Thursday, August 06, 2009

Versioning & p2, slides from EclipseCon

This year at EclipseCon, I presented a 10 minute talk on the importance of versioning with p2. I have posted the slides here. Because this was only a 10 minute talk, there is not a lot of content in the slides, so this post is an attempt at an overview.

id + version == 1 set of bytes

Hopefully by now everyone has heard this before. In the world of p2, an id and a version represent a particular set of bytes. If multiple copies of a given artifact exist in multiple places, and they all have the same id and version, they are assumed to be the same (or equivalent) bytes.

In particular, this means that if your user already has a bundle org.foo_1.0.0 on his machine, and you are trying to deliver an update, then the user will not download your updated if the version number is still 1.0.0.

So make sure you increase your version numbers, it is hard to support a customer when you can't tell if his org.foo_1.0.0 is the broken version or the fixed version!

We tend to version sources

The process we follow in Eclipse tends to version sources. We tag our source code with the version qualifier which determines the version of the resulting binary. The idea is this leads to reproducible builds.

However, we must realize that there is more than just the source code that affects what the resulting binary looks like. These are things like what compiler was used, the build scripts, and most importantly the dependencies that were on our build-time classpath.

If some bundle B depends on bundle A and A changes in some way, then recompiling B can potentially result in different byte-code even though the source code did not change. In this case, B deserves to have its version number increased.

Mirroring with a baseline and comparator

This scenario of a bundle changing without having its version incremented has happened more than once in the Eclipse SDK releng builds. In order to detect this, we use a previous build as a baseline in a mirror operation. If the baseline already contains a bundle with the same version as the one that was just built, then we mirror the old bundle and discard the new one.

We install our product using the mirrored results which ensures that our new install contains the same old bundles that already exist out on user's machines. The unit tests run against these old bundles.

In order to detect when a bundle actually changes and needs its version number increased, the mirror operation supports a comparator. Olivier kindly contributed some code to p2 that can do a semantic compare on java class files to see if they are equivalent. Similarly, things like manifests and properties files can be compared for semantic instead of bitwise equivalence.

Example Build

I put together an example feature build that uses a comparator to detect if a bundle has changed when the version hasn't. The projects are in CVS under There are 4 projects to check out:
  • example.Builder - the builder project, contains a runBuild.xml
  • org.example.a - a project that we will change between builds
  • org.example.b - depends on a, won't change source between builds
  • org.example.feature - the feature to build
To run the example, just run runBuild.xml script as an Ant Build using the same JRE as the workspace. The build will mirror the results into example.Builder/composite/I<timestamp>. You can run the build multiple times, and each time the results will be added to the composite repository.

The mirror call is done in example.Builder/customTargets.xml/postBuild. It looks like this:

<target name="postBuild">
<antcall target="gatherLogs" />

<!-- mirror from build results, comparing against previous builds that are in the composite repo -->
<source location="file:${assemblyTempDir}/${buildLabel}"/>
<destination location="file:${builder}/composite/${buildLabel}"/>
<comparator comparator="" comparatorLog="${buildDirectory}/comparator.log">
<repository location="file:${builder}/composite" />

<!-- add the new build to the composite -->
<p2.composite.repository destination="file:${builder}/composite">
<repository location="file:${builder}/composite/${buildLabel}"/>

Each build uses the previous contents of the composite repository as a baseline, and then adds itself to the composite.

To see the comparator in action, edit org.example.a.Sub and uncomment the doSomething method:

public void doSomething(List o) {
for (Iterator iterator = o.iterator(); iterator.hasNext();) {

This change to the source code of org.example.a will change which doSomething B ends up calling:

public class B {
public void method() {
ArrayList list = new ArrayList();

Sub d = new Sub();

That is, recompiling B against the new A results in different byte-code for B. If we run the build again with the changed source, then it will now fail with an error message:

[p2.mirror] Messages while mirroring artifact descriptors.
[p2.mirror] Compare and download of canonical: osgi.bundle,org.example.b,1.0.0 from baseline.
[p2.mirror] Difference found for org/example/b/B.class within [canonical: osgi.bundle,org.example.b,1.0.0]
from file:/C:/workspace/example.Builder/composite/I20090806060019/

