问题复盘｜Spring Boot 项目启动时避免空指针异常的解决方案

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。