jboss,tomcat,jetty 容器的classloader机制

背景

     前段时间一直在做应用容器的迁移，将公司的应用容器从jboss,tomcat统一迁移到jetty。在整个迁移过程中遇到最多的潜在问题还是在classloader机制上，这里记录一下希望能对大家有所帮助，避免重复走弯路。

 

啥都不说，先来看下遇到的几个问题，比较纠结的问题。

问题1： (jar sealed问题)

Caused by: java.lang.SecurityException: sealing violation: 
package
 com.sun.media.jai.util is sealed
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:
234

)
        at java.net.URLClassLoader.access$
000

(URLClassLoader.java:
58

)
        at java.net.URLClassLoader$
1

.run(URLClassLoader.java:
197

)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:
190

)
        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
419

)
        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
381

)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(ClassLoader.java:
632

)

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

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

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) 

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:
39

)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25

)
    at java.lang.reflect.Method.invoke(Method.java:
597

)
    at org.eclipse.jetty.start.Main.invokeMain(Main.java:
490

)
    at org.eclipse.jetty.start.Main.start(Main.java:
634

)
    at org.eclipse.jetty.start.Main.parseCommandLine(Main.java:
280

)
    at org.eclipse.jetty.start.Main.main(Main.java:
82

)
Caused by: javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.SAXParserFactoryImpl not found
    at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:
134

)
    at org.eclipse.jetty.xml.XmlParser.<init>(XmlParser.java:
68

)
    at org.eclipse.jetty.xml.XmlConfiguration.initParser(XmlConfiguration.java:
79

)
    at org.eclipse.jetty.xml.XmlConfiguration.<init>(XmlConfiguration.java:
112

)
    at org.eclipse.jetty.xml.XmlConfiguration$
1

.run(XmlConfiguration.java:
1028

)
    at java.security.AccessController.doPrivileged(Native Method)
    at org.eclipse.jetty.xml.XmlConfiguration.main(XmlConfiguration.java:
983

)
    ... 
8

 more

java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.eclipse.jetty.start.Main.invokeMain(Main.java:490)
	at org.eclipse.jetty.start.Main.start(Main.java:634)
	at org.eclipse.jetty.start.Main.parseCommandLine(Main.java:280)
	at org.eclipse.jetty.start.Main.main(Main.java:82)
Caused by: javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.SAXParserFactoryImpl not found
	at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:134)
	at org.eclipse.jetty.xml.XmlParser.<init>(XmlParser.java:68)
	at org.eclipse.jetty.xml.XmlConfiguration.initParser(XmlConfiguration.java:79)
	at org.eclipse.jetty.xml.XmlConfiguration.<init>(XmlConfiguration.java:112)
	at org.eclipse.jetty.xml.XmlConfiguration$1.run(XmlConfiguration.java:1028)
	at java.security.AccessController.doPrivileged(Native Method)
	at org.eclipse.jetty.xml.XmlConfiguration.main(XmlConfiguration.java:983)
	... 8 more

说明：　jboss容器运行正常 , jetty容器运行异常

 

问题4：(mail问题) 

Caused by: java.lang.ClassNotFoundException: javax.mail.event.TransportListener
        at java.net.URLClassLoader$
1

.run(URLClassLoader.java:
202

)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:
190

)
        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
419

)
        at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
381

)
        ... 
78

 more

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

 

 

<
server
>

   
<!-- Tomcat 5 Service-->
   
<
mbean


 
code

=
"org.jboss.web.tomcat.tc5.Tomcat5"
      
name

=
"jboss.web:service=WebServer"

 
xmbean-dd

=
"META-INF/webserver-xmbean.xml"

>
......

<!-- Get the flag indicating if the normal Java2 parent first class
           loading model should be used over the servlet 2.3 web container first
           model.
      --
>
      
<
attribute


 
name

=
"Java2ClassLoadingCompliance"

>


false
</
attribute
>
      <!-- A flag indicating if the JBoss Loader should be used. This loader
           uses a unified class loader as the class loader rather than the tomcat
           specific class loader.
           The default is false to ensure that wars have isolated class loading
           for duplicate jars and jsp files.
      --
>
      
<
attribute


 
name

=
"UseJBossWebLoader"

>


true
</
attribute
>
      <!-- The list of package prefixes that should not be loaded without
         delegating to the parent class loader before trying the web app
         class loader. The packages listed here are those tha are used by
         the web container implementation and cannot be overriden. The format
         is a comma separated list of the package names. There cannot be any
         whitespace between the package prefixes.
         This setting only applies when 
UseJBossWebLoader

=
false

.
      --
>
      
<
attribute


 
name

=
"FilteredPackages"

>


javax.servlet,org.apache.commons.logging
</
attribute
>

.....

