Java Servlet上下文参数初始化问题

Question

我是Java servlet的新手，并且一直在关注本教程。 我有几个初始化参数的问题； 从web.xml检索的所有值似乎都是NULL。 引发异常的代码是：

public class LoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public void init() throws ServletException {
        //we can create DB connection resource here and set it to Servlet context
        System.out.println("Check the string getServletContext().getInitParameter(\"dbURL\") = " + getServletContext().getInitParameter("dbURL")); // <== exception is thrown at this line
        if(getServletContext().getInitParameter("dbURL").equals("jdbc:mysql://localhost/mysql_db") &&
            getServletContext().getInitParameter("dbUser").equals("mysql_user") &&
            getServletContext().getInitParameter("dbUserPwd").equals("mysql_pwd"))
            getServletContext().setAttribute("DB_Success", "True");
        else throw new ServletException("DB Connection error");
    }
    ...
}

getServletContext.getInitParameter（“ anyParameter”）为每个传递的参数值返回NULL字符串。

web.xml文件具有以下条目：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <display-name>DynamicWebProject1</display-name>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
        <welcome-file>login.html</welcome-file>
    </welcome-file-list>
    <context-param>
        <param-name>dbURL</param-name>
        <param-value>jdbc:mysql://localhost/mysql_db</param-value>
    </context-param>
    <context-param>
        <param-name>dbUser</param-name>
        <param-value>mysql_user</param-value>
    </context-param>
    <context-param>
        <param-name>dbUserPwd</param-name>
        <param-value>mysql_pwd</param-value>
    </context-param>
</web-app>

我目前在Win 7 x64平台上使用eclipse kepler，jdk 7和apache-tomcat-7.0.47。

编辑：

堆栈跟踪：

javax.servlet.ServletException: Servlet.init() for servlet sebi.first.LoginServlet threw exception
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)
       org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
       java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
       java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
       java.lang.Thread.run(Unknown Source)

根本原因

java.lang.NullPointerException
javax.servlet.GenericServlet.getServletContext(GenericServlet.java:125)
sebi.first.LoginServlet.init(LoginServlet.java:32)
javax.servlet.GenericServlet.init(GenericServlet.java:160)

org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)

                                                                   org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)
org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
   java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
   java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
   java.lang.Thread.run(Unknown Source)

Answer 1

我不确定为什么在堆栈跟踪中会得到org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) 。 但是，如果您要覆盖init（config），请尝试以下操作：

 public void init(ServletConfig config) throws ServletException { super.init(config); /* and rest of the things here */ }