Resource files bundled with Jenkins and plugins are by default exposed through several different mechanisms.
Jenkins exposes static resource files of allowed file types (.js, .css, .jpeg, .jpg, .png, .gif, .html, .htm as of Jenkins 2.290) in core and plugins at the /resources/ URL.
This is implemented by Jenkins#doResources.
In views, the URL prefix to use is available as app.VIEW_RESOURCE_PATH.
The full path can be computed using h.getViewResource(…).
https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/lib/layout/breadcrumb.gif is exposed at, e.g., https://ci.jenkins.io/resources/cachekey/lib/layout/breadcrumb.gif.
The URL should not be hardcoded, as headers controlling caching are set; and the computed cache key will result in different Jenkins versions having a different prefix.
These resources are available to users with Overall/Read permissions.
The Stapler web framework provides a mechanism called adjuncts that is intended to be used to include CSS and JS files related to a view being rendered.
Using this mechanism ensures that each adjunct file is only included once.
Through the same URL space, some static resource files like images are accessible.
See AdjunctManager#allowResourceToBeServed for supported file types (.css, .js, .gif, .png as of Jenkins 2.290).
These resources are typically exposed through the st:adjunct tag in Jelly or Groovy views, but the URLs are predictable:
https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/lib/layout/breadcrumb.gif is exposed at, e.g., https://ci.jenkins.io/adjuncts/cachekey/lib/layout/breadcrumb.gif.
The URL should not be hardcoded, as headers controlling caching are set; and the computed cache key will result in different Jenkins versions having a different prefix.
These resources are available to any user without authentication, not requiring Overall/Read permission.
Messages_??.properties (including Messages.properties) files contain localized message strings.
For the classic Jenkins UI, they are generally used directly as described in Internationalization and Localization.
Additionally for modern web UIs, localized resources are exposed at /i18n/resourceBundle/, at URLs like https://ci.jenkins.io/i18n/resourceBundle?baseName=hudson.Messages.
See I18n for details.
These resources are available to users with Overall/Read permissions.
Descriptor#doHelp will serve corresponding help.jelly views, if they exist, and lets Stapler localize them (i.e. uses help_??.jelly with locale suffix, if it exists).
An example of such a Stapler view is https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/hudson/tasks/Maven/help.jelly available at https://ci.jenkins.io/descriptor/hudson.tasks.Maven/help.
These are usually used by the automatically determined help URL corresponding to the descriptor (and optionally form field name), and a help link will be shown on the UI if this file exists.
These help views are available to users with Overall/Read permissions.
Descriptor#doHelp looks up com/acme/package/MyDescribable/help-fieldname_??.html HTML files and serves them at /descriptor/myDescriptorSymbol/help/fieldname, typically corresponding to specific fields in views. It also supports com/acme/package/MyDescribable/help_??.html at /descriptor/myDescriptorSymbol/help.
An example of such an HTML file is https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/hudson/tasks/Shell/help.html, which is exposed at https://ci.jenkins.io/descriptor/hudson.tasks.Shell/help.
These are usually used by the automatically determined help URL corresponding to the descriptor (and optionally form field name), and a help link will be shown on the UI if this file exists.
These help files are available to users with Overall/Read permissions.
Jenkins core webapp/ resources are exposed at their expected path relative to the context root directory.
Example: https://github.com/jenkinsci/jenkins/blob/master/war/src/main/webapp/robots.txt is exposed at https://ci.jenkins.io/robots.txt
This is implemented by Stapler#service invoking #openResourcePathByLocale.
This in turn ends up invoking LocaleDrivenResourceProvider#lookupResource, an SPI implemented in Jenkins core as MetaLocaleDrivenResourceProvider from 2.173.
This in turn looks up implementations of PluginLocaleDrivenResourceProvider in plugins.
Localization Support Plugin provides PluginLocaleDrivenResourceProviderImpl.
These resources are available to any user without authentication, not requiring Overall/Read permission.
Plugins expose src/main/webapp/ resource files directly packaged into the jpi file via Plugin#doDynamic at /plugin/namehere/ invoking StaplerResponse#serveLocalizedFile.
This calls #selectResourceByLocale which ends up invoking LocaleDrivenResourceProvider#lookupResource.
See the section on core webapp resources for further details.
These resources are available to users with Overall/Read permissions.
Jenkins core exposes assets (any static resource files visible to the Jenkins core classloader with the asset/ package prefix) at the /assets/ URL.
This is implemented by the AssetManager class.
These resources are available to any user without authentication, not requiring Overall/Read permission.
This was used from Jenkins 2.0 to 2.288 (inclusive) to serve JavaScript assets (jQuery, Handlebars, etc.). The infrastructure remains in place after 2.288.
Assets from plugins are served by reusing the plugin webapp resources serving feature Plugin#doDynamic.
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