Using maven to output the version number to a text file

What you are referring to is called filtering

You need to enable filtering on a particular resource, and then use ${project.version} which will be substituted as part of your build

Sure. Create a text file somewhere in src/main/resources, call it version.txt (or whatever)

File content:

${project.version}

now in your pom.xml, inside the build element, put this block:

<build>
  <resources>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>true</filtering>
      <includes>
        <include>**/version.txt</include>
      </includes>
    </resource>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>false</filtering>
      <excludes>
        <exclude>**/version.txt</exclude>
      </excludes>
    </resource>
    ...
  </resources>
</build>

after every build, the file (which you can find in target/classes) will contain the current version.

Now if you want to move the file somewhere else automatically, you are probably going to need to execute an ant task via the maven-antrun-plugin.

Something like this:

  <build>
    ...
    <plugins>
      <plugin>
        <artifactId>maven-antrun-plugin</artifactId>
         <version>1.4</version>
         <executions>
          <execution>
            <phase>process-resources</phase>
            <configuration>
               <tasks>
                 <copy file="${project.build.outputDirectory}/version.txt"
                   toFile="..." overwrite="true" />
              </tasks>
            </configuration>
            <goals>
              <goal>run</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
   </plugins>
 </build>

For a Spring Boot application, follow the accepted answer from above however substituting

${project.version}

with

@project.version@

Here's the link to the documentation https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-1.3-Release-Notes#maven-resources-filtering

Use standard META-INF\MANIFEST.MF (Then you can use Java code getClass().getPackage().getImplementationVersion() to get version)

For .war use this configuration:

<plugin>
    <artifactId>maven-war-plugin</artifactId>
    <version>2.1</version>
    <configuration>
        <archive>                   
            <manifest>
                <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
                <addDefaultSpecificationEntries>true</addDefaultSpecificationEntries>
            </manifest>
        </archive>
    </configuration>
</plugin>

That will add manifest during build, or you can call mvn war:manifest

See also How to get package version at running Tomcat?