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.
