Spring学习--通过注解配置 Bean (三)

组件装配：

<context:component-sacan> 元素还会自动注册 AutowiredAnnotationBeanPostProcesser 实例 , 该实例可以自动装配具有 @Autowired 和 @Resource 、 @Inject 注解的属性。

使用 @Autowired 自动装配 Bean：

@Autowired 注解自动装配具有兼容类型的单个 Bean 属性：

构造器 , 普通字段(即使是非 public) , 一切具有参数的方法都可以用 @Autowired 注解。
默认情况下 , 所有使用 @Autowired 注解的属性都需要被设置。当 Spring 找不到匹配的 bean 装配属性时 , 会抛出异常 , 若某一属性允许不被设置 , 可以设置 @Autowired 注解的 required 属性为 false。
默认情况下 , 当 IOC 容器里存在多个类型兼容的 bean 时 , 通过类型的自动装配将无法工作 , 此时可以在 @Qualifier 注解里提供 bean 的名称。Spring 允许对方法的入参标注 @Qualifiter 已指定注入 bean 的名称。或在需要自动装配的注解中通过 value 属性值标识组件的名称。
@Autowired 注解也可以应用在数组类型的属性上 , 此时 Spring 将会把所有匹配的 bean 进行自动装配。
@Autowired 注解也可以应用在集合属性上 , 此时 Spring 读取该集合的类型信息 , 然后自动装配所有与之兼容的 bean。
@Autowired 注解用在 java.util.Map 上时 , 若该 Map 的键值为 String , 那么 Spring 将自动装配与之 Map 值类型兼容的 bean , 此时 bean 的名称作为键值。

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <beans xmlns="http://www.springframework.org/schema/beans"
 3        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 4        xmlns:context="http://www.springframework.org/schema/context"
 5        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
 6        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
 7 
 8     <!-- 指定 Spring IOC 容器扫描的包 -->
 9     <context:component-scan base-package="com.itdjx.spring.annotation"/>
10 
11 </beans>

 1 package com.itdjx.spring.annotation.controller;
 2 
 3 import com.itdjx.spring.annotation.service.UserService;
 4 import org.springframework.stereotype.Controller;
 5 
 6 /**
 7  * @author Wáng Chéng Dá
 8  * @create 2017-03-03 9:28
 9  */
10 @Controller
11 public class UserController {
12     private UserService userService;
13 
14     public void execute() {
15         System.out.println("I am UserController's execute method...");
16         userService.add();
17     }
18 }

 1 package com.itdjx.spring.annotation.service;
 2 
 3 import com.itdjx.spring.annotation.repository.UserRepository;
 4 import org.springframework.stereotype.Service;
 5 
 6 /**
 7  * @author Wáng Chéng Dá
 8  * @create 2017-03-03 9:26
 9  */
10 @Service
11 public class UserService {
12 
13     private UserRepository userRepository;
14 
15     public void add() {
16         System.out.println("I am UserService's add method...");
17         userRepository.sava();
18     }
19 }

 1 package com.itdjx.spring.annotation.repository;
 2 
 3 /**
 4  * @author Wáng Chéng Dá
 5  * @create 2017-03-03 9:23
 6  */
 7 public interface UserRepository {
 8 
 9     void sava();
10 
11 }

 1 package com.itdjx.spring.annotation.repository;
 2 
 3 import org.springframework.stereotype.Repository;
 4 
 5 /**
 6  * @author Wáng Chéng Dá
 7  * @create 2017-03-03 9:24
 8  */
 9 @Repository("userRepository")
10 public class UserRepositoryImpl implements UserRepository {
11 
12     @Override
13     public void sava() {
14         System.out.println("I am UserRepositoryImpl's sava method...");
15     }
16 }

 1 package com.itdjx.spring.annotation;
 2 
 3 import com.itdjx.spring.annotation.controller.UserController;
 4 import org.springframework.context.ApplicationContext;
 5 import org.springframework.context.support.ClassPathXmlApplicationContext;
 6 
 7 /**
 8  * @author Wáng Chéng Dá
 9  * @create 2017-03-03 9:34
10  */
11 public class Main {
12 
13     public static void main(String[] args) {
14 
15         ApplicationContext cxt = new ClassPathXmlApplicationContext("beans-annotation.xml");
16         UserController userController = (UserController) cxt.getBean("userController");
17         userController.execute();
18 
19     }
20 }

控制台输出：

Exception in thread "main" java.lang.NullPointerException
at com.itdjx.spring.annotation.controller.UserController.execute(UserController.java:16)
at com.itdjx.spring.annotation.Main.main(Main.java:17)

