细说tomcat之类加载器

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$JAVA_HOME/jre/lib/ext/目录下的所有jar）
/|\ /|\
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AppClassLoader（加载应用程序classpath目录下的所有jar和class文件） System（加载$CATALINA_HOME/bin/bootstrap.jar，$CATALINA_BASE/bin/tomcat-juli.jar，$CATALINA_HOME/bin/commons-daemon.jar）
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|
Common（加载$CATALINA_BASE/lib和$CATALINA_HOME/lib下的class，资源和jar文件）
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WebAppClassLoader（加载WebApp/WEB-INF/classes，WebApp/WEB-INF/lib）

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

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@379619aajava.net.URLClassLoader@379619aa
sun.misc.Launcher$AppClassLoader@764c12b6
sun.misc.Launcher$ExtClassLoader@1a93a7ca
null

@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());
}

null（JVM Bootstrap ClassLoader）
sun.misc.Launcher$ExtClassLoader@1a93a7ca
sun.misc.Launcher$AppClassLoader@764c12b6
java.net.URLClassLoader@379619aa
org.apache.catalina.loader.ParallelWebappClassLoader

（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

（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

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.