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Note
This feature was reintroduced in Jetty 9.0.4
The Jetty Overlay Deployer allows multiple WAR files to be overlaid so that a web application can be customized, configured, and deployed without unpacking, modifying and repacking the WAR file. This has the following benefits:
This tutorial describes how to configure Jetty to use the Overlay Deployer, and how to deploy multiple instances of a web application using the JTrac application in the example.
Customizing, configuring and deploying a web application bundled as a WAR file frequently includes some or all of these steps:
WEB-INF/web.xml
file to set init parameters, add filters/servlets or to configure JNDI resources.WEB-INF/
.WEB-INF/
(for example, jetty-web.xml
or jboss-web.xml
).The result is that the customizations and configurations blend into both the container and the WAR file. If either the container or the base WAR file is upgraded to a new version, it can be a very difficult and error prone task to identify all the changes that have been made and to reapply them to a new version.
To solve the problems highlighted above, Jetty introduced WAR overlays (a concept borrowed from the Maven WAR plugin).
An overlay is basically just another WAR file, whose contents merge on top of the original WAR so that filed can be added or replaced.
Jetty overlays also allow fragments of web.xml
to be mixed in, which means the configuration can be modified without replacing it.
The JTrac issue tracking web application is a good example of a typical web application, as it uses the usual suspects of libs: spring, hibernate, dom4j, commons-*, wicket, etc. The files for this demonstration are available in overlays-demo.tar.gz. The demonstration can be expanded on top of the Jetty distribution; this tutorial expands it to /tmp and installs the components step-by-step:
$ cd /tmp $ wget http://webtide.com/wp-content/uploads/2011/05/overlays-demo.tar.gz $ tar xfvz overlays-demo.tar.gz $ export OVERLAYS=/tmp/overlays
Overlays support is included in jetty distributions from 7.4.1-SNAPSHOT onwards, which can be downloaded from oss.sonatype.org or Maven Central and unpack into a directory.
The start.ini
file needs edited so that it includes the overlay option and configuration file.
The resulting file should look like:
OPTIONS=Server,jsp,jmx,resources,websocket,ext,overlay
etc/jetty.xml
etc/jetty-deploy.xml
etc/jetty-overlay.xml
The mechanics of this are in etc/jetty-deploy.xml, which installs the OverlayedAppProvider
into the DeploymentManager
.
Jetty can then be started normally:
$ java -jar start.jar
Jetty is now listening on port 8080, but with no webapp deployed.
Important
You should conduct the rest of the tutorial in another window with the JETTY_HOME environmental variable set to the Jetty distribution directory.
The WAR file for this demo can be downloaded and deployed the using the following commands, which downloads and extracts the WAR file to the $JETTY_HOME/overlays/webapps directory.
$ cd /tmp $ wget -O jtrac.zip http://sourceforge.net/projects/j-trac/files/jtrac/2.1.0/jtrac-2.1.0.zip/download $ jar xfv jtrac.zip jtrac/jtrac.war $ mv jtrac/jtrac.war $JETTY_HOME/overlays/webapps
When these commands (or equivalent) have been executed, a message that the OverlayedAppProvider
has extracted and loaded the WAR file will be displayed in the Jetty server window:
2011-05-06 10:31:54.678:INFO:OverlayedAppProvider:Extract jar:file:/tmp/jetty-distribution-7.4.1-SNAPSHOT/overlays/webapps/jtrac-2.1.0.war!/ to /tmp/jtrac-2.1.0_236811420856825222.extract
2011-05-06 10:31:55.235:INFO:OverlayedAppProvider:loaded jtrac-2.1.0@1304641914666
Unlike the normal webapps dir, loading a WAR file from the overlays/webapp dir does not deploy the web application, it simply makes it available to use as the basis for templates and overlays.
A template overlay is a WAR structured directory/archive that contains the files that have been added or modified to customize/configure the web application for all instances planned for deployment.
The demo template can be installed from the downloaded files with the command:
$ mv $OVERLAYS/jtracTemplate\=jtrac-2.1.0 $JETTY_HOME/overlays/templates/
In the Jetty server window, a message similar to this will be displayed confirmed that the template is loaded:
2011-05-06 11:00:08.716:INFO:OverlayedAppProvider:loaded jtracTemplate=jtrac-2.1.0@1304643608715
The contents of the loaded template are as follows:
templates/jtracTemplate=jtrac-2.1.0
|__ WEB-INF
|__ classes
| |__ jtrac-init.properties
|__ log4j.properties
|__ overlay.xml
|__ template.xml
|__ web-overlay.xml
ContextHandler
for each instance. \
In this case it sets up the context path:<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">
<Configure class="org.eclipse.jetty.webapp.WebAppContext">
<Set name="contextPath">/</Set>
</Configure>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">
<Configure class="org.eclipse.jetty.overlays.TemplateContext">
<Get name="resourceCache">
<Set name="useFileMappedBuffer">true</Set>
<Set name="maxCachedFileSize">10000000</Set>
<Set name="maxCachedFiles">1000</Set>
<Set name="maxCacheSize">64000000</Set>
</Get>
</Configure>
web.xml
fragment that Jetty overlays on top of the web.xml
from the base WAR file; it can set init parameters and add/modify filters and
servlets.
In this example it sets the application home and springs webAppRootKey
:<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
version="2.4">
<context-param>
<param-name>jtrac.home</param-name>
<param-value>/tmp/jtrac-${overlay.instance.classifier}</param-value>
</context-param>
<context-param>
<param-name>webAppRootKey</param-name>
<param-value>jtrac-${overlay.instance.classifier}</param-value>
</context-param>
<filter>
</web-app>
Notice the parameterization of values such as ${overlays.instance.classifier}
, as this allows the configuration to be in the template, and not customized for each instance.
Without the Overlay Deployer, all of the above would still need to have configure , but rather than being in a single clear structure the configuration elements would have been either in the server’s common directory, the server’s webdefaults.xml
(aka server.xml
), or baked into the WAR file of each application instance using copied/modified files from the original.
The Overlay Deployer allows all these changes to be made in one structure; moreover it allows for the parameterization of some of the configuration, which facilitates easy multi-tenant deployment.
Now that the template is installed, one or more instance overlays can be implemented to deploy the actual web applications:
$ mv /tmp/overlays/instances/jtracTemplate\=blue $JETTY_HOME/overlays/instances/ $ mv /tmp/overlays/instances/jtracTemplate\=red $JETTY_HOME/overlays/instances/ $ mv /tmp/overlays/instances/jtracTemplate\=blue $JETTY_HOME/overlays/instances/
As each instance moves into place, the Jetty server window reacts and deploys the instance. Within each instance, there is the structure:
instances/jtracTemplate=red/
|__ WEB-INF
| |__ overlay.xml
|__ favicon.ico
|__ resources
|__ jtrac.css
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">
<Configure class="org.eclipse.jetty.webapp.WebAppContext">
<Set name="virtualHosts">
<Array type="String">
<Item>127.0.0.2</Item>
<Item>red.myVirtualDomain.com</Item>
</Array>
</Set>
</Configure>
Deployed instances can be vied by pointing a browser at http://127.0.0.1:8080, http://127.0.0.2:8080 and http://127.0.0.3:8080. The default username/password for JTrac is admin/admin.
ResourceCache
so only a single instance of each static content is loaded into memory.Jetty hot deploys all overlays and tracks dependencies.
overlays/webapps
and then rename jtracTemplate\=jtrac-2.1.0
to jtracTemplate\=jtrac-2.2.0
.options=jndi
, annotations and jetty-plus.xml
in start.ini
) and shows how additional JARs can be added in the overlays.