How to use addInstance method of org.testng.junit.JUnitTest class

You need to add the alwaysRun attribute to your @BeforeClass and @AfterClass annotiations:

From the testNG docs: alwaysRun For before methods (beforeSuite, beforeTest, beforeTestClass and beforeTestMethod, but not beforeGroups): If set to true, this configuration method will be run regardless of what groups it belongs to. For after methods (afterSuite, afterClass, ...): If set to true, this configuration method will be run even if one or more methods invoked previously failed or was skipped.

Maven has since been updated and can use categories.

An example from the Surefire documentation:

<plugin>
      <artifactId>maven-surefire-plugin</artifactId>
      <version>2.11</version>
      <configuration>
        <groups>com.mycompany.SlowTests</groups>
      </configuration>
</plugin>

This will run any class with the annotation @Category(com.mycompany.SlowTests.class)

Instead of exclude, you may use include. It'll exactly what you want. Only this test will be executed.

  <classes>
    <class name="test.IndividualMethodsTest">
      <methods>
        <include name="testMethod" />
      </methods>
    </class>
  </classes>

You can create a listener implementing IMethodInterceptor interface. Which will give you an ability to access groups list from your @Test and manage your "tests to execute list" as you need. At same time ITestContext parameter allows you to access the data from testNg xml. So, you can set groups to run in default testNg manner (suite xml file); but run them depending on algorithm you implement. Something like:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.testng.IMethodInstance;
import org.testng.IMethodInterceptor;
import org.testng.ITestContext;
import org.testng.annotations.Test;

public class Interceptor implements IMethodInterceptor
{

    @Override
    public List<IMethodInstance> intercept(List<IMethodInstance> methods, ITestContext context)
    {
        int methCount = methods.size();
        List<IMethodInstance> result = new ArrayList<IMethodInstance>();

        for (int i = 0; i < methCount; i++)
        {
            IMethodInstance instns = methods.get(i);
            List<String> grps = Arrays.asList(instns.getMethod().getConstructorOrMethod().getMethod().getAnnotation(Test.class).groups());
//get these groups from testng.xml via context method parameter                         
            if (grps.contains("A") && grps.contains("B"))
            {
                result.add(instns);
            }                       
        }                       
        return result;
    }
}

The "problem" is somehow related to the fact that you are compiling your tests using the jsr14 target mode which causes the compiler to emit JDK 1.4-compatible bytecode so I think that annotations are removed and TestNG doesn't detect anything to run.

My suggestion would be to configure the maven-compiler-plugin to use different compliance level when compiling code code from main and test as shown below:

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <source>1.6</source>
    <target>jsr14</target>
  </configuration>
  <executions>
    <execution>
      <id>test-compile</id>
      <phase>process-test-sources</phase>
      <goals>
        <goal>testCompile</goal>
      </goals>
      <configuration>
        <source>1.6</source>
        <target>1.6</target>
      </configuration>
    </execution>
  </executions>
</plugin>

I did a quick test, it seems to work.

Well, uh, it was more simple than I thought, and as is often the case I find the solution only once I've asked the question...

Anyway, the solution is to create an abstract base class which initializes the toolkit for you and it is as "easy" as this:

@Test
public abstract class JavafxViewTest
{
    @BeforeClass
    public static void initToolkit()
        throws InterruptedException
    {
        final CountDownLatch latch = new CountDownLatch(1);
        SwingUtilities.invokeLater(() -> {
            new JFXPanel(); // initializes JavaFX environment
            latch.countDown();
        });

        // That's a pretty reasonable delay... Right?
        if (!latch.await(5L, TimeUnit.SECONDS))
            throw new ExceptionInInitializerError();
    }
}

taskkill is a standard Windows utility. The fact that Selenium cannot find it means that the environment variable PATH does not include the directory that contains standard system utilities. It is C:\Windows\system32 for modern Windows versions.

Add this directory to the PATH variable (follow this instruction to modify the PATH variable: http://www.computerhope.com/issues/ch000549.htm) and restart the console or IDE where you run Selenium scripts to apply this environment change.

@BeforeClass
public void setUP() {   
    System.out.println("Thread = " + Thread.currentThread().getId() + " - object hash = " + this.hashCode());
    testRunner = new TestNGCucumberRunner(TestRunner.class);
    ExtentProperties extentProperties = ExtentProperties.INSTANCE;
    extentProperties.setReportPath("output/" + this.hashCode() + "-report.html");
}

This setup method is invoked twice in parallel. It you are changing setReportPath on ExtentProperties.INSTANCE which is shared between these threads. So the threads are overwriting each others results. You can verify this by printing out the hash of ExtentProperties.INSTANCE.

I used the following in my main() method and it worked.

    CommandLineOptions options = new CommandLineOptions();
    JCommander jCommander = new JCommander(options, args);

    XmlSuite suite = new XmlSuite();
    suite.setName("MyTestSuite");
    suite.setParameters(options.convertToMap());

    List<XmlClass> classes = new ArrayList<XmlClass>();
    classes.add(new XmlClass("com.some.path.tests.MyTests"));

    XmlTest test = new XmlTest(suite);
    test.setName("MyTests");
    test.setXmlClasses(classes);

    List<XmlSuite> suites = new ArrayList<XmlSuite>();
    suites.add(suite);

    TestNG testNG = new TestNG();
    testNG.setXmlSuites(suites);
    testNG.run();