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This article outlines the class loader hierarchy employed by SonarQube and is intended to assist plugin authors in understanding what classes/resources are accessible to their plugins at runtime.

SonarQube Classloader

The SonarQube classloader:

Plugin Classloader

For each plugin, a classloader is created as a child of the SonarQube classloader. By default, the plugin classloader uses parent-first delegation model.

Search order for parent-first model:

  1. parent class loader
  2. classes exported from other plugins (see next section)
  3. classes in plugin

A plugin can explicitly use the child-first model (see the property useChildFirstClassLoader). In this case, the search order is :

  1. classes exported from other plugins (see next section)
  2. classes in plugin
  3. parent class loader

How to Share Classes Between Plugins 

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This feature is implemented since version 2.4 for Maven 2 builds, and since version 2.5 for Maven 3 builds.

Each plugin has its own class loader, so by default a plugin cannot use a class defined in other plugins. This constraint can be bypassed if a plugin explicitly exports a subset of classes. These classes must be defined in the package "org.sonar.plugins.<plugin key>.api" or any of its potential sub-packages. For this reason this feature is reserved to the plugins hosted in the forge of open-source plugins.

For example the DBCleaner plugin shares the class org.sonar.plugins.dbcleaner.api.PeriodCleaner. Any plugin can use this class.

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