The Loader Component

Table of Contents

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 the ClassLoader HowTo.

The description below uses the variable name $CATALINA_BASE to refer the base directory against which most relative paths are resolved. If you have not configured Tomcat for multiple instances by setting a CATALINA_BASE directory, then $CATALINA_BASE will be set to the value of $CATALINA_HOME, the directory into which you have installed Tomcat.

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

NOTE - This attribute has been deprecated in Tomcat 9 and will be removed in Tomcat 10. Use the equivalent attribute on the surrounding Context component.

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
loaderClass

Java class name of the java.lang.ClassLoader implementation class to use. Custom implementations must extend org.apache.catalina.loader.WebappClassLoaderBase.

If not specified, the default value is org.apache.catalina.loader.ParallelWebappClassLoader. The default loaderClass is parallel capable, which means that multiple threads may load difference classes in parallel. A non-parallel capable loaderClass is available and can be used by specifying org.apache.catalina.loader.WebappClassLoader.

Nested Components

No components may be nested inside a Loader element.

Special Features

Logging

A loader is associated with the log category based on its classname.