1.Caused by: java.lang.SecurityException: sealing violation: package com.sun.media.jai.util is sealed  2.        at java.net.URLClassLoader.defineClass(URLClassLoader.java:234)  3.        at java.net.URLClassLoader.access$000(URLClassLoader.java:58)  4.        at java.net.URLClassLoader$1.run(URLClassLoader.java:197)  5.        at java.security.AccessController.doPrivileged(Native Method)  6.        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)  7.        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:419)  8.        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:381)  9.        at java.lang.ClassLoader.defineClass1(Native Method)  10.        at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)

说明：　jboss容器运行正常 , jetty容器运行出错

问题2:  (xml解析) 

1.Caused by: java.lang.NoSuchMethodError: javax.xml.parsers.SAXParserFactory.newInstance(Ljava/lang/String;Ljava/lang/ClassLoader;)Ljavax/xml/parsers/SAXParserFactory;

说明：　jetty容器运行正常 , tomcat容器运行正常，jboss容器运行异常 

问题3:  (xml解析) 

1.java.lang.reflect.InvocationTargetException  2.    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)  3.    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)  4.    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)  5.    at java.lang.reflect.Method.invoke(Method.java:597)  6.    at org.eclipse.jetty.start.Main.invokeMain(Main.java:490)  7.    at org.eclipse.jetty.start.Main.start(Main.java:634)  8.    at org.eclipse.jetty.start.Main.parseCommandLine(Main.java:280)  9.    at org.eclipse.jetty.start.Main.main(Main.java:82)  10.Caused by: javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.SAXParserFactoryImpl not found  11.    at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:134)  12.    at org.eclipse.jetty.xml.XmlParser.
     
      (XmlParser.java:68)  13.    at org.eclipse.jetty.xml.XmlConfiguration.initParser(XmlConfiguration.java:79)  14.    at org.eclipse.jetty.xml.XmlConfiguration.
      
       (XmlConfiguration.java:112)  15.    at org.eclipse.jetty.xml.XmlConfiguration$1.run(XmlConfiguration.java:1028)  16.    at java.security.AccessController.doPrivileged(Native Method)  17.    at org.eclipse.jetty.xml.XmlConfiguration.main(XmlConfiguration.java:983)  18.    ... 8 more
      
     

说明：　jboss容器运行正常 , jetty容器运行异常

问题4：(mail问题) 

1.Caused by: java.lang.ClassNotFoundException: javax.mail.event.TransportListener  2.        at java.net.URLClassLoader$1.run(URLClassLoader.java:202)  3.        at java.security.AccessController.doPrivileged(Native Method)  4.        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)  5.        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:419)  6.        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:381)  7.        ... 78 more

说明：　jboss容器运行正常 , jetty容器运行异常 

可以说基本都是对应的class , method等找不到，或者类冲突等问题，一看就是比较典型的classloader引发的问题。

下面就来看看对容器classloader机制的分析和对比，相信大家了解了相关classloader机制微妙的区别后，基本也能解析这一类问题了

jboss4.05 classloader机制

这里早期对jboss classloader几点配置做了下说明，可以参见： http://agapple.iteye.com/blog/791940

为了和tomcat,jetty有更明显的对比，这里就主要介绍三个参数代码层面上的实现：

1.
     
         2.     3.   
        4.   
      
         6.......  7.  8.
         12.      
       
        false
         13.      
         19.      
       
        true
         20.      
         28.      
       
        javax.servlet,org.apache.commons.logging
         29.  30......  31.  32.
      
     

相信这几个参数大家都应该比较熟知，配置项在 deploy/jbossweb-tomcat55.sar/META-INF/jboss-service.xml中

下面循着代码来看一下jboss的相关实现：

1. 代码入口： org.jboss.web.tomcat.tc5.Tomcat5， 这里的三个配置都对应于Tomcat5类的属性，默认值就是当前配置的值。
2. Tomcat5会创建一个Deploy进行war包的装载，TomcatDeployer(继承于AbstractWebDeployer)

