Spring bean 创建但未自动装配

Question

我知道有很多类似的问题。我看了很多，但问题仍然存在。

我有一个已创建但未自动装配的服务。没有（！）手动启动，无论是在项目中还是在测试中（如this question）

Tomcat 输出显示，两个 bean 都已找到并创建。至少第一个服务没有注入它应该在的地方。我不知道第二个。

服务（接口）：

public interface SchoolService {
  public School getSchool(String id);
}

服务（实施）：

@Service
@Transactional
public class SchoolServiceImpl implements SchoolService {

  @Autowired
  private SchoolDAO schoolDAO;

  public School getSchool(String id) {
    //database things
    return school;
  }

}

它在哪里被“调用”

public class SchoolMenu implements Serializable {

  @Autowired
  private SchoolService schoolService;

  public SchoolMenu () {
    //here schoolService is null
    School school = schoolService.getSchool("id");
  }

}

应用程序上下文.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:p="http://www.springframework.org/schema/p"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
       http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd
       http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd">


    <bean id="SchoolServiceImpl" class="content_management.School.service.SchoolServiceImpl"/>
    <bean id="SchoolDAOImpl" class="content_management.School.dao.SchoolDAOImpl"/>
</beans>

Tomcat 输出：

org.springframework.beans.factory.support.DefaultListableBeanFactory: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@57003970: defining beans [SchoolServiceImpl,SchoolDAOImpl]; root of factory hierarchy
org.springframework.beans.factory.support.DefaultListableBeanFactory: Creating shared instance of singleton bean 'SchoolServiceImpl'
org.springframework.beans.factory.support.DefaultListableBeanFactory: Creating instance of bean 'SchoolServiceImpl'
org.springframework.beans.factory.support.DefaultListableBeanFactory: Eagerly caching bean 'SchoolServiceImpl' to allow for resolving potential circular references
org.springframework.beans.factory.support.DefaultListableBeanFactory: Finished creating instance of bean 'SchoolServiceImpl'
org.springframework.beans.factory.support.DefaultListableBeanFactory: Creating shared instance of singleton bean 'SchoolDAOImpl'
org.springframework.beans.factory.support.DefaultListableBeanFactory: Creating instance of bean 'SchoolDAOImpl'
org.springframework.beans.factory.support.DefaultListableBeanFactory: Eagerly caching bean 'SchoolDAOImpl' to allow for resolving potential circular references
org.springframework.beans.factory.support.DefaultListableBeanFactory: Finished creating instance of bean 'SchoolDAOImpl'

我的错误在哪里？

Answer 1

为了@Autowired 工作，SchoolMenu 还必须是一个 Spring bean。如果不是，您可以从应用程序上下文中获取schoolService。像这样的

ApplicationContext appContext = new ClassPathXmlApplicationContext("application-context.xml");
SchoolService schoolService = (SchoolService) appContext.getBean(SchoolService.class);

Answer 2

你在SchoolMenu的构造函数中访问schoolService：

public SchoolMenu () {
    //here schoolService is null
    School school = schoolService.getSchool("id");
}

此时，schoolService 尚未注入。不可能——在构造函数被调用之前什么都不会发生。

您可以在 bean 初始化后声明要调用的方法，方法是使用 javax.annotation.PostConstruct 注释对其进行注释。在该方法中执行需要注入依赖项的操作，而不是在构造函数中。

（还有其他几种实现方式，比如实现InitializingBean接口，但是这些都有些过时了）

例子：

public SchoolMenu () {
}

@PostConstruct
public void init() {
    // schoolService is NOT null here
    School school = schoolService.getSchool("id");
}

Answer 3

您可以通过将此配置添加到应用程序上下文中来使用组件扫描，而不是在 xml 中定义 bean，Spring 将自动为您扫描和检测 bean。

<context:component-scan base-package="content_management.School" />

您还需要添加上下文命名空间，以便 application-context.xml 看起来像这样：

<?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:p="http://www.springframework.org/schema/p"
   xmlns:aop="http://www.springframework.org/schema/aop"
   xmlns:tx="http://www.springframework.org/schema/tx"
   xmlns:context="http://www.springframework.org/schema/context"
   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
   http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd
   http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

 <context:component-scan base-package="content_management.School" />

 </beans>