Does Maven always download from remote repositories even if an artifact is already available in the local repository? To answer this question correctly, we need to dig deep into how we define remote repositories in Maven.

Remote repositories can be further divided into three: release, snapshot, and plugin.

A release repository holds artifacts that have a fixed version. An artifact with the given groupId, artifactId, and version tags (GAV coordinates) is the same all the time. The following is an example of a released dependency. If you download this dependency today and then again in a month, both will be the same artifact:

<dependency>
  <groupId>com.googlecode.json-simple</groupId>
  <artifactId>json-simple</artifactId>
  <version>1.1</version>
</dependency>

A snapshot repository holds artifacts that have a special version, which ends with SNAPSHOT. Any artifact that has the SNAPSHOT version can change over time. What you download from the repository might not be the same if you download it again in a month. The following is an example of a SNAPSHOT dependency. You add a SNAPSHOT version to the project artifacts, which are still under development. As a convention, the version to be released will be postfixed by the keyword SNAPSHOT:

<dependency>
  <groupId>org.apache.axis2</groupId>
  <artifactId>axis2-kernel</artifactId>
  <version>1.7.0-SNAPSHOT</version>
</dependency>

A plugin repository is a remote repository that holds plugins. A plugin repository can be a release repository or a snapshot repository.

Maven knows about remote repository locations from the project POM file. By default, the Maven super POM file defines a set of repositories. Even if you do not define any repositories in your application POM, you will inherit what is defined in the super POM.

Repositories that are defined in the super POM file are shown in the following configuration. Here, it's the same repository that acts as the release repository as well as the plugin repository. If we set <snapshots><enabled>true</enabled></snapshots>, then the corresponding repository is treated as a snapshot repository; if set to false, then it's a release-only repository:

<repositories>
  <repository>
    <snapshots>
      <enabled>false</enabled>
    </snapshots>
    <id>central</id>
    <name>Central Repository</name>
    <url>http://repo.maven.apache.org/maven2</url>
  </repository>
</repositories>

<pluginRepositories>
  <pluginRepository>
    <releases>
      <updatePolicy>never</updatePolicy>
    </releases>
    <snapshots>
      <enabled>false</enabled>
    </snapshots>
    <id>central</id>
    <name>Central Repository</name>
    <url>http://repo.maven.apache.org/maven2</url>
  </pluginRepository>
</pluginRepositories>

Let's get back to our original question: does Maven always download artifacts from remote repositories, even if an artifact is already available in the local repository? This relies on the repository configuration. In the preceding code snippet from the super POM file, you will find the following under the pluginRepository section:

<releases>
  <updatePolicy>never</updatePolicy>
</releases>

The updatePolicy element can carry any of the values from always, daily, interval:X, or never. In this case, it is set to never, which means that the artifacts from this repository will be downloaded only if they are not available in the local repository. This is a perfectly valid configuration for a release repository. If this is a snapshot repository, then it won't work, and you might have to work with stale artifacts. For a snapshot repository, you have to set the value to always, daily or interval:X.

The always value means that Maven will always download the artifacts in every build. If the updatePolicy element says daily, then Maven will download the artifacts from the remote repository only once for a given day during the build. It compares the metadata associated with the local POM file with the remote one to see which has the latest timestamp. If the value is set to interval:X where X is an integer value in minutes, Maven will download the artifact only after this time interval.