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Jetty/Feature/Jetty OSGi

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Revision as of 02:26, 23 September 2012 by Eclipse.rc.gmail.com (Talk | contribs)



Introduction

Jetty on OSGi consists of bootstrapping a standard jetty server from OSGi. It supports the deployment of traditional J2EE web-applications as well as web-applications contained inside an OSGi bundle (RFC66).

The goal of Jetty on OSGi is to offer a migration path for traditional J2EE applications to run inside an OSGi container. It favours J2EE applications currently deployed on jetty: developers and IT administrators familiar with a jetty deployment will find the same folder structure and configuration files.

Jetty-on-OSGi also provides an SDK for Eclipse-PDE that supports the development and debugging of a mix of traditional web-applications and web-applications contained in an OSGi bundle: Jetty on OSGi SDK for Eclipse PDE.

This document describes the internal architecture of Jetty-OSGi and describes the features supported. It assumes the reader is interested into taking advantage of OSGi.

Feature

Jetty bootstrapping in OSGi

A single bundle 'org.eclipse.jetty.osgi.boot' acts as an alternative to Jetty's Start.jar. It wires up all the jetty jars that can be located. It locates the ${jetty.home} folder where the configuration files are located. It starts a jetty server according to these configuration files:

It locates a ${jetty.home} folder (or generates one by default) and starts a jetty server according to the jetty.xml file. If you add extra configuration files, then please ensure that the first element configures the server instance with the id of "Server":

 <Configure id="Server" class="org.eclipse.jetty.server.Server">

The classloader inside which the jetty server is executing is a mix of the OSGi classloader provided by the 'org.eclipse.jetty.osgi.boot' bundle and also the jars and folders found inside ${jetty.home}/lib/ext and ${jetty.home}/resources.

The jetty.xml provided by default in jetty will deploy all web-applications found inside the ${jetty.home}/webapps and will read the jetty context files found inside ${jetty.home}/contexts.

OSGi's HttpService via equinox servlet bridge

Equinox packages a servlet that supports HttpService. Jetty-OSGi is distributed with a bundle that deploys this servlet and hence provides an implementation of HttpService. The URL mapping to this servlet is "/*": it will catch any request not caught by other handlers.

Deployment of web-applications via OSGi

Support for RFC66

RFC66 standardize the format of a bundle that contains a web-application and its lifecycle. The basic use-case is this one: When a bundle is started and contains "WEB-INF/web.xml", an RFC66 implementation will deploy the web-application contained in that bundle. The RFc66 implementation reads the manifest headers of the bundle for non-default deployment parameters of that web-application.

Jetty-OSGi takes advantage of jetty's native web-app deployer and context deployers and exposes those as extensions to RFC66.

Manifest headers and access to OSGI in the servlet context

The mandatory manifest header of RFC66 is supported:

Web-ContextPath: /theContextPath<br>

The default behavior when it is missing is also supported: the default context path is the symbolic name of the bundle that contains WEB-INF/web.xml.

The optional header "Jsp-ExtractLocation" is not supported currently.

A web-application deployed by OSGi contains a pointer to the org.osgi.BundleContext object as the attribute 'osgi-bundlecontext' of the servlet context.

Jetty-OSGi does not currently enforce that the WEB-INF/classes folder and the WEB-INF/lib/*.jar must be part of the Manifest entry "Bundle-Classpath". If they are not present, jetty-osgi will still load them in the classloader of the web-application.

"war" URL scheme

The "war" URL scheme specified by RFC66 support the installation of J2EE web archives - "war" files - into an OSGi environment. On the OSGi shell, it is possible to install a war file with a command like this one:

>install war:file://path/to/pet.war<br>

The corresponding war file is transformed into a suitable OSGi bundle and installed on to the OSGi container.

The support for this protocol is mostly complete. The query string parameters that enable overriding the manifest headers and other settings have not been extensively tested.

Also RFC66 specifies a factory class that create URLs with the war scheme. This factory is not implemented currently. The factory is not specific to Jetty so hopefully it will be packaged with the OSGi APIs.

The support for the war scheme is almost fully decoupled from Jetty APIs. It could be used in any RFC66 implementation. It is in the bundle org.eclipse.jetty.osgi.warurl.

Internals and extensions to RFC66

Manifest headers specific to jetty

Jetty-WarFolderPath: /path/to/a/webapp

This entry let's the user define a comma separated list of folder paths relative to the base folder of the bundle. Each folder will be deployed as a web-application. It is required that each folder contains WEB-INF/web.xml Each of those web-application will use its own classloader which is a child of the bundle's classloader that contains them. Hence each webapp can eventually contain WEB-INF/lib and WEB-INF/classes for libraries specific to that web-app.

Optionally, you can also include a context xml file in the META-INF of your bundle called jetty-webapp-context.xml which will be applied to each webapp within the bundle to preconfigure it. Some settings within the webapp can be overridden by the manifest headers (eg Web-ContextPath). Of course you can also use jetty's WEB-INF/jetty-web.xml file to post-configure the webapp too.

Jetty-ContextFilePath: /path/to/a/context/file.xml,/path/to/another/context/file.xml

This header is a comma separated list of context xml files that will be executed by jetty. These files can configure a webapp, or any type of ContextHandler supported by Jetty.

