https://github.com/shahrukhamd/SampleGradlePlugin/blob/master/src/main/groovy/com/project/sdk/

ServicesPlugin.groovy

package com.project.sdk

import org.gradle.api.Plugin
import org.gradle.api.Project

import java.util.regex.Matcher
import java.util.regex.Pattern

class ServicesPlugin implements Plugin<Project> {

    public final static String JSON_FILE_NAME = "example.json"
    public final static Pattern VARIANT_PATTERN = ~/(?:([^\p{javaUpperCase}]+)((?:\p{javaUpperCase}[^\p{javaUpperCase}]*)*)\/)?([^\/]*)/
    public final static Pattern FLAVOR_PATTERN = ~/(\p{javaUpperCase}[^\p{javaUpperCase}]*)/

    @Override
    void apply(Project project) {

        project.android.applicationVariants.all { variant ->
            handleVariantOpenFile(project, variant)
        }
    }

    static handleVariantOpenFile(Project project, def variant) {
        File quickstartFile = null
        List<String> fileLocations = getJsonLocations("$variant.dirName", project)
        String searchedLocation = System.lineSeparator()
        for (String location : fileLocations) {
            File jsonFile = project.file(location + '/' + JSON_FILE_NAME)
            searchedLocation = searchedLocation + jsonFile.getPath() + System.lineSeparator()
            if (jsonFile.isFile()) {
                quickstartFile = jsonFile
                break
            }
        }

        if (quickstartFile == null) {
            quickstartFile = project.file(JSON_FILE_NAME)
        }

        File outputDir = project.file("$project.buildDir/generated/res/example-plugin/$variant.dirName")

        ServicesTask task = project.tasks.create("${variant.name.capitalize()}YourPluginTaskName", ServicesTask)

        task.jsonFile = quickstartFile
        task.newFile = outputDir
    }

    static List<String> getJsonLocations(String variantDirname, Project project) {
        Matcher variantMatcher = VARIANT_PATTERN.matcher(variantDirname)
        List<String> fileLocations = new ArrayList<>()
        if (!variantMatcher.matches()) {
            project.getLogger().warn("$variantDirname failed to parse into flavors. Please start " +
                    "all flavors with a lowercase character")
            fileLocations.add("src/$variantDirname")
            return fileLocations
        }
        List<String> flavorNames = new ArrayList<>()
        if (variantMatcher.group(1) != null) {
            flavorNames.add(variantMatcher.group(1).toLowerCase())
        }
        flavorNames.addAll(splitVariantNames(variantMatcher.group(2)))
        String buildType = variantMatcher.group(3)
        String flavorName = "${variantMatcher.group(1)}${variantMatcher.group(2)}"
        fileLocations.add("src/$flavorName/$buildType")
        fileLocations.add("src/$buildType/$flavorName")
        fileLocations.add("src/$flavorName")
        fileLocations.add("src/$buildType")
        fileLocations.add("src/$flavorName${buildType.capitalize()}")
        fileLocations.add("src/$buildType")
        String fileLocation = "src"
        for(String flavor : flavorNames) {
            fileLocation += "/$flavor"
            fileLocations.add(fileLocation)
            fileLocations.add("$fileLocation/$buildType")
            fileLocations.add("$fileLocation${buildType.capitalize()}")
        }
        fileLocations.unique().sort{a,b -> countSlashes(b) <=> countSlashes(a)}
        return fileLocations
    }

    private static List<String> splitVariantNames(String variant) {
        if (variant == null) {
            return Collections.emptyList()
        }
        List<String> flavors = new ArrayList<>()
        Matcher flavorMatcher = FLAVOR_PATTERN.matcher(variant)
        while (flavorMatcher.find()) {
            String match = flavorMatcher.group(1)
            if (match != null) {
                flavors.add(match.toLowerCase())
            }
        }
        return flavors
    }

    private static long countSlashes(String input) {
        return input.codePoints().filter{x -> x == '/'}.count()
    }
}

ServicesTask.java

package com.project.sdk;

import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import org.gradle.api.DefaultTask;
import org.gradle.api.GradleException;
import org.gradle.api.tasks.InputFile;
import org.gradle.api.tasks.Optional;
import org.gradle.api.tasks.OutputDirectory;
import org.gradle.api.tasks.TaskAction;

import java.io.*;
import java.util.Map;
import java.util.TreeMap;

public class ServicesTask extends DefaultTask {

    private File jsonFile;
    private File newFile;

    @InputFile
    @Optional
    public File getJsonFile() {
        return jsonFile;
    }

    public void setJsonFile(File jsonFile) {
        this.jsonFile = jsonFile;
    }

    @OutputDirectory
    public File getNewFile() {
        return newFile;
    }

    public void setNewFile(File newFile) {
        this.newFile = newFile;
    }

    @TaskAction
    public void action() throws IOException {
        JsonElement root = new JsonParser().parse(new BufferedReader(new InputStreamReader(
                new FileInputStream(jsonFile), "UTF-8")));
        JsonObject rootObject = root.getAsJsonObject();

        Map<String, String> resValues = new TreeMap<>();

        // set value map from config json
        for (String key: rootObject.keySet()) {
            resValues.put(key, rootObject.get(key).getAsString());
        }

        // write the values file.
        File values = new File(newFile, "values");
        if (!values.exists() && !values.mkdirs()) {
            throw new GradleException("Failed to create folder: " + values);
        }

        PrintWriter fileWriter = new PrintWriter(new OutputStreamWriter(
                new FileOutputStream(new File(values, "values.xml")), "UTF-8"));
        fileWriter.print(mapToString(resValues));
        fileWriter.flush();
        fileWriter.close();
    }

    /**
     * This will get the map and convert it into Android String resource format string.
     */
    private static String mapToString(Map<String, String> values) {
        StringBuilder sb = new StringBuilder();
        sb.append("<?xml version=\"1.0\" encoding=\"utf-8\"?>\n" + "<resources>\n");

        for (Map.Entry<String, String> entry : values.entrySet()) {
            String name = entry.getKey();
            sb.append("    <string name=\"").append(name).append("\" translatable=\"false\">")
                    .append(entry.getValue()).append("</string>\n");
        }

        sb.append("</resources>\n");

        return sb.toString();
    }
}