Converting your TestNG assertions to AssertJ

This page will help you converting your existing TestNG assertions to AssertJ ones. Note that both types of assertions can coexist, you don't have to migrate all at once.

The idea here is to convert code like :

assertEquals(expected, actual);
to :

assertThat(actual).isEqualTo(expected);

There are several ways to perform the conversion :

  • automatically with a shell script.
  • manually using the regexps described in this page.
  • With IntelliJ IDEA, using the Structured Search and Replace (SSR) feature.

Automatically converting your TestNG assertions to AssertJ

It is simple : use the following script (unix and windows only, osx to be done) : convert-testng-assertions-to-assertj.sh

The script is based on sed stream editor and regexps. It looks recursively at all *Test.java files and performs search and replace to convert TestNG assertions to AssertJ ones (if *Test.java file pattern does not suit you, just change it according to your needs in the script).

Usage : execute the script in the base directory containing the test files

# in the directory containing the test files
convert-testng-assertions-to-assertj.sh

It is not perfect (multiline assertions are not converted) but should do most of the job. After executing it, you will need to :

  • optimize imports with your IDE to remove unused imports.
  • if you were using assertEquals with a delta to compare numbers, you will need to statically import org.assertj.core.api.Assertions.offset which is how you express deltas in AssertJ (see number_assertions_with_offset_examples() test at the end of NumberAssertionsExamples).

The script handles the cases where you use an assertion description, for example :

assertEquals("test context", "a", "a");
will be replaced by :

assertThat("a").as("test context").isEqualTo("a");

See below the output of the script execution :

Converting TestNG assertions to AssertJ assertions on files matching pattern : *Test.java

 1 - Replacing : assertEquals(0, myList.size()) ................. by : assertThat(myList).isEmpty()
 2 - Replacing : assertEquals(expectedSize, myList.size()) ...... by : assertThat(myList).hasSize(expectedSize)
 3 - Replacing : assertEquals(expectedDouble, actual, delta) .... by : assertThat(actual).isEqualTo(expectedDouble, offset(delta))
 4 - Replacing : assertEquals(expected, actual) ................. by : assertThat(actual).isEqualTo(expected)
 5 - Replacing : assertArrayEquals(expectedArray, actual) ....... by : assertThat(actual).isEqualTo(expectedArray)
 6 - Replacing : assertNull(actual) ............................. by : assertThat(actual).isNull()
 7 - Replacing : assertNotNull(actual) .......................... by : assertThat(actual).isNotNull()
 8 - Replacing : assertTrue(logicalCondition) ................... by : assertThat(logicalCondition).isTrue()
 9 - Replacing : assertFalse(logicalCondition) .................. by : assertThat(logicalCondition).isFalse()
10 - Replacing : assertSame(expected, actual) ................... by : assertThat(actual).isSameAs(expected)
11 - Replacing : assertNotSame(expected, actual) ................ by : assertThat(actual).isNotSameAs(expected)

12 - Replacing TestNG static import by AssertJ ones, at this point you will probably need to :
12 --- optimize imports with your IDE to remove unused imports
12 --- add "import static org.assertj.core.api.Assertions.offset;" if you were using TestNG number assertions with delta

On windows you might have permission problems as sed creates temp files. If you get this error, you have to grant write permissions to the current user for the folder where your test files live.

Converting TestNG assertions to AssertJ assertions on files matching pattern : *Test.java

 1 - Replacing : assertEquals(0, myList.size()) ................. by : assertThat(myList).isEmpty()
sed: preserving permissions for `./sed005904': Permission denied
sed: preserving permissions for `./sed004220': Permission denied

Converting your TestNG assertions to AssertJ with regexp

Here's a list of find/replace expressions that allow to change TestNG assertions into AssertJ assertions (don't forget to check regexp mode in your editor replace window).

The order of find/replace is important, so that you can benefit from the most relevant AssertJ assertions. For example you should convert assertEquals(0, myList.size()) to assertThat(myList).isEmpty() instead of assertThat(myList.size()).isEqualTo(0)

1 - Converting assertEquals(0, myList.size()) to assertThat(myList).isEmpty()

Find/replace Regexp :

assertEquals(0,(.*).size()); -> assertThat(\1).isEmpty();

It's important to run this one before the assertEquals -> isEqualTo conversion to avoid ending with : assertThat(myList.size()).isEqualTo(0)

2 - Converting assertEquals(expectedSize, myList.size()) to assertThat(myList).hasSize(expectedSize)

Find/replace Regexp :

assertEquals((.*),(*.).size()); -> assertThat(\2).hasSize(\1);

It's important to run this one before the assertEquals -> isEqualTo conversion to avoid ending with : assertThat(myList.size()).isEqualTo(expectedSize)

3 - Converting assertEquals(value, valueUnderTest) to assertThat(valueUnderTest).isEqualTo(value)

Find/replace Regexp :

assertEquals((.*),(.*)); -> assertThat(\2).isEqualTo(\1);
4 - Converting assertNull(objectUnderTest) to assertThat(objectUnderTest).isNull()

Find/replace Regexp :

assertNull((.*)); -> assertThat(\1).isNull();
5 - Converting assertNotNull(objectUnderTest) to assertThat(objectUnderTest).isNotNull()

Find/replace Regexp :

assertNotNull((.*)); -> assertThat(\1).isNotNull();
6 - Converting assertTrue(logicalCondition) to assertThat(logicalCondition).isTrue()

Find/replace Regexp :

assertTrue((.*)); -> assertThat(\1).isTrue();
7 - Converting assertFalse(logicalCondition) to assertThat(logicalCondition).isFalse()

Find/replace Regexp :

assertFalse((.*)); -> assertThat(\1).isFalse();