有可能是我在这里失踪的东西,但我一直没能弄明白。错误说“java.lang.ClassCastException:java.util.ArrayList不能被转换为”当我甚至没有投入ArrayList到类
所以我写了一个Servlet,它本质上是设计用来帮助我从数据库或书本下载文件,如果您愿意的话。现在我知道了这个错误:
22:05:18,349 ERROR [io.undertow.request] (default task-6) UT005023: Exception handling request to /PubHub/DownloadBook: java.lang.ClassCastException: java.util.ArrayList cannot be cast to examples.pubhub.model.Book at examples.pubhub.servlets.DownloadBookServlet.doGet(DownloadBookServlet.java:37) at javax.servlet.http.HttpServlet.service(HttpServlet.java:687) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:85) at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:129) at org.apache.logging.log4j.web.Log4jServletFilter.doFilter(Log4jServletFilter.java:71) at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:61) at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:84) at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62) at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(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 io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46) at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64) at io.undertow.servlet.handlers.security.ServletSecurityConstraintHandler.handleRequest(ServletSecurityConstraintHandler.java:59) at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60) at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77) at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50) at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61) at io.undertow.server.handlers.PredicateHandler.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.call(ServletInitialHandler.java:135) at 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.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.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$1.run(HttpServerExchange.java:805) at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) at java.lang.Thread.run(Unknown Source)
意味着我不安全铸造ArrayList的一个对象,它的ArrayList中不是一个子类。问题是,我甚至没有将ArrayList投射到该对象,在这种情况下,Book。
我不知道是什么原因造成的问题,我已经检查的实体,甚至通过其他地方一样铸造搜索:
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();
}
}
代码为方法,该方法导致错误:
@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;
}
代码实际的类/对象/实体,图书:
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,我没有将数组列表(我拼写这种方式来拼写检查)。但也许有一些东西我没有得到,当试图将其投入到这个实体。我使用的休眠版本是5.1.6(5.2有其问题)。
任何意见将不胜感激。
DownloadBookServlet中的某行已关闭37行中的内容不是您发布的代码中的可执行行。猜测,getBookByISBN()会返回一个List,但是您显示的代码并不是真正执行的代码。重新检查你的构建环境 - 你没有使用你认为你的类。 –
stdunbar
那么,它会返回一个包含书中所有属性的记录。这在技术上是否被视为一个列表?它不应该是一个对象吗? –