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Session 严重：为servlet目录页java.lang.InstanceionException分配异常

session servlets

Session 严重：为servlet目录页java.lang.InstanceionException分配异常,session,servlets,tomcat6,Session,Servlets,Tomcat6,我正在练习“使用购物车和会话跟踪的在线商店”或coreservlet教程。我在运行CatalogPage servlet时发现异常。stacktrace如下所示： SEVERE: Allocate exception for servlet CatalogPage java.lang.InstantiationException at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(Instant

我正在练习“使用购物车和会话跟踪的在线商店”或coreservlet教程。我在运行CatalogPage servlet时发现异常。stacktrace如下所示：

SEVERE: Allocate exception for servlet CatalogPage
java.lang.InstantiationException
    at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:30)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at java.lang.Class.newInstance0(Class.java:355)
    at java.lang.Class.newInstance(Class.java:308)
    at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1149)
    at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:827)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:861)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:606)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)

<servlet>
      <servlet-name>CatalogPage</servlet-name>
      <servlet-class>com.servletcontroller.CatalogPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>CatalogPage</servlet-name>
      <url-pattern>/CatalogPage</url-pattern>
  </servlet-mapping> 
  <servlet>
      <servlet-name>KidsBooksPage</servlet-name>
      <servlet-class>com.servletcontroller.KidsBooksPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>KidsBooksPage</servlet-name>
      <url-pattern>/KidsBooksPage</url-pattern>
  </servlet-mapping>
  <servlet>
      <servlet-name>TechBooksPage</servlet-name>
      <servlet-class>com.servletcontroller.TechBooksPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>TechBooksPage</servlet-name>
      <url-pattern>/TechBooksPage</url-pattern>
  </servlet-mapping>
java是一个抽象的servlet类，由两个类扩展，每个类分别有init（）定义。代码如下所示：

public abstract class CatalogPage extends HttpServlet {
    private Item[] items;
    private String[] itemIDs;
    private String title;

    protected void setItems(String[] itemIDs) {
        this.itemIDs = itemIDs;
        items = new Item[itemIDs.length];
    }

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

    @Override
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        if (items == null) {
            response.sendError(response.SC_NOT_FOUND, "Missing Items.");
            return;
        }
        PrintWriter out = response.getWriter();
        out.println(ServletUtilities.headWithTitle(title)
                + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=\"CENTER\">"
                + title + "</H1>");
        Item item;
        for (int i = 0; i < items.length; i++) {
            out.println("<HR>");
            item = items[i];
            // Show error message if subclass lists item ID
            // that’s not in the catalog.
            if (item == null) {
                out.println("<FONT COLOR=\"RED\">" + "Unknown item ID "
                        + itemIDs[i] + "</FONT>");
            } else {
                out.println();
                String formURL = "OrderPage";
                // Pass URLs that reference own site through encodeURL.
                formURL = response.encodeURL(formURL);
                out.println("<FORM ACTION=\"" + formURL + "\">\n"
                        + "<INPUT TYPE=\"HIDDEN\" NAME=\"itemID\" "
                        + "       VALUE=\"" + item.getItemID() + "\">\n"
                        + "<H2>" + item.getShortDescription() + " ($"
                        + item.getCost() + ")</H2>\n"
                        + item.getLongDescription() + "\n" + "<P>\n<CENTER>\n"
                        + "<INPUT TYPE=\"SUBMIT\" "
                        + "VALUE=\"Add to Shopping Cart\">\n"
                        + "</CENTER>\n<P>\n</FORM>");
            }
        }
        out.println("<HR>\n</BODY></HTML>");

    }
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}
public class KidsBooksPage extends CatalogPage {
    public void init() {
        String[] ids = { "lewis001", "alexander001", "rowling001" };
        setItems(ids);
        setTitle("All-Time Best Children’s Fantasy Books");
    }
}
public class TechBooksPage extends CatalogPage {
    public void init() {
        String[] ids = { "hall001", "hall002" };
        setItems(ids);
        setTitle("All-Time Best Computer Books");
    }
}

公共抽象类CatalogPage扩展了HttpServlet{
私人物品[]项；
私有字符串[]itemIDs；
私有字符串标题；
受保护的无效集合项（字符串[]项ID）{
this.itemIDs=itemIDs；
items=新项目[ItemId.length]；
}
受保护的无效集合标题（字符串标题）{
this.title=标题；
}
@凌驾
受保护的无效数据集（HttpServletRequest请求，
HttpServletResponse响应）引发ServletException，IOException{
response.setContentType（“text/html”）；
if（items==null）{
response.sendError（response.SC_未找到，“缺少项”）；
回来
}
PrintWriter out=response.getWriter（）；
out.println（ServletUtilities.headWithTitle（title）
+“\n”+”
+标题+“”）；
项目；
对于（int i=0；i”）；
项目=项目[i]；
//如果子类列出项目ID，则显示错误消息
//那不在目录里。
如果（项==null）{
out.println（“+”未知项目ID”
+itemIDs[i]+“”）；
}否则{
out.println（）；
字符串formURL=“OrderPage”；
//通过encodeURL传递引用自己站点的URL。
formURL=response.encodeURL（formURL）；
out.println（“\n”
+“\n”
+“+项。getShortDescription（）+”（$”
+item.getCost（）+“”\n“
+item.getLongDescription（）+“\n”+“
\n\n”
+“\n”
+“\n
\n”）；
}
}
out.println（“
\n”）；
}
@凌驾
受保护的void doPost（HttpServletRequest请求、HttpServletResponse响应）
抛出ServletException、IOException{
//TODO自动生成的方法存根
doGet（请求、响应）；
}
}
公共类KidsBooksPage扩展了CatalogPage{
公共void init（）{
字符串[]id={“lewis001”、“alexander001”、“rowling001”}；
设置项目（ID）；
setTitle（“所有时间最佳儿童幻想书”）；
}
}
公共类TechBookPage扩展了CatalogPage{
公共void init（）{
字符串[]id={“hall001”，“hall002”}；
设置项目（ID）；
setTitle（“所有时间最好的计算机书籍”）；
}
}
web.xml中的sevlet映射如下所示：

SEVERE: Allocate exception for servlet CatalogPage
java.lang.InstantiationException
    at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:30)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at java.lang.Class.newInstance0(Class.java:355)
    at java.lang.Class.newInstance(Class.java:308)
    at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1149)
    at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:827)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:861)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:606)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)

<servlet>
      <servlet-name>CatalogPage</servlet-name>
      <servlet-class>com.servletcontroller.CatalogPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>CatalogPage</servlet-name>
      <url-pattern>/CatalogPage</url-pattern>
  </servlet-mapping> 
  <servlet>
      <servlet-name>KidsBooksPage</servlet-name>
      <servlet-class>com.servletcontroller.KidsBooksPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>KidsBooksPage</servlet-name>
      <url-pattern>/KidsBooksPage</url-pattern>
  </servlet-mapping>
  <servlet>
      <servlet-name>TechBooksPage</servlet-name>
      <servlet-class>com.servletcontroller.TechBooksPage</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>TechBooksPage</servlet-name>
      <url-pattern>/TechBooksPage</url-pattern>
  </servlet-mapping>


目录页
com.servletcontroller.CatalogPage
目录页
/目录页
儿童书页
com.servletcontroller.KidsBooksPage
儿童书页
/儿童书页
TechBooksPage
com.servletcontroller.TechBooksPage
TechBooksPage
/TechBooksPage
请帮助我找到该异常的根本原因。

CatalogPage.java是一个抽象类，因此不能以这种方式实例化和显示它。根据的第281页，不应显示，因为“…每个显示页面的代码列出了页面标题和项目标识符…”，这是CatalogPage.java不能做的。因此，唯一的显示页面是KidsBooksPage.java和TechBooksPage.java