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错误说“java.lang.ClassCastException:java.util.ArrayList无法转换为”当我甚至没有将ArrayList强制转换为类时

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我可能在这里缺少一些东西,但我无法弄明白 .

所以我编写了一个Servlet,它本质上是为了帮助我从数据库或书中下载文件,如果你愿意的话 . 现在我知道这个错误:

22:05:18,349 ERROR [io.undertow.request](默认任务-6)UT005023:对/ PubHub / DownloadBook的异常处理请求:java.lang.ClassCastException:java.util.ArrayList无法强制转换为examples.pubhub . 在javax.servlet.http.HttpServlet.service(HttpServlet.java:687)的javax.servlet.http.HttpServlet.service(HttpServlet . )上的examples.pubhub.servlets.DownloadBookServlet.doGet(DownloadBookServlet.java:37)上的model.Book . java:790)ato.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:85)ato.undertow.servlet.handlers.FilterHandler $ FilterChainImpl.doFilter(FilterHandler.java:129)org.apache.logging .log4j.web.Log4jServletFilter.doFilter(Log4jServletFilter.java:71)at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:61)at io.undertow.servlet.handlers.FilterHandler $ FilterChainImpl.doFilter(FilterHandler) .java:131)at io.undertow.servlet.handler.FilterHandler.handleRequest(FilterHandler.java:84)at io.undertow.servlet.handlers.security.ServletSecu位于org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest的io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36)中的rityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62)(SecurityContextAssociationHandler.java:78) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:131)at io.undertow.servlet.handlers.security .servletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57)at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)at io.undertow.security.handlers.AuthenticationConstraintHandler.handleRequest(AuthenticationConstraintHandler.java:53)at at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46)at io.undertow.ser vlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)at io.undertow.servlet.security.ServletSecurityConstraintHandler.handleRequest(ServletSecurityConstraintHandler.java:59)at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest( AuthenticationMechanismsHandler.java:60)io.undertow.servlet.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77)at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50)at io.undertow位于org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest的io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43).security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43) (JACCContextIdHandler.java:61)at io.undertow.server.handlers.Predicat eHandler.handleRequest(PredicateHandler.java:43)at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:292)at io .undertow.servlet.handlers.ServletInitialHandler.access $ 100(ServletInitialHandler.java:81)at io.undertow.servlet.handlers.ServletInitialHandler $ 2.call(ServletInitialHandler.java:138)at io.undertow.servlet.handlers.ServletInitialHandler $ 2 . 调用(ServletInitialHandler.java:135)io.undertow.servlet.core.ServletRequestContextThreadSetupAction $ 1.call(ServletRequestContextThreadSetupAction.java:48)at io.undertow.servlet.core.ContextClassLoaderSetupAction $ 1.call(ContextClassLoaderSetupAction.java:43)at io .undertow.servlet.api.LegacyThreadSetupActionWrapper $ 1.call(LegacyThreadSetupActionWrapper.java:44)at io.undertow.servlet.api.LegacyThreadSetupActionWrapper $ 1.call(LegacyThreadSetupActionWrapper.java:44)at io.undertow.servlet.api.LegacyThreadS etupActionWrapper $ 1.call(LegacyThreadSetupActionWrapper.java:44)atio.undertow.servlet.api.LegacyThreadSetupActionWrapper $ 1.call(LegacyThreadSetupActionWrapper.java:44)at io.undertow.servlet.api.LegacyThreadSetupActionWrapper $ 1.call(LegacyThreadSetupActionWrapper.java:44)at io.undertow.servlet.api.LegacyThreadSetupActionWrapper $ 1 .call(LegacyThreadSetupActionWrapper.java:44)at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:272)at io.undertow.servlet.handlers.ServletInitialHandler.access $ 000(ServletInitialHandler.java:81)at io .undertow.servlet.handlers.ServletInitialHandler $ 1.handleRequest(ServletInitialHandler.java:104)at io.undertow.server.Connectors.executeRootHandler(Connectors.java:202)at io.undertow.server.HttpServerExchange $ 1.run(HttpServerExchange.java) :805)java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)at java.util.concurrent.ThreadPoolExecutor $ Worker.run(Unknown Source)at java.lang.Thread.run(Unknown Source)

意味着我不安全地将ArrayList转换为ArrayList不是其子类的对象 . 问题是,我甚至没有将ArrayList转换为该对象,在本例中为Book .

