Is declaring maven "dependencies" in pom.xml really necessary?

I need some verification of how Maven works.

How important is it for us to specify the project dependencies explicitly (<dependencies>) in pom.xml? So开发者_JAVA技巧me said that it's necessary only when we need a specific version of that jar, otherwise Maven will be able to find the jar in your local / Maven's remote repository. However, I find that sometimes I could not build or package a Maven project without specifying/declaring the dependencies.

So.. is the declaration really necessary?

If your code just uses "plain" Java and does not depend on any other libraries you do not need to declare any dependencies (because you do not depend on anything other than the Java runtime).

In most cases you will use some 3rd party libraries - thus you have to declare them as dependencies in your project to let maven construct a valid classpath which lets your build work (transitive dependencies will be resolved automatically - as already mentioned).

Regarding to the specific version of a jar have a look at the Project Dependencies section of the "Maven: The Complete Reference" book provided by Sonatype. You have several options to declare the version you need (including version ranges).

Do not expect that the declaration

<version>1.2.4</version>

will force Maven to use that version. That is only meant as "allow anything, but prefer 1.2.4". If you need to force maven to use a specific version and nothing else you have to use

<version>[1.2.4]</version>

Yes, the dependencies are needed. Most plugins use them to construct the necessary classpath, or to determine what to include in the artifact. Maven is declarative - you are declaring what you need, not how and where to find them locally.

You need not to declare Transitive dependencies of a JAR. Other than that, everything must be declared. Here is a good read on how maven mananges dependencies. http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html

You always need to specify the dependencies. Maven can't predict, which libraries you need. What you in most times don't need to specify, are additional Maven repositories. You need that only when you have libraries as dependencies, which are not contained in Maven Central.

What you also can eliminate in your projects, are the version numbers of your dependencies, if you have a parent POM, where the versions are specified.