为什么我不能在Servlet中动态加载类?

Question

我最近想做一个Servlet演示，其中用户可以将他们的类文件上传到我的服务器，然后我使用Servlet读取上传的类中的方法。但是我被无法动态读取Servlet的问题卡住了。下面是我的代码:

package TestReflect;
@WebServlet("/showMsg")
public class showMsg extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public showMsg() {
super();
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
try {
out.println(ReadClass.getProperty());
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}       

out.flush();
out.close();
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
}

package ReadClass;
public class ReadClass {
public static String getProperty() throws ClassNotFoundException {
try {
URL url = new URL(null, "file:C:/TestClass/");
ClassLoader classLoader = new URLClassLoader(new URL[]{url}, ReadClass.class.getClassLoader());
Class<?> thisClass = classLoader.loadClass("add");
return thisClass.getMethods()[0].getName();
}
catch (Exception e)
{
e.printStackTrace();
}
return "";
}

我得到错误:

java.lang.ClassNotFoundException: add
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at ReadClass.ReadClass.getProperty(ReadClass.java:12)
at TestReflect.showMsg.doGet(showMsg.java:48)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:494)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:1025)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1137)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:319)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

然而，当我将相同的代码复制到一个普通的java项目中时，这个问题没有发生，程序运行得很好。我检查了很多信息，最类似问题的解决方案是为ClassLoader设置父类。正如您所看到的，我的代码中有设置，但它仍然报告错误，您知道为什么吗?

Answer 1

问题解决了。问题是关于服务器端的路径不是它看起来的样子，所以我认为它需要一个绝对路径，然后才会完成。