我不确定是什么导致了这个问题,我检查了实体,甚至在其他地方搜索了相同的转换:

Servlet代码:

package examples.pubhub.servlets;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import examples.pubhub.dao.BookDAO;
import examples.pubhub.model.Book;
import examples.pubhub.utilities.DAOUtilities;

/**
 * Servlet implementation class DownloadBookservlet
 */
@WebServlet("/DownloadBook")
public class DownloadBookServlet extends HttpServlet {
private static final long serialVersionUID = 1L;



/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse 
response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse 
response) throws ServletException, IOException {

    /* String isbn = request.getParameter("isbn_13"); */

    String isbn = request.getParameter("isbn_13");

    BookDAO dao = DAOUtilities.getBookDAO();
    Book book = dao.getBookByISBN(isbn);

    // In order to download the PDF to the client, we need to add a data stream to the response.
    // First we set the content type to PDF, so the browser knows how to interpret the data it's receiving
    response.setContentType("application/pdf");

    // Then we set the filename to the book's title, so it's not a random string of characters
    response.setHeader("Content-Disposition", "attachment; filename=" + book.getTitle() + ".pdf");

    // Create the input stream (IN to the app FROM the book)
    InputStream is = new ByteArrayInputStream(book.getContent());

    // Create the output stream (OUT of the app TO the client)
    OutputStream os = response.getOutputStream();

    // We're going to read and write 1KB at a time
    byte[] buffer = new byte[1024];

    // Reading returns -1 when there's no more data left to read.
    while (is.read(buffer) != -1) {
        os.write(buffer);
        }


    // Always close your streams!
    os.flush();
    os.close();
    is.close();
    }
}

导致错误的方法的代码:

@Override
public Book getBookByISBN(String isbn) {

    Session session = DAOUtilities.getSessionFactory().openSession();

    session.beginTransaction();

    Query query = session.createQuery("from Book where isbn_13 =:isbn_13");
    query.setParameter("isbn_13", isbn);

    Book book = (Book) query.uniqueResult();

    session.getTransaction().commit();

    return book;

}

实际类/对象/实体的代码,Book:

package examples.pubhub.model;

import java.time.LocalDate;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "books")
public class Book {

@Id
@Column(name="isbn_13")
public String isbn_13;  // International Standard Book Number, unique

@Column(name="title")
private String title;

@Column(name="author")
private String author;

@Column(name="publish_date")
private LocalDate publish_date; // Date of publish to the website

@Column(name="price")
private double price;

@Column(name="content")
private byte[] content;

// Constructor used when no date is specified
public Book(String isbn, String title, String author, byte[] content, double price) {
    super();
    this.isbn_13 = isbn;
    this.title = title;
    this.author = author;
    this.publish_date = LocalDate.now();
    this.content = content;
    this.price = price;
}

// Constructor used when a date is specified
public Book(String isbn, String title, String author, LocalDate publishDate, double price, byte[] content) {
    super();
    this.isbn_13 = isbn;
    this.title = title;
    this.author = author;
    this.publish_date = publishDate;
    this.content = content;
    this.price = price;
}

// Default constructor
public Book() {

}

public String getIsbn_13() {
    return isbn_13;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public String getAuthor() {
    return author;
}

public void setAuthor(String author) {
    this.author = author;
}

public LocalDate getPublish_date() {
    return publish_date;
}


public void setPublish_date(LocalDate publishDate) {
    this.publish_date = publishDate;
}

public double getPrice() {
    return price;
}

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

public byte[] getContent() {
    return content;
}

public void setContent(byte[] content) {
    this.content = content;
}

public void setIsbn_13(String isbn) {
    this.isbn_13 = isbn;

}

正如你所看到的,我并没有将阵列列表(我用这种方式拼写以通过拼写检查) . 但也许在尝试将它投射到这个实体时我没有得到的东西 . 我正在使用的hibernate版本是5.1.6(5.2有它的问题) .

任何建议将不胜感激 .

1 回答

  • 0

    我解决了 . 问题在于如何使用Hibernate映射其他类 .

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