Question

当我尝试将String值从DropDown菜单转换为BoreholeType对象的实例并注入Borehole对象时，我收到此错误（点击＆＃34;提交＆＃34;填写后的形式）：

属性id_borehole_type引发异常;嵌套异常是java.lang.NullPointerException

我根据本教程创建了我的网络应用程序的这一部分：http://howtodoinjava.com/spring/spring-mvc/spring-mvc-populate-and-validate-dropdown-example/

我的代码：

1）Borehole.java

@Entity
@Table(name = "BOREHOLE")
public class Borehole {
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private long id;

   @OneToOne
   BoreholeType id_borehole_type;

   //other fields, getters & setters
}

2）BoreholeType.java

@Entity
@Table(name = "BOREHOLE_TYPE")
public class BoreholeType {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    private String type;

    //setters, getters
}

3）BoreholeTypeRepositoryImpl.java

@Repository
@Transactional
public class BoreholeTypeRepositoryImpl implements BoreholeTypeRepository {

    @PersistenceContext
    private EntityManager em;

    //...

    @Override
    public BoreholeType getBoreholeTypeById(int id) {
        TypedQuery<BoreholeType> boreholeTypeQuery = em.createQuery("SELECT b FROM BoreholeType b WHERE b.id = '" + id + "'", BoreholeType.class);
        BoreholeType boreholeType = boreholeTypeQuery.getSingleResult();
        return boreholeType;
    }
}

4）BoreholeTypeServiceImpl.java

@Service
public class BoreholeTypeServiceImpl implements BoreholeTypeService {
    @Autowired
    private BoreholeTypeRepository boreholeTypeRepository;

    @Override
    public List<BoreholeType> getBoreholeTypes() {
        return boreholeTypeRepository.getAllBoreholeTypes();
    }

    @Override
    public BoreholeType getBoreholeTypeById(int id) {
        return boreholeTypeRepository.getBoreholeTypeById(id);
    }

    //...
}

5）BoreholeTypeEditor.java

public class BoreholeTypeEditor extends PropertyEditorSupport {
    @Autowired
    private BoreholeTypeService boreholeTypeService;

    @Override
    public void setAsText(String id) {
        System.out.println("ID: " + id);
        // <---------------- To this moment everything is fine
        BoreholeType boreholeType = boreholeTypeService.getBoreholeTypeById(Integer.parseInt(id));

        System.out.println("ID: " + boreholeType.getId() + ", Value: " + boreholeType.getType());
        this.setValue(boreholeType);
    }
}

6）BoreholeController.java

@Controller
public class BoreholeController {
    @Autowired
    private BoreholeService boreholeService;

    @Autowired
    private BoreholeTypeService boreholeTypeService;

    @InitBinder
    public void initBinder(WebDataBinder binder) {
        binder.registerCustomEditor(BoreholeType.class, new BoreholeTypeEditor());
    }

    @GetMapping("/add_borehole")
    public String showBoreholeForm(@ModelAttribute("newBorehole") Borehole newBorehole) {
        return "add_borehole";
    }

    @PostMapping("/add_borehole")
    public String processAddNewBoreholeForm(@ModelAttribute("newBorehole") @Valid Borehole boreholeToAdd,
            BindingResult bindingResult, HttpServletRequest request) {
        if (bindingResult.hasErrors()) {
            return "add_borehole";
        }

        try {
            boreholeService.addBorehole(boreholeToAdd);
        } catch (Exception e) {
            e.printStackTrace();
            return "add_borehole";
        }

        return "borehole";
    }
}

7）add_borehole.jsp

<form:form modelAttribute="newBorehole" method="POST"
        action="add_borehole">
        <table>
            <tr>
                <td><spring:message text="_____" /></td>
                <td><form:select path="id_borehole_type">
                    <form:options items="${allBoreholeTypes}" itemValue="id" itemLabel="type" />
                    </form:select></td>
                <td><form:errors path="id_borehole_type" cssClass="error" /></td>
            </tr>
            <tr>
                <td colspan="3"><input type="submit" value="Dodaj odwiert"></td>
            </tr>
        </table>
    </form:form>

应用程序在此行之前正常工作（BoreholeTypeEditor.java）：

BoreholeType boreholeType = boreholeTypeService.getBoreholeTypeById(Integer.parseInt(id));

我犯了错误（这很有可能，因为我是Spring Framework的新手）？

Answer 1

解决此类问题的步骤（使用转换器）：

1）创建一个新的转换器类：

public class StringToBoreholeTypeConverter implements Converter<String, BoreholeType> {
    @Autowired
    private BoreholeTypeService boreholeTypeService;

    @Override
    public BoreholeType convert(String source) {
        if (source == null || source.isEmpty()) {
            return null;
        }
        BoreholeType boreholeType = boreholeTypeService.getBoreholeTypeById(Integer.parseInt(source));
        return boreholeType;
    }

}

2）将转换器bean添加到上下文文件中：

<mvc:annotation-driven conversion-service="conversionService" />

<bean id="conversionService"
        class="org.springframework.context.support.ConversionServiceFactoryBean">
    <property name="converters">
        <set>
            <bean class="com.example.converter.StringToBoreholeTypeConverter" />
        </set>
    </property>
</bean>

Spring MVC - 嵌套异常是java.lang.NullPointerException

1 个答案: