java - 结合使用AJAX和Servlet

我刚开始使用AJAX。我不确定很多事情，尽管我在网上找到了一些教程并开始工作。对于粘贴大量代码，我深表歉意，但是对于我哪里出错了，我并没有太多想法。谢谢您的帮助！ :-)

这是我的JSP（仅javascript部分）和servlet代码：（之后是异常，后跟等效的是我将用于普通Java应用程序的Java代码）：

JSP：

//--Function to get the xmlhttp object
function getHttpObject(){
    var xmlhttp = null;
    if (window.XMLHttpRequest){xmlhttp=new XMLHttpRequest();}
    else if (window.ActiveXObject){xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");}
    else {alert("Your browser does not support XMLHTTP!");}
    return xmlhttp;
}

function populateReply(str){
    xmlhttp = getHttpObject();
    xmlhttp.onreadystatechange=function(){
        if(xmlhttp.readyState==4){
            ajxfrm.chatresponse.value=xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET","NewServlet?chatinput="+str,true);
    xmlhttp.send(null);
}

Servlet：

public class NewServlet extends HttpServlet implements Servlet {

NetworkAimlFacade aiml = null;
int status = 0;
String botName;
String param[];
GraphBuilder builder;


public NewServlet() throws Exception{
    super();

    aiml = new NetworkAimlFacade(param);
    builder = aiml.getNetworkGraphBuilder();
    builder.addDirectoryUnlessAlreadyAdded
    (
        "C:\\Program Files\\RebeccaAIML\\aiml\\annotated_alice"
    );
    builder.createGraph();
    botName = builder.getBotPredicate("name");

}

private String handleRequest(String param){
    return null;
}

public void destroy()
{
    try{
    aiml.destroy();
    }catch(Exception e){
        e.printStackTrace();
    }
}

protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{

    res.setContentType("text/xml");
    res.setHeader("Cache-Control", "no-store, no-cache");
    String input = "";
    if(req.getParameter("chatinput") != null)
        input = req.getParameter("chatinput");
    try{
        String response = builder.getResponse(input);
        if(response!=null)    res.getWriter().write(response);
            else res.getWriter().write("No");
    }catch(Exception e){
        e.printStackTrace();
    }

}

protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

}}

这是我得到的错误，而不是从servlet获得响应：

Apache Tomcat / 7.0.11-错误报告HTTP状态500-类型异常reportmessage描述服务器遇到内部错误（），阻止其满足此请求。

    javax.servlet.ServletException: Error instantiating servlet class NewServlet
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:498)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:394)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:243)
    org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    java.lang.Thread.run(Thread.java:619)

root cause

java.lang.NullPointerException  Ice.PropertiesI.<init>(PropertiesI.java:250)
    Ice.Util.createProperties(Util.java:29)
    Ice.Util.initialize(Util.java:70)
    Ice.Util.initialize(Util.java:49)
    Ice.Util.initialize(Util.java:56)
    rebecca.NetworkGraphBuilderAIML.<init>(Unknown Source)
    rebecca.NetworkAimlFacade.<init>(Unknown Source)
    NewServlet.<init>(NewServlet.java:27)
    sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)   sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    java.lang.Class.newInstance0(Class.java:355)
    java.lang.Class.newInstance(Class.java:308)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:498)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:394)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:243)
    org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    java.lang.Thread.run(Thread.java:619)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.11 logs.

Apache Tomcat/7.0.11

Here's the equivalent Java code, that I use in a normal Java application, and it works fine!

  public class Console {

  public static void main(String args[]) {
  NetworkAimlFacade aiml = null;
  int status = 0;
  try {
    aiml = new NetworkAimlFacade(args);
    GraphBuilder builder =
        aiml.getNetworkGraphBuilder();
    builder.addDirectoryUnlessAlreadyAdded
    (
      "C:\\Program Files\\RebeccaAIML\\aiml\\annotated_alice"
    );
    builder.createGraph();
    String botName =
        builder.getBotPredicate("name");
    String initialResponse =
        builder.getResponse("connect");
    System.out.println(botName + " says: " +
        initialResponse);
    while(true) {
      System.out.print("You say> ");
      BufferedReader br =
        new BufferedReader(new
            InputStreamReader(System.in));
      String input = br.readLine();
      if(input.equals("/exit")) {
        break;
      } else {

String response =
            builder.getResponse(input);
        System.out.println("=====================");
        //Print out what Rebecca says.
        System.out.println(botName + " says: " +
            response);
        }
    }
    aiml.destroy();
    } catch(NetworkException e) {
      e.printStackTrace();
      status = 1;
    } catch (Exception e) {
      e.printStackTrace();
      status = 1;
    }
    System.exit(status);
  }
}

最佳答案

以下是与您的例外情况相关的摘录：

javax.servlet.ServletException: Error instantiating servlet class NewServlet
  ...
java.lang.NullPointerException
  Ice.PropertiesI.<init>(PropertiesI.java:250)
  Ice.Util.createProperties(Util.java:29)
  Ice.Util.initialize(Util.java:70)
  Ice.Util.initialize(Util.java:49)
  Ice.Util.initialize(Util.java:56)
  rebecca.NetworkGraphBuilderAIML.<init>(Unknown Source)
  rebecca.NetworkAimlFacade.<init>(Unknown Source)
  NewServlet.<init>(NewServlet.java:27)
  ...

换句话说，当容器试图做

Servlet serlvet = new NewServlet();

之所以失败，是因为在第250行的PropertiesI类的构造函数中，某些对象期望某个对象不是null，并尝试直接访问它，而无需事先检查它是否为null。在第250行检查PropertiesI类的源代码，并进行相应的修复。

这些都与ajax或servlet无关。了解如何解释异常和堆栈跟踪。