From: kkolinko Date: Fri, 29 Jul 2011 19:49:58 +0000 (+0000) Subject: Improve class loading documentation X-Git-Url: https://git.internetallee.de/?a=commitdiff_plain;h=736f9e4fa6378d60107254bfaf58211b78ffcdb4;p=tomcat7.0 Improve class loading documentation Review existing documentation. Rephrase and reformat some statements. Documented $CATALINA_BASE/bin/tomcat-juli.jar. Documented new JARs in $CATALINA_HOME/lib, that were not present in Tomcat 6. More changes to this document are to follow. git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1152352 13f79535-47bb-0310-9956-ffa450edef68 --- diff --git a/webapps/docs/changelog.xml b/webapps/docs/changelog.xml index 93509294e..9a08ccfbb 100644 --- a/webapps/docs/changelog.xml +++ b/webapps/docs/changelog.xml @@ -186,6 +186,9 @@ common class loader when separate values are used for $CATALINA_HOME and $CATALINA_BASE. (markt) + + Improve class loading documentation. (kkolinko) + diff --git a/webapps/docs/class-loader-howto.xml b/webapps/docs/class-loader-howto.xml index 9bfa823ac..a8c3539d2 100644 --- a/webapps/docs/class-loader-howto.xml +++ b/webapps/docs/class-loader-howto.xml @@ -41,15 +41,16 @@ 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.

+Servlet Specification, version 2.4 — in particular, Sections 9.4 +and 9.6.

-

In a J2SE 2 (that is, J2SE 1.2 or later) environment, class loaders are +

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. The model for -web application class loaders differs slightly from this, as discussed below, -but the main principles are the same.

+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 @@ -76,83 +77,109 @@ the following section.

As indicated in the diagram above, Tomcat creates the following class loaders as it is initialized:

+ +

  • 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: + following locations in the order they are listed:

    • unpacked classes and resources in $CATALINA_BASE/lib
    • JAR files in $CATALINA_BASE/lib
    • unpacked classes and resources in $CATALINA_HOME/lib
    • JAR files in $CATALINA_HOME/lib
    - By default, this includes the following: +

    By default, this includes the following:

      -
    • annotations-api.jar - JEE annotations classes.
      • -
    • catalina.jar - Implementation of the Catalina servlet +
    • 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.
      • -
    • el-api.jar - EL 2.1 API.
      • -
    • jasper.jar - Jasper 2 Compiler and Runtime.
      • -
    • jasper-el.jar - Jasper 2 EL implementation.
      • -
    • ecj-*.jar - Eclipse JDT Java compiler.
      • -
    • jsp-api.jar - JSP 2.1 API.
      • -
    • servlet-api.jar - Servlet 3.0 API.
      • -
    • tomcat-coyote.jar - Tomcat connectors and utility classes.
      • -
    • tomcat-dbcp.jar - package renamed database connection - pool based on Commons DBCP.
      • -
    • tomcat-i18n-**.jar - Optional JARs containing resource bundles +
    • 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.
    • -
  • WebappX - A class loader is created for each web +

  • 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 archive, plus classes and resources in JAR files - under the /WEB-INF/lib directory of your web application - archive, are made visible to the containing web application, but to - no others.

    • + 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 2 delegation model (in accordance with the recommendations in the -Servlet Specification, version 2.3, section 9.7.2 Web Application Classloader). +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 overriden. For some classes (such as +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 containing servlet API classes will be ignored by the -classloader. +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 @@ -162,18 +189,17 @@ loading looks in the following repositories, in this order:

  • System class loader classes (described above)
  • /WEB-INF/classes of your web application
  • /WEB-INF/lib/*.jar of your web application
    • -
  • $CATALINA_HOME/lib
    • -
  • $CATALINA_HOME/lib/*.jar
    • +
  • Common class loader classes (described above)
    • -
    +
    -

    Among many other changes, the JSE 5 release packages the JAXP APIs, and -a version of Xerces, inside the JRE. This has impacts on applications that -wish to use their own XML parser.

    +

    Among many other changes, the JavaSE 5 release packages the JAXP APIs, and +a version of Apache Xerces, inside the JRE. This has impacts on applications +that wish to use their own XML parser.

    In previous versions of Tomcat, you could simply replace the XML parser in the $CATALINA_HOME/common/lib directory to change the parser