When running tests, it is sometimes necessary to mock certain components within your application context. For example, you may have a facade over some remote service that is unavailable during development. Mocking can also be useful when you want to simulate failures that might be hard to trigger in a real environment. Let's see the following test class where I have used Mocking:

package com.dineshonjava.accountservice; 
 
import static org.hamcrest.CoreMatchers.is; 
import static org.hamcrest.CoreMatchers.notNullValue; 
import static org.junit.Assert.assertFalse; 
import static org.junit.Assert.assertThat; 
import static org.junit.Assert.assertTrue; 
import static org.mockito.Mockito.*; 
 
import org.junit.Test; 
import org.junit.runner.RunWith; 
import org.springframework.boot.test.context.SpringBootTest; 
import org.springframework.boot.test.mock.mockito.MockBean; 
import org.springframework.test.context.junit4.SpringRunner; 
 
import com.dineshonjava.accountservice.domain.Account; 
import com.dineshonjava.accountservice.service.AccountService; 
 
@RunWith(SpringRunner.class) 
@SpringBootTest 
public class AccountControllerTest { 
    
   @MockBean 
   AccountService accountService; 
    
   @Test  
   public void findAllByCustomerId() { 
         assertFalse(accountService.findAllByCustomerId(1000).size() == 
         3); 
   } 
   @Test 
   public void testAddAccount_returnsNewAccount(){ 
         when(accountService.save(any(Account.class))).thenReturn(new 
         Account(200, 200.20, 1000, "SAVING", "SBIWO111", "SBIW")); 
        assertThat(accountService.save(new Account(200, 200.20, 1000, 
        "SAVING", "SBIWO111", "SBIW")), is(notNullValue())); 
   } 
    
   @Test  
   public void findAccountByAccountId() { 
         assertTrue(accountService.findAccountByAccountId(200).getBalance().intValue() == 200); 
   } 
 
} 

Spring Boot includes a @MockBean annotation that can be used to define a Mockito mock for a bean inside your ApplicationContext. You can use the annotation to add new beans or replace a single existing bean definition. The annotation can be used directly on test classes, on fields within your test, or on @Configuration classes and fields. When used on a field, the instance of the created mock is also injected. Mock beans are automatically reset after each test method.