What does the @Valid annotation indicate in Spring?

@Valid in itself has nothing to do with Spring. It's part of Bean Validation specification(there are several of them, the latest one being JSR 380 as of second half of 2017), but @Valid is very old and derives all the way from JSR 303.

As we all know, Spring is very good at providing integration with all different JSRs and java libraries in general(think of JPA, JTA, Caching, etc.) and of course those guys took care of validation as well. One of the key components that facilitates this is MethodValidationPostProcessor.

Trying to answer your question - @Valid is very handy for so called validation cascading when you want to validate a complex graph and not just a top-level elements of an object. Every time you want to go deeper, you have to use @Valid. That's what JSR dictates. Spring will comply with that with some minor deviations(for example I tried putting @Validated instead of @Valid on RestController method and validation works, but the same will not apply for a regular "service" beans).

I wanted to add more details about how the @Valid works, especially in spring.

Everything you'd want to know about validation in spring is explained clearly and in detail in https://reflectoring.io/bean-validation-with-spring-boot/, but I'll copy the answer to how @Valid works incase the link goes down.

The @Valid annotation can be added to variables in a rest controller method to validate them. There are 3 types of variables that can be validated:

• the request body,
• variables within the path (e.g. id in /foos/{id}) and,
• query parameters.

So now... how does spring "validate"? You can define constraints to the fields of a class by annotating them with certain annotations. Then, you pass an object of that class into a Validator which checks if the constraints are satisfied.

For example, suppose I had controller method like this:

@RestController
class ValidateRequestBodyController {

  @PostMapping("/validateBody")
  ResponseEntity<String> validateBody(@Valid @RequestBody Input input) {
    return ResponseEntity.ok("valid");
  }

}

So this is a POST request which takes in a request body, and we're mapping that request body to a class Input.

Here's the class Input:

class Input {

  @Min(1)
  @Max(10)
  private int numberBetweenOneAndTen;

  @Pattern(regexp = "^[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}$")
  private String ipAddress;
  
  // ...
}

The @Valid annotation will tell spring to go and validate the data passed into the controller by checking to see that the integer numberBetweenOneAndTen is between 1 and 10 inclusive because of those min and max annotations. It'll also check to make sure the ip address passed in matches the regular expression in the annotation.

side note: the regular expression isn't perfect.. you could pass in 3 digit numbers that are greater than 255 and it would still match the regular expression.

Here's an example of validating a query variable and path variable:

@RestController
@Validated
class ValidateParametersController {

  @GetMapping("/validatePathVariable/{id}")
  ResponseEntity<String> validatePathVariable(
      @PathVariable("id") @Min(5) int id) {
    return ResponseEntity.ok("valid");
  }
  
  @GetMapping("/validateRequestParameter")
  ResponseEntity<String> validateRequestParameter(
      @RequestParam("param") @Min(5) int param) { 
    return ResponseEntity.ok("valid");
  }
}

In this case, since the query variable and path variable are just integers instead of just complex classes, we put the constraint annotation @Min(5) right on the parameter instead of using @Valid.

I think I know where your question is headed. And since this question is the one that pop ups in google's search main results, I can give a plain answer on what the @Valid annotation does.

I'll present 3 scenarios on how I've used @Valid

Model:

public class Employee{
private String name;
@NotNull(message="cannot be null")
@Size(min=1, message="cannot be blank")
private String lastName;
 //Getters and Setters for both fields.
 //...
}

JSP:

...
<form:form action="processForm" modelAttribute="employee">
 <form:input type="text" path="name"/>
 <br>
 <form:input type="text" path="lastName"/>
<form:errors path="lastName"/>
<input type="submit" value="Submit"/>
</form:form>
...

Controller for scenario 1:

     @RequestMapping("processForm")
        public String processFormData(@Valid @ModelAttribute("employee") Employee employee){
        return "employee-confirmation-page";
    }

In this scenario, after submitting your form with an empty lastName field, you'll get an error page since you're applying validation rules but you're not handling it whatsoever.

Example of said error: Exception page

Controller for scenario 2:

 @RequestMapping("processForm")
    public String processFormData(@Valid @ModelAttribute("employee") Employee employee,
BindingResult bindingResult){
                return bindingResult.hasErrors() ? "employee-form" : "employee-confirmation-page";
            }

In this scenario, you're passing all the results from that validation to the bindingResult, so it's up to you to decide what to do with the validation results of that form.

Controller for scenario 3:

@RequestMapping("processForm")
    public String processFormData(@Valid @ModelAttribute("employee") Employee employee){
                return "employee-confirmation-page";
            }
@ExceptionHandler(MethodArgumentNotValidException.class)
@ResponseStatus(HttpStatus.BAD_REQUEST)
public Map<String, String> invalidFormProcessor(MethodArgumentNotValidException ex){
  //Your mapping of the errors...etc
}

In this scenario you're still not handling the errors like in the first scenario, but you pass that to another method that will take care of the exception that @Valid triggers when processing the form model. Check this see what to do with the mapping and all that.

To sum up: @Valid on its own with do nothing more that trigger the validation of validation JSR 303 annotated fields (@NotNull, @Email, @Size, etc...), you still need to specify a strategy of what to do with the results of said validation.

Hope I was able to clear something for people that might stumble with this.

Another handy aspect of @Valid not mentioned above is that (ie: using Postman to test an endpoint) @Valid will format the output of an incorrect REST call into formatted JSON instead of a blob of barely readable text. This is very useful if you are creating a commercially consumable API for your users.