Spring @Autowired

一时失言乱红尘 2024-03-25 14:07 86阅读 0赞

**`@Autowired` 的使用是有前置条件的 ☞ activate the dependency injection annotations**

**After `enabling` annotation injection, we can use autowiring on `properties`, `setters`, and `constructors`.**

This annotation has the following execution paths, `listed in order of precedence`:  
1、Match by Type  
2、Match by Qualifier  
3、Match by Name

--------------------

## Field Injection ##

### Match by Type ###

@RunWith(SpringJUnit4ClassRunner.class)
    @ContextConfiguration(loader=AnnotationConfigContextLoader.class,classes=ApplicationContextTestAutowiredType.class)
    public class FieldAutowiredIntegrationTest {
        
        @Autowired
        private ArbitraryDependency fieldDependency;
        @Test
        public void givenAutowired_WhenSetOnField_ThenDependencyResolved() {
        
            assertNotNull(fieldDependency);
            assertEquals("Arbitrary Dependency", fieldDependency.toString());
        }
    }
    
    @Configuration
    public class ApplicationContextTestAutowiredType {
        
        @Bean
        public ArbitraryDependency autowiredFieldDependency() {
        
            ArbitraryDependency autowiredFieldDependency =new ArbitraryDependency();
            return autowiredFieldDependency;
        }
    }

类型不存在时 ☞ a `NoSuchBeanDefinitionException` being thrown.

--------------------

### Match by Qualifier ###

@Configuration
    public class ApplicationContextTestAutowiredQualifier {
        
    
        @Bean
        public ArbitraryDependency autowiredFieldDependency() {
        
            ArbitraryDependency autowiredFieldDependency =new ArbitraryDependency();
            return autowiredFieldDependency;
        }
    
        @Bean
        public ArbitraryDependency anotherAutowiredFieldDependency() {
        
            ArbitraryDependency anotherAutowiredFieldDependency =new AnotherArbitraryDependency();
            return anotherAutowiredFieldDependency;
        }
    }
    
    @RunWith(SpringJUnit4ClassRunner.class)
    @ContextConfiguration(
      loader=AnnotationConfigContextLoader.class,
      classes=ApplicationContextTestAutowiredQualifier.class)
    public class FieldQualifierAutowiredIntegrationTest {
        
    
    	# NoUniqueBeanDefinitionException
        @Autowired
        private ArbitraryDependency fieldDependency1;
        @Autowired
        private ArbitraryDependency fieldDependency2;
    
    	# 方案
    	@Autowired
    	@Qualifier("autowiredFieldDependency")
    	private FieldDependency fieldDependency1;
    	@Autowired
    	@Qualifier("anotherAutowiredFieldDependency")
    	private FieldDependency 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());
        }
    }

--------------------

### Match by Name ###

@Component(value="autowiredFieldDependency")
    public class ArbitraryDependency {
        
    
        private final String label = "Arbitrary Dependency";
    
        public String toString() {
        
            return label;
        }
    }
    
    @RunWith(SpringJUnit4ClassRunner.class)
    @ContextConfiguration(loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestAutowiredName.class)
    public class FieldAutowiredNameIntegrationTest {
        
    
        @Autowired
        private ArbitraryDependency autowiredFieldDependency;
    
        @Test
        public void givenAutowired_WhenSetOnField_ThenDependencyResolved(){
        
            assertNotNull(autowiredFieldDependency);
            assertEquals("Arbitrary Dependency",autowiredFieldDependency.toString());
    	}
    }

叫 *`autowiredFieldDependency`* 的bean 不存在 ☞ *`NoUniqueBeanDefinitionException`* is thrown.

--------------------

## Setter Injection ##

public class FooService {
        
        private FooFormatter fooFormatter;
    
    	# 用法如上
        @Autowired
        public void setFormatter(FooFormatter fooFormatter) {
        
            this.fooFormatter = fooFormatter;
        }
    }

--------------------

## Constructors Injection ##

public class FooService {
        
        private FooFormatter fooFormatter;
        @Autowired
        public FooService(FooFormatter fooFormatter) {
        
            this.fooFormatter = fooFormatter;
        }
    }

As of Spring 4.3, classes with `a single constructor` can `omit` the *`@Autowired`* annotation.

--------------------

### @Autowired and Optional Dependencies ###

*`@Autowired`* has a boolean argument called `required` with a default value of `true`.  
It tunes Spring’s behavior when it doesn’t find a suitable bean to wire.  
When true, an exception is thrown when it doesn’t find a suitable bean to wire. otherwise, nothing is wired.

To fix this, we need to declare a bean of the required type:

public class FooService {
        
        @Autowired(required = false)
        private FooDAO dataAccessor; 
    }

--------------------

#### 5.2. Autowiring by Custom Qualifier ####

Spring also allows us to **create our own custom @Qualifier annotation**.

@Qualifier
    @Target({
        ElementType.FIELD, ElementType.METHOD, ElementType.TYPE,ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface FormatterType {
          
        String value();
    }
    
    @FormatterType("Foo")
    @Component
    public class FooFormatter implements Formatter {
        
        public String format() {
        
            return "foo";
        }
    }
    
    @FormatterType("Bar")
    @Component
    public class BarFormatter implements Formatter {
        
        public String format() {
        
            return "bar";
        }
    }
    
    @Component
    public class FooService {
          
        @Autowired
        @FormatterType("Foo")
        private Formatter formatter;
    }

**参考：**  
**[Wiring in Spring: @Autowired, @Resource and @Inject][Wiring in Spring_ _Autowired_ _Resource and _Inject]**  
**[Guide to Spring @Autowired][Guide to Spring _Autowired]**

[Wiring in Spring_ _Autowired_ _Resource and _Inject]: https://www.baeldung.com/spring-annotations-resource-inject-autowire
[Guide to Spring _Autowired]: https://www.baeldung.com/spring-autowire