</
server
>

<server> 
   
   <!-- Tomcat 5 Service-->
   <mbean code="org.jboss.web.tomcat.tc5.Tomcat5"
      name="jboss.web:service=WebServer" xmbean-dd="META-INF/webserver-xmbean.xml">
......

<!-- Get the flag indicating if the normal Java2 parent first class
           loading model should be used over the servlet 2.3 web container first
           model.
      -->
      <attribute name="Java2ClassLoadingCompliance">false</attribute>
      <!-- A flag indicating if the JBoss Loader should be used. This loader
           uses a unified class loader as the class loader rather than the tomcat
           specific class loader.
           The default is false to ensure that wars have isolated class loading
           for duplicate jars and jsp files.
      -->
      <attribute name="UseJBossWebLoader">true</attribute>
      <!-- The list of package prefixes that should not be loaded without
         delegating to the parent class loader before trying the web app
         class loader. The packages listed here are those tha are used by
         the web container implementation and cannot be overriden. The format
         is a comma separated list of the package names. There cannot be any
         whitespace between the package prefixes.
         This setting only applies when UseJBossWebLoader=false.
      -->
      <attribute name="FilteredPackages">javax.servlet,org.apache.commons.logging</attribute>

.....

</server>

 

 

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)
   

   if
    (ctxPath.equals(
   "/"
   ) || ctxPath.equals(
   "/ROOT"
   ) || ctxPath.equals(
   ""
   ))
         {
            log.debug(
   "deploy root context="
    + ctxPath);
            ctxPath = 
   "/"
   ;
            metaData.setContextRoot(ctxPath);
         }
   
         URL url = 
   new
    URL(warUrl);
   
         ClassLoader loader = Thread.currentThread().getContextClassLoader();
         
   /* If we are using the jboss class loader we need to augment its path
         to include the WEB-INF/{lib,classes} dirs or else scoped class loading
         does not see the war level overrides. The call to setWarURL adds these
         paths to the deployment UCL.
         */
         Loader webLoader = 
   null
   ;
         
   if
    (config.isUseJBossWebLoader())  
   // 这里对useJbossWebLoader进行判断，进行不同的classloader处理
         {
            WebCtxLoader jbossLoader = 
   new
    WebCtxLoader(loader);
            jbossLoader.setWarURL(url);
            webLoader = jbossLoader;
         }
         
   else
         {
            String[] pkgs = config.getFilteredPackages();
            WebAppLoader jbossLoader = 
   new
    WebAppLoader(loader, pkgs);
            jbossLoader.setDelegate(getJava2ClassLoadingCompliance());
            webLoader = jbossLoader;
         }

   if (ctxPath.equals("/") || ctxPath.equals("/ROOT") || ctxPath.equals(""))
         {
            log.debug("deploy root context=" + ctxPath);
            ctxPath = "/";
            metaData.setContextRoot(ctxPath);
         }
   
         URL url = new URL(warUrl);
   
         ClassLoader loader = Thread.currentThread().getContextClassLoader();
         /* If we are using the jboss class loader we need to augment its path
         to include the WEB-INF/{lib,classes} dirs or else scoped class loading
         does not see the war level overrides. The call to setWarURL adds these
         paths to the deployment UCL.
         */
         Loader webLoader = null;
         if (config.isUseJBossWebLoader())  // 这里对useJbossWebLoader进行判断，进行不同的classloader处理
         {
            WebCtxLoader jbossLoader = new WebCtxLoader(loader);
            jbossLoader.setWarURL(url);
            webLoader = jbossLoader;
         }
         else
         {
            String[] pkgs = config.getFilteredPackages();
            WebAppLoader jbossLoader = new WebAppLoader(loader, pkgs);
            jbossLoader.setDelegate(getJava2ClassLoadingCompliance());
            webLoader = jbossLoader;
         }