The OSGi service 'org.eclipse.jetty.server.handler.ContextHandler'

The internal mechanism through which jetty manages web-applications is through the OSGi service org.eclipse.jetty.server.handler.ContextHandler. Whenever an instance of that service is registered, jetty-osgi will read the attached service parameters and deploy the corresponding web-application.

When such a service is unregistered, the corresponding web-application is undeployed.

The following static method illustrate how to deploy a web-application in OSGi programmatically:

public static void registerWebapplication(Bundle contributor, String webappFolderPath, String contextPath) throws Exception  {
        WebAppContext contextHandler = new WebAppContext();
 
        Properties dic = new Properties();
        dic.put(OSGiWebappConstants.SERVICE_PROP_WAR, webappFolderPath);
        dic.put(OSGiWebappConstants.SERVICE_PROP_CONTEXT_PATH, contextPath);
 
        contributor.getBundleContext().registerService(ContextHandler.class.getName(), contextHandler, dic);
}

When deploying a web-application, it is necessary to create a WebAppContext rather than a basic ContextHandler.

All possible parameters are defined as constants in the class org.eclipse.jetty.osgi.boot.OSGiWebappConstants:

ParameterDescription
SERVICE_PROP_CONTEXT_PATHThe context path of the installed web-application.
SERVICE_PROP_WARPath to the web-application folder. Those pathes are relative to the bundle location or to the SERVICE_PROP_BUNDLE_INSTALL_LOCATION_OVERRIDE when it is specified.
SERVICE_PROP_CONTEXT_FILE_PATHPath to the jetty context files.
SERVICE_PROP_BUNDLE_INSTALL_LOCATION_OVERRIDEBase folder inside which all pathes are resolved. By default the location where the bundle is installed.
SERVICE_PROP_EXTRA_CLASSPATHExtra jars and folders added to the classloader of the web-application.
SERVICE_PROP_WEB_XML_PATHPath to the web.xml file when we don't default on WEB-INF/web.xml.
SERVICE_PROP_DEFAULT_WEB_XML_PATHPath to the defaultweb.xml file when we don't default on the one provided by Jetty.

Notes: OSGi services versus bundles to manage web-applications.

It is (in my biased opinion) more interesting to manage web-applications via an OSGi service rather than via a bundle:

  • The lifecycle of an OSGi service is a lot finer than the life-cycle of a bundle. Developers can equally create or listen to them. Consoles are developped to administer them: change a parameter for a registered service via an admin console will trigger the reloading of the corresponding web-app with a new deployment parameter.
  • There are many ways to create OSGi services and all of them inter-operable:
    • java code that depends on OSGi API.
    • IOC frameworks including OSGi declarative services, springsource.
  • OSGi services are a promising abstractions for distributed servers etc. Bundles are a lot more coupled to the life-cycle of classloaders and underlying jar files.

The support for RFC66'manifest headers basically consists of listening to the bundles lifecycle and issuing the corresponding registration of the 'org.eclipse.jetty.server.handler.ContextHandler'

Architecture

Classloader tree

'org.eclipse.jetty.osgi.boot' (org.eclipse.jetty.*, javax.*, most optional) + dependencies_injected_by_fragments
      jetty-server (${jetty.home}/resources, ${jetty.home}/lib/ext)
      web-application (OSGi-bundle-that-contains-it, WEB-INF/classes, WEB-INF/lib, extraClasspath)

To be more precise we could mention the extra classloader for jsp that supports jasper and the taglibraries.

Injecting new server-wide features into jetty-osgi

A common situation for framework developers is to make a set of library installed in the server's classloader and to have a hook in the life-cycle of the server.

This is supported in 2 ways:

  • The traditional J2EE way: locate the ${jetty.home} folder and add the libs to lib/ext then configure jetty.xml
  • The OSGi way consists of installing a bundle-fragment hosted by 'org.eclipse.jetty.osgi.boot'

This will make the dependencies of the fragment available in the classloader of the server. In order to hook into the lifecycle of server it is possible to define an implementation of BundleActivator that will be invoked by the bundle 'org.eclipse.jetty.osgi.boot'. In order for this BundleActivator to be invoked it must be named following this convention: ${bundle.id.of.this.fragment}+".FragmentActivator"

Jetty-osgi is distributed with a number of optional bundles that illustrate this technique: Support for jsp with the bundle "org.eclipse.jetty.osgi.boot.jsp" Support for centralized logging on logback; "org.eclipse.jetty.osgi.boot.logback"

Centralized logging support with slf4j and logback-classic

In order to use logback/slf4j, make sure that your target platform contains those libraries. Then set the system property "logback.configurationFile" to point to your logback configuration file (more on Logback's [|documentation]).

JSP support: Jasper integration

The optional bundle 'org.eclipse.jetty.osgi.jasper' adds support for jsp-2.1 and the standard tag libraries. TODO: support custom taglibs; make sure java server faces and other common libraries work well.

Runtime packaging

felix or equinox. The bare framework with jetty*, javax*, slf4j*, logback* installed as well as a folder where new bundles can be hot-deploy simply by droping them in. A jetty.home folder with the expected folder hierarchy.

Next steps

Better SDK with more UI for the config of jetty. Manage multiple instance of jetty. Better support for other frameworks that want to configure jetty themselves (geronimo, spring, eclipse-platform, nuxeo etc)

Additional Resources

Jetty on OSGi SDK for Eclipse PDE.