Parallel Testing in TestNG

What is a Thread?
Java provides built-in support for multi threaded programming. A multi-threaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.

Parallel Testing
Parallelism or multi-threading in software terms is defined as the ability of the software, operating system, or program to execute multiple parts or sub-components of another program simultaneously.

Benefits of having parallel testing

• Reduces execution time – As tests are executed in parallel, multiple tests get executed simultaneously, hence reducing the overall time taken to execute the tests.
• Allows multi-threaded tests – Using this feature, we can write tests to verify certain multi-threaded code in the applications.

There are situations where you want to run multiple tests at the same time.
Situation 1 : If you have 100 test cases and Running these 100 tests sequentially will take you magnitudes of times longer because you can only execute one test at a time.
Situation 2: Cross browser testing - If there are more number of scripts to be executed and executing them on each and every browsers sequentially is time consuming. 
 The above situations can be avoided using a concept in Selenium called Parallel Execution.
 We can reduce the 'execution time' as tests are executed simultaneously in different threads.

In testNG we can achieve parallel execution by two ways.

1. With testng.xml file using parallel attribute of suite tag.
2. We can configure an independent test method to run in multiple threads.

1st Method:
1. With testng.xml file using parallel attribute of suite tag.
Syntax: parellel =“methods”
The parallel attribute of suite tag can accept four values: methods ,classes, tests, instances
Sample XML <?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Suite" parallel=“methods">
  <test name="Test">
    <classes>
      <class name="com.TestPackage.ClassB"/>
    </classes>
  </test> <!-- Test -->
</suite> <!-- Suite -->

Example: I am giving an example here of parallel testing by methods, rest you can watch below youtube video. In below example 4 methods from two classes will be executed in two threads (thread-count="2").

public class ClassA {

 @Test

 public void testcase1() {

  System.out.println("This is testcase1: "+Thread.currentThread().getId());

 }
 @Test

 public void testcase2() {

  System.out.println("This is testcase2: "+Thread.currentThread().getId());
 }
}

public class ClassB {

 @Test
 public void testCase3() {

  System.out.println("This is testcase3: "+Thread.currentThread().getId());

 }
 @Test

 public void testCase4() {

  System.out.println("This is testcase4: "+Thread.currentThread().getId());
 }
}

TestNG.xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">

<suite name="Suite" parallel="methods" thread-count="2">

  <test name="Test">

    <classes>

      <class name="com.parallelDemo.ClassA"/>

      <class name="com.parallelDemo.ClassB"/>

    </classes>

  </test> <!-- Test -->

</suite> <!-- Suite -->

Output:

This is testcase2: 14
This is testcase1: 13
This is testcase3: 14
This is testcase4: 13

2nd Method: 2. We can configure an independent test method to run in multiple threads.

We can configure test method like below and in classC test method testMethod1 will be executed in
two threads (threadPoolSize = 2) and will be executed 3 times(invocationCount = 3)  

public class ClassC {

 @Test(threadPoolSize = 2, invocationCount = 3, timeOut = 1000)

 public void testMethod1() {

  System.out.println("This is testcase1: "+Thread.currentThread().getId());

 }
}

Output:

This is testcase1: 11
This is testcase1: 12
This is testcase1: 11

Please refer below YouTube video on Parallel Testing: