How do I manually invoke the Java Validation API?

Question

Is there any particulars on when to use the annotations in java, or to put it more precisely, is the answer to the following question Yes or No

Only under this condition, can this particular annotations be used.

Obviously, the this in the above statement can be anything or nothing.

The reason I ask this question is that I was given this code base and in their they had used some annotations to validate the classes, properties etc. Now, this is a web application and it makes use of all the spring beans and what not.

I tried the following code. (This is just normal java code, no spring or anything.This is the entire code)

public class AnnotationsTest {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        TestMe t = new TestMe();
        System.out.println(t.getTest1());

    }

}

class TestMe{
    @NotNull
    private String test1;

    public String getTest1() {
        return test1;
    }

    public void setTest1(String test1) {
        this.test1 = test1;
    }
}

This one prints out null as output. Why is not any validation happening?

Well, what I am guessing is that I've provided the annotations for this, but have not really validated. So I went to this link. Therein he had all sorts of ValidationFactory and what not.

So I wrote,

public class AnnotationsTest{

    private static Validator validator;

    public static void main(String[] args) {
    ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
    validator = factory.getValidator();
            TestMe t = new TestMe();

        Set<ConstraintViolation<TestMe>> constraintViolations =
                validator.validate(t);

            assertEquals(1, constraintViolations.size());
            assertEquals("may not be null", constraintViolations.iterator().next().getMessage());

    }
}

But this one give the following error

Exception in thread "main" javax.validation.ValidationException: Unable to find a default provider
    at javax.validation.Validation$GenericBootstrapImpl.configure(Validation.java:264)
    at javax.validation.Validation.buildDefaultValidatorFactory(Validation.java:111)
    at AnnotationsTest.main(AnnotationsTest.java:29)

So here's my questions: 1. What exactly happened in the two cases above? 2. I thought that annotations were enough, as in Autowired and stuff, they would just inject the classes on it's own. But here why do I need to do the extra stuff to validate. Why doesnt @NotNull validates the things on it's own.

Thanks.

Answer 1

What you need to understand about annotations is they are just metadata on code. Nothing more, nothing less.

In order for all the black magic to happen (in your case Validation, or in the case of @Autowired as you mentioned the registering into the Spring container), some other piece of code needs to read the class definitions, check and see if the annotations are present and if so, perform a while bunch of steps (which may include simple things like registering objects or more advanced things like creating proxies, manipulating bytecode etc.).

So in your case, although the metadata is present on the class in the form of annotations, however no code is present that actually handles them. In the case of validation, that code would be Hibernate Validator.

In order to use Hibernate Validator you could check out this link