    
3. 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";
   

   if
    (webLoader != 
   null
   )
         {
            server.setAttribute(objectName, 
   new
    Attribute(
   "loader"
   , webLoader));
         }
         
   else
         {
            server.setAttribute(objectName, 
   new
    Attribute(
   "parentClassLoader"
   , loader));
         }
   
         server.setAttribute(objectName, 
   new
    Attribute(
   "delegate"
   , 
   new
    Boolean(getJava2ClassLoadingCompliance()))); 
   // 设置deletegate属性

   if (webLoader != null)
         {
            server.setAttribute(objectName, new Attribute("loader", webLoader));
         }
         else
         {
            server.setAttribute(objectName, new Attribute("parentClassLoader", loader));
         }
   
         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
  

  WebCtxLoader(ClassLoader encLoader)
      {
            
  this
  .encLoader = encLoader;
            
  this
  .ctxLoader = 
  new
   ENCLoader(encLoader);
            ClassLoader parent = encLoader;
            
  while
   ((parent 
  instanceof
   RepositoryClassLoader) == 
  false
   && parent != 
  null
  )
           parent = parent.getParent();
            
  this
  .delegate = (RepositoryClassLoader) parent; 
  //delegate对象设置
      }
  
  
  public
   
  void
   setWarURL(URL warURL) 
  throws
   MalformedURLException
      {
            
  this
  .warURL = warURL;
            String path = warURL.getFile();
            File classesDir = 
  new
   File(path, 
  "WEB-INF/classes"
  );
            
  if
   (classesDir.exists())
            {
           delegate.addURL(classesDir.toURL()); 
  //无非都是委托给delegate loader
           ctxLoader.addURLInternal(classesDir.toURL());
            }
            File libDir = 
  new
   File(path, 
  "WEB-INF/lib"
  );
            
  if
   (libDir.exists())
            {
           File[] jars = libDir.listFiles();
           
  int
   length = jars != 
  null
   ? jars.length : 
  0
  
  ;
           
  for
   (
  int
   j = 
  0
  
  ; j < length; j++)
           {
              delegate.addURL(jars[j].toURL()); 
  //无非都是委托给delegate loader
              ctxLoader.addURLInternal(jars[j].toURL());
           }
            }
         }

  WebCtxLoader(ClassLoader encLoader)
     	{
  	      this.encLoader = encLoader;
  	      this.ctxLoader = new ENCLoader(encLoader);
  	      ClassLoader parent = encLoader;
  	      while ((parent instanceof RepositoryClassLoader) == false && parent != null)
  		 parent = parent.getParent();
  	      this.delegate = (RepositoryClassLoader) parent; //delegate对象设置
  	}
  
  
  public void setWarURL(URL warURL) throws MalformedURLException
     	{
  	      this.warURL = warURL;
  	      String path = warURL.getFile();
  	      File classesDir = new File(path, "WEB-INF/classes");
  	      if (classesDir.exists())
  	      {
  		 delegate.addURL(classesDir.toURL()); //无非都是委托给delegate loader
  		 ctxLoader.addURLInternal(classesDir.toURL());
  	      }
  	      File libDir = new File(path, "WEB-INF/lib");
  	      if (libDir.exists())
  	      {
  		 File[] jars = libDir.listFiles();
  		 int length = jars != null ? jars.length : 0;
  		 for (int j = 0; j < length; j++)
  		 {
  		    delegate.addURL(jars[j].toURL()); //无非都是委托给delegate loader
  		    ctxLoader.addURLInternal(jars[j].toURL());
  		 }
  	      }
  	   }

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

  public
   
  class
   WebAppLoader 
  extends
   org.apache.catalina.loader.WebappLoader
  {
     
  private
   String[] filteredPackages = {
        
  "org.apache.commons.logging"
     };
  
     
  public
   WebAppLoader()
     {
        
  super
  ();
        setLoaderClass(WebAppClassLoader.
  class
  .getName());
     }
  .....
  }

  public class WebAppLoader extends org.apache.catalina.loader.WebappLoader
  {
     private String[] filteredPackages = {
        "org.apache.commons.logging"
     };
  
     public WebAppLoader()
     {
        super();
        setLoaderClass(WebAppClassLoader.class.getName());
     }
  .....
  }

   

