You can invoke TestNG in several different ways:

• With a testng.xml file
• With ant
• From the command line

Assuming that you have TestNG in your class path, the simplest way to invoke TestNG is as follows:

java org.testng.TestNG testng1.xml [testng2.xml testng3.xml ...]

We can execute testng.xml file in different ways.

1. Eclipse
2. InteliJ
3. Ant / Maven
4. Command Line
5. Batch File.

Here is an example testng.xml file:

  <!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
  <suite name="Suite1" verbose="1" >
    <test name="Nopackage" >
      <classes>
        <class name="NoPackageTest" />
      </classes>
    </test>
    <test name="Regression1">
      <classes>
        <class name="test.sample.ParameterSample"/>
        <class name="test.sample.ParameterTest"/>
      </classes>
    </test>
  </suite>

Note: it will throw below warning [TestNGContentHandler] [WARN] It is strongly recommended to add "<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >" at the top of your file, otherwise TestNG may fail or not work as expected., if you don't add the first line <!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >.

You can specify package names instead of class names:

<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" > 
<suite name="Suite1" verbose="1" >
  <test name="Regression1"   >
    <packages>
      <package name="test.sample" />
    </packages>
  </test>
</suite>

In this example, TestNG will look at all the classes in the package test.sample and will retain only classes that have TestNG annotations.

You can also specify groups and methods to be included and excluded:

 <test name="Regression1">
  <groups>
    <run>
      <exclude name="brokenTests"  />
      <include name="checkinTests"  />
    </run>
  </groups>
  <classes>
    <class name="test.IndividualMethodsTest">
      <methods>
        <include name="testMethod" />
      </methods>
    </class>
  </classes>
</test>

You can also define new groups inside testng.xml and specify additional details in attributes, such as whether to run the tests in parallel, how many threads to use, whether you are running JUnit tests, etc...

By default, TestNG will run your tests in the order they are found in the XML file. If you want the classes and methods listed in this file to be run in an unpredictible order, set the preserve-order attribute to false

<test name="Regression1" preserve-order="false">
  <classes>
    <class name="test.Test1">
      <methods>
        <include name="m1" />
        <include name="m2" />
      </methods>
    </class>
    <class name="test.Test2" />
  </classes>
</test>