springboot 项目中在普通类中调用dao层的mapper 出现空指针异常

项目中我遇到同样的问题 特记载一下

有两种方式

一、

该类使用@Component注解

添加一个本类类型的静态字段

创建一个初始化方法，贴上@PostConstruct 标签，用于注入bean

创建方法调用mapper或service接口

最后直接在普通类中调用即可

@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。

个人理解就是mapper 装载失败 重点init()方法中转了一次

@Component
public class TestUtil {
 
    @Autowired
    private DoctorInformationMapper doctorInformationMapper;
 
    public static TestUtil testUtil;
 
    @PostConstruct
    public void init() {
        testUtil = this;
        testUtil.doctorInformationMapper = this.doctorInformationMapper;
    }
 
    //调用
    public static int getOpenId() {
        int count = testUtil.doctorInformationMapper.count("12313");
        return count;
    }
 
}

二。通过spring 容器去拿

2.1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());
    }
 
}

不用调用测试，项目启动就会在后台打印出来

2.2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);
    }
}

注意以上的两种方法都不能在main方法调用，会报空指针异常