📄 classloader in tomcat[转贴加修改]--rosonsandy.htm
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</SPAN>WebApp1<SPAN style="mso-spacerun: yes"> </SPAN>WebApp2<SPAN
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style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">其中：<SPAN
lang=EN-US><BR>- Bootstrap - </SPAN>载入<SPAN lang=EN-US>JVM</SPAN>自带的类和<SPAN
lang=EN-US>$JAVA_HOME/jre/lib/ext/*.jar<BR>- System - </SPAN>载入<SPAN
lang=EN-US>$CLASSPATH/*.class<BR>- Common - </SPAN>载入<SPAN
lang=EN-US>$CATALINA_HOME/common/...</SPAN>，它们对<SPAN
lang=EN-US>TOMCAT</SPAN>和所有的<SPAN lang=EN-US>WEB APP</SPAN>都可见<SPAN
lang=EN-US><BR>- Catalina - </SPAN>载入<SPAN
lang=EN-US>$CATALINA_HOME/server/...</SPAN>，它们仅对<SPAN
lang=EN-US>TOMCAT</SPAN>可见，对所有的<SPAN lang=EN-US>WEB APP</SPAN>都不可见<SPAN
lang=EN-US><BR>- Shared - </SPAN>载入<SPAN
lang=EN-US>$CATALINA_HOME/shared/...</SPAN>，它们仅对所有<SPAN lang=EN-US>WEB
APP</SPAN>可见，对<SPAN lang=EN-US>TOMCAT</SPAN>不可见（也不必见）<SPAN lang=EN-US><BR>-
WebApp - </SPAN>载入<SPAN lang=EN-US>ContextBase?/WEB-INF/...</SPAN>，它们仅对该<SPAN
lang=EN-US>WEB APP</SPAN>可见<SPAN lang=EN-US> </SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US></SPAN></SPAN></P>
<P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-outline-level: 3"
align=left><B><SPAN lang=EN-US
style="FONT-SIZE: 13.5pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">2
- ClassLoader</SPAN></B><B><SPAN
style="FONT-SIZE: 13.5pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">的工作原理<SPAN
lang=EN-US> </SPAN></SPAN></B></P>
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<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">每个运行中的线程都有一个成员<SPAN
lang=EN-US>contextClassLoader</SPAN>，用来在运行时动态地载入其它类<SPAN
lang=EN-US><BR></SPAN>系统默认的<SPAN lang=EN-US>contextClassLoader</SPAN>是<SPAN
lang=EN-US>systemClassLoader</SPAN>，所以一般而言<SPAN
lang=EN-US>java</SPAN>程序在执行时可以使用<SPAN lang=EN-US>JVM</SPAN>自带的类、<SPAN
lang=EN-US>$JAVA_HOME/jre/lib/ext/</SPAN>中的类和<SPAN
lang=EN-US>$CLASSPATH/</SPAN>中的类<SPAN lang=EN-US><BR></SPAN>可以使用</SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">Thread.currentThread().setContextClassLoader(...);</SPAN><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">更改当前线程的<SPAN
lang=EN-US>contextClassLoader</SPAN>，来改变其载入类的行为<SPAN lang=EN-US>
</SPAN></SPAN></P>
<P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">ClassLoader</SPAN><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">被组织成树形，一般的工作原理是：<SPAN
lang=EN-US><BR>1) </SPAN>线程需要用到某个类，于是<SPAN
lang=EN-US>contextClassLoader</SPAN>被请求来载入该类<SPAN lang=EN-US><BR>2)
contextClassLoader</SPAN>请求它的父<SPAN lang=EN-US>ClassLoader</SPAN>来完成该载入请求<SPAN
lang=EN-US><BR>3) </SPAN>如果父<SPAN lang=EN-US>ClassLoader</SPAN>无法载入类，则<SPAN
lang=EN-US>contextClassLoader</SPAN>试图自己来载入<SPAN lang=EN-US> </SPAN></SPAN></P>
<P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><B><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">注意</SPAN></B><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">：<SPAN
lang=EN-US>WebApp?ClassLoader</SPAN>的工作原理和上述有少许不同：<SPAN
lang=EN-US><BR></SPAN>它先试图自己载入类（在<SPAN
lang=EN-US>ContextBase?/WEB-INF/...</SPAN>中载入类），如果无法载入，再请求父<SPAN
lang=EN-US>ClassLoader</SPAN>完成<SPAN lang=EN-US> </SPAN></SPAN></P>
<P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">由此可得：<SPAN
lang=EN-US><BR>- </SPAN>对于<SPAN lang=EN-US>WEB APP</SPAN>线程，它的<SPAN
lang=EN-US>contextClassLoader</SPAN>是<SPAN lang=EN-US>WebApp?ClassLoader<BR>-
</SPAN>对于<SPAN lang=EN-US>Tomcat Server</SPAN>线程，它的<SPAN
lang=EN-US>contextClassLoader</SPAN>是<SPAN lang=EN-US>CatalinaClassLoader
</SPAN></SPAN></P>
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style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US><STRONG>3 类的查找</STRONG></SPAN></SPAN></P>
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style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US>ClassLoader类中loadClass方法为缺省实现，用下面的顺序查找类：</SPAN></SPAN></P>
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style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US>1、调用findLoadedClass方法来检查是否已经被加载。如果没有则继续下面的步骤。</SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US>2、如果当前类装载器有一个指定的委托父装载器，则用委托父装载器的loadClass方法加载类，也就是委托给父装载器加载相应的类。</SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US>3、如果这个类装载器的委托层级体系没有一个类装载器加载该类，则使用类装载器定位类的特定实现机制，调用findClass方法来查找类。</SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><SPAN
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"><SPAN
lang=EN-US></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto"
align=left><B><SPAN lang=EN-US
style="FONT-SIZE: 13.5pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">4 -
</SPAN></B><B><SPAN
style="FONT-SIZE: 13.5pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">部分原代码分析<SPAN
lang=EN-US> </SPAN></SPAN></B></P>
<P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan"
align=left><B><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">4.1
- org/apache/catalina/startup/Bootstrap.java </SPAN></B></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">Bootstrap中定义了三个classloader:commonLoader,catalinaLoader,sharedLoader.三者关系如下：</SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-pagination: widow-orphan"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体"></SPAN></P><SPAN
lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-font-kerning: 0pt; mso-bidi-font-family: 宋体">
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-layout-grid-align: none"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 9pt; COLOR: green; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'">//</SPAN><SPAN
style="FONT-SIZE: 9pt; COLOR: green; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'">注意三个自己定置的<SPAN
lang=EN-US>ClassLoader</SPAN>的层次关系：<SPAN lang=EN-US> </SPAN></SPAN> </P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-layout-grid-align: none"
align=left><SPAN
style="FONT-SIZE: 9pt; COLOR: green; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'"><SPAN
lang=EN-US></SPAN></SPAN><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'"><SPAN
style="mso-spacerun: yes">
</SPAN><SPAN style="COLOR: green">// systemClassLoader
(root) </SPAN></SPAN> </P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: left; mso-layout-grid-align: none"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'"><SPAN
style="COLOR: green"></SPAN></SPAN><SPAN lang=EN-US
style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-font-kerning: 0pt; mso-hansi-font-family: 'Times New Roman'"><SPAN
style="mso-spacerun: yes"> </SPAN><SPAN
style="mso-spacerun: yes"> </SPAN><SPAN
style="COLOR: green">//<SPAN style="mso-spacerun: yes"> </SPAN>+---
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