tomcat7 类加载机制
这边走：http://tomcat.apache.org/tomcat-7.0-doc/class-loader-howto.html
1.Overview
Like many server applications, Tomcat installs a variety of class loaders (that is, classes that implement java.lang.ClassLoader) to allow different portions of the container, and the web applications running on the container, to have access to different repositories of available classes and resources. This mechanism is used to provide the functionality defined in the Servlet Specification, version 2.4 — in particular, Sections 9.4 and 9.6.
In a Java environment, class loaders are arranged in a parent-child tree. Normally, when a class loader is asked to load a particular class or resource, it delegates the request to a parent class loader first, and then looks in its own repositories only if the parent class loader(s) cannot find the requested class or resource. Note, that the model for web application class loaders differs slightly from this, as discussed below, but the main principles are the same.
When Tomcat is started, it creates a set of class loaders that are organized into the following parent-child relationships, where the parent class loader is above the child class loader:
Bootstrap
|
System
|
Common
/ \
Webapp1 Webapp2 ...The characteristics of each of these class loaders, including the source of classes and resources that they make visible, are discussed in detail in the following section.
2.Class Loader Definitions
As indicated in the diagram above, Tomcat creates the following class loaders as it is initialized:
Bootstrap — This class loader contains the basic runtime classes provided by the Java Virtual Machine, plus any classes from JAR files present in the System Extensions directory ($JAVA_HOME/jre/lib/ext). Note: some JVMs may implement this as more than one class loader, or it may not be visible (as a class loader) at all.
System— This class loader is normally initialized from the contents of the CLASSPATH environment variable. All such classes are visible to both Tomcat internal classes, and to web applications. However, the standard Tomcat startup scripts *($CATALINA_HOME/bin/catalina.sh or %CATALINA_HOME%\bin\catalina.bat) totally ignore the contents of the CLASSPATH environment variable itself, and instead build the System class loader from the following repositories:
$CATALINA_HOME/bin/bootstrap.jar — Contains the main() method that is used to initialize the Tomcat server, and the class loader implementation classes it depends on.
CATALINABASE/bin/tomcat−juli.jaror CATALINA_HOME/bin/tomcat-juli.jar — Logging implementation classes. These include enhancement classes to java.util.logging API, known as Tomcat JULI, and a package-renamed copy of Apache Commons Logging library used internally by Tomcat. See logging documentation for more details.
If tomcat-juli.jar is present in CATALINABASE/bin,itisusedinsteadoftheonein CATALINA_HOME/bin. It is useful in certain logging configurations
$CATALINA_HOME/bin/commons-daemon.jar — The classes from Apache Commons Daemon project. This JAR file is not present in the CLASSPATH built by catalina.bat|.sh scripts, but is referenced from the manifest file of bootstrap.jar.
- Common — This class loader contains additional classes that are made
visible to both Tomcat internal classes and to all web applications.
Normally, application classes should NOT be placed here. The locations searched by this class loader are defined by the common.loader property in $CATALINA_BASE/conf/catalina.properties. The default setting will search the following locations in the order they are listed:
unpacked classes and resources in CATALINABASE/libJARfilesin CATALINA_BASE/lib
unpacked classes and resources in CATALINAHOME/libJARfilesin CATALINA_HOME/lib
By default, this includes the following:
annotations-api.jar — JavaEE annotations classes.
catalina.jar — Implementation of the Catalina servlet container portion of Tomcat.
catalina-ant.jar — Tomcat Catalina Ant tasks.
catalina-ha.jar — High availability package.
catalina-tribes.jar — Group communication package.
ecj-*.jar — Eclipse JDT Java compiler.
el-api.jar — EL 2.2 API.
jasper.jar — Tomcat Jasper JSP Compiler and Runtime.
jasper-el.jar — Tomcat Jasper EL implementation.
jsp-api.jar — JSP 2.2 API.
servlet-api.jar — Servlet 3.0 API.
tomcat-api.jar — Several interfaces defined by Tomcat.
tomcat-coyote.jar — Tomcat connectors and utility classes.
tomcat-dbcp.jar — Database connection pool implementation based on package-renamed copy of Apache Commons Pool and Apache Commons DBCP.
tomcat-i18n-**.jar — Optional JARs containing resource bundles for other languages. As default bundles are also included in each individual JAR, they can be safely removed if no internationalization of messages is needed.
tomcat-jdbc.jar — An alternative database connection pool implementation, known as Tomcat JDBC pool. See documentation for more details.
tomcat-util.jar — Common classes used by various components of Apache Tomcat.
tomcat7-websocket.jar — WebSocket 1.1 implementation
websocket-api.jar — WebSocket 1.1 API
- WebappX — A class loader is created for each web application that is
deployed in a single Tomcat instance. All unpacked classes and
resources in the /WEB-INF/classes directory of your web application,
plus classes and resources in JAR files under the /WEB-INF/lib
directory of your web application, are made visible to this web
application, but not to other ones.
As mentioned above, the web application class loader diverges from the default Java delegation model (in accordance with the recommendations in the Servlet Specification, version 2.4, section 9.7.2 Web Application Classloader). When a request to load a class from the web application’s WebappX class loader is processed, this class loader will look in the local repositories first, instead of delegating before looking. There are exceptions. Classes which are part of the JRE base classes cannot be overridden. For some classes (such as the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be used. Last, any JAR file that contains Servlet API classes will be explicitly ignored by the classloader — Do not include such JARs in your web application. All other class loaders in Tomcat follow the usual delegation pattern.
Therefore, from the perspective of a web application, class or resource loading looks in the following repositories, in this order:
Bootstrap classes of your JVM
/WEB-INF/classes of your web application
/WEB-INF/lib/*.jar of your web application
System class loader classes (described above)
Common class loader classes (described above)If the web application class loader is configured with then the order becomes:
Bootstrap classes of your JVM
System class loader classes (described above)
Common class loader classes (described above)
/WEB-INF/classes of your web application
/WEB-INF/lib/*.jar of your web application