摘要：校驗器可以通過對象報告校驗失敗的信息接口定義示例如下，可以使用等工具測試用戶名不能為空常用注解也可以自定義注解及其校驗規則獲取并初始化注解信息設置自定義校驗規則人的姓名不能為空參考

Q: 為什么后端需要校驗，只交給前端校驗行不行
A: 前后端都需要校驗
后端校驗防止請求過來的非法數據，對后端進行破壞

http://commons.apache.org/ 這個地址apache提供了常用的工具類，包括校驗框架

提供了大量的校驗器，如時間、數值、正則表達式、郵箱、URL等待

以IntegerValidator為例

IntegerValidator integerValidator = new IntegerValidator();
        
Integer value = integerValidator.validate("10");
System.out.println(value);
        
value = integerValidator.validate("G");
System.out.println(value);

/**
     * 
Validate/convert an Integer using the default
     *    Locale.
     *
     * @param value The value validation is being performed on.
     * @return The parsed Integer if valid or null
     *  if invalid.
     */
    public Integer validate(String value) {
        return (Integer)parse(value, (String)null, (Locale)null);
    }

在校驗傳入的參數時，當能夠解析成Integer類型就返回這個數字，如果不能就返回null

其它數值類型校驗類似

Spring框架提供的用于校驗對象的Validator接口，在校驗過程中與Errors對象配合。校驗器可以通過Errors對象報告校驗失敗的信息

org.springframework.validation.Validator接口定義

public interface Validator {

    /**
     * Can this {@link Validator} {@link #validate(Object, Errors) validate}
     * instances of the supplied {@code clazz}?
     * 
This method is typically implemented like so:
     * 
return Foo.class.isAssignableFrom(clazz);
     * (Where {@code Foo} is the class (or superclass) of the actual
     * object instance that is to be {@link #validate(Object, Errors) validated}.)
     * @param clazz the {@link Class} that this {@link Validator} is
     * being asked if it can {@link #validate(Object, Errors) validate}
     * @return {@code true} if this {@link Validator} can indeed
     * {@link #validate(Object, Errors) validate} instances of the
     * supplied {@code clazz}
     */
    boolean supports(Class clazz);

    /**
     * Validate the supplied {@code target} object, which must be
     * of a {@link Class} for which the {@link #supports(Class)} method
     * typically has (or would) return {@code true}.
     * 
The supplied {@link Errors errors} instance can be used to report
     * any resulting validation errors.
     * @param target the object that is to be validated (can be {@code null})
     * @param errors contextual state about the validation process (never {@code null})
     * @see ValidationUtils
     */
    void validate(Object target, Errors errors);

示例如下，可以使用POSTMAN等工具測試

package com.adagio.domain;

import javax.validation.constraints.Max;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;

public class Person {

    @NotNull
    private String name;
    
    @Min(0)
    @Max(150)
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
    
    
}

package com.adagio.validator;

import org.springframework.util.StringUtils;
import org.springframework.validation.Errors;
import org.springframework.validation.Validator;

import com.adagio.domain.Person;

public class PersonValidator implements Validator {

    @Override
    public boolean supports(Class clazz) {
        return Person.class.isAssignableFrom(clazz);
    }

    @Override
    public void validate(Object target, Errors errors) {
        Person person = Person.class.cast(target);
        
        String name = person.getName();
        if(!StringUtils.hasLength(name)){
            errors.reject("person.name.not.null", "用戶名不能為空");
        }
    }

}

package com.adagio.controller;

import javax.validation.Valid;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;

import com.adagio.domain.Person;

@Controller 
public class PersonController {

    @PostMapping("/person/save")
    public Person save(@Valid @RequestBody Person person){
        
        return person;
    }
}

常用注解：@Valid、@NotNull、@Null、@Size、@Min、@Max

也可以自定義注解及其校驗規則

package com.adagio.validator;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import javax.validation.Constraint;
import javax.validation.Payload;

@Target({METHOD, FIELD})
@Retention(RUNTIME)
@Documented
@Constraint(validatedBy = {PersonNameConstraintValidator.class})
public @interface PersonName {

    String message() default "{person.name.not.null}";

    Class[] groups() default {};

    Class[] payload() default {};
}

package com.adagio.validator;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class PersonNameConstraintValidator implements ConstraintValidator{

    /**
     * 獲取并初始化注解信息
     */
    @Override
    public void initialize(PersonName constraintAnnotation) {
        
    }

    /**
     * 設置自定義校驗規則
     */
    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        return false;
    }

}

person.name.not.null=人的姓名不能為空

參考：https://segmentfault.com/l/15...