1.if (ctxPath.equals("/") || ctxPath.equals("/ROOT") || ctxPath.equals(""))  2.      {  3.         log.debug("deploy root context=" + ctxPath);  4.         ctxPath = "/";  5.         metaData.setContextRoot(ctxPath);  6.      }  7.  8.      URL url = new URL(warUrl);  9.  10.      ClassLoader loader = Thread.currentThread().getContextClassLoader();  11.      /* If we are using the jboss class loader we need to augment its path 12.      to include the WEB-INF/{lib,classes} dirs or else scoped class loading 13.      does not see the war level overrides. The call to setWarURL adds these 14.      paths to the deployment UCL. 15.      */  16.      Loader webLoader = null;  17.      if (config.isUseJBossWebLoader())  // 这里对useJbossWebLoader进行判断，进行不同的classloader处理  18.      {  19.         WebCtxLoader jbossLoader = new WebCtxLoader(loader);  20.         jbossLoader.setWarURL(url);  21.         webLoader = jbossLoader;  22.      }  23.      else  24.      {  25.         String[] pkgs = config.getFilteredPackages();  26.         WebAppLoader jbossLoader = new WebAppLoader(loader, pkgs);  27.         jbossLoader.setDelegate(getJava2ClassLoadingCompliance());  28.         webLoader = jbossLoader;  29.      }

1. 最后通过MBean调用，将classloader设置给对应的tomcat上下文对象: "org.apache.catalina.core.StandardContext";

1.if (webLoader != null)  2.      {  3.         server.setAttribute(objectName, new Attribute("loader", webLoader));  4.      }  5.      else  6.      {  7.         server.setAttribute(objectName, new Attribute("parentClassLoader", loader));  8.      }  9.  10.      server.setAttribute(objectName, new Attribute("delegate", new Boolean(getJava2ClassLoadingCompliance()))); // 设置deletegate属性

说明：

• WebCtxLoader: 一个对jboss UCL classloader的一个代理而已，setWarUrl也只是将war资源加入到当前jboss的UCL classloader去装载

1.WebCtxLoader(ClassLoader encLoader)  2.    {  3.          this.encLoader = encLoader;  4.          this.ctxLoader = new ENCLoader(encLoader);  5.          ClassLoader parent = encLoader;  6.          while ((parent instanceof RepositoryClassLoader) == false && parent != null)  7.         parent = parent.getParent();  8.          this.delegate = (RepositoryClassLoader) parent; //delegate对象设置  9.    }  10.  11.  12.public void setWarURL(URL warURL) throws MalformedURLException  13.    {  14.          this.warURL = warURL;  15.          String path = warURL.getFile();  16.          File classesDir = new File(path, "WEB-INF/classes");  17.          if (classesDir.exists())  18.          {  19.         delegate.addURL(classesDir.toURL()); //无非都是委托给delegate loader  20.         ctxLoader.addURLInternal(classesDir.toURL());  21.          }  22.          File libDir = new File(path, "WEB-INF/lib");  23.          if (libDir.exists())  24.          {  25.         File[] jars = libDir.listFiles();  26.         int length = jars != null ? jars.length : 0;  27.         for (int j = 0; j < length; j++)  28.         {  29.            delegate.addURL(jars[j].toURL()); //无非都是委托给delegate loader  30.            ctxLoader.addURLInternal(jars[j].toURL());  31.         }  32.          }  33.       }

• WebAppLoader： 对tomcat WebappLoader的一个封装, 同时设置filteredPackages给tomcat WebappLoader进行class控制。

1.public class WebAppLoader extends org.apache.catalina.loader.WebappLoader  2.{  3.   private String[] filteredPackages = {  4.      "org.apache.commons.logging"  5.   };  6.  7.   public WebAppLoader()  8.   {  9.      super();  10.      setLoaderClass(WebAppClassLoader.class.getName());  11.   }  12......  13.}

不过需要注意两点：

1. 目前在jboss4.05上不支持在在war包下WEB-INF/jboss-web.xml的配置，在jboss4.20以后才支持,可见
2. jboss 5.0以后，就没有UseJbossWebLoader这一配置项了，实现方式也不是delegate到tomcat，而是独立的一个classloader，统一处理。

tomcat6.0.30 classloader机制 

tomcat相比于jboss4.05概念上简介了很多，不过tomcat 6版本相比于tomcat 5有一些变化，少了一些shared lib库的概念。

Bootstrap          |       System          |       Common       /     \  Webapp1   Webapp2 ...

