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Expect an exception from the whole test method
Annotate the test with the exception class your code is expected to throw:
import org.testng.annotations.Test;
@Test(expectedExceptions = IllegalArgumentException.class)
public void rejectsInvalidInput() {
service.process(null);
}TestNG passes this test if the method throws the expected exception. It fails if the method returns without throwing or if a different exception escapes. The annotation can accept multiple expected exception classes, but list more than one only when the contract genuinely permits each type. See the TestNG annotation documentation and the TestNG 7.11.0 @Test Javadoc.
Keep this form focused: the expected exception must escape the test method, so avoid unrelated operations that could throw the same type and accidentally satisfy the expectation.
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Check the exception message
TestNG 7.11.0’s @Test annotation also provides expectedExceptionsMessageRegExp. It checks the exception message against a regular expression when expectedExceptions is configured:
@Test(
expectedExceptions = IllegalArgumentException.class,
expectedExceptionsMessageRegExp = ".*must not be null.*"
)
public void rejectsNullInput() {
service.process(null);
}The documented default pattern is .*, which does not meaningfully constrain the message. Supply a pattern that checks the text you care about. This is a regex match, not a plain substring assertion: escape regular-expression metacharacters if you need to match punctuation literally, and avoid depending on message details that can vary dynamically.
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Scope the assertion to one operation
Use Assert.expectThrows when setup or other assertions should run normally and only one operation is expected to throw. It returns the exception, so you can inspect its message or other properties:
import org.testng.Assert;
IllegalArgumentException exception = Assert.expectThrows(
IllegalArgumentException.class,
() -> service.process(null)
);
Assert.assertTrue(exception.getMessage().contains("must not be null"));The API reference for TestNG 7.9.0 says expectThrows runs a ThrowingRunnable, returns the exception of the expected type, and raises AssertionError if nothing is thrown or the wrong type is thrown. That Javadoc marks the method as available since TestNG 6.9.5. Verify that the version in your build includes it before using it; the reference is the TestNG 7.9.0 Assert Javadoc.
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Choose the right form
| Need | Use | Why |
|---|---|---|
| The test method itself should throw the expected type | @Test(expectedExceptions = ...) |
Concise method-wide expectation. |
| Only a particular call should throw | Assert.expectThrows(...) |
Limits the assertion to that operation. |
| You need to inspect the exception object | Assert.expectThrows(...) |
Returns the caught expected exception for further assertions. |
Your project cannot use expectThrows |
Try/catch with Assert.fail() |
Provides a scoped fallback using basic assertions. |
The distinction is scope: an annotation applies to the entire test method, while expectThrows applies to the supplied runnable. TestNG’s book contents include a section on when not to use expectedExceptions; a community example also discusses multi-statement tests and inspecting exception criteria (TestNG documentation; community example).
Use try/catch when necessary
If your TestNG version or project constraints prevent use of expectThrows, explicitly fail when the call returns, then assert on the exception in the catch block:
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try {
service.process(null);
Assert.fail("Expected IllegalArgumentException");
} catch (IllegalArgumentException exception) {
Assert.assertTrue(exception.getMessage().contains("must not be null"));
}Keep the try block limited to the operation being tested. Otherwise, a different statement could throw the same type and make the test misleading.
Common mistakes and fixes
- The expected exception is caught inside the test. If you use
expectedExceptions, do not catch the exception and return normally; TestNG then sees no exception escaping the method and fails the expectation. UseexpectThrowsor try/catch with an explicit failure instead. - The test contains several operations. A matching exception from an unrelated operation can satisfy a method-wide expectation. Move the expected operation into a scoped
expectThrowscall. - The expected type is too broad. Prefer the specific exception promised by the method’s contract. A broad superclass can allow unintended failures to pass if its subtypes are also acceptable to the assertion.
- The message pattern is too loose or malformed. Remember that
expectedExceptionsMessageRegExpuses regex syntax. Use a pattern that constrains the intended text and escape literal regex punctuation. - An assertion failure is mistaken for the application exception. TestNG assertion failures fail the test; they do not demonstrate that the method under test threw the expected application exception.
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