This is not provided by available validation annotations, therefore you have to go for a custom implementation. The task is divided into 2 simple steps:
1. Is a given String is in the JSON format
There are multiple libraries that are able to parse (therefore validate) a String whether follows the JSON syntax standard. Let's use my favourite GSON for example (there are many). It depends on what library do you currently use:
String string = "{\"foo\":\"bar\"}"
JsonParser jsonParser = new JsonParser();
try {
jsonParser.parse(string); // valid JSON
} catch (JsonSyntaxException ex) {
/* exception handling */ // invalid JSON
}
2. Custom validation annotation
Start with providing a dependency enabling validations:
- groupId:
org.hibernate
- artifactId:
hibernate-validator
Create an annotation used for the validation:
@Documented
@Constraint(validatedBy = JsonStringValidator.class)
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface JsonString {
String message() default "The String is not in JSON format";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
... and the validator handling the validation through the annotation:
public class JsonStringValidator implements ConstraintValidator<JsonString, String> {
@Override
public void initialize(JsonString jsonString) { }
@Override
public boolean isValid(String string, ConstraintValidatorContext context) {
// Use an implementation from step 1. A brief example:
try {
new JsonParser().parse(string);
return true; // valid JSON, return true
} catch (JsonSyntaxException ex) {
/* exception handling if needed */
}
return false; // invalid JSON, return false
}
}
The usage is pretty straightforward:
@JsonString
private String expectedJsonString
This implementation is described in detail at Baeldung's.
