Java 指定类时Db4o queryByExample（）方法的结果为空

所以我对db4oapi有一个奇怪的问题。我试图从数据库中检索某些类的对象（取决于用户的查询，但这并不真正相关）。数据库中肯定有我想要检索的类的对象。例如，当我为任何对象查询byExample（）时，然后打印出对象集中每个元素的类，就像这样

public void evaluateQuery(ObjectContainer db) {
    if (this.hasPredicate) {
        ;
    } else {
        if (this.isNode) {
            ObjectSet nodes = db.queryByExample(Object.class);
            ListIterator listIter = nodes.listIterator();
            while (listIter.hasNext()) {
                Object node = listIter.next();
                System.out.println("Object has Class: " + node.getClass());
            }
        }
    }
} 

。。。我得到了这样的结果

Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode

public void evaluateQuery(ObjectContainer db) {
    if (this.hasPredicate) {
        ;
    } else {
        if (this.isNode) {
            ObjectSet<Node> nodes = db.queryByExample(Node.class);
            ListIterator<Node> listIter = nodes.listIterator();
            while (listIter.hasNext()) {
                Node node = listIter.next();
                System.out.println("Object has Class: " + node.getClass());
            }
        }
    }

…如您所见，数据库中的对象的类设置为Node和SubNode。现在，我希望代码能像这样工作

Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class Node
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode
Object has Class: class SubNode

public void evaluateQuery(ObjectContainer db) {
    if (this.hasPredicate) {
        ;
    } else {
        if (this.isNode) {
            ObjectSet<Node> nodes = db.queryByExample(Node.class);
            ListIterator<Node> listIter = nodes.listIterator();
            while (listIter.hasNext()) {
                Node node = listIter.next();
                System.out.println("Object has Class: " + node.getClass());
            }
        }
    }

public void evaluateQuery（ObjectContainer数据库）{
if（this.hasPredicate）{
;
}否则{
if（this.isNode）{
ObjectSet nodes=db.queryByExample（Node.class）；
ListIterator listIter=节点。ListIterator（）；
while（listIter.hasNext（））{
Node=listIter.next（）；
System.out.println（“对象有类：+node.getClass（））；
}
}
}

---

…但当我这样做时，ObjectSet总是空的，我似乎不知道为什么。我在运行时使用ClassLoader将某些类的子集（如上面的节点类）加载到环境中，它们肯定在那里，因为我在其他上下文中实例化/使用它们时没有问题。

使用直接查询对应项。It将检索给定类的所有实例。它与Java泛型的交互也更好=）

List nodes=db.query（Node.class）；
//做事

这实际上是一个包导入问题。出于某种原因，当使用db4o表示

导入com.db4o.*

不解决依赖项，即说

导入com.db4o.query.Predicate

---

…是的。很奇怪…

嗯，这很奇怪。你能提供一个链接，指向一个显示此问题的完全有效的示例吗？