Springboot 项目普通类调用 Mapper 接口使用 MybatisPlus 报错：空指针异常(NullPointerException)

报错

　　开发时，在普通类调用 Mapper 接口使用 MabatisPlus 功能时会报出【空指针异常( NullPointerException )】

分析

　　普通类调用Mapper:

1. @Component 注册该普通Java类
2. @Autowired 注册 Mapper 层接口
3. 声明该类自身的静态类变量
4. 用 @PostConstruct 注入 bean，声明 init() 方法，进行初始化挂载；
5. 使用声明的该类的静态类变量，调用 Mapper 层

   @Component泛指组件，当组件不好归类的时候，我们可以使用这个注解进行标注。

   被@PostConstruct修饰的方法会在服务器加载Servlet的时候运行，并且只会被服务器调用一次，类似于Serclet的inti()方法。被@PostConstruct修饰的方法会在构造函数之后，init()方法之前运行。

解决

方案一：

 代码：

@Slf4j
@NoArgsConstructor
@Component
public class RawMessageHandler extends AbstractMessageHandler {

    @Autowired
    private MessageMapper messageMapper;

    private static RawMessageHandler rawMessageHandler;

    @PostConstruct
    private void init(){
        rawMessageHandler = this;
        rawMessageHandler.messageMapper = this.messageMapper;
    }

    @Override
    public boolean doHandle(ReturnMessage message) {
        
        MessageDO messageDO = new MessageDO();
        messageDO.setIgwId(message.ID);
        messageDO.setSendAt(DateTimeUtils.string2DateTime(message.MessageUTC));
        rawMessageHandler.messageMapper.insert(messageDO);
        return false;
    }
}

方案二：

1、首先打印出Spring容器所有的Bean名称

@Component
public class ApplicationContextBean implements ApplicationContextAware, InitializingBean {
 
    public static ApplicationContext applicationContext;
 
    /**
     * 获取 ApplicationContext
     *
     * @param applicationContext
     * @throws BeansException
     */
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        ApplicationContextBean.applicationContext = applicationContext;
    }
 
    /**
     * 打印IOC容器中所有的Bean名称
     *
     * @throws Exception
     */
    @Override
    public void afterPropertiesSet() throws Exception {
        String[] names = applicationContext.getBeanDefinitionNames();
        for (String name : names) {
            System.out.println(">>>>>>" + name);
        }
       System.out.println("------\nBean 总计:" + applicationContext.getBeanDefinitionCount());
    }
 
}

2、开始写获取bean工具类

@Component
public class SpringUtil implements ApplicationContextAware {
    private static ApplicationContext applicationContext = null;
 
    public SpringUtil() {
    }
 
    public void setApplicationContext(ApplicationContext arg0) throws BeansException {
        if (applicationContext == null) {
            applicationContext = arg0;
        }
 
    }
 
    public static ApplicationContext getApplicationContext() {
        return applicationContext;
    }
 
    public static void setAppCtx(ApplicationContext webAppCtx) {
        if (webAppCtx != null) {
            applicationContext = webAppCtx;
        }
    }
 
    /**
     * 拿到ApplicationContext对象实例后就可以手动获取Bean的注入实例对象
     */
    public static <T> T getBean(Class<T> clazz) {
        return getApplicationContext().getBean(clazz);
    }
 
    public static <T> T getBean(String name, Class<T> clazz) throws ClassNotFoundException {
        return getApplicationContext().getBean(name, clazz);
    }
 
    public static final Object getBean(String beanName) {
        return getApplicationContext().getBean(beanName);
    }
 
    public static final Object getBean(String beanName, String className) throws ClassNotFoundException {
        Class clz = Class.forName(className);
        return getApplicationContext().getBean(beanName, clz.getClass());
    }
 
    public static boolean containsBean(String name) {
        return getApplicationContext().containsBean(name);
    }
 
    public static boolean isSingleton(String name) throws NoSuchBeanDefinitionException {
        return getApplicationContext().isSingleton(name);
    }
 
    public static Class<?> getType(String name) throws NoSuchBeanDefinitionException {
        return getApplicationContext().getType(name);
    }
 
    public static String[] getAliases(String name) throws NoSuchBeanDefinitionException {
        return getApplicationContext().getAliases(name);
    }
 
　　//测试
    public static String getCharId(String chatId) {
        ChatListAssociationMapper associationMapper = (ChatListAssociationMapper) SpringUtil.getBean("chatListAssociationMapper");//这个参数是spring容器里存在的，名字最好复制，大小写要一致，不然会失败
        return associationMapper.chatId(chatId);
    }
 
}