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At the heart of unit tests are assertions which provide a mechanism for comparing expected outcomes with actual outcomes. JUnit provides a large selection of overloaded convenience methods that perform predefined logical assertions, such as testing for equality, negations, and conditions specified by a matcher. MUnit also provides a similar set of assertion capabilities such as to assert two values as equals, validate a logical condition, and a variety of other custom assertions that replicate familiar JUnit assertions.

Assert equality

Asserting equality is the simplest validation to set. JUnit provides a method which accepts three parameters, they are the expected value, followed by actual value, and optionally preceded by a message to output on failure.

public void testAssertEquals() {
  String actual = "animals";
  assertEquals("All animals are equal, some more than others", 
                actual, "animals");

Fig 1: JUnit test showing how to assert equality.

MUnit offers the equivalent. A simple equality comparison of expected with actual values with an optional message parameter that is output when the test fails.

<flow name="assert-equals">
  <set-payload value="animals" doc:name="Set Payload"/>
doc:name="Assert equals" 
message="All animals are equal, some more than others"/>

Fig 2: MUnit test showing how to assert equality.

Assert that a condition is true

A modification of the equality assertion is the Assert That expression which asserts that an actual value satisfies a condition specified by a matcher. The matcher is provided by the hamcrest libraries. Hamcrest is a framework for writing matcher objects allowing ‘match’ rules to be defined declaratively.

public void testAssertThatHasItems() {
  assertThat(Arrays.asList("cat", "dog", "hamster"), 
                           hasItems("cat", "hamster"));

Fig 3: Asserts that the array contains, at least “cat” and “hamster.”

MUint replicates similar functionality with the AssertThat processor. 

<flow name="assert-that">
  <set-payload value='#[["cat","dog","hamster"]]' doc:name="Set Payload" />
  <munit-tools:assert-that doc:name="Assert that" 

Fig 4: Asserts that the array contains at least “hamster.”


MUnit tools provide a set of matchers written as DataWeave functions that provide assertion conditions. They replicate common hamcrest matchers such as String matchers: startsWith, endsWith, containsString, and collection matchers such as everyItem, and hasItem.

Customer assertion and matchers

Some test use cases may not be adequately tested using the out-of-the-box assertions or matchers, which requires a method of creating customer assertion behaviours. This is why Hamcrest provides an abstract class (TypeSafeMatcher) to extend and implement custom matching logic.

public class IsCat extends TypeSafeMatcher<String> {
  protected boolean matchesSafely(String pet) {
    try {
      return pet.equalsIgnoreCase("cat");
    } catch (Exception e){
      return false;
  public void describeTo(Description description) {
    description.appendText("Only cats allowed");
  public static Matcher<String> onlyCats() {
    return new IsCat();


Fig. 5: Custom Hamcrest matcher.

import static com.mulesoft.assertion.IsCat.onlyCats;

public void givenAPet_whenIsOnlyCat_thenCorrect() {
  String pet = "CAT";
  assertThat(pet, onlyCats());

Fig. 6: Import static method to use as Hamcrest match.

MUnit provides two ways to create customer expressions that provide assertion logic: Assert expression and Run Custom. This may be expressed as a simple DataWeave script that makes a comparison, implemented as a custom DataWeave matcher or as a org.mule.munit.assertion.api.MunitAssertion implementation. 

<munit:validation >
  <set-payload value='#["cat"]' doc:name="Set Payload" />
    <munit-tools:assert doc:name="Assert expression" >
      import * from dw::test::Asserts 
      payload must equalTo("cat")

Fig. 7: A custom validation expression written in DataWeave 2.0.

Custom DataWeave matchers

To increase the reusability of assertion logic, custom DataWeave matchers can be created and externalized into a DataWeave script located in test resources directory (src/test/resources).

import * from dw::test::Asserts

  fun beACat(): Matcher<String> =
    (actual:String) -> do {
      var matchesCat = (actual == "Cat")
        matches: matchesCat,
        description: {expected: " a cat.", actual: actual}

Fig 8: Custom DataWeave matcher CustomMatchers.dwl.

<munit-tools:assert doc:name="Assert expression" >
  <munit-tools:that ><![CDATA[#[
    import * from dw::test::Asserts
    payload must CustomMatchers::beACat()

Fig 9: Use the CustomMatchers::beACat() custom matcher.

Custom MUnit assertions

The full power of Java can be unleashed to develop a custom assertion by implementing the org.mule.munit.assertion.api.MunitAssertion interface and execute() method. A dependency on the MUnit::Assert Module is required.

public class CustomAssertion implements MunitAssertion {
  public void execute(TypedValue exp, Object p) throws AssertionError{ 
    if (!"CAT".equals(exp.getValue())) { 
      throw new AssertionError("Error the payload is incorrect");

The custom assertion is referenced in the Run Custom event processor.

  doc:name="Run custom" 
  expression="#[payload]" />

And must also be configured in the mule-artifact.json as an exported package.

  "name": "mule-application",
  "minMuleVersion": "4.2.2",
  "classLoaderModelLoaderDescriptor": {
    "id": "mule",
    "attributes": {
      "exportedPackages": ["com.mulesoft.assertion.custom"]


The assertion capabilities provided by MUnit replicate the most commonly used assertions by developers of JUnit tests. It includes conveniences by predefining assertion types — such as Assert Equals — but also allows the developer to create bespoke assertion testing with the Custom DataWeave Matchers. A Java developer will add ease with the manner in which MUnit provides for assertion testing and thanks to its faithful replication of typical JUnit assertions and HamCrest matchers.

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