AssertJ assert on the cause message

Question

Is there a way when using AssertJ agains a method throwing an excetion to check that the message in the cause is equal to some string.

I'm currently doing something like:

assertThatThrownBy(() -> SUT.method())
            .isExactlyInstanceOf(IllegalStateException.class)
            .hasRootCauseExactlyInstanceOf(Exception.class);

and would like to add an assertion to check the message in the root cause.

Answer 1

AssertJ 3.23

• cause() is favored over getCause():

Throwable runtime = new RuntimeException("no way", 
                                         new Exception("you shall not pass"));

assertThat(runtime).cause()
                   .hasMessage("you shall not pass");

• rootCause() is favored over getRootCause():

Throwable rootCause = new RuntimeException("go back to the shadow!");
Throwable cause = new Exception("you shall not pass", rootCause);
Throwable runtime = new RuntimeException("no way", cause);

assertThat(runtime).rootCause()
                   .hasMessage("go back to the shadow!");

---

AssertJ 3.16

Two new options are available:

• getCause():

Throwable runtime = new RuntimeException("no way", 
                                         new Exception("you shall not pass"));

assertThat(runtime).getCause()
                   .hasMessage("you shall not pass");

• getRootCause():

Throwable rootCause = new RuntimeException("go back to the shadow!");
Throwable cause = new Exception("you shall not pass", rootCause);
Throwable runtime = new RuntimeException("no way", cause);

assertThat(runtime).getRootCause()
                   .hasMessage("go back to the shadow!");

---

AssertJ 3.14

extracting with InstanceOfAssertFactory could be used:

Throwable runtime = new RuntimeException("no way", 
                                         new Exception("you shall not pass"));

assertThat(runtime).extracting(Throwable::getCause, as(THROWABLE))
                   .hasMessage("you shall not pass");

as() is statically imported from org.assertj.core.api.Assertions and THROWABLE is statically imported from org.assertj.core.api.InstanceOfAssertFactories.