Testing that code throws the right exception under the right conditions is a critical part of robust test suites. AssertJ offers two main styles for exception assertions: assertThatThrownBy() for inline assertion chaining, and catchThrowable() for BDD-style separation of the act and assert phases.

assertThatThrownBy

Pass a lambda that is expected to throw. AssertJ captures the thrown exception and lets you assert on its type, message, cause, and more:

assertThatThrownBy(() -> Integer.parseInt("abc"))
    .isInstanceOf(NumberFormatException.class)
    .hasMessageContaining("abc");

catchThrowable (BDD style)

Alternatively, capture the exception into a variable using catchThrowable() and assert separately. This makes the arrange-act-assert pattern clearer:

Throwable thrown = catchThrowable(() -> Integer.parseInt("xyz"));
assertThat(thrown).isInstanceOf(NumberFormatException.class);

assertThatExceptionOfType

A third option is assertThatExceptionOfType(SomeException.class).isThrownBy(...) which is more explicit about the expected type upfront.

Example

package com.logicbig.example;

import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.*;

public class ExceptionAssertionsExample {

    static int divide(int a,
                      int b) {
        if (b == 0) throw new ArithmeticException("Division by zero");
        return a / b;
    }

    @Test
    void myTest() {
        // assertThatThrownBy style
        assertThatThrownBy(() -> divide(10, 0))
                .isInstanceOf(ArithmeticException.class)
                .hasMessage("Division by zero");
        System.out.println("assertThatThrownBy passed");

        // catchThrowable style
        Throwable thrown = catchThrowable(() -> Integer.parseInt("abc"));
        assertThat(thrown).isInstanceOf(NumberFormatException.class)
                          .hasMessageContaining("abc");
        System.out.println("catchThrowable passed");

        // assertThatExceptionOfType style
        assertThatExceptionOfType(ArithmeticException.class)
                .isThrownBy(() -> divide(5, 0))
                .withMessage("Division by zero");
        System.out.println("assertThatExceptionOfType passed");
    }
}

Output

$ mvn clean test -Dtest=*
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< com.logicbig.example:assertj-exception-assertions >----------
[INFO] Building assertj-exception-assertions 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- clean:3.2.0:clean (default-clean) @ assertj-exception-assertions ---
[INFO] 
[INFO] --- resources:3.3.1:resources (default-resources) @ assertj-exception-assertions ---
[INFO] skip non existing resourceDirectory D:\example-projects\assertj\assertj-exception-assertions\src\main\resources
[INFO] 
[INFO] --- compiler:3.11.0:compile (default-compile) @ assertj-exception-assertions ---
[INFO] No sources to compile
[INFO] 
[INFO] --- resources:3.3.1:testResources (default-testResources) @ assertj-exception-assertions ---
[INFO] skip non existing resourceDirectory D:\example-projects\assertj\assertj-exception-assertions\src\test\resources
[INFO] 
[INFO] --- compiler:3.11.0:testCompile (default-testCompile) @ assertj-exception-assertions ---
[INFO] Changes detected - recompiling the module! :source
[INFO] Compiling 1 source file with javac [debug target 17] to target\test-classes
[INFO] 
[INFO] --- surefire:3.2.5:test (default-test) @ assertj-exception-assertions ---
[INFO] Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider
[WARNING] file.encoding cannot be set as system property, use <argLine>-Dfile.encoding=...</argLine> instead
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.logicbig.example.ExceptionAssertionsExample
assertThatThrownBy passed
catchThrowable passed
assertThatExceptionOfType passed
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.092 s -- in com.logicbig.example.ExceptionAssertionsExample
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  2.320 s
[INFO] Finished at: 2026-03-03T13:32:19+08:00
[INFO] ------------------------------------------------------------------------

Conclusion

All three exception-assertion styles succeed, confirming that the targeted code paths throw as expected. The example shows how AssertJ gives fine-grained control over verifying exception type, message text, and message containment — all without try-catch boilerplate.

Example Project

Dependencies and Technologies Used:

• assertj-core 3.27.7 (Rich and fluent assertions for testing in Java)
  
• junit-jupiter-engine 6.0.2 (Module "junit-jupiter-engine" of JUnit)
  
• JDK 17
• Maven 3.9.11