HTTP Status 500 - Error instantiating servlet class

毕老师，这个怎么破？
xml的配置如下

<?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>weixin1</display-name>

  <servlet>

   <servlet-name>weixinServlet</servlet-name>

   <servlet-class>blue.servlet.WeixinServlet</servlet-class>

  </servlet>

  <servlet-mapping>

   <servlet-name>weixinServlet</servlet-name>

   <url-pattern>/wx1.do</url-pattern>

  </servlet-mapping>

  <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-list>

</web-app>