见过最多次的错误：java.lang.NullPointerException

代码：

    @Autowired
    private UserMapper userMapper;
    @Autowired  // 忘记添加
    private RoleMapper roleMapper;
    
    @Override
    public Map<String, Object> selectUserByCondition(Map<String, Object> condition) {
        Map<String, Object> returnUser = userMapper.selectUserByCondition(condition);
        if (condition.get("user_pwd")==null) {
            return returnUser;
        }
        // 先用MD5对密码进行加密，再做比较
        String md5 = Md5Utils.md5((String)condition.get("user_pwd"));
        if(returnUser!=null && md5.equals(returnUser.get("user_pwd"))) {
            return returnUser;
        }
        return null;
    }

    @Override
    public int changePasswordById(Map<String, Object> user) {
        int i = userMapper.changePasswordById(user);
        if (i > 0) {
            return i;
        }
        return 0;
    }
    
    public PageUtils selectByConditionToPage(Map<String, Object>condition, int index) {
        // 1.通过条件查询记录数
        int count = userMapper.selectCountByCondition(condition);
        // 每页的记录数
        int size = Global.PAGE_SIZE;
        LOGGER.debug(count);
        PageUtils pageUtils = null;
        if (count > 0) {
            pageUtils = new PageUtils(index, size, count);
            // 2.通过条件查询数据
            // 注意：开始位置 = 索引*每页记录数
            index = index*size;
            List<Map<String,Object>> page = userMapper.selectByConditionToPage(condition, index, size);
            // 将角色的数据也加入到集合里面
            for (Map<String, Object> user : page) {
                // 第一步：通过 user 中的 role_id
                Object role_id = user.get("role_id");
                Map<String, Object> role = new HashMap<String, Object>();
                role.put("role_id", role_id);
                // 第二步：通过 role_id 获得角色信息
                role = roleMapper.selectRoleByCondition(role); // 报错点
                // 第三步：将角色的字段拼接到 user 里面
                // 使用拼接在同一层的方式，不同表的字段名是不可以重复的
                user.put("role", role);
                LOGGER.debug("管理员拼接后的字段："+role);
            }
            pageUtils.setPage(page);
        }
        return pageUtils;
    }

错误详情：

java.lang.NullPointerException
    at cn.mgy.service.impl.UserServiceImpl.selectByConditionToPage(UserServiceImpl.java:72)
    at cn.mgy.controller.UserController.toUserList(UserController.java:129)
    at cn.mgy.controller.UserController$$FastClassBySpringCGLIB$$25cf3127.invoke(<generated>)
    at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:736)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
    at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:133)
    at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:121)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:671)
    at cn.mgy.controller.UserController$$EnhancerBySpringCGLIB$$5f531980.toUserList(<generated>)
    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.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

怎么样去解决错误：第一：先看控制台，报错的原因，看到空指针异常，有点蒙，不知去哪找错了，并且报错的地方好像有点没对象。

　　　　　　　　　第二：加了日志，可日志好像并没有打印出来，联想到可能调用的方法没有去执行。那为什么会不执行呢？

　　　　　　　　　第三：应该马上联想到该对象没有注入，然后去瞄一眼，哎，真的是，真没有注入，马上注入，立即执行，啊，成功了