看到这里大家相信应该差不多了解了，总结一下： 

1. java2ClassLoadingCompliance是针对useJbossWebLoader=false时而言，是通过设置tomcat WebappClassloader的是否delegate进行控制classloader，实现child first/parent first。
2. java2ClassLoadingCompliance在useJBossWebLoader=true时，不会生效，会被强制设置为false，具体可看WebCtxLoaders，实现了Loader接口,getClassLoader()返回的是一个ctxLoader对jboss WebLoader的一个包装。
3. filteredPackages目前是通过tomcat的classloader机制上进行控制，所以只能是在useJbossWebLoader=false时有效，因为依赖了tomcat的实现。

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https://jira.jboss.org/browse/JBAS-3047?subTaskView=all
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。

jboss classloader机制大家也不必太刻意的去学习，只要适当的了解基本使用。只能说一切都是浮云浮云，随时都可能被改变。

---

 

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()
   

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

   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
   

   // Construct a class loader based on our current repositories list
           
   try
    {
   
               classLoader = createClassLoader();
               classLoader.setResources(container.getResources());
               classLoader.setDelegate(
   this
   .delegate);
               classLoader.setSearchExternalFirst(searchExternalFirst);
               .....
               
   for
    (
   int
    i = 
   0
   
   ; i < repositories.length; i++) {
                   classLoader.addRepository(repositories[i]);
               }
   }<SPAN style=
   "WHITE-SPACE: normal"
   > </SPAN>

   // Construct a class loader based on our current repositories list
           try {
   
               classLoader = createClassLoader();
               classLoader.setResources(container.getResources());
               classLoader.setDelegate(this.delegate);
               classLoader.setSearchExternalFirst(searchExternalFirst);
               .....
               for (int i = 0; i < repositories.length; i++) {
                   classLoader.addRepository(repositories[i]);
               }
   } 
   

4. 最后就是WebappClassLoader的loadClass方法了，具体的类装载策略。

下面来看一下，loadClass的相关逻辑： 

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。
   

相关代码：

// (0.1) Check our previously loaded class cache
    clazz = findLoadedClass(name);

// (0.2) Try loading the class with the system class loader, to prevent the webapp from overriding J2SE classes
    clazz = system.loadClass(name);

 
boolean
 delegateLoad = delegate || filter(name);
// (1) Delegate to our parent if requested
if
 (delegateLoad) {
    ClassLoader loader = parent;
            
if
 (loader == 
null
)
                loader = system;
            
try
 {
                clazz = loader.loadClass(name);
        ....
        }
     .....
}

// (2) Search local repositories
clazz = findClass(name);

// (3) Delegate to parent unconditionally
if
 (!delegateLoad) {
    ClassLoader loader = parent;
            
if
 (loader == 
null
)
                loader = system;
            
try
 {
                clazz = loader.loadClass(name);
        ....
        }
     .....
}

// (0.1) Check our previously loaded class cache
	clazz = findLoadedClass(name);

// (0.2) Try loading the class with the system class loader, to prevent the webapp from overriding J2SE classes
	clazz = system.loadClass(name);

 boolean delegateLoad = delegate || filter(name);
// (1) Delegate to our parent if requested
if (delegateLoad) {
	ClassLoader loader = parent;
            if (loader == null)
                loader = system;
            try {
                clazz = loader.loadClass(name);
	    ....
 	    }
     .....
}

// (2) Search local repositories
clazz = findClass(name);

// (3) Delegate to parent unconditionally
if (!delegateLoad) {
	ClassLoader loader = parent;
            if (loader == null)
                loader = system;
            try {
                clazz = loader.loadClass(name);
	    ....
 	    }
     .....
}

 

最后总结一下, 两个参数配置：

1. delegate(child first/parent first)，默认值为false，即为child first
2. packageTriggers，执行child first时，排除的package列表，如果匹配了package，即时为delegate=false，也会优先执行parent first策略。
3. tomcat的classloader机制在处理class装载时，会优先尝试使用system进行装载，这样可以优先使用jdk的相关类，这也是问题2的原因。

---

 

jetty7.1.20 classloader机制

jetty与tomcat相比，没有太大上的区别，也是有parent first/child first的概念，filter package等。但还是有些小区别，那先看一下代码：

