Filtering

Variables can be included in your resources. These variables, denoted by the ${...} delimiters, can come from the System properties, your project properties, from your filter resources and from the command lines.

For example, if we have a resource src/main/resources/hello.txt containing

Hello ${name}

And a pom like this

<project>
  [...]
  <name>My Resources Plugin Practice Project</name>
  [...]
  <build>
    [...]
    <resources>
      [...]
      <resource>
        <directory>src/main/resources</directory>
      </resource>
      [...]
    </resources>
  [...]
  </build>
  [...]
</project>

Upon calling

mvn resources:resources

This will create a resource output in target/class/hello.txt which contains exactly the same.

Hello ${name}

However, if we add a filtering tag to our pom and set it to true such as this

      [...]
      <resource>
        <directory>src/main/resources</directory>
        <filtering>true</filtering>
      </resource>
      [...]

Our target/class/hello.txt after calling

mvn resources:resources 

would be

Hello My Resources Plugin Practice Project

That's because the name variable was replaced by the value of the project's name (which was specified in the pom).

Moreover, we can also assign values through the command line using the "-D" tag. For example, to change the value for the variable name to "world", we can simply invoke this command

mvn resources:resources -Dname="world"

And the output in target/class/hello.txt would be

Hello world

Furthermore, we are not limited to use pre-defined project variables. We can specify our own variables and their values under the properties tag. For example, if we want to change the variable from "name" to "your.name", we can do so by adding a "your.name" tag under the "properties" tag.

<project>
  [...]
  <properties>
    <your.name>world</your.name>
  </properties>
  [...]
</project>

But to organize your project, you may want to put all your variables and their values on a separate file so that you will not have to rewrite your pom, or set their values all the time with every build. This can be done by adding a filter.

<project>
  [...]
  <name>My Resources Plugin Practice Project</name>
  [...]
  <build>
    [...]
    <filters>
      <filter> [a filter property] </filter>
    </filters>
    [...]
  [...]
  </build>
  [...]
</project>

For example, we can separate "your.name" from pom by specifying a filter, "my filter values.properties" containing

your.name = world

and adding that to our pom

    [...]
    <filters>
      <filter>my filter values.properties</filter>
    </filters>
    [...]