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Fine Grain Classloader Control

Mule artifacts use a particular type of classloaders that provide a way to change the default classloading behaviour. By default, these fine grain classloaders behave using a parent first schema, but they can be configured  to define how specific classes and packages have to managed during the classloading process.

Each classloader is configured passing a list of classes or packages fully-qualified names.

Each class/package can be declared in one of two available modes:

Override

Overriding a class/package means that this entry attempts to load from the current fine grain classloader before attempting to load from the parent classloader. Basically the override mode applies a child-first classloading schema for the given resources.

For example, an override that includes a class (com.example.MyProvider) and a package (com.sun.jersey) would be specified as follows:

loader.override=com.example.MyProvider, com.sun.jersey

Notes:

  • Overriding a package means that all of its subpackages are also overridden. For example, com.sun.jersey also includes com.sun.jersey.impl.

  • System packages can’t be overriden in this way. This restriction covers packages who’s names start with java., javax., org.mule., com.mulesoft., and com.mulesource.

  • Put the loader.override statement in the mule-deploy.properties file.

Blocking

Blocking a class/package means that this entry attempts to load from the current fine grain classloader only, and never attempts to load from the parent classloader.

Blocking is configured similarly to class/package override, the only difference is that names must be prefixed with a - (dash/minus sign).

Here, is an example of a blocking specification:

loader.override=-com.example.AnotherProvider

Note that if a class/package is blocked and its not found in the current fine grain classloader, then a ClassNotFoundException is thrown, even if Mule does have such a class on a system level.

Configuring the Fine Grain Classloader

This type of classloader is available in different types of Mule artifacts, the configuration usually implies adding a property value in some artifact configuration file. Following section describes how to configure fine grain classloading for each type of artifact.

Mule Application: add the following property in the application’s mule-deploy.properties file like this:

loader.override=<comma-separated list of classes or packages>

Application Plugin: add the following property in the plugin’s plugin.properties file like this:

loader.override=<comma-separated list of classes or packages>

Mule Plugin: add the following property in the plugin’s mule-plugin.properties file like this:

loader.overrides=<comma-separated list of classes or packages>

Put the loader.override statement in the mule-deploy.properties file.

Classloader Override/Blocking Diagram

The following diagram illustrates how class loading is handled for class overrides and blocking. When you view the diagram note that every class loader has a parent class loader. Unless loader override is configured, a classloader first delegates the search for a class to its parent classloader before attempting to find the class itself (the classloader is the child of its parent classloader).

chart

Note that when using an override for a package (and no blocking) every class from that package attempts to load from the application classloader. If a class is not found, then the application’s parent classloader is used instead.