java为什么会出现类强制转换错误？（冬眠）

Question

我有一个Product类，它有一个子类InventoryProduct。它们通过数据库中的upc（通用产品代码）连接。这是产品类

@Entity
@Table(name = "PRODUCT")
@Inheritance(strategy = InheritanceType.JOINED)
public class Product implements java.io.Serializable {

/**
 * 
 */
private static final long serialVersionUID = 1L;

private long id;

private String upc;

private Category category;

private String name;

private String description;

private BigDecimal price;

public Product(){

}

public Product(String upc){
    this.upc = upc;
}

public Product(String upc, String name, BigDecimal price){
    this.upc = upc;
    this.name = name;
    this.price = price;
}

public Product(String upc, Category category, String name, String description, BigDecimal price) {
    super();
    this.upc = upc;
    this.category = category;
    this.name = name;
    this.description = description;
    this.price = price;
}

public void setId(long id) {
    this.id = id;
}

@Id @GeneratedValue
@Column(name = "id")
public long getId() {
    return id;
}

@Column(name = "upc")
public String getUpc() {
    return upc;
}

public void setUpc(String upc) {
    this.upc = upc;
}

@OneToOne
@JoinColumn(name = "category_id", referencedColumnName = "category_id", insertable = false, updatable = false)
public Category getCategory() {
    return category;
}

public void setCategory(Category category) {
    this.category = category;
}

@Column(name = "name")
public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

@Column(name = "description")
public String getDescription() {
    return description;
}

public void setDescription(String description) {
    this.description = description;
}

@Column(name = "price")
public BigDecimal getPrice() {
    return price;
}

public void setPrice(BigDecimal price) {
    this.price = price;
}

}

这是InventoryProduct类

@Entity
@Table(name = "PRODUCT_INVENTORY")
@Polymorphism(type = PolymorphismType.EXPLICIT)
public class InventoryProduct extends Product {

/**
 * 
 */
private static final long serialVersionUID = 1L;

private long id;

private Product product;

private int stock;

public InventoryProduct(){

}

public InventoryProduct(Product product) {
    super(product.getUpc(), product.getCategory(), product.getName(), product.getDescription(), product.getPrice());
    // TODO Auto-generated constructor stub
}

public InventoryProduct(Product product, int stock) {
    super(product.getUpc(), product.getCategory(), product.getName(), product.getDescription(), product.getPrice());
    this.stock = stock;
}

public InventoryProduct(String upc, int stock){
    setUpc(upc);
    this.stock = stock;
}

@Id @GeneratedValue
@Column(name = "id")
public long getId() {
    return id;
}

public void setId(long id) {
    this.id = id;
}

@Column(name = "stock")
public int getStock() {
    return stock;
}

public void setStock(int stock) {
    this.stock = stock;
}

@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name="upc", referencedColumnName = "upc", insertable = false, updatable = false)
public Product getProduct() {
    return product;
}

public void setProduct(Product product) {
    this.product = product;
}
}

我想做的是在这两个对象之间做一个内部连接。我的问题是：

public static final String HQL_GET_INVENTORY_PRODUCT = "from InventoryProduct as ip inner join ip.product as p where ip.upc = :upc";

这里是我使用这个查询的地方：

@Override
public InventoryProduct getInventoryProductByUpc(String upc) {

    InventoryProduct product = null;

    Session session = sessionFactory.openSession();

    Transaction transaction = null;

    try {
        transaction = session.beginTransaction();

        Query query = session.createQuery(DAOQuery.HQL_GET_INVENTORY_PRODUCT)
                             .setParameter("upc", upc);

        product = (InventoryProduct) query.getSingleResult();

        transaction.commit();
    } catch (HibernateException e) {
        if (transaction != null)
            transaction.rollback();

        e.printStackTrace();
    } finally {
        session.close();
    }

    return product;
}

我不知道为什么会出现这种类型的类强制转换错误：

Oct 19, 2016 11:06:33 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [dispatcher] in context with path [/spring-machine-problem] threw exception [Request processing failed; nested exception is java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.qbryx.domain.InventoryProduct] with root cause
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.qbryx.domain.InventoryProduct
at com.qbryx.dao.ProductDaoHQLImpl.getInventoryProductByUpc(ProductDaoHQLImpl.java:104)
at com.qbryx.service.CustomerServiceImpl.addProductInCart(CustomerServiceImpl.java:44)
at com.qbryx.controller.CustomerController.productCart(CustomerController.java:91)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:180)
at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:440)
at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:428)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.orm.hibernate5.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:151)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1100)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:687)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

有人能帮我吗？谢谢我已经被困在这里好几个小时了

编辑

这里是例外情况指向的地方：

product = (InventoryProduct) query.getSingleResult();

Answer 1

getSingleResult返回对象的数组，我想这个数组的第一个元素是您的产品

Object[] arr = (Object[]) query.getSingleResult();
if(arr.length == 0) {
    product = null;
} else {
    product = (InventoryProduct)arr[0];
}

您可以使用返回列表的方法（getResult据我记忆所及），它可以为您节省一次强制转换