官网：http://tomcat.apache.org/tomcat-7.0-doc/class-loader-howto.html
Java类加载与Tomcat类加载器层级关系对比
Java ClassLoader：
Bootstrap ClassLoader（加载$JAVA_HOME/jre/lib/目录下核心类库：resources.jar，rt.jar，sunrsasign.jar，jsse.jar，jce.jar，charsets.jar，jfr.jar，以及jre/classes目录下的class）
/|\
| Tomcat ClassLoader：
ExtClassLoader（加载$JAVA_HOME/jre/lib/ext/目下的所有jar） -------- Bootstrap（加载$JAVA_HOME/jre/lib/ext/目录下的所有jar）
/|\ /|\
| |
AppClassLoader（加载应用程序classpath目录下的所有jar和class文件） System（加载$CATALINA_HOME/bin/bootstrap.jar，$CATALINA_BASE/bin/tomcat-juli.jar，$CATALINA_HOME/bin/commons-daemon.jar）
/|\
|
Common（加载$CATALINA_BASE/lib和$CATALINA_HOME/lib下的class，资源和jar文件）
/|\
|
WebAppClassLoader（加载WebApp/WEB-INF/classes，WebApp/WEB-INF/lib）
Java ClassLoader验证：
public class ClassLoaderTest {
public static void main(String[] args) {
ClassLoader appClassLoader = ClassLoaderTest.class.getClassLoader();
ClassLoader extClassLoader = appClassLoader.getParent();
ClassLoader bootstrapClassLoader = extClassLoader.getParent();
System.out.println(appClassLoader);
System.out.println(extClassLoader);
System.out.println(bootstrapClassLoader);
}
}
输出：
sun.misc.Launcher$AppClassLoader@73d16e93
sun.misc.Launcher$ExtClassLoader@15db9742
nul
Tomcat ClassLoader验证：
public class ClassLoaderServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ClassLoader loader = this.getClass().getClassLoader();
while(loader != null) {
System.out.println(loader);
loader = loader.getParent();
}
System.out.println(loader);
}
}
输出：
ParallelWebappClassLoader
context: test-web
delegate: false
----------> Parent Classloader:
java.net.URLClassLoader@379619aa
java.net.URLClassLoader@379619aa
sun.misc.Launcher$AppClassLoader@764c12b6
sun.misc.Launcher$ExtClassLoader@1a93a7ca
null
跟踪org.apache.catalina.loader.ParallelWebappClassLoader源码发现，org.apache.catalina.loader.WebappClassLoaderBase重写了toString()方法：
@Override
public String toString() { StringBuilder sb = new StringBuilder(this.getClass().getSimpleName());
sb.append("\r\n context: ");
sb.append(getContextName());
sb.append("\r\n delegate: ");
sb.append(delegate);
sb.append("\r\n");
if (this.parent != null) {
sb.append("----------> Parent Classloader:\r\n");
sb.append(this.parent.toString());
sb.append("\r\n");
}
if (this.transformers.size() > 0) {
sb.append("----------> Class file transformers:\r\n");
for (ClassFileTransformer transformer : this.transformers) {
sb.append(transformer).append("\r\n");
}
}
return (sb.toString());
}
也就是说，Tomcat的ClassLoader结构为：
null（JVM Bootstrap ClassLoader）
sun.misc.Launcher$ExtClassLoader@1a93a7ca
sun.misc.Launcher$AppClassLoader@764c12b6
java.net.URLClassLoader@379619aa
org.apache.catalina.loader.ParallelWebappClassLoader
Tomcat Common ClassLoader默认的搜索顺序为：
（1）unpacked classes and resources in $CATALINA_BASE/lib
（2）JAR files in $CATALINA_BASE/lib
（3）unpacked classes and resources in $CATALINA_HOME/lib
（4）JAR files in $CATALINA_HOME/lib
Tomcat classloader类图
总结：
1. Java装载类使用“全盘负责委托机制”。
“全盘负责”是指当一个ClassLoder装载一个类时，除非显示地使用另外一个ClassLoder，否则该类所依赖及引用的类也由这个ClassLoder载入；
“委托机制”是指先委托父类装载器寻找目标类，只有在找不到的情况下才从自己的类路径中查找并装载目标类。
2. Tomcat的WebAppClassLoade默认不使用“委托机制”，查找class和资源的顺序如下：
（1）Bootstrap classes of your JVM
（2）/WEB-INF/classes of your web application
（3）/WEB-INF/lib/*.jar of your web application
（4）System class loader classes
（5）Common class loader classes
If the web application class loader is configured with <Loader delegate="true"/> then the order becomes:
（1）Bootstrap classes of your JVM
（2）System class loader classes
（3）Common class loader classes
（4）/WEB-INF/classes of your web application
（5）/WEB-INF/lib/*.jar of your web application
3. Tomcat中变量CATALINA_BASE和CATALINA_HOME的含义
Throughout the docs, you'll notice there are numerous references to $CATALINA_HOME的含义.
This represents the root of your Tomcat installation. When we say, "This information can be found in your $CATALINA_HOME/README.txt file" we mean to look at the README.txt file at the root of your Tomcat install.
Optionally, Tomcat may be configured for multiple instances by defining $CATALINA_BASE for each instance. If multiple instances are not configured, $CATALINA_BASE is the same as $CATALINA_HOME.
一言以蔽之：如果没有明确设置CATALINA_BASE变量，则CATALINA_BASE与CATALINA_HOME值相同，都是值tomcat安装目录。
【参考】
http://www.hollischuang.com/archives/199 深度分析Java的ClassLoader机制（源码级别）
http://blog.csdn.net/xyang81/article/details/7292380 深入分析Java ClassLoader原理