Most of the times Mockito when()
method is good enough to mock an object’s behavior. But when we have to mock a void method, we can’t use when()
.
Mockito provides following methods that can be used to mock void methods.
doAnswer()
: We can use this to perform some operations when a mocked object method is called that is returning void.doThrow()
: We can use doThrow() when we want to stub a void method that throws exception.Let’s create a simple class with a void method that we will mock in our test classes.
package com.journaldev;
public class Employee {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
if (name == null)
throw new IllegalArgumentException("Employee Name can't be null");
this.name = name;
}
}
Mockito doAnswer() method takes Answer
as argument. It’s a functional interface so we can use lambda expression for its implementation.
doAnswer((i) -> {
System.out.println("Employee setName Argument = " + i.getArgument(0));
assertTrue("Pankaj".equals(i.getArgument(0)));
return null;
}).when(emp).setName(anyString());
Notice that return null
statement is required since we are mocking void method.
Below code snippet shows how to use doThrow() method to mock void methods with the exception.
doThrow(IllegalArgumentException.class).when(emp).setName(null);
Here is a complete example in JUnit where I am using Mockito to mock void method.
package com.journaldev.mockito.voidmethod;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.Mockito.doAnswer;
import static org.mockito.Mockito.doThrow;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
import org.junit.jupiter.api.Test;
import com.journaldev.Employee;
class JUnitMockitoVoidMethod {
@Test
void test_mockito_void() {
Employee emp = mock(Employee.class);
doThrow(IllegalArgumentException.class).when(emp).setName(null);
doAnswer((i) -> {
System.out.println("Employee setName Argument = " + i.getArgument(0));
assertTrue("Pankaj".equals(i.getArgument(0)));
return null;
}).when(emp).setName(anyString());
when(emp.getName()).thenReturn("Pankaj");
assertThrows(IllegalArgumentException.class, () -> emp.setName(null));
emp.setName("Pankaj");
assertEquals("Pankaj", emp.getName());
}
}
Since JUnit 5 and TestNG annotations are so similar, we don’t have to any code specific changes in above class to switch from JUnit 5 to TestNG. Just remove the JUnit 5 import statements and add below imports to change testing framework from JUnit to TestNG.
import static org.testng.Assert.assertEquals;
import static org.testng.Assert.assertThrows;
import static org.testng.Assert.assertTrue;
import org.testng.annotations.Test;
You can download the complete project code from our GitHub Repository.
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I want to mock a void method whose one input state got changed in method body like public void doSomething(SomeBusinessClass1 input1,SomeBusinessClass2 input2,SomeBusinessClass3 input3) { if (openartion on input1){ input3.setOneFiled(calcValue(input2)); } } How can do this ?
- shyam