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# The Loader Component

Introduction

> The Loader element represents the web application class loader that will be used to load Java classes and resources for your web application. Such a class loader must follow the requirements of the Servlet Specification, and load classes from the following locations:

> - From the `/WEB-INF/classes` directory inside your web application.
> - From JAR files in the `/WEB-INF/lib` directory inside your web application.
> - From resources made available by Catalina to all web applications globally.

> A Loader element MAY be nested inside a Context component. If it is not included, a default Loader configuration will be created automatically, which is sufficient for most requirements.

> For a more in-depth description of the class loader hierarchy that is implemented by Catalina, see FIXME - Reference.

> > The description below uses the variable name $CATALINA_HOME to refer to the directory into which you have installed Tomcat 4, and is the base directory against which most relative paths are resolved. However, if you have configured Tomcat 4 for multiple instances by setting a CATALINA_BASE directory, you should use $CATALINA_BASE instead of $CATALINA_HOME for each of these references.

Attributes

> Common Attributes

> > All implementations of Loader support the following attributes:

> > Attribute | Description
> > --- | ---
> > `className` | Java class name of the implementation to use. This class must implement the `org.apache.catalina.Loader` interface. If not specified, the standard value (defined below) will be used.
> > `delegate` | Set to `true` if you want the class loader to follow the standard Java2 delegation model, and attempt to load classes from parent class loaders before looking inside the web application. Set to `false` (the default) to have the class loader look inside the web application first, before asking parent class loaders to find requested classes or resources.

> > `reloadable`

> > Set to `true` if you want Catalina to monitor classes in `/WEB-INF/classes/` and `/WEB-INF/lib` for changes, and automatically reload the web application if a change is detected. This feature is very useful during application development, but it requires significant runtime overhead and is not recommended for use on deployed production applications. You can use the Manager web application, however, to trigger reloads of deployed applications on demand.

> > NOTE - The value for this property will be inherited from the `reloadable` attribute you set on the surrounding Context component, and any value you explicitly set here will be replaced.

> Standard Implementation

> > The standard implementation of Loader is org.apache.catalina.loader.WebappLoader. It supports the following additional attributes (in addition to the common attributes listed above):

> > Attribute | Description
> > --- | ---
> > `checkInterval` | The number of seconds between checks for modified classes and resources, if `reloadable` has been set to `true`. The default is 15 seconds.
> > `debug` | The level of debugging detail logged by this Engine to the associated Logger. Higher numbers generate more detailed output. If not specified, the default debugging detail level is zero (0).
> > `loaderClass` | Java class name of the `java.lang.ClassLoader` implementation class to use. If not specified, the default value is `org.apache.catalina.loader.WebappClassLoader`.

> > `workDir`

> > Pathname to a scratch directory to be provided by this Context for temporary read-write use by servlets within the associated web application. This directory will be made visible to servlets in the web application by a servlet context attribute (of type `java.io.File`) named `javax.servlet.context.tempdir` as described in the Servlet Specification. If not specified, a suitable directory underneath `$CATALINA_HOME/work` will be provided.

Nested Components

> No components may be nested inside a Loader element.

Special Features

> No special features are associated with a Loader element.

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