jackson-validation

Integrates Jakarta Bean Validation with Jackson object deserialization.

Limitations

Note: static methods annotated with @JsonCreator will not be validated. This is due to Bean Validation itself not supporting them.

As an alternative, consider switching to a @JsonCreator constructor.

Usage

Register ValidationModule with the ObjectMapper

var mapper = new ObjectMapper();
var validator = Validation.buildDefaultValidatorFactory().getValidator();
mapper.registerModule(new ValidationModule(validator));

Annotate class with constraint annotations

You can apply constraints to constructor arguments:

public class MyBeanForValidation {

    private final String name;
    private final Integer age;

    @JsonCreator
    public MyBeanForValidation(@NotBlank String name, @NotNull @PositiveOrZero Integer age) {
        this.name = Objects.requireNonNull(name);
        this.age = Objects.requireNonNull(age);
    }
}

Call ObjectMapper.readValue

Now, an attempt to deserialize an incorrect value will fail with FailedValidationException:

ObjectMapper mapper = // get mapper from somewhere...
try {
    // omit the `age` property on purpose
    mapper.readValue("""
        {
            "name": "the-name"
        }
        """)
} catch (FailedValidationException e) {
    JsonObjectViolations violations = e.getJsonViolations();
    violations.getNode("name").getViolations(); // contains "must not be blank" violation
    violations.getNode("age").getViolations(); // contains "must not be null" violation
}

Rationale ("Why should I use this?")

Strongly typed languages like Java generally put a lot of emphasis on creating types that abstract away their internals, and checking that you don't mix apples and oranges; for example, although URIs can be easily represented as strings, the standard java.net.URI type guarantees that it contains a valid URI and provides a special API for accessing its components:

private void foo(URI uri) {
    // ...
}

URI uri = URI.create("https://example.com");
foo(uri); // compiles!

String uriString = "https://example.com";
foo(uriString); // compiler error!

However, this strong enforcement of types often clashes with the need to parse JSON, which uses a limited set of types to represent data; The uri property in the example below is technically a JSON string but contains a URI:

{
    "uri": "https://example.com"
}

Traditionally, this has been overcome by the use of frameworks such as Jackson, that provide facilities to "bind" JSON data into a class structure:

public class MyObject {

    @JsonProperty
    private URI uri;
}

While this works well for the happy path where JSON values match the expected structure, it doesn't work so well when they don't; consider the folowing JSON object:

{
    "uri": "431"
}

The value of the uri property is clearly not a URI, so parsing will fail, generally by throwing an exception.

To be continued...