Tomcat源码分析(类加载器)

妖怪幻

　　1.Tomcat共有五类ClassLoader:
  commonLoader:
      用于加载公共jar包，比如servlet规范包,servlet-api.jar，位于/common
  catalinaLoader:
      用于加载Tomcat的实现jar包，比如catalina.jar，位于/server
  sharedLoader:
      用于加载所有应用的共享jar包，比如struts核心包就可以放在下面，位于/shared
  webappClassLoader
      用于加载web应用下的class，一个web应用有一个webappClassLoader
  JasperLoader
      用于加载web应用下由jsp编译后的servlet，一个web应用有一个JasperLoader

它们的父子代理关系如下:

                     JREClassLoader
                               |
                     commonLoader
                     /                       \ 
          catalinaLoader        sharedLoader  
                                              /                      \
                          webappClassLoader ........webappClassLoader
                                     |                                         |
                           JasperLoader                   JasperLoader

2.WebAppClassLoader加载策略
  1.首先判断class是否已经加载，若已加载，则直接返回，若没加载，则进行第2步
  2.使用系统加载器加载class，若加载成功，则返回，若失败，则进行第3步
  3.如果webAppClassLoader设置了代理模式，则用父加载器进行加载；否则直接加载
  4.如果webAppClassLoader未设置代理模式，则自己先加载，失败后再使用父加载器加载

WebappClassLoader类loadClass方法

public Class loadClass(String name, boolean resolve)
throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class clazz = null;
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
// (0) Check our previously loaded local class cache
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug("  Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (0.1) Check our previously loaded class cache
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug("  Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (0.2) Try loading the class with the system class loader, to prevent
//       the webapp from overriding J2SE classes
try {
clazz = system.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
// (0.5) Permission to access this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
securityManager.checkPackageAccess(name.substring(0,i));
} catch (SecurityException se) {
String error = "Security Violation, attempt to use " +
"Restricted Class: " + name;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name);
// (1) Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug("  Delegating to parent classloader1 " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug("  Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
// (2) Search local repositories
if (log.isDebugEnabled())
log.debug("  Searching local repositories");
try {
clazz = findClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug("  Loading class from local repository");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
// (3) Delegate to parent unconditionally
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug("  Delegating to parent classloader at end: " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug("  Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
throw new ClassNotFoundException(name);
}