Resource(name="namedFile")
private File defaultFile;
@Test
public void givenResourceAnnotation_WhenOnField_ThenDependencyValid(){
assertNotNull(defaultFile);
assertEquals("namedFile.txt", defaultFile.getName());
}
}
Let’s go through the code. In the FieldResourceInjectionTest integration test, at line 7, the resolution of the dependency by name is achieved by passing in the bean name as an attribute value to the @Resource annotation:
@Resource(name="namedFile") private File defaultFile;
This configuration will resolve dependencies using the match-by-name execution path. The bean namedFile must be defined in the ApplicationContextTestResourceNameType application context.
Note that the bean id and the corresponding reference attribute value must match:
@Configuration
public class ApplicationContextTestResourceNameType {
@Bean(name="namedFile")
public File namedFile() {
File namedFile = new File("namedFile.txt");
return namedFile;
}
}
Failure to define the bean in the application context will result in a org.springframework.beans.factory.NoSuchBeanDefinitionException being thrown. This can be demonstrated by changing the attribute value passed in to the @Bean annotation, in the ApplicationContextTestResourceNameType application context; or changing the attribute value passed in to the @Resource annotation, in the FieldResourceInjectionTest integration test.
2.1.2. Match by Type
To demonstrate the match-by-type execution path, just remove the attribute value at line 7 of the FieldResourceInjectionTest integration test so that it looks as follows:
@Resource private File defaultFile;
and run the test again.
The test will still pass because if the @Resource annotation does not receive a bean name as an attribute value, the Spring Framework will proceed with the next level of precedence, match-by-type, in order to try resolve the dependency.
2.1.3. Match by Qualifier
To demonstrate the match-by-qualifier execution path, the integration testing scenario will be modified so that there are two beans defined in the ApplicationContextTestResourceQualifier application context:
@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;
}
}
The QualifierResourceInjectionTest integration test will be used to demonstrate match-by-qualifier dependency resolution. In this scenario, a specific bean dependency needs to be injected into each reference variable:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestResourceQualifier.class)
public class QualifierResourceInjectionTest {
@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());
}
}
Run the integration test, and a org.springframework.beans.factory.NoUniqueBeanDefinitionException is thrown. This exception is thrown because the application context has found two bean definitions of type File, and it is confused as to which bean should resolve the dependency.
To resolve this issue, please refer to line 7 to line 10 of the QualifierResourceInjectionTest integration test:
@Resource private File dependency1; @Resource private File dependency2;
and add the following lines of code:
@Qualifier("defaultFile")
@Qualifier("namedFile")
so that the code block looks as follows:
@Resource
@Qualifier("defaultFile")
private File dependency1;
@Resource
@Qualifier("namedFile")
private File dependency2;
Run the integration test again, this time round it should pass. The objective of this test was to demonstrate that even if there are multiple beans defined in an application context, the @Qualifier annotation clears any confusion by allowing specific dependencies to be injected into a class.
2.2. Setter Injection
The execution paths taken when injecting dependencies on a field are applicable to setter-based injection.
2.2.1. Match by Name
The only difference is the MethodResourceInjectionTest integration test has a setter method:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestResourceNameType.class)
public class MethodResourceInjectionTest {
private File defaultFile;
@Resource(name="namedFile")
protected void setDefaultFile(File defaultFile) {
this.defaultFile = defaultFile;
}
@Test
public void givenResourceAnnotation_WhenSetter_ThenDependencyValid(){
assertNotNull(defaultFile);
assertEquals("namedFile.txt", defaultFile.getName());
}
}
Resolving dependencies by setter injection is done by annotating a reference variable’s corresponding setter method. Pass the name of the bean dependency as an attribute value to the @Resource annotation:
private File defaultFile;
@Resource(name="namedFile")
protected void setDefaultFile(File defaultFile) {
this.defaultFile = defaultFile;
}
The namedFile bean dependency will be reused in this example. The bean name and the corresponding attribute value must match.
Run the integration test as-is and it will pass.
To see that the dependency was indeed resolved by the match-by-name execution path, change the attribute value passed to the @Resource annotation to a value of your choice and run the test again. This time, the test will fail with a NoSuchBeanDefinitionException.
