标签：ima extend tar 参考 src gets final url builder
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_BASE/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_BASE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
标签：ima extend tar 参考 src gets final url builder
原文地址：http://www.cnblogs.com/nuccch/p/7071446.html