1. jetty启动入口org.eclipse.jetty.server.Server，通过DeploymentManager进行webapps目录的加载
2. 具体war包加载是通过WebAppProvider,通过createContextHandler创建应用上下文WebAppContext。
3. WebAppContext在初始化时，创建WebAppClassLoader
   

   public
    
   void
    preConfigure() 
   throws
    Exception
       {
           
   // Setup configurations
           loadConfigurations();
   
           
   // Setup system classes
           loadSystemClasses();
   
           
   // Setup server classes
           loadServerClasses();
   
           
   // Configure classloader
           _ownClassLoader=
   false
   ;
           
   if
    (getClassLoader()==
   null
   )
           {
               WebAppClassLoader classLoader = 
   new
    WebAppClassLoader(
   this
   );
               setClassLoader(classLoader);
               _ownClassLoader=
   true
   ;
           }
   .....
   }

   public void preConfigure() throws Exception
       {
           // Setup configurations
           loadConfigurations();
   
           // Setup system classes
           loadSystemClasses();
           
           // Setup server classes
           loadServerClasses();
   
           // Configure classloader
           _ownClassLoader=false;
           if (getClassLoader()==null)
           {
               WebAppClassLoader classLoader = new WebAppClassLoader(this);
               setClassLoader(classLoader);
               _ownClassLoader=true;
           }
   .....
   }

4. 最后就是WebAppClassLoader的loadclass方法了。

理解了这些基本概念后，jetty的loadclass方法，逻辑比较好理解：

1. 如果属于system_class或者_parentLoaderPriority=true，并且不是server_class，优先采用parent first进行装载
   

   (_context.isParentLoaderPriority() || system_class) && !server_class)

   (_context.isParentLoaderPriority() || system_class) && !server_class)

2. 否则采用child first，先由WebAppClassLoader进行装载，没找到class再交由父CL装载

里面的有几个概念：

•  _parentLoaderPriority: 对应child first/parent first。
• _serverClasses ： jetty系统实现类，jetty认为服务实现类，必须在WEB-INF/lib , WEB-INF/classes优先加载。
  

  public final static String[] __dftServerClasses =
      {
          
  "-org.eclipse.jetty.continuation."
  , // don't hide continuation classes
          
  "-org.eclipse.jetty.jndi."
  ,         // don't hide naming classes
          
  "-org.eclipse.jetty.plus.jaas."
  ,    // don't hide jaas classes
          
  "-org.eclipse.jetty.websocket."
  ,    // don't hide websocket extension
          
  "-org.eclipse.jetty.servlet.DefaultServlet"
  , // don't hide default servlet
          
  "org.eclipse.jetty."
                  // hide other jetty classes
      } ; <SPAN style=
  "WHITE-SPACE: normal"
  > </SPAN>

  public final static String[] __dftServerClasses = 
      {
          "-org.eclipse.jetty.continuation.", // don't hide continuation classes
          "-org.eclipse.jetty.jndi.",         // don't hide naming classes
          "-org.eclipse.jetty.plus.jaas.",    // don't hide jaas classes
          "-org.eclipse.jetty.websocket.",    // don't hide websocket extension
          "-org.eclipse.jetty.servlet.DefaultServlet", // don't hide default servlet
          "org.eclipse.jetty."                // hide other jetty classes
      } ;  
  

• __dftSystemClasses :  系统类，类似于tomcat的filterPackage，优先采取parent first进行装载。
  

  public final static String[] __dftSystemClasses =
      {
          
  "java."
  ,                            // Java SE classes (per servlet spec v2.
  5
  
   / SRV.
  9.7
  
  .
  2
  
  )
          
  "javax."
  ,                           // Java SE classes (per servlet spec v2.
  5
  
   / SRV.
  9.7
  
  .
  2
  
  )
          
  "org.xml."
  ,                         // needed by javax.xml
          
  "org.w3c."
  ,                         // needed by javax.xml
          
  "org.apache.commons.logging."
  ,      // TODO: review if special case still needed
          
  "org.eclipse.jetty.continuation."
  ,  // webapp cannot change continuation classes
          