2.2.2. Match by Type
To demonstrate setter-based, match-by-type execution, we will use the MethodByTypeResourceTest integration test:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestResourceNameType.class)
public class MethodByTypeResourceTest {
private File defaultFile;
@Resource
protected void setDefaultFile(File defaultFile) {
this.defaultFile = defaultFile;
}
@Test
public void givenResourceAnnotation_WhenSetter_ThenValidDependency(){
assertNotNull(defaultFile);
assertEquals("namedFile.txt", defaultFile.getName());
}
}
Run this test as-is, and it will pass.
In order to verify that the File dependency was indeed resolved by the match-by-type execution path, change the class type of the defaultFile variable to another class type like String. Execute the MethodByTypeResourceTest integration test again and this time a NoSuchBeanDefinitionException will be thrown.
The exception verifies that match-by-type was indeed used to resolve the File dependency. The NoSuchBeanDefinitionException confirms that the reference variable name does not need to match the bean name. Instead, dependency resolution depends on the bean’s class type matching the reference variable’s class type.
2.2.3. Match by Qualifier
The MethodByQualifierResourceTest integration test will be used to demonstrate the match-by-qualifier execution path:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestResourceQualifier.class)
public class MethodByQualifierResourceTest {
private File arbDependency;
private File anotherArbDependency;
@Test
public void givenResourceQualifier_WhenSetter_ThenValidDependencies(){
assertNotNull(arbDependency);
assertEquals("namedFile.txt", arbDependency.getName());
assertNotNull(anotherArbDependency);
assertEquals("defaultFile.txt", anotherArbDependency.getName());
}
@Resource
@Qualifier("namedFile")
public void setArbDependency(File arbDependency) {
this.arbDependency = arbDependency;
}
@Resource
@Qualifier("defaultFile")
public void setAnotherArbDependency(File anotherArbDependency) {
this.anotherArbDependency = anotherArbDependency;
}
}
The objective of this test is to demonstrate that even if multiple bean implementations of a particular type are defined in an application context, a @Qualifier annotation can be used together with the @Resource annotation to resolve a dependency.
Similar to field-based dependency injection, if there are multiple beans defined in an application context, a NoUniqueBeanDefinitionException is thrown if no @Qualifier annotation is used to specify which bean should be used to resolve dependencies.
3. The @Inject Annotation
The @Inject annotation belongs to the JSR-330 annotations collection. This annotation has the following execution paths, listed by precedence:
- Match by Type
- Match by Qualifier
- Match by Name
These execution paths are applicable to both setter and field injection. In order to access the @Inject annotation, the javax.inject library has to be declared as a Gradle or Maven dependency.
For Gradle:
testCompile group: 'javax.inject', name: 'javax.inject', version: '1'
For Maven:
<dependency>
<groupId>javax.inject</groupId>
<artifactId>javax.inject</artifactId>
<version>1</version>
</dependency>
3.1. Field Injection
3.1.1. Match by Type
The integration test example will be modified to use another type of dependency, namely the ArbitraryDependency class. The ArbitraryDependency class dependency merely serves as a simple dependency, and holds no further significance. It is listed as follows:
@Component
public class ArbitraryDependency {
private final String label = "Arbitrary Dependency";
public String toString() {
return label;
}
}
The FieldInjectTest integration test in question is listed as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestInjectType.class)
public class FieldInjectTest {
@Inject
private ArbitraryDependency fieldInjectDependency;
@Test
public void givenInjectAnnotation_WhenOnField_ThenValidDependency(){
assertNotNull(fieldInjectDependency);
assertEquals("Arbitrary Dependency",
fieldInjectDependency.toString());
}
}
Unlike the @Resource annotation, which resolves dependencies by name first; the default behavior of the @Inject annotation resolves dependencies by type.
This means that even if a class reference variable name differs from the bean name, the dependency will still be resolved, provided that the bean is defined in the application context. Note how the reference variable name in the following test:
@Inject private ArbitraryDependency fieldInjectDependency;
differs from the bean name configured in the application context:
@Bean
public ArbitraryDependency injectDependency() {
ArbitraryDependency injectDependency = new ArbitraryDependency();
return injectDependency;
}
and when the test is executed, it is able to resolve the dependency.
