suppose I have a project say A which is dependent on B. so when I build project A. does maven generates the artifact of A by bundling with project B artifact?
and suppose if project B is dependent on C. then when I build project A, will it by default takes the transitive dependency c to generate the artifact? and even if it takes, what will happen if I add C as a dependency in project A pom.xml? will maven takes the C artifact for two times to build A and generates a bigger artifact file?
what will happen if I add C as a dependency in project A pom.xml?
If C's dependency has already been resolved, maven will simply ignore it if it is declared elsewhere.
will maven takes the C artifact for two times to build A and generates a
bigger artifact file?
No.
Please check https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Transitive_Dependencies for more details.
Transitivity brings a very serious problem when different versions of the same artefacts are included by different dependencies. It may cause version mismatch issue in runtime. In this case, dependency:tree command is very useful in dealing with conflicts of JARs.
$ mvn dependency:tree
Check https://howtodoinjava.com/maven/maven-dependency-management/#dependency-tree for more details.
this question is a little bit broad, things can change a little with you start to talk about Maven multi-module projects (so there is a parent-child relationship between modules). I will assume that multi-module setups are off-topic for this question.
Also: IDEs have tight integration with Maven and might add additional automatic processes on top of it. I assume this is plain Maven as it would work from a shell.
finally: there is a big difference from working with dependencies you pull in from the internet and projects you are building yourself with Maven. I am going to limit this answer to only building your own projects when it comes to dependency management. Otherwise I'd be writing the Maven manual here.
suppose I have a project say A which is dependent on B. so when I
build project A. does maven generates the artifact of A by bundling
with project B artifact?
Maven will include whatever you put in the dependencies listing; if it cannot it will fail the build. If project B is your own project, it will not automatically build B for you when you are building project A, it will only try to include whatever jar is already in your local maven repository. If there is no jar in your local repository, it will try to download it from all the Maven repositories that are known within the project. This will fail if Project B is just a project living on your harddrive, the only way then to make the jar of Project B available for usage in Project A is to actually build and install Project B first so a copy of the jar is put in your local maven repository.
Similarly if project B changes but you do not change its Maven version number, it is your own responsibility to rebuild and install it so the existing jar in your local maven repository gets overwritten.
It is possible that the dependency jar of B is pushed to a remote repository; it could be that your company is hosting or licensing a Nexus or an Artifactory for example and development builds are pushed to a snapshot repository so it can be used by several developers on different machines without them having to checkout the project and build it on their own machine. Then it depends on how Maven is configured if it will eagerly download updated versions of the jar when building project A. If you are working with a remote repository and you suspect that during the build an older version of a dependent jar is being used, this existing question's answers detail a few ways to force dependency jars to be updated when they were already downloaded before. But you should hardly ever need to use that.
and suppose if project B is dependent on C. then when I build project
A, will it by default takes the transitive dependency c to generate
the artifact?
Yes it will walk the entire dependency tree and include all transitive dependencies.
and even if it takes, what will happen if I add C as a
dependency in project A pom.xml? will maven takes the C artifact for two times to build A and generates a bigger artifact file?
Well first of all: dependencies are not included in the jar of project A by default so the size of the jar does not depend on the number of dependencies the project has. You would have to specifically use a plugin that has this functionality, often called "one jarring", "creating a fat jar" or creating an "uber jar". The Maven Shade plugin is a common way to do this.
Regardless, Maven does not allow multiple copies of the same dependency groupId+artifactId pair to exist on the classpath and will filter out duplicate copies when it finds them. This also happens when you add the same artifact multiple times within the same pom, Maven will call you out on it and include only one copy.
So no, in the end there will be only one copy of C.jar. Even if the pom of A refers to C version 1.0 and the pom of B refers to C version 1.1.
Related
Looking for the best option, or a solution I am unaware of:
(probably unnecessary context) I developed "project A", a 1990's RPG video game ported to Java. Currently starting on "project B", a server to co-ordinate project A to run as a MMORPG.
Project A, lives in gitHub, compiles in maven and has no dependencies on project B.
Project B, (will) live in gitHub, has dependencies on "project A", and aspires to compile in maven
I would like both to sit in separate github repositories
I would like project B's pom.xml to specify the version of 'project A' needed.
No preference if this is achieved via specifying a git branch/version or maven artifactId/version.
I would like running maven for project B to find either:
(if available) Project A's last local build of specified version.
(fallback) Projects A's latest version on gitHub
I would prefer not to host project A Jars in some url (or git repo from project A, or B), as they will become large due to graphics. Also that type of thing rubs me the wrong way.
I would like "project B" to "just work" via a 'git clone ...', followed by a 'mvn package'.
