private void initClassLoaders() {
 try {
 commonLoader = createClassLoader("common", null);
 if( commonLoader == null ) {
  // no config file, default to this loader - we might be in a 'single' env.
  commonLoader=this.getClass().getClassLoader();
 }
 // 指定仓库路径配置文件前缀和父加载器，创建 ClassLoader 实例
 catalinaLoader = createClassLoader("server", commonLoader);
 sharedLoader = createClassLoader("shared", commonLoader);
 } catch (Throwable t) {
 log.error("Class loader creation threw exception", t);
 System.exit(1);
 }
}

private ClassLoader createClassLoader(String name, ClassLoader parent)
 throws Exception {

 String value = CatalinaProperties.getProperty(name + ".loader");
 if ((value == null) || (value.equals("")))
  return parent; // 如果没有配置，则返回传入的父加载器
 ArrayList repositoryLocations = new ArrayList();
 ArrayList repositoryTypes = new ArrayList();
 ...
 // 获取资源仓库路径
 String[] locations = (String[]) repositoryLocations.toArray(new String[0]);
 Integer[] types = (Integer[]) repositoryTypes.toArray(new Integer[0]);
 // 创建一个 StandardClassLoader 对象
 ClassLoader classLoader = ClassLoaderFactory.createClassLoader
   (locations, types, parent);
 ...
 return classLoader;
}

public synchronized Class loadClass(String name, boolean resolve)
 throws ClassNotFoundException {
 ...
 Class clazz = null;
 // (0) 检查自身内部缓存中是否已经加载
 clazz = findLoadedClass0(name);
 if (clazz != null) {
 if (log.isDebugEnabled())
  log.debug(" Returning class from cache");
 if (resolve) resolveClass(clazz);
 return (clazz);
 }
 // (0.1) 检查 JVM 的缓存中是否已经加载
 clazz = findLoadedClass(name);
 if (clazz != null) {
 if (log.isDebugEnabled())
  log.debug(" Returning class from cache");
 if (resolve) resolveClass(clazz);
 return (clazz);
 }
 // (0.2) 尝试使用系统类加载加载，防止覆盖 J2SE 类
 try {
 clazz = system.loadClass(name);
 if (clazz != null) {
  if (resolve) resolveClass(clazz);
  return (clazz);
 }
 } catch (ClassNotFoundException e) {// Ignore}
 // (0.5) 使用 SecurityManager 检查是否有此类的访问权限
 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) 是否委托给父类，这里默认为 false
 if (delegateLoad) {
  ...
 }
 // (2) 尝试查找自己的存储库并加载
 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) 如果此时还加载失败，那么将加载请求委托给父加载器
 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) {}
 }
 // 最后加载失败，抛出异常
 throw new ClassNotFoundException(name);
}

在防止覆盖 J2SE 类的时候，版本 Tomcat 6，使用的是 AppClassLoader，rt.jar 核心类库是由 Bootstrap Classloader 加载的，但是在 Java 代码是获取不了这个加载器的，在高版本做了以下优化：
ClassLoader j = String.class.getClassLoader();
if (j == null) {
 j = getSystemClassLoader();
 while (j.getParent() != null) {
 j = j.getParent();
 }
}
this.javaseClassLoader = j;

ClassLoader j = String.class.getClassLoader();
if (j == null) {
 j = getSystemClassLoader();
 while (j.getParent() != null) {
 j = j.getParent();
 }
}
this.javaseClassLoader = j;