细说Tomcat如何打破双亲委派(续1)

恒晖瑶

　　在上一篇文章里面分析WebappClassLoader.loadClass,了解了Tomcat是如何打破双亲委派的. 有兴趣的可以参考. 细说Tomcat如何打破双亲委派(有源码和图) 
　　但是上面的分析偏重于流程,没有分析ClassLoader是如何加按照顺序寻址,然后加载对应的Class到二进制流中.
　　WebappClassLoader.loadClass在违背双亲委派规则,自己寻址class文件时,就会调用下面的函数,函数已经添加了注释. 此函数依旧没有分析是如何寻址文件的,寻址文件的代码在findResourceInternal()中,它同时包括了对Jar,class, properties的寻址, 稍后有空在对此函数进行分析.

    /**
* Find the specified class in our local repositories, if possible.  If
* not found, throw <code>ClassNotFoundException</code>.
*
* @param name Name of the class to be loaded
*
* @exception ClassNotFoundException if the class was not found
*/
@Override
public Class<?> findClass(String name) throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("    findClass(" + name + ")");
// Cannot load anything from local repositories if class loader is stopped
if (!started) {
throw new ClassNotFoundException(name);
}
//--默认为NULL, security获取package.definition的定义,默认的定义可以在如下地址找到
//--${TOMCAT_HOME}\bin\bootstrap.jar\org\apache\catalina\startup\catalina.properties//--package.definition=sun.,java.,org.apache.catalina.,org.apache.coyote.,org.apache.tomcat.,org.apache.jasper.
//--如果加载的package被包含了,则抛出异常,加载失败
// (1) Permission to define this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
if (log.isTraceEnabled())
log.trace("      securityManager.checkPackageDefinition");
securityManager.checkPackageDefinition(name.substring(0,i));
} catch (Exception se) {
if (log.isTraceEnabled())
log.trace("      -->Exception-->ClassNotFoundException", se);
throw new ClassNotFoundException(name, se);
}
}
}
//-- 从父类查找Class(默认会跳过), 因为hasExternalRepositories && searchExternalFirs为false
//-- Ask our superclass to locate this class, if possible
// (throws ClassNotFoundException if it is not found)
Class<?> clazz = null;
try {
if (log.isTraceEnabled())
log.trace("      findClassInternal(" + name + ")");
if (hasExternalRepositories && searchExternalFirst) {
try {
clazz = super.findClass(name);
} catch(ClassNotFoundException cnfe) {
// Ignore - will search internal repositories next
} catch(AccessControlException ace) {
log.warn("WebappClassLoader.findClassInternal(" + name
+ ") security exception: " + ace.getMessage(), ace);
throw new ClassNotFoundException(name, ace);
} catch (RuntimeException e) {
if (log.isTraceEnabled())
log.trace("      -->RuntimeException Rethrown", e);
throw e;
}
}
//-开始从本地加载Class
if ((clazz == null)) {
try {
clazz = findClassInternal(name);
} catch(ClassNotFoundException cnfe) {
if (!hasExternalRepositories || searchExternalFirst) {
throw cnfe;
}
} catch(AccessControlException ace) {
log.warn("WebappClassLoader.findClassInternal(" + name
+ ") security exception: " + ace.getMessage(), ace);
throw new ClassNotFoundException(name, ace);
} catch (RuntimeException e) {
if (log.isTraceEnabled())
log.trace("      -->RuntimeException Rethrown", e);
throw e;
}
}
//--hasExternalRepositories:如果之前添加了外部路径,可以尝试从外部路径查找Class
if ((clazz == null) && hasExternalRepositories && !searchExternalFirst) {
try {
clazz = super.findClass(name);
} catch(AccessControlException ace) {
log.warn("WebappClassLoader.findClassInternal(" + name
+ ") security exception: " + ace.getMessage(), ace);
throw new ClassNotFoundException(name, ace);
} catch (RuntimeException e) {
if (log.isTraceEnabled())
log.trace("      -->RuntimeException Rethrown", e);
throw e;
}
}
//--如果都尝试了依旧找不到,就抛出异常了
if (clazz == null) {
if (log.isDebugEnabled())
log.debug("    --> Returning ClassNotFoundException");
throw new ClassNotFoundException(name);
}
} catch (ClassNotFoundException e) {
if (log.isTraceEnabled())
log.trace("    --> Passing on ClassNotFoundException");
throw e;
}
// Return the class we have located
if (log.isTraceEnabled())
log.debug("      Returning class " + clazz);
if (log.isTraceEnabled()) {
ClassLoader cl;
if (Globals.IS_SECURITY_ENABLED){
cl = AccessController.doPrivileged(
new PrivilegedGetClassLoader(clazz));
} else {
cl = clazz.getClassLoader();
}
log.debug("      Loaded by " + cl.toString());
}
return (clazz);
}

　　总结一下,就是这个函数默认的是直接在Tomcat本地查找Class,但是提供了参数可以控制从父类,或者指定的目录去查找Class.