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Bean Validation 约束与约束分组

使用分组应用 Jakarta Bean Validation 注解，强制执行依赖上下文的规则。

Bean Validation 约束与约束分组 是 CoddyKit 上的免费 Spring Boot 4 Complete Guide 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Spring Boot 4 Complete Guide 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Spring Boot 4 Complete Guide 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Why Bean Validation?

In a Spring Boot 4 application you constantly receive untrusted data: request bodies, query params, form submissions. Jakarta Bean Validation (the jakarta.validation API) lets you declare rules as annotations directly on your model fields instead of writing manual if checks everywhere.

Declarative — the rule lives next to the field it protects.
Centralized — Spring triggers validation automatically at the controller boundary.
Consistent — the same annotated class can be validated in the web layer, service layer, or persistence layer.

The reference implementation behind the API is Hibernate Validator, pulled in by the spring-boot-starter-validation dependency.

Adding the Starter

Validation is not bundled with the web starter anymore, so you must add it explicitly. Once present, Hibernate Validator is auto-configured and Spring wires a Validator bean for you.

Add the dependency to your pom.xml:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Annotating a DTO

You attach constraints to the fields of a Data Transfer Object. Each annotation carries an optional message and validation parameters.

@NotBlank — the string must contain at least one non-whitespace character.
@Email — must look like a valid email address.
@Size(min, max) — length must fall within range.
@Min / @Max — numeric bounds.

public class UserRegistrationRequest {

    @NotBlank(message = "Username is required")
    @Size(min = 3, max = 20)
    private String username;

    @NotBlank
    @Email(message = "Provide a valid email")
    private String email;

    @Min(value = 18, message = "Must be at least 18")
    private int age;

    // getters and setters
}

Triggering Validation with @Valid

Annotating the DTO alone does nothing. You must tell Spring to validate the argument by placing @Valid on the controller method parameter. If validation fails, Spring throws a MethodArgumentNotValidException before your method body ever runs, and returns a 400 Bad Request.

@RestController
@RequestMapping("/api/users")
public class UserController {

    @PostMapping
    public ResponseEntity<String> register(
            @Valid @RequestBody UserRegistrationRequest request) {
        // reaches here only if all constraints pass
        return ResponseEntity.ok("Registered " + request.getUsername());
    }
}

The Problem: One DTO, Many Contexts

Imagine the same UserRegistrationRequest is reused for two operations:

Create — the client must NOT send an id (the server generates it).
Update — the client MUST send an existing id to know what to modify.

A field that should be @Null on create but @NotNull on update cannot be expressed with plain annotations, because they always fire. This is exactly the problem constraint groups solve.

Defining Constraint Groups

A constraint group is just a marker interface — an empty interface used purely as a tag. You create one per validation context.

Every constraint annotation accepts a groups attribute. When you assign a constraint to a group, that constraint only runs when validation is requested for that group.

public interface OnCreate {}

public interface OnUpdate {}

Assigning Constraints to Groups

Now tag each constraint with the group(s) it belongs to. A constraint with no groups attribute implicitly belongs to the built-in Default group and runs unless you switch groups.

Here the id field obeys opposite rules depending on context:

public class ProductRequest {

    @Null(groups = OnCreate.class,
          message = "id must be empty when creating")
    @NotNull(groups = OnUpdate.class,
             message = "id is required when updating")
    private Long id;

    @NotBlank(groups = {OnCreate.class, OnUpdate.class})
    private String name;

    @Positive(groups = {OnCreate.class, OnUpdate.class})
    private BigDecimal price;

    // getters and setters
}

Selecting a Group with @Validated

Here is the crucial distinction: @Valid always validates the Default group only and cannot select a group. To activate a specific group you must use Spring's @Validated annotation, which accepts the target group class.

Each endpoint picks the group that matches its operation:

@RestController
@RequestMapping("/api/products")
public class ProductController {

    @PostMapping
    public ResponseEntity<Void> create(
            @Validated(OnCreate.class) @RequestBody ProductRequest req) {
        // @Null id, @NotBlank name, @Positive price enforced
        return ResponseEntity.status(HttpStatus.CREATED).build();
    }

    @PutMapping
    public ResponseEntity<Void> update(
            @Validated(OnUpdate.class) @RequestBody ProductRequest req) {
        // @NotNull id enforced instead
        return ResponseEntity.ok().build();
    }
}

@Valid vs @Validated

This pair trips up many developers, so commit it to memory:

@Valid — comes from jakarta.validation. Works on fields for nested/cascading validation, but cannot specify a group (uses Default).
@Validated — comes from org.springframework.validation.annotation. Accepts group classes, and also enables method-level validation on Spring beans. Cannot be placed on a field for cascading.

Rule of thumb: use @Validated(Group.class) at the controller parameter to pick a group; use @Valid on inner object fields to cascade into them.

Cascading and Group Sequences

Two advanced needs come up often:

Cascading — to validate a nested object, mark its field with @Valid. Without it, the nested object's constraints are skipped.
Ordering — a @GroupSequence runs groups in order and stops at the first group that fails, so you can enforce cheap checks before expensive ones.

@GroupSequence({First.class, Second.class})
public interface OrderedChecks {}

public class OrderRequest {

    @NotEmpty(groups = First.class)
    private List<@Valid LineItem> items; // @Valid cascades into each item

    @AssertTrue(groups = Second.class,
                message = "Total must match line items")
    public boolean isTotalConsistent() {
        return /* expensive cross-field check */ true;
    }
}

Reading the Errors

When a grouped validation fails, the resulting MethodArgumentNotValidException still carries the standard BindingResult. You can translate it into a clean JSON response with a @RestControllerAdvice handler, mapping each field to its message.

@RestControllerAdvice
public class ValidationExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String, String> handle(MethodArgumentNotValidException ex) {
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(err ->
                errors.put(err.getField(), err.getDefaultMessage()));
        return errors;
    }
}

Quick Check

Test your understanding of how Spring selects a constraint group.

Recap

You learned how to enforce context-specific validation rules with Jakarta Bean Validation in Spring Boot 4:

Add spring-boot-starter-validation, then annotate DTO fields with constraints like @NotBlank, @Email, @Size, @Min.
Trigger validation at the controller boundary; a failure yields a 400 via MethodArgumentNotValidException.
Constraint groups are marker interfaces assigned via each annotation's groups attribute, letting one DTO carry different rules per operation (e.g. @Null on create vs @NotNull on update).
Use @Validated(Group.class) to select a group — @Valid only runs the Default group and is for cascading into nested objects.
@GroupSequence orders groups and short-circuits on the first failure; @Valid on a collection cascades into each element.

常见问题解答

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