我的 Facade 中的空指针:(

Question

我写了一个任务管理器，好吧，这是一个很长的故事......顺便说一句，全部都是用 Java 编写的。所以我写了一个 Facade，你可以在下面看到 HashMap 有一个问题，我怀疑我在构造过程中尝试添加到 HashMap 中的值进展得不太顺利。触发空指针异常的方法是create方法。该方法的输入参数已由我和我值得信赖的调试器验证并填充。

这里的任何帮助都会很棒...我确信我忘记提及一些事情，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

要使 Class.forName("RepeatingTask") 返回类，您必须有一个类 persistence.RepeatingTask。但在你的评论中你说 obj.getClass() 返回了domain.RepeatingTask 所以在我看来你有2个“RepeatingTask”类或domain.RepeatingTask 是一个子类型。

Answer 2

我的猜测是您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) {}

您默默地忽略了 ClassNotFOoundException。如果您向捕获添加日志记录，我希望它会告诉您未找到类 SingleTask，因为我希望您没有将这些类放入默认包中。

鉴于您对评论的回复，这些类位于 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);
    }

顺便说一句，向代码添加日志记录将有助于找到意外行为背后的原因。

Answer 3

Class.forName("SingleTask"); 抛出 ClassCastException，因此映射器不会被填充。由于您在构造函数中忽略了 ClassCastExeption ，因此您似乎错过了该错误。