  "org.eclipse.jetty.jndi."
  ,          // webapp cannot change naming classes
          
  "org.eclipse.jetty.plus.jaas."
  ,     // webapp cannot change jaas classes
          
  "org.eclipse.jetty.websocket."
  ,     // WebSocket is a jetty extension
          
  "org.eclipse.jetty.servlet.DefaultServlet"
    // webapp cannot change default servlets
      } ;

  public final static String[] __dftSystemClasses = 
      {
          "java.",                            // Java SE classes (per servlet spec v2.5 / SRV.9.7.2) 
          "javax.",                           // Java SE classes (per servlet spec v2.5 / SRV.9.7.2)
          "org.xml.",                         // needed by javax.xml
          "org.w3c.",                         // needed by javax.xml
          "org.apache.commons.logging.",      // TODO: review if special case still needed 
          "org.eclipse.jetty.continuation.",  // webapp cannot change continuation classes
          "org.eclipse.jetty.jndi.",          // webapp cannot change naming classes
          "org.eclipse.jetty.plus.jaas.",     // webapp cannot change jaas classes
          "org.eclipse.jetty.websocket.",     // WebSocket is a jetty extension
          "org.eclipse.jetty.servlet.DefaultServlet"  // webapp cannot change default servlets
      } ;

相关配置：　

• _parentLoaderPriority的变量，可以通过环境变量org.eclipse.jetty.server.webapp.parentLoaderPriority=false/true
• server_class和system_class，可以通过设置Server attributes
  

  <
  Call
  
  
   
  name
  
  =
  "setAttribute"
  
  >
        
  <
  Arg
  >
  
  
  org.eclipse.jetty.webapp.systemClasses
  </
  Arg
  >
        
  <
  Arg
  >
            
  <
  Array
  
  
   
  type
  
  =
  "java.lang.String"
  
  >
                
  <
  Item
  >
  
  
  java.
  </
  Item
  >
            ......
        
  </
  Array
        
  </
  Arg
  >
  </
  Call
  >

  <Call name="setAttribute">
        <Arg>org.eclipse.jetty.webapp.systemClasses</Arg>
        <Arg>
            <Array type="java.lang.String">
                <Item>java.</Item>
  	      ......
  	  </Array
        </Arg>
  </Call>

---

 

总结和对比一下(jboss,tomcat,jetty)容器的classloader机制

容器	jboss(4.05)	tomcat(6.0.30)	jetty(7.1.20)
支持child/parent first设置(默认值)	Java2ClassLoadingCompliance=false	delegate=false	_parentLoaderPriority=false
过滤package配置	FilteredPackages 默认值：　javax.servlet,org.apache.commons.logging	packageTriggers 默认配置：org.apache.commons.logging	systemClasses 默认配置：java.  javax. org.xml. org.w3c. org.apache.commons.logging. org.eclipse.jetty.continuation. org.eclipse.jetty.jndi. org.eclipse.jetty.plus.jaas. org.eclipse.jetty.websocket. org.eclipse.jetty.servlet.DefaultServlet.
特殊性	1. UseJBossWebLoader=false时,过滤packages才能生效 2. UseJBossWebLoader=true时,不支持过滤packages 3. jboss 5.0以后UseJBossWebLoader参数将不支持	1. 在执行child/parent判断之前，会委托system classloader装载系统class，比如jdk的lib库	1. 多了一个serverclass配置，如果是serverclass优先采用child first 2. systemclass默认的配置，多了javax,org.xml,org.w3c配置。
相关文档	svn url : http://anonsvn.jboss.org/repos/jbossas/tags/JBoss_4_0_5_GA_CP18 jboss社区classloader文档: http://community.jboss.org/wiki/ClassLoadingConfiguration	svn url : http://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk 官方classloader机制： http://tomcat.apache.org/tomcat-6.0-doc/class-loader-howto.html	svn url : http://dev.eclipse.org/svnroot/rt/org.eclipse.jetty/jetty/tags/jetty-7.2.0.v20101020/ classloader 官方文档： http://docs.codehaus.org/display/JETTY/Classloading

 

 

---

 

最后(相关问题分析)

 

问题1：

　　是一个jar sealed问题，　官方说明：　http://download.oracle.com/javase/tutorial/deployment/jar/sealman.html

 

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.
A package might be sealed to ensure version consistency among the classes in your software or as a security measure.
To seal a package, a Name header needs to be added for the package, followed by a Sealed header, similar to this:
Name: myCompany/myPackage/
Sealed: true

The Name header's value is the package's relative pathname. Note that it ends with a '/' to distinguish it from a filename.   

  


  
    
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