Testing Error Scenarios in Java Applications

Testing Error Scenarios in Java Applications

Robust Java applications need thorough error scenario testing. Here's how to test failure paths effectively with JUnit 5 and Mockito.

Assert Exceptions with JUnit 5

@Test
void getUser_withInvalidId_throwsNotFoundException() {
    assertThrows(ResourceNotFoundException.class, () -> {
        userService.getUser("nonexistent-id");
    });
}

@Test
void getUser_withInvalidId_hasCorrectMessage() {
    var exception = assertThrows(ResourceNotFoundException.class, () -> {
        userService.getUser("invalid");
    });
    assertEquals("User with id invalid not found", exception.getMessage());
}

Simulate Failures with Mockito

@Test
void createOrder_whenDatabaseFails_throwsServiceException() {
    when(orderRepository.save(any(Order.class)))
        .thenThrow(new DataAccessException("Connection refused") {});

    assertThrows(ServiceException.class, () -> {
        orderService.createOrder(validOrderRequest);
    });
}

@Test
void fetchData_whenServiceTimesOut_returnsDefault() {
    when(externalClient.getData(anyString()))
        .thenThrow(new SocketTimeoutException("Read timed out"));

    var result = dataService.fetchWithFallback("key");
    assertEquals(DataService.DEFAULT_VALUE, result);
}

Spring Boot Controller Error Tests

@WebMvcTest(UserController.class)
class UserControllerErrorTest {

    @Autowired MockMvc mockMvc;
    @MockBean UserService userService;

    @Test
    void getUser_notFound_returns404() throws Exception {
        when(userService.getUser("123"))
            .thenThrow(new ResourceNotFoundException("User", "123"));

        mockMvc.perform(get("/api/users/123"))
            .andExpect(status().isNotFound())
            .andExpect(jsonPath("$.errorCode").value("RESOURCE_NOT_FOUND"));
    }

    @Test
    void createUser_invalidBody_returns400() throws Exception {
        mockMvc.perform(post("/api/users")
                .contentType(MediaType.APPLICATION_JSON)
                .content("{}"))
            .andExpect(status().isBadRequest());
    }
}

Error Scenarios to Cover

• Null and empty inputs — boundary conditions
• Concurrent modification — OptimisticLockException handling
• External service failures — timeouts, connection refused
• Data integrity violations — duplicate key, constraint violations
• Authentication and authorization failures — expired tokens, missing roles

Complement your test suite with Bugsly's production error tracking. Every unhandled exception Bugsly catches should become a new test case in your suite.