There seems to be a multitude of approaches:
via 3rd party: eg jitpack
via maven reactor / modules (with what appear to be several sub approaches)
running 'mvn install' from project A
nexus, artifactory
via git submodules
I would like both to sit in separate github repositories
that's trivial
I would like project B's pom.xml to specify the version of 'project A' needed.
you can use version to distinguish your jars
I would like running maven for project B to find either:
(if available) Project A's last local build of specified version.
(fallback) Projects A's latest version on gitHub
that's maven by design
I would prefer not to host project A Jars in some url (or git repo from project A, or B), as they will become large due to graphics. Also that type of thing rubs me the wrong way.
have you consider to separate resources from core? ie put them in separate project given that they least likely to change
now solution, in my opionion what you need is remote maven repository (by remote i mean opposite to local which will be accessable whenever you want to work on your project. personally i would recommend nexus, but i might be biased here, given that i have most experience with it.
other solution might be maven repository on github. never tried it, but it looks ok
You can
check out both projects on your computer, build project A with version x.y.z and then build B against A with that version. This works through the local Maven repository on your computer.
put both B and A into a multi-module project so that you can build everything at once. This means that both projects have to be in subdirectories of some parent directory.
Use your own Nexus/Artifactory. Then you can mvn deploy B in a version to this repository and resolve it in A through the same one. Works more or less like the local repository, but is much more convenient to use and search in.
I have 3 projects, A->B->C in that dependency order. Currently everytime I make a change to B or C I have to go to the directory and do a mvn clean install in order to install it into the local repository. It is troublesome if I have to do this every time the projects updates.
How can I do it such that every time I do a mvn clean package on A, it will automatically build and install my dependent projects B and C into the local repository?
Create a parent project for all your projects A,B,C and then add all your child project on the parent pom.xml file something like this
<modules>
<module>A</module>
<module>B</module>
<module>C</module>
</modules>
Its called maven multi module project mentioned by #khmarbaise
Here are some example for this
How do I create a multi-module project in Eclipse?
Maven Multi module tutorial
Guide to Working with Multiple Modules
By use of multi module project you will get plenty of benefits like
Anytime you can add any new project with all of the current project
Separation of project is good for code cleanup
You can build Single project or You can build all project in one go.
Duplicacy of jar can be easily ignore .
Maven take care of the build order for you.
One single Jenkins job to build everything.
Plenty of other benefits.But remember if there will some pros then cons also there,its totally now what you want to use .
You can follow the solution I provided to the question Maven 2 Projects, since it is the pattern I usually use when building project with a certain complexity.
Summarizing you would have to create a main Maven project which has three submodules, say master, platform and parent.
The main project has simply the order in which the other projects will be evaluated by Maven
The master pom contains the list of project to be built and their order (aka Reactor order)
The platform pom contains all information about your platform, like JDK version, maven plugin versions, encoding and so on.
The parent pom has the platform pom as a parent and contains all global GAV information about the dependencies you are going to use in your project (Spring, CXF, junit, log4j etc.)
I have a maven project that depends on another maven project which is not available on a public repo. Let's call them projects A and B, and A depends on B.
I'm currently pulling in B in project A by declaring it as an external dependency which I've placed inside an in project repo. (I call mvn bundle:bundle in project B and just copy the generated jar over to project A) The problem is, by doing this maven can't infer which dependencies B relies on since it's a bundled jar.
Let's suppose both projects A and B rely on another maven project C which is available on the remote maven repository. Project A uses project C, version 0.2 and project B uses project C, version 0.1. However, since project B is being loaded in project A as an external bundled jar, maven doesn't know this. During run time, project A tries to call a method which exists in C 0.2 but not in C 0.1. However maven built the project by importing project B first then C as the remote dependency, so C 0.2 is not in the bundled project A jar, and we get a runtime methodNotFound error.
Any idea how I can deal with this? Is it possible to just include a pom.xml file with the bundled project B jar somehow so maven knows that B relies on C as a dependency and can sort the issue out itself?
The way I'm currently working around this is just by manually setting the build order outside of maven and not using it at all for compilation.
EDIT: OK I got it to work by just including a (artifactId)-(version).pom file in the same directory as the jar for maven to get a dependency list
If you would use maven as per tutorials (e.g. here) it just works. Why are you doing bundle? Why do you need copy something? Just declare B as dependency in A's pom.xml and do mvn install for B, then A will use it from your local maven repo.
Also, if you just add both modules into single project (I'm using IDEA), IDE recognises the dependency.
What is an artifact and why does Maven need it?
An artifact is a file, usually a JAR, that gets deployed to a Maven repository.
A Maven build produces one or more artifacts, such as a compiled JAR and a "sources" JAR.
