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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。