Tomcat类装载的问题.

竹子开花的时

昨天发布一个web application的时候发现一个奇怪的问题, 明明jar包里面有的类说找不到, 真是奇怪. 不过要下班了. 明天再说吧.
java.lang.NoClassDefFoundError: com/netsboss/util/dbobject/GeneralEntity
  java.lang.ClassLoader.defineClass1(Native Method)
  java.lang.ClassLoader.defineClass(ClassLoader.java:620)
  java.lang.ClassLoader.defineClass(ClassLoader.java:465)
  com.netsboss.util.proxy.ProxyClassLoader.defineClass(ProxyClassLoader.java:75)
  com.netsboss.util.proxy.Proxy.getProxyClass(Proxy.java:216)
  com.netsboss.util.dbobject.DatabaseManager.generateClass(DatabaseManager.java:1161)
  com.netsboss.util.dbobject.DatabaseManager.<init>(DatabaseManager.java:240)
  com.netsboss.util.dbobject.DatabaseManager.getDatabaseManager(DatabaseManager.java:98)
  com.aft.constants.SystemConstants.getDefaultAFTDatabaseManager(SystemConstants.java:258)
  com.aft.database.retrieve.CAFTDBGetWebUserInfo.<init>(CAFTDBGetWebUserInfo.java:64)
  com.aft.database.retrieve.CAFTDBGetWebUserInfo.getCAFTDBGetWebUserInfo(CAFTDBGetWebUserInfo.java:51)
  com.aft.servlets.actions.RegisterAction.perform(RegisterAction.java:43)
  org.apache.struts.action.Action.execute(Action.java:420)
  org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
  org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
  org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
  org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
  javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
  javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
上面的是异常的信息, 我明明把含有这个类的jar包放到了WEB-INF/lib下面. 怎么办,还能这么办, trace code.
结果发现在了一个自定义的类装载器com.netsboss.util.proxy.ProxyClassLoader,他在构着函数里面没有传他的parent classloader(tomcat Webapp classloader)进去, 所以他的parent classloader是systemclassloader, 我想假如ProxyClassLoader的构着函数里面调用了super(ProxyClassLoader.class.getClassLoader());的话,
 应该能解决这个问题的. 我想这个就是这个java.lang.NoClassDefFoundError异常的原因吧.     
这里有篇介绍类装载的问题, 也介绍了Tomcat类装器的机制.
http://blog.csai.cn/blog/number01/2005628165135.html
添加一些: 这个是自定义的classloader, 他在构着函数里面没有传他的parent classloader(tomcat Webapp classloader)进去, 所以他的parent classloader是systemclassloader,具体代码看java.lang.ClassLoader.     /**
     * Creates a new class loader using the specified parent class loader for
     * delegation.
     *
     *　　If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * checkCreateClassLoader} method is invoked.  This may result in
     * a security exception.  

     *
     * @param  parent
     *         The parent class loader
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          checkCreateClassLoader method doesn&apost allow creation
     *          of a new class loader.
     *
     * @since  1.2
     */
    protected ClassLoader(ClassLoader parent) {
SecurityManager security = System.getSecurityManager();
if (security != null) {
     security.checkCreateClassLoader();
}
this.parent = parent;
initialized = true;
    }    /**
     * Creates a new class loader using the ClassLoader returned by
     * the method {@link #getSystemClassLoader()
     * getSystemClassLoader()} as the parent class loader.
     *
     *　　If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * checkCreateClassLoader} method is invoked.  This may result in
     * a security exception.  

     *
     * @throws  SecurityException
     *          If a security manager existsand its
     *          checkCreateClassLoader method doesn&apost allow creation
     *          of a new class loader.
     */
    protected ClassLoader() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
     security.checkCreateClassLoader();
}
this.parent = getSystemClassLoader();
initialized = true;
    } package com.netsboss.util.proxy; import java.io.File;import java.io.FileOutputStream;import java.io.IOException;
import java.io.UnsupportedEncodingException;import java.net.URL;
import java.net.URLDecoder;
import java.net.URLEncoder;import java.util.HashMap;
import java.util.Locale;import org.apache.log4j.Category; public class ProxyClassLoader extends ClassLoader {  private static Category log = Category.getInstance(ProxyClassLoader.class); private HashMap classes = new HashMap(); private boolean saveToClassPath = false;  public ProxyClassLoader(boolean saveToClassPath){  this.saveToClassPath = saveToClassPath; }  /**  * @see ClassLoader#findClass(String)  */ protected Class findClass(String name) throws ClassNotFoundException {   Class find = super.findClass(name);  if(find != null) return find;   find = (Class)classes.get(name);  if(find != null) return find;  else throw new ClassNotFoundException(name);  }  public void defineClass(byte classfile[], String classname){   Class clazz = defineClass(classname, classfile,0, classfile.length);  classes.put(classname,clazz);   if(saveToClassPath){   saveToClassPath(classfile, classname);  }  }  /**  * return a class which implements all the interfaces or inherit from one of class  * currently, just return null.  */ public Class search(Class interfaces[]){  return null; }  private void saveToClassPath(byte classfile[], String classname){  String filename = classname + ".class";   URL url = getClass().getProtectionDomain().getCodeSource().getLocation();  if( "file".equals(url.getProtocol()) && (url.getHost() == null || url.getHost().equals("")) ){      String dir = null;      try{
        dir = URLDecoder.decode(url.getFile(), "UTF-8");
      }catch(UnsupportedEncodingException ex){
        log.error("Encoding not supported", ex);
      }    File classpath = new File(dir);   if(classpath.isDirectory() == false){    log.info("Classpath error for ProxyClassLoader:" + dir + " not a directory");    return;   }   File file = new File(classpath, filename);   try {    FileOutputStream os = new FileOutputStream(file);    os.write(classfile);    os.close();   }   catch(IOException ex){
        log.error("Error writing Proxy Class file", ex);
    ex.printStackTrace();   }  }  } }参考这篇文章http://www.cjsdn.net/post/view?bid=29&id=162060&sty=1&tpg=1&age=0