What's the difference between these Maven dependency scopes: provided/compile/system/import

Question

I have read the documentation and have some understanding.

Please correct or inform me of the truth; as per my understanding:

• provided
  The dependencies must will be on the machine you run the code on, and must be included in the path

• compile
  The dependencies will not be on the machine that runs the code, so include them in the build

• system
  Exactly the same as provided, but you need the dependencies to be present in a jar file strictly

• import
  Seems like it should import the dependencies from some other POM file but I don't know how/why, so a little elaboration would be appreciated

Answer 1

You are wrong/ambiguous about provided. It means, "This jar should be compiled against locally, but it will be provided on the classpath by something else during runtime, so don't include it in the classpath for me." For example, all web containers (eg: tomcat) include the jars for servlets. You should use provided for the servlet classes so you can compile your code locally, but you don't want to override the servlet classes that tomcat provides for you when you deploy to it.

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system means, "These dependencies are on my system and I want to point to them directly". You want to avoid this if you can, because another person on another computer won't necessarily have these dependencies.

The difference between provided is easier to show:

<dependency>
  <groupId>javax.sql</groupId>
  <artifactId>jdbc-stdext</artifactId>
  <version>2.0</version>
  <scope>system</scope>
  <systemPath>${java.home}/lib/rt.jar</systemPath>
</dependency>

See how it has that <systemPath>? That's the difference. You don't specify the path with provided, provided knows how to get the dependency from a repository. system gets it from your file system only.

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I've never even heard of import. @JigarJoshi linked to http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html which says

import (only available in Maven 2.0.9 or later)

This scope is only used on a dependency of type pom in the <dependencyManagement> section. It indicates that the specified POM should be replaced with the dependencies in that POM's section. Since they are replaced, dependencies with a scope of import do not actually participate in limiting the transitivity of a dependency.

I think this is saying, "take all the dependencies this project has and inline them in this <dependencyManagement> section." Someone correct me if I'm wrong.