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我们有一个类 User 与 Preference 具有一对一的关系（该属性称为 storedPreference）。假设用户将 CustomPreference（Preference 的子类）的一个实例保存为它的 storedPreference。获取 storedPreference 并将其转换为 CustomPreference 会导致 ClassCastException 但为什么呢？

(CustomPreference)theUser.getStoredPreference()

原因

java.lang.ClassCastException: core.preferences.Preferences_$$_javassist_82 cannot be cast to hb.core.preferences.CustomPreferences
    at hb.base.user.user.HadithUser.getPreferences(HadithUser.java:39)
    at hb.base.user.user.HadithUser.getPreferences(HadithUser.java:23)
    at core.form.base.ThemeForm.getTheme(ThemeForm.java:116)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at javax.el.BeanELResolver.getValue(BeanELResolver.java:87)
    at javax.el.CompositeELResolver.getValue(CompositeELResolver.java:67)
    at org.apache.myfaces.el.unified.resolver.FacesCompositeELResolver.getValue(FacesCompositeELResolver.java:179)
    at org.apache.el.parser.AstValue.getValue(AstValue.java:183)
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:185)
    at org.primefaces.renderkit.HeadRenderer.encodeBegin(HeadRenderer.java:85)
    at javax.faces.component.UIComponentBase.encodeBegin(UIComponentBase.java:587)
    at javax.faces.component.UIComponentBase.encodeAll(UIComponentBase.java:517)
    at javax.faces.component.UIComponentBase.encodeAll(UIComponentBase.java:541)
    at org.apache.myfaces.view.facelets.FaceletViewDeclarationLanguage.renderView(FaceletViewDeclarationLanguage.java:1981)
    at org.apache.myfaces.application.ViewHandlerImpl.renderView(ViewHandlerImpl.java:298)
    at javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java:59)
    at org.apache.myfaces.tomahawk.application.ResourceViewHandlerWrapper.renderView(ResourceViewHandlerWrapper.java:93)
    at org.springframework.faces.webflow.FlowViewHandler.renderView(FlowViewHandler.java:99)
    at org.apache.myfaces.lifecycle.RenderResponseExecutor.execute(RenderResponseExecutor.java:116)
    at org.apache.myfaces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:241)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:199)
    ... 84 more

以下是实体定义：

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Preferences {
   ...
}


@Entity
public class CustomPreferences extends Preferences {
   ...
}


@Entity
public class User {
    @OneToOne(fetch = FetchType.LAZY)
    public Preferences getStoredPreferences() {
        return storedPreferences;
    }

    ...
}

我检查了数据库以确保此实例的 DTYPE 值是 CustomPreference 并且它是正确的

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1 回答 1

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这与 JPA 本身无关。这是计划java，超类引用可以引用子类对象，但超类对象不能被子类引用。

(CustomPreference)theUser.getStoredPreference();

Preference 对象可以由 Preference 类型或其超类型引用，但不能由其子类型（即 CustomPreference）引用。如果您尝试这样做，它会给出 ClassCasteException。

于 2013-07-10T07:29:01.657 回答