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

问题

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

回答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.

---

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.

---

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.

回答2:

Please find the exact meaning of scopes in Maven

I refered to Maven http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html

Dependency scope is used to limit the transitivity of a dependency, and also to affect the classpath used for various build tasks.

There are 6 scopes available:

• compile:

  This is the default scope, used if none is specified. Compile dependencies are available in all classpaths of a project. Furthermore, those dependencies are propagated to dependent projects.

• provided:

  This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.

• runtime:

  This scope indicates that the dependency is not required for compilation, but is for execution. It is in the runtime and test classpaths, but not the compile classpath.

• test:

  This scope indicates that the dependency is not required for normal use of the application, and is only available for the test compilation and execution phases.

• system:

  This scope is similar to provided except that you have to provide the JAR which contains it explicitly. The artifact is always available and is not looked up in a repository.

• import (only available in Maven 2.0.9 or later):

  This scope is only used on a dependency of type pom in the 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.

来源：https://stackoverflow.com/questions/16907682/whats-the-difference-between-these-maven-dependency-scopes-provided-compile-sy