Exercise_5_ValidationAndExceptions.md

[Optional] Exercise 5: Introduce Validations and Exception Handler

Learning Goal

After this exercise you will know how to validate method arguments and properties automatically using annotations defined as part of the Java Validation API (a.k.a. JSR-303), like @NotNull or @Size. Furthermore, you will learn how exceptions can be mapped to specific http status codes or error messages.

The task of this exercise is to add validation for one possible error case, and show a corresponding error message to the user.

Prerequisite

Continue with your solution of the last exercise. If this does not work, you can checkout the branch origin/solution-4-2-DeleteUpdate.

Step 1: Add Maven Dependencies

The Hibernate Validator is the reference implementation of JSR-303/JSR-349 bean validation (not to be confused with Hibernate JPA implementation). In order to make Java Validation work in Spring MVC you need to make sure that an implementation of the Java API, such as Hibernate Validator, is in the project’s classpath and

Add the following dependencies to your pom.xml using the XML view of Eclipse:

Add the hibernate-validator dependency:

<!-- Bean validation (@NotNull etc.) -->
<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>5.2.1.Final</version>
</dependency>

Add the javax.el-api dependency

<!-- MethodValidationPostProcessor relies on Expression Language -->
<dependency>
	<groupId>javax.el</groupId>
	<artifactId>javax.el-api</artifactId>
	<version>3.0.1-b04</version>
</dependency>

• After you've changed the Maven settings, don't forget to update your Eclipse project! To do so, right click on your Eclipse project and select Maven - Update Project ... (Alt+F5)

• Furthermore there need to be a MethodValidationPostProcessor bean registered in the application context that delegates to a JSR-303 provider for performing method-level validation. You can add the bean for example as part of the WebAppContextConfig class:

@Bean
public MethodValidationPostProcessor methodValidationPostProcessor() {
   return new MethodValidationPostProcessor();
}

Step 2: Implement REST Argument Validation Test-driven

• Extend the AdvertisementControllerTest class and add a test which does a GET request using the ID "-1". Your expectation is that the server responds with an HTTP status code 400 (Bad Request). But actually the test fails either because of HTTP status code 404 (as no advertisement with ID -1 exists), or HTTP status code 204 (No Content).

• To implement a validation of the ID path parameter you need to add the @Min(0) annotation to your ID argument in the AdvertisementController class. This indicates that only non-negative IDs are valid for this method. And the class needs to be annotated with @Validated annotation, for their methods to be searched for inline constraint annotations.

Notice that the test still fails as the server responds now with a javax.validation.ConstraintViolationException exception (500, Internal Server Error), which needs now to be mapped to 400 as explained in the next step.

Step 3: Use ExceptionMapper

Currently an Internal Server Error (HTTP code 5xx) is returned when requesting using a negative ID. But, even more appropriate would be a client error (HTTP code 4xx)! Thus, change your test and assert that HTTP code 400 (Bad Request) is returned.

• To fix the failing test, we want to implement an exception handler which automatically transforms exceptions of type ConstraintViolationException to respond with an HTTP code 400 (Bad Request). This response should also include a meaningful error message.

• Create a class CustomExceptionMapper in the package com.sap.bulletinboard.ads.controllers and copy the code from here.

• Then, make sure the test succeeds. Furthermore when triggering a request in Postman, check that the error message contains a helpful description and indicates the erroneous value.

• the user-facing message SHOULD be localizable and

• next to the message field also a code field MUST be provided that is "a technical code of the error situation to be used for support purposes".

Step 4: Implement Entity Validation Test-driven

Similar to the @Min annotation we can also use @NotNull or @NotBlank (Hibernate Validator).

• Write a test creating a new advertisement with title set to null or even "". Your expectation is that the server responds with an HTTP status code 400 (Bad Request).
• To fix the test, extend the definition of the title field in your Advertisement class using @NotBlank. Furthermore you need to add the @Valid annotation to the argument of the POST request method, to trigger the verification of the passed argument.

Detail Notes

Annotation	JSR-303 standard	Description
@NotNull	yes	The CharSequence, Collection, Map or Array object is not null, but can be empty.
@NotEmpty	Hibernate	The CharSequence, Collection, Map or Array object is not null and size > 0.
@NotBlank	Hibernate	The string is not null and the trimmed length is greater than zero.

---

© 2018 SAP SE

---