For some maven POM.xml, I found there are extensions in the build section. What's the purpose to include build extensions?
Takes less than 6 second in Google and read. See here http://maven.apache.org/pom.html#Extensions
Extensions are a list of artifacts that are to be used in this build. They will be included in the running build's classpath. They can enable extensions to the build process (such as add an ftp provider for the Wagon transport mechanism), as well as make plugins active which make changes to the build lifecycle. In short, extensions are artifacts that activated during build. The extensions do not have to actually do anything nor contain a Mojo. For this reason, extensions are excellent for specifying one out of multiple implementations of a common plugin interface.
Related
I have a Maven-managed project which contains a few modules, one of which is the actual library of interest. The other modules are just add-ons or examples that build off of the library. I'm looking to generate the Maven site for this library and have it automatically deployed (as a standalone site and not as part of a multi-module site) but I am having trouble with the Javadoc plugin.
When executing the javadoc:javadoc goal, the javadoc plugin is attempting to access the jar for the other modules causing a failure.
I have created a simple example which demonstrates this phenomenon. Make sure you run the clean goal before any others so that the flaw be shown. Though executing the packaging first would solve this error, this cannot be done because the use case occurs during the Maven-managed release process which starts from a clean state.
Is there a way for me to disable this functionality in the javadoc plugin so I only get the documetation for the library module?
I can think of two options depending on your preference. Both include using profiles. If you want the default build to create the javadocs for your library of interest. Make the other modules use a property inside of the default profile in order to skip the javadocs.
If you are okay with passing in a profile, just have the javadocs only run in the profile.
I want to deliver a single .jar file to my clients, but my project is currently built with Maven, and I have several modules that generate a single .jar each.
I know nesting different .jar files is not a great idea, so I am not sure how can I achieve this.
Any ideas?
If you really want to go this direction, there are several ways to do that:
with the Maven Assembly Plugin and maybe the jar-with-dependencies predefined assembly descriptor (that will unpack dependencies)
with the Maven Shade Plugin (similar to the above one but gives more flexibility)
with the Maven One-Jar Plugin (that uses One-JAR and its custom classloader to allow nesting of JARs)
Depending on your exact requirements and constraints, you might prefer one or the other.
First of all, ask yourself if you have a really good reason for packaging your application and all of its dependencies in to a single jar. I haven't found a very many good reason for this at all (with most reasons being related to organizational policy foolishness or just plain ignorance). The way to go is to keep libraries in their own jars and supplying a .zip/.tar.gz containing all of your libraries and your application with either
An executable .jar with the
classpath setup appropriately in
your MANFIEST.MF file
a .bat/.sh
script that invokes java and builds
an appropriate classpath based on
your deps
Conversely, use JNLP (better known as Java Web Start).
If you really want to have maven bundle all of your dependencies and your application under a single jar, what you want to use is the "jar-with-dependencies" predefined assembly. The maven assembly plugin usage page also shows how you might this up as well.
You can try Maven Shade plugin too.
http://maven.apache.org/plugins/maven-shade-plugin/
General instructions on how to use the Shade Plugin can be found on the usage page. Some more specific use cases are described in the examples given below. Last but not least, users occasionally contribute additional examples, tips or errata to the plugin's wiki page.
In case you still have questions regarding the plugin's usage, please feel free to contact the user mailing list. The posts to the mailing list are archived and could already contain the answer to your question as part of an older thread. Hence, it is also worth browsing/searching the mail archive.
If you feel like the plugin is missing a feature or has a defect, you can fill a feature request or bug report in our issue tracker. When creating a new issue, please provide a comprehensive description of your concern. Especially for fixing bugs it is crucial that the developers can reproduce your problem. For this reason, entire debug logs, POMs or most preferably little demo projects attached to the issue are very much appreciated. Of course, patches are welcome, too. Contributors can check out the project from our source repository and will find supplementary information in the guide to helping with Maven.
Actually, nesting .jar files is not possible. A jar can't have other jars in it.
.war and .ear files can contain jars, and that's a good solution if you're delivering a J2EE application.
If your app is just J2SE, however, I recommend looking at the Maven Assembly plugin. As the name implies, it allows you to create a single binary distribution of your build.
I have some basic questions in eclipse plugin development, can anyone give clarification of the following questions,
When should we have to add features in our plugin development ?
What is the difference between feature and plugin.xml ?
Regards
Mathan
As mentioned in this thread:
A plugin is the eclipse "unit of work". An OSGi bundle that supplies a classpath and can contribute to eclipse through extensions.
A fragment points to a host plugin, and anything it provides (classpath, extensions, etc) are "sucked" into the host plugin. A fragment is also a more specialized OSGi bundle.
A feature represents a versioned collection of plugins, and is used for configuration management in eclipse. They can be deployed manually or through the update manager. If you want to deploy through the update manager, then you need to use features to represent your plugins.
So if you want to manage your plugin or plugins through the update manager, a feature is in order.
You can find more in the Eclipse Help:
Feature
Features do not contain any code.
They merely describe a set of plug-ins that provide the function for the feature and information about how to update it.
Features are packaged in a feature archive file and described using a feature manifest file, feature.xml.
Plugin
While features are organized for the purposes of distributing and updating products, plug-ins are organized to facilitate the development of the product function among the product team. The development team determines when to carve up program function into a separate plug-in.
Plug-ins are packaged in a plug-in archive file and described using a plug-in manifest file, plugin.xml.
Can I make a single maven project that can be included as a dependency (to reference Java classes inside) and executed as a plugin?
I'm working on a library to help with hosting GWT on a LAMP stack. For someone to use this, they need to extend some Java classes (so it must be a dependency) and they need to invoke a maven plugin (so it needs to be a plugin). The plugin code references the same Java classes, so if they are seperate projects, the plugin one must depend on the library one.
As is, I have the library as a normal maven project, and the plugin as a maven plugin that depends on the library. This means that to do a release, I have to release two different artifacts, and the dependent project must update both version numbers for both artifacts. It'd be nice to have a single project.
You'd be better of by doing the following
project for the jar, Foo:Foo.jar
project that uses Foo:Foo.jar as a
dependency that builds the plugin
Maven parent project that
builds 1&2
The directory structure would look like this
\project\pom.xml
\project\foo\pom.xml
\project\foo\src\main\java\foo.java
\project\plugin\pom.xml
\project\plugin\src\main\resources
\project\plugin\src\main\java
From \project you can do a mvn clean package to build \project\foo\target\foo.jar and \project\plugin\target\plugin.jar
Hope this helps.
If you create a maven plugin it still has a artifactId/groupId/version. There's no reason it can't be references both in your section and in your section. On the other hand, if thats ugly, why not just make a library with the common code that both your main project and your maven plugin project depend on?
EDIT:
Sorry, wasn't clear on the second part. Look into composite maven projects, where there is a top level pom that defines a number of child modules. In this case, the maven plugin and the common library code could be separate children producing separate artifacts, but you only need one version number and one release command executed from the top level. I haven't done this but there are any number of open source projects that do. its often used as an idiom to put testing code into a single module that can be referenced by all the others, without having it go out in any distributable jar.
The best practice is to not do what you're suggesting. Examples of this include PMD, BND, JUnit/TestNG, and so on - no serious projects seem to package the maven plugin with the library proper.
One way to get both alternatives is to use maven assemblies to have two seperate maven projects for each the library proper and the plugin and then a separate packaging as a jar containing the classes from both.
I am working on a fairly large project (with a number of modules, a bunch of external libraries etc.) and we are now considering switching from Ant to Maven. I understand the differences between the two, but I am not convinced that it is really worth spending time converting the project layout, setting up all the dependencies, teaching developers and configuration managers doing things "the new way" etc.
There are a lot of resources on the web describing how to migrate from Ant to Maven, but I haven't found that many that say why :-)
Before changing your build system, ask yourself (and the group) why you're changing? If you're changing just because Maven is the "new thing", don't. If you actually see a technical reason to migrate, do it.
In general, unless there's a major compelling reason to do so (new capabilities or much simpler management), I'd say stay with what you have for the current project, but consider Maven for future projects.
Have you read chapter 1 of "Maven, the definitive guide"? In particular, 1.7 Comparing Maven with Ant has an interesting discussion.
I agree with the other answers that advise caution. Maven has strong points, but nothing that can't be done by an Ant build process:
dependency management: Ant has the Ivy subproject, which can interact with Maven repositories.
convention over configuration: you can also do that with Ant, it's just a matter of establishing the rules and enforcing them.
build lifecycle: same as above, you can enforce a convention over the tasks exposed by each build.
build logic reuse (Maven plugins): you can also achieve that in Ant with macrodefs and task libraries.
The thing is, with Maven you get these features out-of-the-box, while with Ant you need a rock-solid build, a very strict set of rules and a way to enforce them (for instance, make sure that everyone follows the conventions when they create a new subproject, that they reuse the existing blocks instead of doing everything from scratch, etc.).
Personally, I would see how well the existing process addresses the issues above: how are dependencies managed, is there a central repository? Are the project structures uniform (when I checkout a project I don't know, how long does it take to figure out how to build it)? Is there some form of build logic reuse, or does each project reinvent the wheel? Which of these features are needed?
Then I would try to balance the cost of adding the missing features to the existing Ant script, against the cost of migrating to Maven (if you don't know Maven, that also includes the cost of learning it).
In any case, I suggest you build a small Maven prototype (5 to 10 projects) illustrating the common cases in your build. You can test a lot of Maven's features with dummy projects containing little java logic (use the archetype plugin to generate them).
Before Maven we were checking dependency libraries (typically third-party, open source variety) into source control - so that we could insure our components compiled and got packaged with the precise versions intended.
Now with Maven in place, we're relying on artifact repositories to hold those versions and we let our pom.xml dependency declarations be the official means of defining version dependencies. This has proven to be a simplifying approach that makes project organization in version control repositories (and their Hudson build projects) much easier to devise. Our local artifact repository is under backup policy along with our source control repositories. It's nice to use the Maven tools to go and search and specify a needed library version. We also use parent pom files to specify dependencies that other project poms inherit by default. So if you want all projects to use the same log4j version, then that is specified in one place in the parent pom file. (But any project can at anytime override and specify a specific version instead of just accepting the default from the parent pom.)
Here is the secret to a successful adoption of Maven:
Use Maven project build approach for
your new greenfield projects
Modify existing legacy projects that
use ant build.xml files to incorporate Maven task
for managing depenedencies (a hybrid
approach)
The benefit is that you can then get all of your projects under Maven dependency management, which is of course it's greatest benefit.
The nice thing about the Maven task for ant, where you specify all dependencies in a pom.xml file, is that it involves just modest modification of the existing ant build.xml file to incorporate Maven for this. From the ant file's perspective, Maven is just a means for defining classpath definitions, which are subsequently used by the various ant build task.
The Maven scope classifier of dependencies can be utilized when defining classpaths such that a suitable classpath can be set for compiling, running unit test, packaging, et al. Other definitions in the pom can also be accessed as ant property definitions.
A lot of existing ant build files are rather complex. It can be a formidable undertaking to convert such projects to a full Maven build process. This hybrid approach of having Maven manage all the dependencies and leave the bulk of the ant build.xml file as is, is most pragmatic.
First, like I'm sure a lot of people will mention, Ant and Maven are not exactly intended to solve the same goals. Since you said you understand what each provides, I won't get into the details of that, so suffice it to say that Ant lets you define the details of how to build individual components, while Maven manages the dependencies between components plus Maven lets you define a complete project build cycle from compile through test and deploy in a programmatic way.
I've used Maven on a couple projects in the past, and I just started using it on another one recently. There are plenty of articles on the net that compare Ant and Maven, so you can look at those, but from my experience, its always worthwhile to consider how you can improve a project. Dependency management and build lifecycle are two important aspects of any large project, and Maven helps in both those areas. If you already have a good build system in place using ant, and your dependencies are kept in a easy to access central location, and you don't plan on extending your build process to include any more advanced build management, then maybe you should stay with what you have.
On the other hand, if you want to use a continuous integration server like hudson or an artifact repository like nexus, then moving your project to maven can really help with build efficiency and automation. You probably would like maven in those situations because the full cycle from dependency to build to artifact can be achieved using those types of tools and you'll be able to better control your builds and releases. On my current project we have many modules and dependencies, like you mention. Migrating to maven so we could use hudson and nexus really helped because we could drop all those 3rd party jars into a nexus repository and stop having to check them into version control or email them around. Also, builds were out of hand because the CM people had a build plan as a document that they would sometimes follow, but making that part of your project (i.e., the pom.xml) defines how you are supposed to build and lets you enforce it. Maven is the glue that holds all of those things together.
In the end, its a matter of how long you expect the project to last, how good your process is now, whether you want to clean up your dependencies, whether you want to enforce your build plan, and whether you want to have the option to use continuous integration and artifact management. If you any of those things, Maven is a strong candidate.