细说tomcat之类加载器

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("
  context: ");
    sb.append(getContextName());
    sb.append("
  delegate: ");
    sb.append(delegate);
    sb.append("
");
    if (this.parent != null) {
        sb.append("----------> Parent Classloader:
");
        sb.append(this.parent.toString());
        sb.append("
");
    }
    if (this.transformers.size() > 0) {
        sb.append("----------> Class file transformers:
");
        for (ClassFileTransformer transformer : this.transformers) {
            sb.append(transformer).append("
");
        }
    }
    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