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在 Eclipse 中，我使用 DAO 制作了一个项目，它运行良好。然后我创建了一个新的 JPA 项目并将其添加到初始项目的部署程序集中。然后，我将一个 DAO 转换为与 JPA 一起使用，并将其放入 JPA 项目中。当我测试应用程序时，它给出了一个 Java 错误 NoClassDefFoundError（见底部）。我不知道为什么。我到处检查过，但找不到错误。JPA 在 servlet 中被调用：

package eshop;
....
import model.ProductModelDAO;
import model.ProductModelDAOImpl;


ProductModelDAO dao4 = new ProductModelDAOImpl("Product");
ArrayList products = dao4.getProductsByCategory(categoryId);

然后在 JPA 项目中，我有 Product.java，它是从“产品”表生成的。我有persistence.xml。我有两个原始的 DAO：

package model;

import java.util.*;

public interface ProductModelDAO {  
public ArrayList searchProducts(String keyword);
public ArrayList getProductsByCategory(String categoryId);
public Product getProductById(String productID);
}

和：

package model;

import java.util.ArrayList;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.PersistenceContext;


public class ProductModelDAOImpl implements ProductModelDAO {

@PersistenceContext
private EntityManager em;

// this is the default constructor
public ProductModelDAOImpl()
{
this("Product");
}

  public ProductModelDAOImpl(String unitName)
  {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory(unitName);
    em = emf.createEntityManager();
  }

  // this is the special constructor to set the entity manager.
  // this is used when we run with EJB3
  public ProductModelDAOImpl(EntityManager em)
  {
    this.em = em;
  }

  public void setEntityManager(EntityManager em)
  {
    this.em = em;
  }


@Override
public ArrayList<Product> searchProducts(String keyword) {
    @SuppressWarnings("unchecked")
    ArrayList<Product> results = (ArrayList<Product>)
      em.createQuery("select A from products A where (A.product_name = ?1 or A.descr = ?1)")
          .setParameter(1, keyword)
          .getResultList();
    return results; 
}

@Override
public ArrayList <Product> getProductsByCategory(String categoryId) {
    @SuppressWarnings("unchecked")
    ArrayList<Product> results = (ArrayList<Product>)
            em.createQuery("select A from products A where A.category_id = ?1")
              .setParameter(1, categoryId)
              .getResultList();
        return results; 
}

@Override
public Product getProductById(String productID) {
    Product results = 
    (Product) em.createQuery("select A from products A where A.product_id = ?1")
      .setParameter(1, productID)
      .getResultList();
    return results; 

}

}

持久性xml如下：

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
xmlns="http://java.sun.com/xml/ns/persistence" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence 
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
<persistence-unit name="abook">
<provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider>
<class>model.Product</class>
<properties>
<property name="openjpa.ConnectionDriverName" value="com.mysql.jdbc.Driver" />
<property name="openjpa.ConnectionURL" value="jdbc:mysql://localhost:3306/shop" />
<property name="openjpa.ConnectionUserName" value="root" />
<property name="openjpa.ConnectionPassword" value="" />
<property name="openjpa.Log" value="DefaultLevel=WARN, Tool=INFO" />
</properties>
</persistence-unit>
</persistence>

错误如下

java.lang.NoClassDefFoundError: model/ProductModelDAOImpl
at eshop.Servlet.doPost(Servlet.java:112)
at eshop.Servlet.doGet(Servlet.java:57)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:300)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:183)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.doIt(WebAppServletContext.java:3686)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3650)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:121)
at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2268)
at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2174)
at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1446)
at weblogic.work.ExecuteThread.execute(ExecuteThread.java:201)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:173)
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1 回答 1

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当 Java 虚拟机无法在运行时找到在编译期间可用的特定类时，Java 中的 NoClassDefFoundError 就会出现。查看ProductModelDAOImplweb-app 类路径中是否可用。

在这里阅读更多

于 2012-11-02T11:43:47.557 回答