Each artifact has a group ID (usually a reversed domain name, like com.example.foo), an artifact ID (just a name), and a version string. The three together uniquely identify the artifact.
A project's dependencies are specified as artifacts.
In general software terms, an "artifact" is something produced by the software development process, whether it be software related documentation or an executable file.
In Maven terminology, the artifact is the resulting output of the maven build, generally a jar or war or other executable file. Artifacts in maven are identified by a coordinate system of groupId, artifactId, and version. Maven uses the groupId, artifactId, and version to identify dependencies (usually other jar files) needed to build and run your code.
I know this is an ancient thread but I wanted to add a few nuances.
There are Maven artifacts, repository manager artifacts and then there are Maven Artifacts.
A Maven artifact is just as other commenters/responders say: it is a thing that is spat out by building a Maven project. That could be a .jar file, or a .war file, or a .zip file, or a .dll, or what have you.
A repository manager artifact is a thing that is, well, managed by a repository manager. A repository manager is basically a highly performant naming service for software executables and libraries. A repository manager doesn't care where its artifacts come from (maybe they came from a Maven build, or a local file, or an Ant build, or a by-hand compilation...).
A Maven Artifact is a Java class that represents the kind of "name" that gets dereferenced by a repository manager into a repository manager artifact. When used in this sense, an Artifact is just a glorified name made up of such parts as groupId, artifactId, version, scope, classifier and so on.
To put it all together:
Your Maven project probably depends on several Artifacts by way of its <dependency> elements.
Maven interacts with a repository manager to resolve those Artifacts into files by instructing the repository manager to send it some repository manager artifacts that correspond to the internal Artifacts.
Finally, after resolution, Maven builds your project and produces a Maven artifact. You may choose to "turn this into" a repository manager artifact by, in turn, using whatever tool you like, sending it to the repository manager with enough coordinating information that other people can find it when they ask the repository manager for it.
Hope that helps.
Maven organizes its build in projects.
An artifact in maven is a resource generated by a maven project. Each maven project can have exactly one artifact like a jar, war, ear, etc.
The project's configuration file "pom.xml" describes how the artifact is build, how unit tests are run, etc.
Commonly a software project build with maven consists of many maven-projects that build artifacts (e.g. jars) that constitute the product.
E.g.
Root-Project // produces no artifact, simply triggers the build of the other projects
App-Project // The application, that uses the libraries
Lib1-Project // A project that creates a library (jar)
Lib2-Project // Another library
Doc-Project // A project that generates the user documentation from some resources
Maven artifacts are not limited to java resources. You can generate whatever resource you need. E.g. documentation, project-site, zip-archives, native-libraries, etc.
Each maven project has a unique identifier consiting of [groupId, artifactId, version]. When a maven project requires resources of another project a dependency is configured in it's pom.xml using the above-mentioned identifier. Maven then automatically resolves the dependencies when a build is triggered. The artifacts of the required projects are then loaded either from the local repository, which is a simple directory in your user's home, or from other (remote) repositories specified in you pom.xml.
Q. What is Artifact in maven?
ANS: ARTIFACT is a JAR,(WAR or EAR), but it could be also something else. Each artifact has,
a group ID (like com.your.package),
an artifact ID (just a name), and
a version string. The three together uniquely identify the artifact.
Q.Why does Maven need them?
Ans: Maven is used to make them available for our applications.
An artifact is a JAR or something that you store in a repository. Maven gets them out and builds your code.
To maven, the build process is arranged as a set of artifacts. Artifacts include:
The plugins that make up Maven itself.
Dependencies that your code depends on.
Anything that your build produces that can, in turn be consumed by something else.
Artifacts live in repositories.
usually we talking Maven Terminology about Group Id , Artifact Id and Snapshot Version
Group Id:identity of the group of the project
Artifact Id:identity of the project
Snapshot version:the version used by the project.
Artifact is nothing but some resulting file like Jar, War, Ear....
simply says Artifacts are nothing but packages.
Usually, when you create a Java project you want to use functionalities made in another Java projects.
For example, if your project wants to send one email you dont need to create all the necessary code for doing that. You can bring a java library that does the most part of the work.
Maven is a building tool that will help you in several tasks. One of those tasks is to bring these external dependencies or artifacts to your project in an automatic way ( only with some configuration in a XML file ).
Of course Maven has more details but, for your question this is enough.
And, of course too, Maven can build your project as an artifact (usually a jar file ) that can be used or imported in other projects.
This website has several articles talking about Maven :
https://connected2know.com/programming/what-is-maven/
https://connected2know.com/programming/maven-configuration/
An artifact is an element that a project can either use or produce. In Maven terminology, an artifact is an output generated after a Maven project build. It can be, for example, a jar, war, or any other executable file.