3.1.2. Match by Qualifier
But what if there are multiple implementations of a particular class type, and a certain class requires a specific bean? Let us modify the integration testing example so that another dependency is required.
In this example, we subclass the ArbitraryDependency class, used in the match-by-type example, to create the AnotherArbitraryDependency class:
public class AnotherArbitraryDependency extends ArbitraryDependency {
private final String label = "Another Arbitrary Dependency";
public String toString() {
return label;
}
}
The objective of each test case is to ensure that each dependency is injected correctly into each reference variable:
@Inject private ArbitraryDependency defaultDependency; @Inject private ArbitraryDependency namedDependency;
The FieldQualifierInjectTest integration test used to demonstrate match by qualifier is listed as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestInjectQualifier.class)
public class FieldQualifierInjectTest {
@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());
}
}
If there are multiple implementations of a particular class in an application context and the FieldQualifierInjectTest integration test attempts to inject the dependencies in the manner listed below:
@Inject private ArbitraryDependency defaultDependency; @Inject private ArbitraryDependency namedDependency;
a NoUniqueBeanDefinitionException will be thrown.
Throwing this exception is the Spring Framework’s way of pointing out that there are multiple implementations of a certain class and it is confused about which one to use. In order to elucidate the confusion, go to line 7 and 10 of the FieldQualifierInjectTest integration test:
@Inject private ArbitraryDependency defaultDependency; @Inject private ArbitraryDependency namedDependency;
pass the required bean name to the @Qualifier annotation, which is used together with the @Inject annotation. The code block will should now look as follows:
@Inject
@Qualifier("defaultFile")
private ArbitraryDependency defaultDependency;
@Inject
@Qualifier("namedFile")
private ArbitraryDependency namedDependency;
The @Qualifier annotation expects a strict match when receiving a bean name. Ensure that the bean name is passed to the Qualifier correctly, otherwise a NoUniqueBeanDefinitionException will be thrown. Run the test again, and this time it should pass.
3.1.3. Match by Name
The FieldByNameInjectTest integration test used to demonstrate match by name is similar to the match by type execution path. The only difference is now a specific bean is required, as opposed to a specific type. In this example, we subclass the ArbitraryDependency class again to produce the YetAnotherArbitraryDependency class:
public class YetAnotherArbitraryDependency extends ArbitraryDependency {
private final String label = "Yet Another Arbitrary Dependency";
public String toString() {
return label;
}
}
In order to demonstrate the match-by-name execution path, we will use the following integration test:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestInjectName.class)
public class FieldByNameInjectTest {
@Inject
@Named("yetAnotherFieldInjectDependency")
private ArbitraryDependency yetAnotherFieldInjectDependency;
@Test
public void givenInjectQualifier_WhenSetOnField_ThenDependencyValid(){
assertNotNull(yetAnotherFieldInjectDependency);
assertEquals("Yet Another Arbitrary Dependency",
yetAnotherFieldInjectDependency.toString());
}
}
The application context is listed as follows:
@Configuration
public class ApplicationContextTestInjectName {
@Bean
public ArbitraryDependency yetAnotherFieldInjectDependency() {
ArbitraryDependency yetAnotherFieldInjectDependency =
new YetAnotherArbitraryDependency();
return yetAnotherFieldInjectDependency;
}
}
Run the integration test as-is, and it will pass.
In order to verify that the dependency was indeed injected by the match-by-name execution path, change the value, yetAnotherFieldInjectDependency, that was passed in to the @Named annotation to another name of your choice. Run the test again – this time, a NoSuchBeanDefinitionException is thrown.
3.2. Setter Injection
Setter-based injection for the @Inject annotation is similar to the approach used for @Resource setter-based injection. Instead of annotating the reference variable, the corresponding setter method is annotated. The execution paths followed by field-based dependency injection also apply to setter based injection.
4. The @Autowired Annotation
The behaviour of @Autowired annotation is similar to the @Inject annotation. The only difference is that the @Autowired annotation is part of the Spring framework. This annotation has the same execution paths as the @Inject annotation, listed in order of precedence:
- Match by Type
- Match by Qualifier
- Match by Name
These execution paths are applicable to both setter and field injection.
