TestNG Parallel Execution Modes
The parallel attribute in testng.xml determines WHAT runs in parallel.
1. parallel="methods" — Most Granular
Each @Test method runs in its own thread. All @Test methods across all classes run simultaneously.
XML
<suite name="Suite" parallel="methods" thread-count="5">
<test name="Tests">
<classes>
<class name="tests.LoginTest"/>
<class name="tests.CartTest"/>
</classes>
</test>
</suite>
- ✓Thread 1:
LoginTest.validLogin() - ✓Thread 2:
LoginTest.invalidLogin() - ✓Thread 3:
CartTest.addItem() - ✓Thread 4:
CartTest.removeItem() - ✓Thread 5: etc.
Requirement: Each test method must be completely independent. Use ThreadLocal<WebDriver>.
2. parallel="tests" — Most Common for Cross-Browser
Each <test> block in testng.xml runs in its own thread. Ideal for cross-browser testing.
XML
<suite name="CrossBrowserSuite" parallel="tests" thread-count="3">
<test name="ChromeTests">
<parameter name="browser" value="chrome"/>
<classes><class name="tests.LoginTest"/></classes>
</test>
<test name="FirefoxTests">
<parameter name="browser" value="firefox"/>
<classes><class name="tests.LoginTest"/></classes>
</test>
<test name="EdgeTests">
<parameter name="browser" value="edge"/>
<classes><class name="tests.LoginTest"/></classes>
</test>
</suite>
All 3 <test> blocks run simultaneously — Chrome, Firefox, Edge in parallel.
3. parallel="classes" — Class Level
Each test class gets its own thread. All methods within a class run sequentially.
XML
<suite name="Suite" parallel="classes" thread-count="3">
<test name="Tests">
<classes>
<class name="tests.LoginTest"/> <!-- Thread 1: all LoginTest methods -->
<class name="tests.CartTest"/> <!-- Thread 2: all CartTest methods -->
<class name="tests.CheckoutTest"/> <!-- Thread 3: all CheckoutTest methods -->
</classes>
</test>
</suite>
Use when: Test methods within a class share state (same login session) but classes are independent.
4. parallel="instances" — Instance Level
Each instance of a class runs in its own thread. Used with @Factory:
XML
<suite name="Suite" parallel="instances" thread-count="3">
<test name="Tests">
<classes>
<class name="factory.LoginTestFactory"/>
</classes>
</test>
</suite>
Use when: @Factory creates multiple class instances that should run concurrently.
Comparison Table
| Mode | Unit of Parallelism | Within-Class Methods | Best For |
|---|---|---|---|
methods | Each @Test method | Parallel | Independent unit tests |
tests | Each <test> block | Sequential | Cross-browser (different configs) |
classes | Each test class | Sequential | Classes sharing a session |
instances | Each class instance | Sequential | @Factory-created instances |
thread-count Setting
XML
<!-- Max threads in pool — set to (available CPUs × 2) or Grid node count -->
<suite parallel="tests" thread-count="4">
Critical: Always Use ThreadLocal for parallel="methods" or "instances"
Java
// Without ThreadLocal, threads share and overwrite the driver
private static ThreadLocal<WebDriver> driver = new ThreadLocal<>();