上一节讲到了基本的基于注解的配置Bean,但是每个Bean之间是没有关联的,现在我们想实现下面的功能。
基本目录:
UserController.java
在这里调用UserService中的add方法。
package com.gong.spring.beans.annotation.controller;
import org.springframework.stereotype.Controller;
import com.gong.spring.beans.annotation.service.UserService;
@Controller
public class UserController {
private UserService userService;
public void execute() {
System.out.println("UserController的execute方法");
userService.add();
}
}
UserService.java
在这里调用UserRepository中的save方法。
package com.gong.spring.beans.annotation.service;
import org.springframework.stereotype.Service;
import com.gong.spring.beans.annotation.repository.UserRepository;
@Service
public class UserService {
private UserRepository userRepository;
public void add() {
System.out.println("UserService中的add方法");
userRepository.save();
}
}
UserRepository.java
package com.gong.spring.beans.annotation.repository;
public interface UserRepository {
public void save();
}
UserRepositoryImpl.java
package com.gong.spring.beans.annotation.repository;
import org.springframework.stereotype.Repository;
@Repository(value="userRepository")
public class UserRepositoryImpl implements UserRepository{
@Override
public void save() {
// TODO Auto-generated method stub
System.out.println("UserReposityImpl的save方法");
}
}
Main.java
package com.gong.spring.beans.annotation;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.gong.spring.beans.annotation.controller.UserController;public class Main {
public static void main(String[] args) {
//1.创建spring的IOC容器对象
ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-annotation.xml");
//2.从容器中获取Bean实例
UserController userController = (UserController) ctx.getBean("userController");
System.out.println(userController);
userController.execute();
}
}
beans-annotation.xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd">
<!-- 配置springIOC容器扫描的包 -->
<context:component-scan base-package="com.gong.spring.beans.annotation">
</context:component-scan>
</beans>
按照上节正常的配置,打印下输出结果:
会报空指针异常,是因为我们还没有在UserController中装配userService属性。
组件装配:<context:component-scan> 还会自动注册AutowiredAnnotationBeanPostProcessor实例,该实例可以自动装配具有@Autowired、@Resource和@INject注解的属性。
使用@Autowired注解自动装配具有类型兼容的单个Bean属性:
讲了这么多,就是在要用@Autowired让属性自动装配到相应的bean上,即变成:
@Autowired
private UserService userService;
@Autowired
private UserRepository userRepository;
那么,就可以正常运行了,输出:
当然,还有另一种方式就是将@Autowired对setter方法进行注解,而不是属性,即:
private UserRepository userRepository;
@Autowired
public void setUserRepository(UserRepository userRepository) {
this.userRepository = userRepository;
}
这样也是可以的。
TestObject.java
package com.gong.spring.beans.annotation;
import org.springframework.stereotype.Component;
@Component
public class TestObject {
}
接下来我们在userRepositoryImpl中加入:
package com.gong.spring.beans.annotation.repository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;
import com.gong.spring.beans.annotation.TestObject;
@Repository(value="userRepository")
public class UserRepositoryImpl implements UserRepository{
@Autowired
private TestObject testObject;
@Override
public void save() {
// TODO Auto-generated method stub
System.out.println("UserReposityImpl的save方法");
System.out.println(testObject);
}
}
程序是可以运行的:
我们将TestObjec.java中的@Component注解去掉:
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 method 'setUserRepository' parameter 0; 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.gong.spring.beans.annotation.TestObject' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
因为我们设置了:
@Autowired
private TestObject testObject;
没有找到该Bean,就会抛出异常。
我们再加上:说明不必要装配该属性
@Autowired(required=false)
private TestObject testObject;
此时即使没有装配上TestObject也不会抛出异常了,即
(我们删除掉了@Autowired(required=false)private TestObject testObject;System.out.println(testObject);)我们再新建一个UserJdbcRepository.java
package com.gong.spring.beans.annotation.repository;
import org.springframework.stereotype.Repository;
@Repository
public class UserJdbcRepository implements UserRepository {
@Override
public void save() {
// TODO Auto-generated method stub
System.out.println("UserJdbcRepository的save方法");
}
}
同时删除掉@Repository(value="userRepository")中的value属性,运行会抛出异常:没有一个唯一的bean。
因此存在多个bean的实现类时,我们需要指定bean的名字,在调用时调用该名字,即:
private UserRepository userRepository;
@Autowired
public void setUserRepository(UserRepository userRepository) {
this.userRepository = userRepository;
}
或者,我们这么指定要装配的bean的名字:
private UserRepository userRepository;
@Autowired
@Qualifier("userJdbcRepository")
public void setUserRepository(UserRepository userRepository) {
this.userRepository = userRepository;
}
输出:
也可以将该注解放入的setter方法中:
private UserRepository userRepository;
@Autowired
public void setUserRepository(@Qualifier("userJdbcRepository") UserRepository userRepository) {
this.userRepository = userRepository;
}
@Resource和@Inject和@Autowired类似,一般使用@Autowired就足够了。