Spring學習--通過注解配置 Bean (三)

本文轉載自查看原文 2017-03-03 15:17 1943 Spring

組件裝配：

<context:component-sacan> 元素還會自動注冊 AutowiredAnnotationBeanPostProcesser 實例 , 該實例可以自動裝配具有 @Autowired 和 @Resource 、 @Inject 注解的屬性。

使用 @Autowired 自動裝配 Bean：

@Autowired 注解自動裝配具有兼容類型的單個 Bean 屬性：

構造器 , 普通字段(即使是非 public) , 一切具有參數的方法都可以用 @Autowired 注解。
默認情況下 , 所有使用 @Autowired 注解的屬性都需要被設置。當 Spring 找不到匹配的 bean 裝配屬性時 , 會拋出異常 , 若某一屬性允許不被設置 , 可以設置 @Autowired 注解的 required 屬性為 false。
默認情況下 , 當 IOC 容器里存在多個類型兼容的 bean 時 , 通過類型的自動裝配將無法工作 , 此時可以在 @Qualifier 注解里提供 bean 的名稱。Spring 允許對方法的入參標注 @Qualifiter 已指定注入 bean 的名稱。或在需要自動裝配的注解中通過 value 屬性值標識組件的名稱。
@Autowired 注解也可以應用在數組類型的屬性上 , 此時 Spring 將會把所有匹配的 bean 進行自動裝配。
@Autowired 注解也可以應用在集合屬性上 , 此時 Spring 讀取該集合的類型信息 , 然后自動裝配所有與之兼容的 bean。
@Autowired 注解用在 java.util.Map 上時 , 若該 Map 的鍵值為 String , 那么 Spring 將自動裝配與之 Map 值類型兼容的 bean , 此時 bean 的名稱作為鍵值。

 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 容器掃描的包 -->
 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 }

控制台輸出：

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 容器中雖然每個 bean 節點都存在 , 但是沒有引入到需要引入的 bean 節點中 , 需要將需要引入的 bean 節點引入相應的 bean 節點中。

 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 }

控制台輸出：

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

假如我們將 IOC 容器中不存在的 bean 節點引入到 bean 節點中 , 會出現什么狀況呢?

 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 }

控制台輸出：

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

解決方法：

 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 }

控制台輸出：

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

使用 @Resource 或 @Inject 自動裝配 Bean：

Spring 還支持 @Resource 和 @Inject 注解 , 這兩個注解和 @Autowired 注解的功能類似。

@Resource 注解要求提供一個 bean 名稱的屬性 , 若該屬性為空 , 則自動采用標注出的變量或方法名作為 bean 的名稱。

@Inject 和 @Autowired 注解一樣也是按類型匹配注入的 bean , 但沒有 required 屬性。

建議使用 @Autowired 注解。

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