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 line.

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/classes/hello.txt which contains exactly the same text.

Hello ${name}

However, if we add a <filtering> tag to our POM and set it to true like this:

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

Our target/classes/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" option. 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/classes/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 in the <properties> element. For example, if we want to change the variable from "name" to "your.name", we can do so by adding a <your.name> element within the <properties> element.

<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 in 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 the POM by specifying a filter file my-filter-values.properties containing:

your.name = world

and adding that to our POM

    ...
    <filters>
      <filter>my-filter-values.properties</filter>
    </filters>
    ...

Warning: Do not filter files with binary content like images! This will most likely result in corrupt output.

If you have both text files and binary files as resources it is recommended to have two separated folders. One folder src/main/resources (default) for the resources which are not filtered and another folder src/main/resources-filtered for the resources which are filtered.

<project>
  ...
  <build>
    ...
    <resources>
      <resource>
        <directory>src/main/resources-filtered</directory>
        <filtering>true</filtering>
      </resource>
      ...
    </resources>
    ...
  </build>
  ...
</project>

Now you can put those files into src/main/resources which should not filtered and the other files into src/main/resources-filtered.

As already mentioned filtering binary files like images,pdf`s etc. could result in corrupted output. To prevent such problems you can configure file extensions which will not being filtered.