4.1. Field Injection
4.1.1. Match by Type
The integration testing example used to demonstrate the @Autowired match-by-type execution path will be similar to the test used to demonstrate the @Inject match-by-type execution path. The FieldAutowiredTest integration test used to demonstrate match-by-type using the @Autowired annotation is listed as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestAutowiredType.class)
public class FieldAutowiredTest {
@Autowired
private ArbitraryDependency fieldDependency;
@Test
public void givenAutowired_WhenSetOnField_ThenDependencyResolved() {
assertNotNull(fieldDependency);
assertEquals("Arbitrary Dependency", fieldDependency.toString());
}
}
The application context for this integration test is listed as follows:
@Configuration
public class ApplicationContextTestAutowiredType {
@Bean
public ArbitraryDependency autowiredFieldDependency() {
ArbitraryDependency autowiredFieldDependency =
new ArbitraryDependency();
return autowiredFieldDependency;
}
}
The objective of the integration test is to demonstrate that match-by-type takes first precedence over the other execution paths. Notice in line 8 of the FieldAutowiredTest integration test how the reference variable name:
@Autowired private ArbitraryDependency fieldDependency;
is different to the bean name in the application context:
@Bean
public ArbitraryDependency autowiredFieldDependency() {
ArbitraryDependency autowiredFieldDependency =
new ArbitraryDependency();
return autowiredFieldDependency;
}
When the test is run, it will pass.
In order to confirm that the dependency was indeed resolved using the match-by-type execution path, change the type of the fieldDependency reference variable and run the integration test again. This time round the FieldAutowiredTest integration test must fail, with a NoSuchBeanDefinitionException being thrown. This verifies that match-by-type was used to resolve the dependency.
4.1.2. Match by Qualifier
What if faced with a situation where multiple bean implementations have been defined in the application context, like the one listed below:
@Configuration
public class ApplicationContextTestAutowiredQualifier {
@Bean
public ArbitraryDependency autowiredFieldDependency() {
ArbitraryDependency autowiredFieldDependency =
new ArbitraryDependency();
return autowiredFieldDependency;
}
@Bean
public ArbitraryDependency anotherAutowiredFieldDependency() {
ArbitraryDependency anotherAutowiredFieldDependency =
new AnotherArbitraryDependency();
return anotherAutowiredFieldDependency;
}
}
If the FieldQualifierAutowiredTest integration test, listed below, is executed:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestAutowiredQualifier.class)
public class FieldQualifierAutowiredTest {
@Autowired
private ArbitraryDependency fieldDependency1;
@Autowired
private ArbitraryDependency fieldDependency2;
@Test
public void givenAutowiredQualifier_WhenOnField_ThenDep1Valid(){
assertNotNull(fieldDependency1);
assertEquals("Arbitrary Dependency", fieldDependency1.toString());
}
@Test
public void givenAutowiredQualifier_WhenOnField_ThenDep2Valid(){
assertNotNull(fieldDependency2);
assertEquals("Another Arbitrary Dependency",
fieldDependency2.toString());
}
}
a NoUniqueBeanDefinitionException will be thrown.
The exception is due to the ambiguity caused by the two beans defined in the application context. The Spring Framework does not know which bean dependency should be autowired to which reference variable. Resolve this issue by adding the @Qualifier annotation to lines 7 and 10 of the FieldQualifierAutowiredTest integration test:
@Autowired private FieldDependency fieldDependency1; @Autowired private FieldDependency fieldDependency2;
so that the code block looks as follows:
@Autowired
@Qualifier("autowiredFieldDependency")
private FieldDependency fieldDependency1;
@Autowired
@Qualifier("anotherAutowiredFieldDependency")
private FieldDependency fieldDependency2;
Run the test again, and this time it will pass.