When the build fails in this manner, it means we need to increment the version for org.example.b

Build Notes

  1. The builder's sets pluginPath and elementPath to enable the build to use the sources in the workspace without copying them to the buildDirectory. elementPath points to the top level feature that we are building.
  2. The build is using p2.gathering=true, for feature builds, this groups the configurations, we set archivesFormat to leave the results as a folder.
  3. runBuild.xml automatically refreshes the workspace when it finishes, however when the build fails after changing the source in org.example.a, this refresh doesn't happen and you need to refresh the workspace manually to see the new results.
  4. In customTargets.xml/preGenerate we create an empty composite repository if there wasn't already one there.
  5. We changed the default customTargets.xml/gatherLogs to use elementPath since the top level feature is not under buildDirectory.
  6. org.example.a has version 1.0.0.qualifier where the qualifier gets replaced each build with the timestamp. org.example.b just has version 1.0.0. In a releng build, the CVS tag from a map file would be used instead of the timestamp.

Wednesday, July 08, 2009

ADT Part 2: More like the EPP

Yesterday, I posted an example of how to compose a product made up out of updatable sub-components. The first question there was about how this relates to the packages built by the EPP. The EPP packages are just products, much like my ADT. They use their own feature and plugin to brand the Eclipse package and do things like open a different default perspective.

The EPP packages are built slightly different from my first example. They use feature requirements to get version ranges for the sub-components, but they don't currently perform the extra director calls to make those sub-components into updatable roots.

Here is another example that is structured a bit more like the EPP packages.
This example is also available in cvs with the first (

[Edit 2013/07/03: Eclipse CVS has migrated to git starting in 2010.  The examples are now available under the examples folder in The example has not been updated to work with git and may require modifications.]

Features instead of p2.inf

In our first example, we used a p2.inf file to add requirements with version ranges to our product. I did it this way because I didn't have anything to add to the product, and didn't want to bother creating a feature.

Perhaps a more familiar way of doing things would have been to use a feature. So we create a feature "adt.feature", and instead of including our sub-components, we require them using a "compatible" match rule:

I also made my feature include the "org.eclipse.platfom" feature. The adt.product file then just includes adt.feature. We no longer need the p2.inf to add requirements to our product.

Adding Branding

Since we created our own feature for this product, we may as well take the next step and add our own branding plug-in. The first thing I did way create my own product extension in the plugin.xml:
    <extension id="product" point="org.eclipse.core.runtime.products">
      <product application="org.eclipse.ui.ide.workbench" name="ADT Product">
         <property name="aboutText" value="Andrew's Development Tools"/>
         <property name="windowImages" value="icons/icon.gif"/>
         <property name="aboutImage" value="product.gif"/>

Note that the word 'product' here is slightly overloaded. There is a 'org.eclipse.core.runtime.products' extension point that defines branding for eclipse, and there is the product itself which is what we are building using the .product file.

This product is just running the normal workbench application, but it uses my own icons and images. We then change our adt.product file to use adt.plugin.product instead of the org.eclipse.platform.ide product extension we were using before.

In my adt.feature I set the branding plug-in to be my new adt.plugin. I also provide the normal about.* files so that my feature shows up in the about dialog.

As a final touch, I made my own splash screen.

Building the new Product

The build script is essentially unchanged from the first example. So I won't bother explaining the details again. The only difference is that I made some minor changes to the builder's file so that can find our feature and plugin.

Note that PDE/Build does not follow feature requirements when performing a build and publishing metadata. This means that for the director install to work, you need to have pre-existing metadata for the things that you are requiring. This same requirement exists with the first example where we used a p2.inf file.

Tuesday, July 07, 2009

Composing and updating custom Eclipse distros

I've recently seen a couple of different posts to the newsgroups dealing with problems updating RCP applications using p2. [edit 2009/10/21, update links to forums]

As an example, I've created my own Eclipse product. It is composed of the Eclipse Platform, CVS support, the CDT and Mylyn. I'm calling it the ADT (Andrew's Development Tools).

It's not hard to create a feature based product that includes these things, and do a product build to end up with something like this:

As explained in this newsgroup post, there are two kinds of things that are included in an Eclipse install:
  1. Things that are explicitly installed
  2. Things that are required by the things that are installed.
Here in my example, only my development tools "org.example.adt" is installed, the rest (CDT, CVS, Mylyn) are required by my product.

Only things that are explicitly installed will be searched for when you look for updates. Also, the installed things generally specify the versions of things they require, which makes it hard to install/update those required items independently of the root product. In both the newsgroup postings I referred to above, the problem was trying to install/update one of the required items without updating the root product.

So the question becomes how to allow updating sub-components of the product without updating the product itself.

Composing for Updatability

What we want to do is to update sub-components of the product without updating the root product itself. In this example we do not to allow updating the Eclipse Platform independently, to do that, the user will need to update the product itself.

I have created a example builder to do this. Get it from cvs (

[Edit 2013/07/03: Eclipse CVS has migrated to git starting in 2010.  The examples are now available under the examples folder in The example has not been updated to work with git and may require modifications to run properly.]

We need to do two things:
  1. Use version ranges to include sub-components in our product so that we allow upgrading those components.
  2. Explicitly install those sub-components so they will be found when checking for updates. This is essentially a book-keeping step.

The ADT .product File

There is a adt.builder/product/adt.product file which we will use to run a product build. If we were to include the features for our sub-components in the .product file, then we would end up with requirements on specific versions of those components. Instead we only include the platform feature [1].

To get requirements to our sub-components, we use a p2.inf file to customize the metadata. We add requirements with entries that look like this:
requires.1.namespace = org.eclipse.equinox.p2.iu =
requires.1.range = [1.1.100, 1.2.0)

requires.2.namespace = org.eclipse.equinox.p2.iu =
requires.2.range = [3.2.0, 3.3.0)


The suffix is the name of the p2 Installable Unit corresponding to the features we are interested in. We specify the version ranges in which we will allow those components to be updated.

The ADT Builder

The adt.builder project includes a buildADT.xml ant script which will run a headless product build for us. The first thing it does is download zips containing the things we need. This example illustrates three different ways of reconsuming metadata.
  1. The CDT and CVS both come as zipped p2 repositories. Things that are not referenced directly by the .product file only need to be available as repositories. We can reuse these zips directly by specifying them as context repositories using jar: urls. See the p2.context.repos property in the adt.builder/ file.
  2. Mylyn is not a p2 repository, it is a zipped old style update site. For this, we use a publisher task to generate p2 metadata for it. [2]
  3. The Eclipse Platform is a p2 repository just like the CDT and CVS. It is similar to the delta pack in that it contains the org.eclipse.equinox.executable feature that is need to get launchers in product builds. Because the platform feature is included directly in the product, we can't just specify the platform as a context repository, we need the bundles available to like in a normal headless build. To do this we transform the repository using the p2.repo2runnable task. See the transformedRepoLocation and repoBaseLocation properties in the file. The transformed repository automatically gets included along with the pluginPath property used by

Adding additional director calls

In order for our sub-components to be independently updatable, they need to be explicitly installed in our resulting product. By default PDE/Build performs a director install for just the product being built. We can use a customAssembly.xml script to perform additional director[3] calls before the final archive is created.

It looks like this:
<target name="pre.archive">
<ant antfile="${genericTargets}" target="runDirector" inheritAll="true">
<property name="p2.repo" value="${}"/>
<property name="p2.director.iu" value=""/>
<property name="p2.director.installPath" value="${eclipse.base}"/>
We make director calls for each of the sub components we allow to be updated. In the example we do CVS, Mylyn, CDT, and the CDT-Mylyn bridge.

The final result

Run the adt.builder by right-clicking on buildADT.xml and choosing Run As -> Ant Build... Be sure to run in the same JRE as the workspace. After running the build, the results are available under adt.builder/buildDirectory/I.<timstamp>.

Running the resulting product, we see that the CDT, Mylyn and CVS are all showing up as installed roots, and are therefore independently updatable.


  1. PDE/Build will automatically generate start level configuration information, but only for things that are included in the .product file. If we didn't include the platform feature, or at least the bundles that need start level information, then this would not happen automatically and we would need to handle start levels ourselves. See the help page here for more information of configuring start levels.
  2. We publish the p2 metadata for mylyn into ${}. This property specifies the location of the p2 repository that will be used internally by the build. Publishing the mylyn metadata here instead of some location specified as a context repository saves the build from mirroring the required IUs into the build repository.
  3. PDE/Build provides a "runDirector" target that can be used to invoke the director. This works by executing the director application in a new process. Normally, this requires setting the "equinoxLauncherJar" property specifying the location of the equinox launcher to use, but because we are calling the director from customAssembly.xml, we inherit this property from the generated assembly scripts.
  4. Running this build produces a properly p2 enabled product. It does not produce a corresponding repository for that product other than the build time repository ${}. To produce a final repository containing the final product, define the properties p2.metadata.repo and p2.artifact.repo in the The product and its requirements will then be automatically mirrored into that repo.

Thursday, June 25, 2009

Using the deltapack in Eclipse 3.5

Eclipse Galileo was released yesterday and I have already seen a couple of questions on the newsgroups regarding how to install the delta pack.

What's a delta pack?

For those who don't know what it is, the delta pack is a zip file provided by the Eclipse Platform and it is used for developing RCP applications for multiple platforms. The delta pack archive contains all the platform specific fragments from the Eclipse SDK. It also includes a feature "org.eclipse.equinox.executable" which contains binary launchers for all the platforms. The archive is available for download through the Eclipse Classic "Other downloads" page.

The delta pack is required if you want to do headless product builds, or to export products from the UI:

If you don't have the delta pack installed, then the option to export your product for multiple platforms will not appear in the wizard.

How to install the delta pack

Ian Bull blogged about the improved Target Platform management system as #3 in his Galileo Feature top ten list. This is the recommended method of installing the delta pack.
  1. Extract the delta pack archive into its own directory on disk.
  2. Open the Target Platform preferences (Window -> Preferences -> Plug-in Development -> Target Platform).
  3. Add a new target, or edit the active target.
  4. Add the directory where you extracted the delta pack.

Monday, June 15, 2009

Patching Features (Part 2)

In my last post I showed how to create a simple feature patch. However, this patch leaves us in a state where we can no longer upgrade the platform because the patch itself requires a specific version of the feature it is patching.

Making the patch Optional

If we make the inclusion of the patch in our org.example.feature optional, then we no longer block upgrading the platform underneath us.

When a new version of the platform is available, there is a conflict between the version of the p2.user.ui feature required by the patch and the one required by the new platform. By making the patch optional, p2 will not install the patch if there is a conflict.

This is fine if the new version of the platform includes the fix we want. But what if it doesn't? What if we need to have our patch apply to several different versions of the platform?

Relaxing version ranges

Unfortunately, currently the only way to do the following is to edit the metadata by hand. (Paul Webster uses XSLT transforms, I have also raised a bug to allow changing things using the p2.inf file.)

Looking at the metadata for the patch, we see a few interesting sections:

The Patch Scope

The scope section of the patch's metadata specifies which feature(s) the patch applies to:

<requires size='1'>
<required namespace='org.eclipse.equinox.p2.iu'

Here we see the patch applies to a specific version of the org.eclipse.equinox.p2.user.ui feature. By relaxing the version range here we can make the patch apply to other versions of the platform. For example, say we knew our bug was fixed on June 11, but there were several version between June 5th and the 10th that still contained the bug. We could instead use a version range like [1.1.0.v20090605-1440,1.1.0.v20090611) (the upper end of the range is open, it includes everything up to but not including v20090611).

Note that the patch Installable Unit also has normal requirements on the feature it is patching. When widening the scope range, the regular requirements should also be widened to match.

Change From / Change To

The 'changes' section of the patch's metadata specifies which plug-in we are patching, and the version to change it to.
<required namespace='org.eclipse.equinox.p2.iu'
<required namespace='org.eclipse.equinox.p2.iu'

In the above example, we are changing the org.eclipse.equinox.p2.touchpoint.eclipse bundle from any version ("0.0.0") to version 1.0.101.v20090611.
We could imagine changing these ranges in the following ways:
  • Change the "scope" range to "0.0.0" to match any version of the p2.user.ui feature, and then change the "change from" range to [1.0.101, 1.0.101.v20090611). This would result in patching any version of the p2.user.ui feature that includes any 1.0.101 versioned bundle up to v20090611 (which is the version with the proper fix).
  • Change the "change to" range to something like [1.0.101.v20090611, 1.0.102). We know v20090611 contains the fix we want, but the in the future there will be more bug fixes before the maintenance stream is released. By widening the range here, we allow future bug fixes to also be included in our patch.

Friday, June 12, 2009

Patching features with p2

The Galileo release of Eclipse is in its final days. The eclipse SDK itself is effectively done and the current build is under consideration to be the final release.

Imagine This...

Say I have a feature that I build, it runs on top of the Eclipse Platform. I know Eclipse Galileo is coming out soon, so I download it and try it out with my feature.

Oh No! It doesn't work! I've found a bug in the platform, and its a major blocker for me. (Oops, maybe I should have tried this a couple of months ago when there still would have been time to fix the bug.)

Meta-Comment: The Eclipse Platform Project has a reputation for shipping on time (even if that means there are unresolved bugs). There is a strict end game plan that is followed for the release, lock down started back at the beginning of May. The further we are along in the plan, the harder it is to get a fix approved for release. A lot of people don't seem to relealize this happens, and perhaps wonder why their important bugs are defered with the comment "Its too late".

Ok, I need to patch the platform. How do I do this?

The Example Feature

To work through the steps of patching the platform, I created my own example feature:

  1. Create a New Plug-in Project "org.example.plugin". The plug-in is not a "Rich client Application". On the templates page of the wizard I chose "Plug-in with a view".
  2. Create a New Feature "org.example.feature". Add org.example.plugin to the feature.
  3. Export the feature, and on the Options tab in the export wizard check "Generate metadata repository".
For the purposes of the example, I'm starting from the Eclipse Platform Runtime 3.5RC4. Because I want to strictly control the upgrade path in the example, I removed the Galileo and Eclipse Project repositories from the available sites (Window -> Preferences -> Install/Update->Available Software Sites).

Unzip the platform, and installed the feature from the exported repository. I didn't create any categories for the feature, so "Group items by category" must be unchecked.

The Patch

For the example, I am inspired by bug 279542 which came in very late and almost was not fixed in 3.5. The bundle org.eclipse.equinox.p2.touchpoint.eclips has the bug and this plugin is included by the feature org.eclipse.equinox.p2.user.ui.

Say we have a fix with touchpoint.eclipse version 1.0.101.v20090611. (When following the eclipse versioning guidelines, the maintenance version of this bundle should be 1.0.101 and the version for the next year's release will probably be 1.0.200).

We can easily create a new Feature Patch with the wizard:
We specify the version of p2.user.ui that we are patching (here the one shipped in 3.5RC4). In the feature patch, include org.eclipse.equinox.p2.touchpoint.eclipse with the specific version containing the fix. Then, add the feature patch as an inclusion in our org.example.feature:

<feature id="org.example.feature" label="Feature" version="1.0.0.qualifier">
<includes id="org.example.patch" version="0.0.0" />
<plugin id="org.example.plugin" version="0.0.0" unpack="false"/>

<feature id="org.example.patch" label="Patch" version="1.0.0">
<import feature="org.eclipse.equinox.p2.user.ui" version="1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl" patch="true"/>
<plugin id="org.eclipse.equinox.p2.touchpoint.eclipse" version="1.0.101.v20090611" unpack="false"/>

If we have the bundle containing the fix in our target platform, we can now export a new version of our example feature that includes it. And we can update the platform where our feature is installed to get the new version that brings the patch with it.

The metadata for the Patch installable unit looks like this:

<unit id='' version='1.0.0' singleton='false'>
<requires size='1'>
<required namespace='org.eclipse.equinox.p2.iu' name='' range='[1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl,1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl]'/>
<required namespace='org.eclipse.equinox.p2.iu' name='org.eclipse.equinox.p2.touchpoint.eclipse' range='0.0.0'/>
<required namespace='org.eclipse.equinox.p2.iu' name='org.eclipse.equinox.p2.touchpoint.eclipse' range='[1.0.101.v20090611,1.0.101.v20090611]'/>
<requires size='2'>
<required namespace='org.eclipse.equinox.p2.iu' name='' range='[1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl,1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl]' greedy='false'/>

This says, in version 1.1.0.v20090605-1440-7u6Fb3FbPbJP5MiKiZgpdl, change whatever version of org.eclipse.equinox.p2.touchpoint.eclipse is there to version 1.0.101.v20090611.

It works! Except...

It works, except that we notice that the patch IU has hard requirements on the version of the p2.user.ui feature. This would prevent upgrading the underlying platform because the feature patch has locked down the version of the p2.user.ui feature.

p2 is much more flexible about patches than the old Update manager was, so there are a few different ways to address this, and I will write another post covering this soon.

(Right now its Friday after 5pm, and time to go home for the weekend :) )

Thursday, March 26, 2009

Building p2 RCP products in Eclipse 3.5M6

We can also call this "Building for the Cloud" just to take advantage of the newest buzz word.

Susan Franklin McCourt put together a nice wiki page covering how to add p2 self-updating support to your RCP application. One of those examples was called "Updating from the Cloud".

As part of preparing for EclipseCon, I put together a simple releng product build that builds this cloud product for p2. The cloud bundle "" is available in CVS together with a releng project that I created.

[Edit 2013/07/03: Eclipse CVS has migrated to git starting in 2010.  The examples are now available under the examples folder in The example has not been updated to work with git and has an obsolete CVS entry in the .map file.]

To run this example, check both projects out into your workspace, and read the

Builder Setup

The releng project serves at the builder for a headless build. The file is a copy of the template provided by PDE/Build with some things we don't need removed. We modified the following properties:
  • product : This defines the .product file which specifies how to create the cloud product. The first segment of this path is the plug-in id for the cloud project that contains the file.
  • configs : Here we define the platforms we are building for. This is currently set to build windows, other platforms can be added. (The default here is *,*,* which is platform independent pieces, which doesn't make sense for a product).
  • J2SE-1.5 : The cloud project has a Bundle-RequiredExecutionEnvironment of 1.5. We set this property to be the bootclasspath to use for compiling 1.5. See the eclipse help for more details on these properties. We also set CDC-1.1/Foundation-1.1 because we had the org.eclipse.osgi bundle in our workspace (and so were compiling it for the product) and osgi requires that EE defined in order to compile properly.
  • p2.gathering : We set this to true to use the new support for publishing metadata directly from source. This will also automatically install the product using the p2 director. More details on this below.
  • p2.metadata.repo, p2.artifact.repo : In addition to the installed product, we also want a p2 repository.

We also provide a simple ant script named "buildProduct.xml". This is a very simple script that does the following:
  1. Set the baseLocation. This is the location of your target binaries against which you want to compile. In the script we set it to ${eclipse.home} which is a property that is automatically set to be the eclipse that is running the build.
  2. Find the delta pack. The delta pack is required for headless product builds because it contains all the platform specific fragments, and org.eclipse.equinox.executable feature which contains launchers for all the platforms. Download the 3.5M6 deltapack from here. The deltapack location will be added to the pluginPath property.
  3. builder : the location of the build configuration files (ie: We set it to be the directory containing the buildProduct.xml script.
  4. buildDirectory: the directory where everything happens, set it to a subfolder of our builder.
  5. pluginPath: the location to find more plugins and features. Here we add the workspace (one level up from the builder), and the deltapack.
  6. Run the build. The property ${eclipse.pdebuild.scripts} is automatically set to the location of the PDE/Build scripts directory. Here we call the product build.
  7. Victory! : copy the resulting archive into the root of the releng product.
The results of this build is a fully provisioned, p2 enabled product with an accompanying repository.


In 3.4, p2 metadata was generated by running the metadata generator on the binary jars that were the result of the build. At the end of the build you were left with a repository and you need to perform a director install for get a fully p2-enabled product.

In 3.5 we have this new property p2.gathering. Setting this to true does a couple of things:
  • Generate metadata from source. PDE/Build extends the p2 publisher to publish metadata and artifacts directly from source (and the compiled .class files) into a repository. There are no intermediate steps. (This also has the side effect of increased performance).
  • Feature Builds result in a repository that is a group of all the platforms that were built. (This is an implicit groupConfigurations=true).
  • Product Builds result in a fully installed p2 enabled product. This removes the need for a manual director call after the build is complete. The product build can also create a repository containing the IUs required to install the product.
The 3.4 properties still work for doing the old style generation.

Wednesday, February 25, 2009

Installing and running plugins in Eclipse applications

A common question on the newsgroups is: "I copied new plug-ins into my RCP application but they are getting ignored, what's the problem?". The answer to this is really quite fundamental and is something I think every RCP developer should understand.

My answer here is about the Equinox runtime which is used in Eclipse and is the reference implementation of the OSGi spec.

Installing into the OSGi Runtime

Every plug-in (or bundle, I use the terms interchangeably), must be installed into the OSGi runtime before you can even think about it running. There are two main ways to install a bundle: programmatically through the BundleContext API, or declaratively through the osgi.bundles property in the config.ini file.

The osgi.bundles property is read at startup. It is a list of bundles that are automatically installed and optionally started once the framework is up and running. The format of each entry is
<URL | simple bundle location>[@ [<start-level>] [":start"]]

Starting your bundles

It is not enough to simply have your plug-in installed, it must also be started. When the OSGi framework starts up, it increments through the start levels starting bundles as it goes. A bundle won't be started until its start level is reached, and to get started it must be marked as such, or be marked as lazy start. A lazy start bundle will be started when something tries to load a class from that bundle (and once the start level has been reached for that bundle).

[Edit:] In most cases, marking a bundle as Lazy-Start is sufficient and one need not worry about starting bundles explicitly. See Neil's comment attached to this post.

The default start level for bundles is 4.

Managing installed bundles

There are 2 strategies here, one is to list all of your bundles on the osgi.bundles list, the other is to list only a few bundles that can bootstrap the rest of the application. I will call this kind of bootstrap bundle a "configurator".

If you list all your plug-ins on the osgi.bundles property, then it can be painful to maintain and add new plug-ins to your application. I won't try to make arguments for or against any particular way of managing your system, but it a large way it often comes back to the differences between the old update manager and the new p2 provisioning system.

Update Configurator

In Eclipse 3.3 and earlier, there was the update configurator (org.eclipse.update.configurator). It was installed on the osgi.bundles list and started at level 3. It would scan the plugins directory and automatically install everything it found there.

This is where the magic ability to just copy things into the plugins directory and have them work came from. However, update configurator is essentially forcing the found bundles into the framework with no regard to any conflicts or unresolved dependencies that may result.

Simple Configurator

In Eclipse 3.4, with the advent of p2, update configurator is replace with simple configurator (org.eclipse.equinox.simpleconfigurator). As the name suggests, it is quite simple: it installs everything it finds listed in its configuration file (specified by the org.eclipse.equinox.simpleconfigurator.configUrl property, usually

Editing the file to add plug-ins to your product is not really any easier than editing the osgi.bundles list. The difference here is that p2 manages for you. In fact, if your product is p2 enabled for updating and installing plug-ins, then any changes made manually are likely to be lost when p2 updates or installs new software.


Whatever the other problems update has with respect to avoiding conflicts and ensuring that dependencies are satisfied, it is hard to ignore the simplicity of just copying new bundles into your install. p2 has support for dropins (provided by org.eclipse.equinox.p2.reconciler.dropins).

Like the update configurator, the dropins reconciler will automatically watch a directory and install things it finds there. However, unlike the update configurator, it will ensure that the bundles found will not conflict with others in the system and that all dependencies are satisfied.