 一个树状结构，相信大家差不多都知道tomcat默认是以child first装载class，优先载入web中的class类，找不到才会去装载公用类。

下面就来看一下代码的一些实现: 

1. tomcat启动入口，通过分析StandardContext.start()

   1.public synchronized void start() {  2......  3.  4.if (getLoader() == null) {  5.            WebappLoader webappLoader = new WebappLoader(getParentClassLoader());  6.            webappLoader.setDelegate(getDelegate());  7.            setLoader(webappLoader);  8.        }  9......  10.}

   public synchronized void start() {.....if (getLoader() == null) {            WebappLoader webappLoader = new WebappLoader(getParentClassLoader());            webappLoader.setDelegate(getDelegate());            setLoader(webappLoader);        }.....}

2. 如果getLoader为空，则创建一个新的WebappLoader，注意这也是jboss设置tomcat loader的入口，从而替换默认的tomcat classloader。
3. WebappLoader通过在startInternal启动了一个新的WebappClassLoader
   1.// Construct a class loader based on our current repositories list  2.        try {  3.  4.            classLoader = createClassLoader();  5.            classLoader.setResources(container.getResources());  6.            classLoader.setDelegate(this.delegate);  7.            classLoader.setSearchExternalFirst(searchExternalFirst);  8.            .....  9.            for (int i = 0; i < repositories.length; i++) {  10.                classLoader.addRepository(repositories[i]);  11.            }  12.} 
4. 最后就是WebappClassLoader的loadClass方法了，具体的类装载策略。

1. (0.1)先检查class是否已经被装载过，findLoadedClass
2. (0.2)通过system classloader装载系统class，所以这里会优先装载jdk的相关代码，这个很重要，也很特别。
3. 如果是delegate=true并且不是filterPackage列表，则采用parent first，否则采用child first。

1.// (0.1) Check our previously loaded class cache  2.    clazz = findLoadedClass(name);  3.  4.// (0.2) Try loading the class with the system class loader, to prevent the webapp from overriding J2SE classes  5.    clazz = system.loadClass(name);  6.  7. boolean delegateLoad = delegate || filter(name);  8.// (1) Delegate to our parent if requested  9.if (delegateLoad) {  10.    ClassLoader loader = parent;  11.            if (loader == null)  12.                loader = system;  13.            try {  14.                clazz = loader.loadClass(name);  15.        ....  16.        }  17.     .....  18.}  19.  20.// (2) Search local repositories  21.clazz = findClass(name);  22.  23.// (3) Delegate to parent unconditionally  24.if (!delegateLoad) {  25.    ClassLoader loader = parent;  26.            if (loader == null)  27.                loader = system;  28.            try {  29.                clazz = loader.loadClass(name);  30.        ....  31.        }  32.     .....  33.}

1.Package Sealing: A package within a JAR file can be optionally sealed, which means that all classes defined in that package must be archived in the same JAR file.   2.A package might be sealed to ensure version consistency among the classes in your software or as a security measure.  3.To seal a package, a Name header needs to be added for the package, followed by a Sealed header, similar to this:  4.Name: myCompany/myPackage/  5.Sealed: true  6.  7.The Name header's value is the package's relative pathname. Note that it ends with a '/' to distinguish it from a filename.   8.Any headers following a Name header, without any intervening blank lines, apply to the file or package specified in the Name header.   9.In the above example, because the Sealed header occurs after the Name: myCompany/myPackage header,   10.with no blank lines between, the Sealed header will be interpreted as applying (only) to the package myCompany/myPackage.   11.  12.This code doesn't work.

1.
            2.
          
           com.sun.media.jai.
            3.
            4.
          
           com.sun.media.imageio.
            5.
          
           com.sun.media.imageioimpl.
            6.
          
           jj2000.j2k.
          

1.return (SAXParserFactory) FactoryFinder.find(  2.                /* The default property name according to the JAXP spec */  3.                "javax.xml.parsers.SAXParserFactory",  4.                /* The fallback implementation class name */  5.                "com.sun.org.apache.xerces.internal.jaxp.SAXParserFactoryImpl");

1.-Djavax.xml.parsers.SAXParserFactory=org.apache.xerces.jaxp.SAXParserFactoryImpl