How to replace a string for a buildvariant with gradle in android studio?

I solved the problem on my own, so here is the solution “step by step” – perhaps it will help some other newbies to gradle 🙂

• Copy Task in General:

  copy{
      from("pathToMyFolder"){
          include "my.file"
      }
      // you have to use a new path for youre modified file
      into("pathToFolderWhereToCopyMyNewFile")
  }
  

• Replace a line in General:

  copy {
     ...
     filter{
         String line -> line.replaceAll("<complete line of regular expression>",
                                        "<complete line of modified expression>")
     }
  }
  

• I think the biggest problem was to get the right paths, because I had to make this dynamically (this link was very helpful for me). I solved my problem by replacing the special lines in the manifest and not in the String-file.

• The following example shows how to replace the “meta-data”-tag in the manifest to use youre google-maps-api-key (in my case there are different flavors that use different keys ):

  android.applicationVariants.each{ variant -> 
      variant.processManifest.doLast{ 
          copy{
              from("${buildDir}/manifests"){
                  include "${variant.dirName}/AndroidManifest.xml"
              }
              into("${buildDir}/manifests/$variant.name")
  
              // define a variable for your key:
              def gmaps_key = "<your-key>"
  
              filter{
                  String line -> line.replaceAll("<meta-data android:name=\"com.google.android.maps.v2.API_KEY\" android:value=\"\"/>",
                                                 "<meta-data android:name=\"com.google.android.maps.v2.API_KEY\" android:value=\"" + gmaps_key + "\"/>")
              }
  
              // set the path to the modified Manifest:
              variant.processResources.manifestFile = file("${buildDir}/manifests/${variant.name}/${variant.dirName}/AndroidManifest.xml")
          }    
     }
  }