原因：IOC 容器中虽然每个 bean 节点都存在 , 但是没有引入到需要引入的 bean 节点中 , 需要将需要引入的 bean 节点引入相应的 bean 节点中。

 1 package com.itdjx.spring.annotation.controller;
 2 
 3 import com.itdjx.spring.annotation.service.UserService;
 4 import org.springframework.beans.factory.annotation.Autowired;
 5 import org.springframework.stereotype.Controller;
 6 
 7 /**
 8  * @author Wáng Chéng Dá
 9  * @create 2017-03-03 9:28
10  */
11 @Controller
12 public class UserController {
13     @Autowired
14     private UserService userService;
15 
16     public void execute() {
17         System.out.println("I am UserController's execute method...");
18         userService.add();
19     }
20 }

 1 package com.itdjx.spring.annotation.service;
 2 
 3 import com.itdjx.spring.annotation.repository.UserRepository;
 4 import org.springframework.beans.factory.annotation.Autowired;
 5 import org.springframework.stereotype.Service;
 6 
 7 /**
 8  * @author Wáng Chéng Dá
 9  * @create 2017-03-03 9:26
10  */
11 @Service
12 public class UserService {
13 
14     @Autowired
15     private UserRepository userRepository;
16 
17     public void add() {
18         System.out.println("I am UserService's add method...");
19         userRepository.sava();
20     }
21 }

控制台输出：

I am UserController's execute method...
I am UserService's add method...
I am UserRepositoryImpl's sava method...

假如我们将 IOC 容器中不存在的 bean 节点引入到 bean 节点中 , 会出现什么状况呢?

 1 package com.itdjx.spring.annotation;
 2 
 3 
 4 /**
 5  * @author Wáng Chéng Dá
 6  * @create 2017-03-03 8:44
 7  */
 8 
 9 public class TestObject {
10 }

 1 package com.itdjx.spring.annotation.repository;
 2 
 3 import com.itdjx.spring.annotation.TestObject;
 4 import org.springframework.beans.factory.annotation.Autowired;
 5 import org.springframework.stereotype.Repository;
 6 
 7 /**
 8  * @author Wáng Chéng Dá
 9  * @create 2017-03-03 9:24
10  */
11 @Repository("userRepository")
12 public class UserRepositoryImpl implements UserRepository {
13 
14     @Autowired
15     private TestObject testObject;
16     @Override
17     public void sava() {
18         System.out.println("I am UserRepositoryImpl's sava method...");
19         System.out.println(testObject);
20     }
21 }

控制台输出：

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userController': Unsatisfied dependency expressed through field 'userService'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService': Unsatisfied dependency expressed through field 'userRepository'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userRepository': Unsatisfied dependency expressed through field 'testObject'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.itdjx.spring.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userController': Unsatisfied dependency expressed through field 'userService'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService': Unsatisfied dependency expressed through field 'userRepository'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userRepository': Unsatisfied dependency expressed through field 'testObject'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.itdjx.spring.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at com.itdjx.spring.annotation.Main.main(Main.java:15)
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService': Unsatisfied dependency expressed through field 'userRepository'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userRepository': Unsatisfied dependency expressed through field 'testObject'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.itdjx.spring.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:208)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1138)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
... 15 more
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userRepository': Unsatisfied dependency expressed through field 'testObject'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.itdjx.spring.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:208)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1138)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
... 28 more
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.itdjx.spring.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1486)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1104)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
... 41 more

解决方法：

 1 package com.itdjx.spring.annotation.repository;
 2 
 3 import com.itdjx.spring.annotation.TestObject;
 4 import org.springframework.beans.factory.annotation.Autowired;
 5 import org.springframework.stereotype.Repository;
 6 
 7 /**
 8  * @author Wáng Chéng Dá
 9  * @create 2017-03-03 9:24
10  */
11 @Repository("userRepository")
12 public class UserRepositoryImpl implements UserRepository {
13 
14     @Autowired(required = false)
15     private TestObject testObject;
16 
17     @Override
18     public void sava() {
19         System.out.println("I am UserRepositoryImpl's sava method...");
20         System.out.println(testObject);
21     }
22 }

控制台输出：

I am UserController's execute method...
I am UserService's add method...
I am UserRepositoryImpl's sava method...
null

使用 @Resource 或 @Inject 自动装配 Bean：

Spring 还支持 @Resource 和 @Inject 注解 , 这两个注解和 @Autowired 注解的功能类似。

@Resource 注解要求提供一个 bean 名称的属性 , 若该属性为空 , 则自动采用标注出的变量或方法名作为 bean 的名称。

@Inject 和 @Autowired 注解一样也是按类型匹配注入的 bean , 但没有 required 属性。

建议使用 @Autowired 注解。