4.1.3. Match by Name
The same integration test scenario will be used to demonstrate the match-by-name execution path when using the @Autowired annotation to inject a field dependency. When autowiring dependencies by name, the @ComponentScan annotation must be used with the application context, ApplicationContextTestAutowiredName:
@Configuration
@ComponentScan(basePackages={"com.baeldung.dependency"})
public class ApplicationContextTestAutowiredName {
}
The @ComponentScan annotation will search packages for Java classes that have been annotated with the @Component annotation. For example, in the application context, the com.baeldung.dependency package will be scanned for classes that have been annotated with the @Component annotation. In this scenario, the Spring Framework must detect the ArbitraryDependency class, which has the @Component annotation:
@Component(value="autowiredFieldDependency")
public class ArbitraryDependency {
private final String label = "Arbitrary Dependency";
public String toString() {
return label;
}
}
The attribute value, autowiredFieldDependency, passed into the @Component annotation, tells the Spring Framework that the ArbitraryDependency class is a component named autowiredFieldDependency. In order for the @Autowired annotation to resolve dependencies by name, the component name must correspond with the field name defined in the FieldAutowiredNameTest integration test; please refer to line 8:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
loader=AnnotationConfigContextLoader.class,
classes=ApplicationContextTestAutowiredName.class)
public class FieldAutowiredNameTest {
@Autowired
private ArbitraryDependency autowiredFieldDependency;
@Test
public void givenAutowiredAnnotation_WhenOnField_ThenDepValid(){
assertNotNull(autowiredFieldDependency);
assertEquals("Arbitrary Dependency",
autowiredFieldDependency.toString());
}
}
When the FieldAutowiredNameTest integration test is run as-is, it will pass.
But how do we know that the @Autowired annotation really did invoke the match-by-name execution path? Change the name of the reference variable autowiredFieldDependency to another name of your choice, then run the test again.
This time, the test will fail and a NoUniqueBeanDefinitionException is thrown. A similar check would be to change the @Component attribute value, autowiredFieldDependency, to another value of your choice and run the test again. A NoUniqueBeanDefinitionException will also be thrown.
This exception is proof that if an incorrect bean name is used, no valid bean will be found. Therefore, the match-by-name execution path was invoked.
4.2. Setter Injection
Setter-based injection for the @Autowired annotation is similar the approach demonstrated for @Resource setter-based injection. Instead of annotating the reference variable with the @Inject annotation, the corresponding setter is annotated. The execution paths followed by field-based dependency injection also apply to setter based injection.
5. Applying These Annotations
This raises the question, which annotation should be used and under what circumstances? The answer to these questions depends on the design scenario faced by the application in question, and how the developer wishes to leverage polymorphism based on the default execution paths of each annotation.
5.1. Application-Wide use of Singletons Through Polymorphism
If the design is such that application behaviors are based on implementations of an interface or an abstract class, and these behaviors are used throughout the application, then use either the @Inject or @Autowired annotation.
The benefit of this approach is that when the application is upgraded, or a patch needs to be applied in order to fix a bug; then classes can be swapped out with minimal negative impact to the overall application behavior. In this scenario, the primary default execution path is match-by-type.
5.2. Fine-Grained Application Behavior Configuration Through Polymorphism
If the design is such that the application has complex behavior, each behavior is based on different interfaces/abstract classes, and usage of each of these implementations varies across the application, then use the @Resource annotation. In this scenario, the primary default execution path is match-by-name.
5.3. Dependency Injection Should be Handled Solely by the Java EE Platform
If there is a design mandate for all dependencies to be injected by the Java EE Platform and not Spring, then the choice is between the @Resource annotation and the @Inject annotation. You should narrow down the final decision between the two annotations, based on which default execution path is required.
5.4. Dependency Injection Should be Handled Solely by the Spring Framework
If the mandate is for all dependencies to be handled by the Spring Framework, the only choice is the @Autowired annotation.
5.5. Discussion Summary
The table below summarizes the discussion.
| Scenario | @Resource | @Inject | @Autowired |
|---|---|---|---|
| Application-wide use of singletons through polymorphism | ✗ | ✔ | ✔ |
| Fine-grained application behaviour configuration through polymorphism | ✔ | ✗ | ✗ |
| Dependency injection should be handled solely by the Java EE platform | ✔ | ✔ | ✗ |
| Dependency injection should be handled solely by the Spring Framework | ✗ | ✗ | ✔ |
6. Conclusion
The article aimed to provide a deeper insight into the behavior of each annotation. Understanding how each annotation behaves will contribute to better overall application design and maintenance.
The code used during the discussion can be found on github.