Question

我已经写了一个任务经理，而且很好;这是一个很长的故事......顺便说一下，所有这些都是用Java编写的。所以我写了一个Facade，你可以在下面看到HashMap存在问题，我怀疑在构建过程中我试图添加到HashMap中的值并不是那么好。触发空指针异常的方法是create方法。我已经验证了该方法的输入参数，并且我的可信调试器已被填充。

这里的任何帮助都会很棒......我确定我忘了提一些东西所以我会尽快回复评论，因为我现在需要完成这件事。

package persistence;

import java.util.UUID;
import java.util.HashMap;

import persistence.framework.ComplexTaskRDBMapper;
import persistence.framework.IMapper;
import persistence.framework.RepeatingTaskRDBMapper;
import persistence.framework.SingleTaskRDBMapper;

public class PersistanceFacade {

    @SuppressWarnings("unchecked")
    private static Class SingleTask;
    @SuppressWarnings("unchecked")
    private static Class RepeatingTask;
    @SuppressWarnings("unchecked")
    private static Class ComplexTask;

    private static PersistanceFacade uniqueInstance = null;
    @SuppressWarnings("unchecked")
    private HashMap<Class, IMapper> mappers;

    public PersistanceFacade() {
        mappers = new HashMap<Class, IMapper>();
        try {
            SingleTask = Class.forName("SingleTask");
            RepeatingTask = Class.forName("RepeatingTask");
            ComplexTask = Class.forName("ComplexTask");
            mappers.put(SingleTask, new SingleTaskRDBMapper());
            mappers.put(RepeatingTask, new RepeatingTaskRDBMapper());
            mappers.put(ComplexTask, new ComplexTaskRDBMapper());
        }
        catch (ClassNotFoundException e) {}

    }

    public static synchronized PersistanceFacade getUniqueInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new PersistanceFacade();
            return uniqueInstance;
        }
        else return uniqueInstance;
    }

    public void create(UUID oid, Object obj) {
        IMapper mapper = (IMapper) mappers.get(obj.getClass());
        mapper.create(oid, obj);
    }

    @SuppressWarnings("unchecked")
    public Object read(UUID oid, Class type) {
        IMapper mapper = (IMapper) mappers.get(type);
        return mapper.read(oid);
    }

    public void update(UUID oid, Object obj) {
        IMapper mapper = (IMapper) mappers.get(obj.getClass());
        mapper.update(oid, obj);
    }

    @SuppressWarnings("unchecked")
    public void destroy(UUID oid, Class type) {
        IMapper mapper = (IMapper) mappers.get(type);
        mapper.destroy(oid);
    }


}

Answer 1

要让Class.forName("RepeatingTask")返回课程，您必须拥有课程persistence.RepeatingTask。但是在您的评论中，您说obj.getClass()会返回domain.RepeatingTask，因此我认为您有2个“RepeatingTask”类或domain.RepeatingTask是子类型。

Answer 2

我的猜测是你的问题在于构造函数：

    try {
        SingleTask = Class.forName("SingleTask");
        RepeatingTask = Class.forName("RepeatingTask");
        ComplexTask = Class.forName("ComplexTask");
        mappers.put(SingleTask, new SingleTaskRDBMapper());
        mappers.put(RepeatingTask, new RepeatingTaskRDBMapper());
        mappers.put(ComplexTask, new ComplexTaskRDBMapper());
    }
    catch (ClassNotFoundException e) {}

您默默忽略ClassNotFOundException。如果你向catch添加日志记录我希望它告诉你找不到类SingleTask，因为我希望你没有把这些类放在默认包中。

鉴于您对评论的回复，这些课程都在domain.包中，因此您可以尝试更改为：

    try {
        SingleTask = Class.forName("domain.SingleTask");
        RepeatingTask = Class.forName("domain.RepeatingTask");
        ComplexTask = Class.forName("domain.ComplexTask");
        mappers.put(SingleTask, new SingleTaskRDBMapper());
        mappers.put(RepeatingTask, new RepeatingTaskRDBMapper());
        mappers.put(ComplexTask, new ComplexTaskRDBMapper());
    }
    catch (ClassNotFoundException e) {
        log.warn("Cannot load class", e);
    }

顺便说一下，在代码中添加日志记录将有助于找出意外行为背后的原因。

Answer 3

Class.forName("SingleTask");抛出了ClassCastException，因此mappers不会被填充。由于您忽略了构造函数中的ClassCastExeption，因此您似乎错过了该错误。

在我的门面空指针:(

3 个答案: