I have a Maven project that depends on generated sources. These sources need to be generated by a java program built by another maven pom. (In this case the sources are generated by the greendao source generator, but they could be generated by any generic java executable.)
I haven't dealt with this sort of interdependency between maven projects before. Assuming I want to use a reactor to build these two submodules, how can I ensure that the first module is built AND executed and generates its source files to be included in the second module?
Module A generates sources. Module B depends on Module A. If you say that Module B depends on Module A in Module B's pom, I think the reactor will figure out it needs to build Module A first.
Related
I am thinking about creating a multi-module project in maven, with one plugin module (i.e. this module is used as a plugin in other projects, not a dependency).
Question: Is it feasible to have a plugin as a module in a multi module maven project?
It is feasible and it is also documented on the official maven multi-module/reactor page concerning how having a plugin as module is handled by the build (bold is mine):
Because modules within a multi-module build can depend on each other, it is important that The reactor sorts all the projects in a way that guarantees any project is built before it is required.
The following relationships are honoured when sorting projects:
a project dependency on another module in the build
a plugin declaration where the plugin is another modules in the build
a plugin dependency on another module in the build
[..]
Once the plugin is installed and deployed, it will then not bring with it any knowledge of its module nature, that is, it will be seen as a normal plugin by the projects that will use it via its unique maven coordinates (GAV).
The same is applied to archetypes, which can be modules of a multi module project and then be used individually.
Also note that you can have - as an example - an aggregator project of projects (then modules) completely unrelated between each other, and, say, just aggregate then because you want to build them all together (often not really meaningful, but useful in some cases).
Very new to Maven, can someone please explain to me the difference between using maven modules vs just adding a dependency to your maven project to another maven project in your workspace? When would you use one over the other?
A dependency is a pre-built entity. You get the artifact for that dependency from Maven Central (or Nexus or the like.) It is common to use dependencies for code that belongs to other teams or projects. For example, suppose you need a CSV library in Android. You'd pull it as a dependency.
A Maven module gets built just like your project does. It is common to use Maven modules for components that the project owns. For example, maybe your project creates three jar files.
A dependency can be thought of as a lib/jar (aka Artifact in Maven parlance) that you need to use for building and/or running your code.
This artifact can either be built by your one of the modules of your multi module project or a third party pre-build library (for example log4j).
One of the concepts of maven is that each module is going to output a single artifact (say a jar). So in case of a complex project it is good idea to split your project to multiple modules. And these modules can be dependent on each other via declared dependencies.
See http://books.sonatype.com/mvnex-book/reference/multimodule-sect-intro.html for example of how a web app is split to parent and child modules and how they are linked.
One of the most confusing aspects of Maven is the fact that the parent pom can act as both a parent and as an aggregator.
99% of the functionality you think about in Maven is the parent pom aspect, where you inherit things like repositories, plugins, and most importantly, dependencies.
Dependencies are hard, tangible relationships between your libs that are evaluated during each build. If you think of your software as a meal, it's basically saying A requires ingredient B.
So let's say you're preparing lasagne. Then your dependency chain would look something like this:
lasagne
<- meatSauce
<- groundBeef
<- tomatoPaste
<- cheese
<- noodles
The key thing is, each of the above items (meatSause, groundBeef, cheese, etc) are individual builds that have their individual set of dependencies.
By contrast, the only section of your pom that pertains to aggregation is the modules section:
<modules>
<module>meatSauce</module>
<module>groundBeef</module>
<module>tomatoPaste</module>
<module>cheese</module>
<module>noodles</module>
</modules>
Aggregation simply tells your build engine that it should run these 5 builds in rapid succession:
groundBeef -> tomatoPaste -> cheese -> noodles -> meatSauce
The main benefit of aggregation is the convenience (just click build once) and ensuring the builds are in the correct order (e.g. you wouldn't want to build meatSauce before tomatoPaste).
Here's the thing though: even if you organize the libs as standalone projects without module aggregation, your build will still come out the same provided you build in the correct order.
Moreover, both Jenkins and Eclipse have mechanisms for triggering builds if a dependent project has changed (e.g. changing groundBeef will automatically trigger meatSauce).
Therefore if you're building out of Jenkins or Eclipse, there is no need for aggregation
I am trying to build a maven project that has several modules. So I expect them to be built in the order given in the pom.xml ,but It can be seen that the order of the modules in build is not same as the order mentioned in the pom.xml file. What can be the reason for that ?
My Maven version : Apache Maven 3.0.4
Maven decides the order based on the dependencies. So if you have 3 submodules: A, B an C, you need to go into each submodule and make the dependency explicit. For example, if you go to the pom.xml of B and declare that it depends on A and C, maven will build A and C in some random order, and will build B at the end.
Order that you mentioned in the parent pom file is also relevant in the case when there is no dependency crash between modules , that means if any module present above in the list and it depends on the module that is below it , then in this case order mentioned in POM file won't be used , Maven will use his brain and first build all the modules that are going to need by some other modules to build.
For more on build order , please have a look at this question and Maven Spec on same.
From the specs :
Reactor Sorting
Because modules within a multi-module build can depend on each other,
it is important that The reactor sorts all the projects in a way that
guarantees any project is built before it is required.
The following relationships are honoured when sorting projects:
1. project dependency on another module in the build
2. plugin declaration where the plugin is another modules in the build
3. plugin dependency on another module in the build
4. build extension declaration on another module in the build
5. the order declared in the modules element (if no other rule applies)
Note that only "instantiated" references are used - dependencyManagement
and pluginManagement elements will not cause a change to the reactor
sort order.
Here is a my file system setup:
.m2/ <--- Local Directory
app/
pom.xml
module1/
module2/
module3/
target/ <--- Package directory
Question A
In the parent pom.xml, it has a dependency that all the modules depend upon. Though, when it goes into compile phase, module2 will need to depend on something that just compiled in module1 and module3 needs to depend on something in module2. How would you go about doing this without making two or three "maven projects"? Is it possible to run the compile, install, and package phases on each module individually one at a time (I'd rather do this solution)?
Question B
Also, I know when you install the module, all the "stuff" is updated and added to the local repo. When you are in the compile phase, by default, it grabs all the stuff it needs to depend on from the local repo (Correct if I'm wrong). Also, if I were able to achieve Question A, instead of using the local repo to obtain all the information needed for my dependencies, it possible use those same files from the packaged target directory instead?
Can these questions be answered by a simple pom manipulation and/or adding a plug-in? If this is possible, how would I go about doing this?
Maven is built with the idea of one project-one artifact. What you would normally do in the situation you describe is create a separate POM in each of the modules with the top level POM specified as the parent. Each of the sub modules would then inherit the dependencies of te parent and be free to add additional module specific dependencies. In the top level Pom, you then use the declaration to declare the three sub modules.
One key advantage is that if there are dependencies between the sub modules (like you describe), then maven will automatically figure out the correct order to build them in based in those dependencies.
I am a bit curious about how this works - if I have 5 module projects in a Maven multi-module project, can you import the content and packages into other modules without adding that project as a dependency? Or do you in-fact need the .jar (or a snapshot jar) in order to use structures/functions from other modules?
Thank you,
Inter-module dependencies (adding one module as a dependency in another module's pom.xml) is meaningful in the world of Maven (as a build tool, not an IDE). When you build your multi-module projects from command-line, you don't need concern the dependencies between each module yourself, Maven will topologically sort the modules such that dependencies are always build (hence generate the jar file) before dependent modules (so that it can reference generated jar file as a dependency).
This is not necessary (in theory) if you use IDE like Eclipse, as you can achieve the same result (adding one project as a dependency of another) by some manual settings in Eclipse Right click project and choose Properties -> Java Build Path -> Projects -> Add, which is automatically handled when Eclipse import your multi-module project, if you define inter-module dependency in pom.xml.
The question is why you want to do something unusual (without adding that project as a dependency) that you cannot gain any benefit from it.