Also, Maven artifacts include five key elements, groupId, artifactId, version, packaging, and classifier. Those are the elements we use to identify the artifact and are known as Maven coordinates.
read it from here
Our software is written in Java and comprise many (7) projects.
These projects are Netbeans ant projects.
I'm considering to converting them to maven2.
Where can I find some hints for doing such thing?
Don't read that book. It will only make you confused. Read this book instead: "Maven - The definitive guide" http://www.sonatype.com/books/maven-book/reference/ .
Also, the maven site has a lot of information, but the structure is terrible so you'll need to use google to navigate in it.
Here is my suggestion:
Do this by hand, not with "automagic" "help" from the IDE. Maven integration doesn't work that good yet, not in any IDE.
Make sure you program project is divided into modules under a common umbrella module, so that each module produces a single binary artifact (jar, war,...) possibly accompanied by the javadoc of the source code behind that artifact, a zip with the source code etc. The basic principle is that each module produces a single artifact, containing all the non-test-code under that module. You can do this while the project is still built by ant.
Each module should conform to the standard maven directory layout. The build destination is under [module]/target/[output-type, e.g. "classes"]. The source code is under [module]/src/main/[src-type e.g. "java"] and [module]/test/[src-type]. The artifact consists of all the code under src/main, and none of the code under src/test, as it built to the target directories. You can do this while the is still built by ant.
Start by transforming the sub-module that has no dependencies on other modules in the project.
Now you can create the parent maven module pom.xml with artifact type "pom", consisting of one of the modules below. Make a child module for the first submodule (the one with only external dependencies), using the umbrella module as "parent". Remember that you need to specify version for the parent. Remember to add the child module as a "module" in the parent too. Always use ${project.version} as version in the child modules when you create multi-module projects like this. All modules under a parent must be released simultaneously in a single operation, and if you use this setting maven will make sure the version fields stay the same across all modules and gets updated everywhere during the release. This may make it difficult to re-use the existing numbering scheme, but that doesn't matter. You are never going to run out of version numbers anyway.
Add the necessary dependencies, and make sure you can build the parent and the child module together using the command "mvn clean install" from the parent module.
Proceed with the rest of the modules the same way. Dependencies to other modules under the same parent project should also use ${project.version} as the "version" they are depending on, meaning "the same version as this". NOTE THAT in order to build, the module you are depending on must be built using "mvn install", so that it gets deployed to you local (computer) repository. Otherwise the depending module will not be able to find the classes. There are NO source-code dependencies between modules in maven, only dependencies to built and packed versions installed in local and remote repositories. This can be very confusing if you come from ant-projects. Build from the root module until you get comfortable with this. It takes two days.
Don't use maven integration in IDEs. It is a bad idea. Use "mvn idea:idea" or "mvn eclipse:eclipse" to set up your workspace as a non-maven ordinary IDE project. The inter-module dependencies mechanisms in maven and the IDE aren't identical and will never be. Also, if you have several mavenized projects with dependencies in between, you want to have several of these in your workspace with dependencies set up between. You can do this with mvn idea:idea / eclipse:eclipse if you create a separate maven project file called "workspace.xml" (or whatever) in the same directory as parent module, set up as a multi-module project containing modules "." and "../otherproject" (only one-way reference here, no parent ref back). If you run "mvn idea:idea / eclipse:eclipse -f workspace.xml" you get a workspace with all these modules linked together. No IDE integration lets you do that. This sound like a lot of extra work, but the workspace.xml-file is really small. It doesn't have to contain all that dependency stuff and all that, only the reference to the modules you want to bind together in your IDE.
I did a succeful migration of NetBeans Ant project to Maven project using the instruccions by Joseph Mocker here: http://forums.netbeans.org/ptopic55953.html
I cite the important part:
close the project
rename the build.xml, nbproject files/folders to something so NB won't recognize them.
close and restart NB (so any memory cache knowledge of the project is gone)
copy in an empty pom from some other project.
open the project back up in NB (NB should now identify it as a maven project)
rearrange the files to follow the maven way (™)
This won't be an easy task since Maven2 expects the files to be organized in a specific way. Anyway Better Builds with Maven is a free book that should get you started. It will help you understand Maven and it also has a chapter on migration.
I discovered that the migration is not necessary. The real requirements that I need was automatic download of dependencies (libraries).
This is also achieved by Ivy which nonetheless uses maven repositories.
I solved converting project from ant to ant+ivy with IvyBeans.
I have built a script to migrate Ant builds to Maven. You can find more information here:
https://github.com/ewhauser/ant2maven
It won't help you with fixing your directory structure and or any additional Ant tasks, but it removes a lot of the tedious steps to get started.