问题复盘|Spring Boot 项目启动时避免空指针异常的解决方案

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猴君
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Hello大家好,我是阿月,坚持学习,老年痴呆追不上我。
在Spring Boot项目中,空指针异常(NullPointerException, NPE)是常见的问题之一,尤其是在Spring容器尚未完全初始化时试图获取Bean对象。今天我们将探讨在Spring Boot项目启动时如何避免此类异常,并提供几种有效的解决方案。

问题描述

在Spring Boot项目启动时,以下代码导致了空指针异常:

DepartmentMapper departmentMapper = ApplicationContextHolder.getApplicationContext().getBean(DepartmentMapper.class); 

该代码片段位于一个标注为@Component的类中:

@Component public class ClusterConfig {      public static List<ClusterItemConfig> clusterItemConfigList;      DepartmentMapper departmentMapper = ApplicationContextHolder.getApplicationContext().getBean(DepartmentMapper.class);      @Value("${clusterConfigList:[]}")     private void setValue(String clusterConfigList) {         List<ClusterItemConfig> result = new Gson().fromJson(clusterConfigList, new TypeToken<ArrayList<ClusterItemConfig>>() {         }.getType());         result.forEach(clusterBaseConfig ->                 clusterBaseConfig.setDepartmentCodeList(                         departmentMapper.getCodeListByName(clusterBaseConfig.getDepartmentNameList())));         ClusterConfig.clusterItemConfigList = result;     } } 

问题出在Spring容器尚未完全初始化时,ApplicationContextHolder.getApplicationContext()返回null,导致departmentMapper为空。

解决方案

1. 使用 @Autowired 注入

使用@Autowired注解将DepartmentMapper注入到ClusterConfig类中,让Spring自动管理DepartmentMapper的初始化和注入。

@Component public class ClusterConfig {      public static List<ClusterItemConfig> clusterItemConfigList;      @Autowired     private DepartmentMapper departmentMapper;      @Value("${clusterConfigList:[]}")     private void setValue(String clusterConfigList) {         List<ClusterItemConfig> result = new Gson().fromJson(clusterConfigList, new TypeToken<ArrayList<ClusterItemConfig>>() {         }.getType());         result.forEach(clusterBaseConfig ->                 clusterBaseConfig.setDepartmentCodeList(                         departmentMapper.getCodeListByName(clusterBaseConfig.getDepartmentNameList())));         ClusterConfig.clusterItemConfigList = result;     } } 

2. 延迟初始化

在方法内部延迟获取Bean,只在需要时通过ApplicationContextHolder获取DepartmentMapper

@Component public class ClusterConfig {      public static List<ClusterItemConfig> clusterItemConfigList;      @Value("${clusterConfigList:[]}")     private void setValue(String clusterConfigList) {         DepartmentMapper departmentMapper = ApplicationContextHolder.getApplicationContext().getBean(DepartmentMapper.class);         List<ClusterItemConfig> result = new Gson().fromJson(clusterConfigList, new TypeToken<ArrayList<ClusterItemConfig>>() {         }.getType());         result.forEach(clusterBaseConfig ->                 clusterBaseConfig.setDepartmentCodeList(                         departmentMapper.getCodeListByName(clusterBaseConfig.getDepartmentNameList())));         ClusterConfig.clusterItemConfigList = result;     } } 

3. 使用 @PostConstruct 注解

使用@PostConstruct注解确保在Spring容器初始化完成后执行特定的初始化逻辑。

@Component public class ClusterConfig {      public static List<ClusterItemConfig> clusterItemConfigList;      @Autowired     private DepartmentMapper departmentMapper;      @Value("${clusterConfigList:[]}")     private String clusterConfigList;      @PostConstruct     private void init() {         List<ClusterItemConfig> result = new Gson().fromJson(clusterConfigList, new TypeToken<ArrayList<ClusterItemConfig>>() {         }.getType());         result.forEach(clusterBaseConfig ->                 clusterBaseConfig.setDepartmentCodeList(                         departmentMapper.getCodeListByName(clusterBaseConfig.getDepartmentNameList())));         ClusterConfig.clusterItemConfigList = result;     } } 

4. 确保 ApplicationContextHolder 初始化正确

确保ApplicationContextHolder能正确获取ApplicationContext,并在所有Bean初始化后能够正常工作。

@Component public class ApplicationContextHolder implements ApplicationContextAware {      private static ApplicationContext context;      @Override     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {         context = applicationContext;     }      public static ApplicationContext getApplicationContext() {         return context;     } } 

结论

通过以上方法之一,可以确保在Spring容器完全初始化后,正确地获取DepartmentMapper Bean,从而避免空指针异常。在实际项目中,根据具体情况选择合适的方案,确保代码的稳定性和可维护性。
如果还有任何疑问或建议,欢迎在评论区留言讨论。

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