Spring5学习JdbcTemplate时遇到的问题

今天在学习Spring5的JdbcTemplate部分时，做前期准备的过程中遇到一个坑，记录一下

首先，导入好JdbcTemplate所需要的jar包

然后，在Spring配置文件中配置好数据库连接池和创建JdbcTemplate对象

然后，根据数据库表的结构创建好对应的实体类，并且也创建好了Service层和Dao层

//实体类
@Component
public class User {
    private Integer id;
    private String username;
    private String password;
    private String status;
    //省略set、get、toString方法
}

//service层
@Service
public class UserService {
    //注入UserDao
    @Autowired
    private UserDao userDao;
    
    public void add(User user){}
}

//dao层
@Repository
public class UserDaoImpl implements UserDao {
    //注入jdbcTemplate
    @Autowired
    private JdbcTemplate jdbcTemplate;
    
    public void insert(User user){}
}

然后，在这里我没有选择使用xml文件来配置开启组件扫描，而是使用创建配置类的方式来开启组件扫描（这儿就是踩坑的原因）

@Configuration
@ComponentScan(basePackages = {"com.spring5"})
public class SpringConfig {
}

最后，编写好测试添加数据的操作

public class TestSpring5 {
    @Test
    public void test(){
        ApplicationContext context = new AnnotationConfigApplicationContext(SpringConfig.class);
        UserService userService = context.getBean("userService", UserService.class);
        User user = new User();
        user.setId(1);
        user.setUsername("admin1");
        user.setPassword("12345");
        user.setStatus("1");
        System.out.println(user);
        userService.addUser(user);
    }
}

当我运行测试代码时，出现了报错：

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userDaoImpl': Unsatisfied dependency expressed through field 'jdbcTemplate'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.jdbc.core.JdbcTemplate' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userDaoImpl': Unsatisfied dependency expressed through field 'jdbcTemplate'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.jdbc.core.JdbcTemplate' 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$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:659)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:639)
	at org.springframework.beans.factory.annotation.Injectionmetadata.inject(Injectionmetadata.java:119)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1431)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:619)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:953)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.(AnnotationConfigApplicationContext.java:93)
	at TestSpring5.test(TestSpring5.java:23)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.frameworkMethod$1.runReflectiveCall(frameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.frameworkMethod.invokeExplosively(frameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.jdbc.core.JdbcTemplate' 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:1799)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1355)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1309)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:656)
	... 40 more

大致看了一下，应该是xml配置文件没有被加载导致在xml文件中配置创建DataSource和JdbcTemplate对象的过程没有执行，从而UserDaoImpl类中需要自动装载的JdbcTemplate对象就不能被创建

由此就可想出解决方法：加载xml配置文件创建JdbcTemplate对象即可

//在UserDaoImpl中将jdbcTemplate用final修饰，然后在代码块中读取xml配置文件来创建JdbcTemplate对象
private final JdbcTemplate jdbcTemplate;

{
    ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
    jdbcTemplate = context.getBean("jdbcTemplate", JdbcTemplate.class);
}

当然，还有一种简单的方法就是不使用配置类的方式来开启组件扫描，而在xml配置文件中开启，然后在测试加载xml配置文件，反正在使用时也会通过加载配置文件或配置类的方式获取一个调用dao层方法的service对象

在想出上述解决方法后又遇到一个疑惑，不管是使用xml配置文件还是使用配置类的方式开启组件扫描，为什么最开始都得利用ClassPathXmlApplicationContext类或AnnotationConfigApplicationContext类的对象通过getBean()方法或使用new来获取service对象呢，而如果使用@Autowired注解自动装载的话在使用service对象调用自身的方法时报出空指针异常，但是当在service类中调用dao的方法时需要用到的dao对象又可以自动装载

希望有大佬能替我解决疑惑或提出更好的解决办法，有理解不对的地方也请大佬帮我指出