How can I dynamically assign test groups to test methods and @BeforeXXX and @AfterXXX annotated methods during runtime?

Here's how you do it.

1. You first build an implementation of org.testng.IAnnotationTransformer and org.testng.IAlterSuiteListener
2. Within this implementation's constructor, you read/parse the JSON/CSV/XML/YAML file that contains the method to group mapping.
3. Within the transform() method, you filter out methods that match the incoming method and then add the groups as found in the data source file from (2).
4. Within the alter() you read a JVM argument that hints at which groups to be filtered and then apply that filter accordingly.

Here's a full fledged sample that uses the Google Gson library for json parsing.

Here's my test class

package com.rationaleemotions.runtime;

import java.util.Arrays;
import java.util.stream.Collectors;
import org.testng.ITestResult;
import org.testng.Reporter;
import org.testng.annotations.Test;

public class DemoClass1 {

  @Test
  public void testMethod1() {
    printer();
  }

  @Test
  public void testMethod2() {
    printer();
  }

  private static void printer() {
    ITestResult itr = Reporter.getCurrentTestResult();
    System.err.println(itr.getMethod().getQualifiedName() + " belongs to the groups " +
        Arrays.stream(itr.getMethod().getGroups()).collect(Collectors.toList()));
  }
}

Here's how the listener would look like

package com.rationaleemotions.runtime;

import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonParser;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.util.List;
import java.util.stream.StreamSupport;
import org.testng.IAlterSuiteListener;
import org.testng.IAnnotationTransformer;
import org.testng.annotations.ITestAnnotation;
import org.testng.xml.XmlGroups;
import org.testng.xml.XmlRun;
import org.testng.xml.XmlSuite;

public class GroupChanger implements IAnnotationTransformer, IAlterSuiteListener {

  private JsonArray json;

  public GroupChanger() throws FileNotFoundException {
    String mapping = System.getProperty("mapping.file", "src/test/resources/file.json");
    if (!mapping.trim().isEmpty()) {
      json = JsonParser.parseReader(new FileReader(mapping))
          .getAsJsonArray();
    }
  }

  @Override
  public void alter(List<XmlSuite> suites) {
    String groupsToRun = System.getProperty("groups", "g1");
    if (groupsToRun.equalsIgnoreCase("*")) {
      //Execute everything. So don't add groups filtering in the suite file
      return;
    }
    for (XmlSuite suite: suites) {
      XmlGroups xmlGroups = new XmlGroups();
      XmlRun xmlRun = new XmlRun();
      for (String group : groupsToRun.split(",")) {
        xmlRun.onInclude(group);
      }
      xmlGroups.setRun(xmlRun);
      suite.setGroups(xmlGroups);
    }
  }

  @Override
  public void transform(ITestAnnotation annotation, Class testClass, Constructor testConstructor,
      Method testMethod) {
    if (testMethod == null) {
      return;
    }
    if (json == null) {
      return;
    }
    String fqmn = testMethod.getDeclaringClass().getCanonicalName() + "." + testMethod.getName();
    StreamSupport.stream(json.spliterator(), true)
        .filter(each -> methodName(each).equalsIgnoreCase(fqmn))
        .findFirst()
        .ifPresent(each -> {
          System.err.println("Found " + each);
          annotation.setGroups(groups(each));
        });
  }

  private static String methodName(JsonElement each) {
    return each.getAsJsonObject().get("method").getAsString();
  }

  private static String[] groups(JsonElement each) {
    return each.getAsJsonObject().get("groupName").getAsString().split(",");
  }
}

Here's how the suite file would look like

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="groups_suite" verbose="2">
  <listeners>
    <listener class-name="com.rationaleemotions.runtime.GroupChanger"/>
  </listeners>
  <test name="groups_test" verbose="2">
    <classes>
      <class name="com.rationaleemotions.runtime.DemoClass1"/>
    </classes>
  </test>
</suite>

Here's how the json mapping would look like

[
  {
    "method": "com.rationaleemotions.runtime.DemoClass1.testMethod1",
    "groupName": "g1"
  },
  {
    "method": "com.rationaleemotions.runtime.DemoClass1.testMethod2",
    "groupName": "g2"
  }
]