@NotNull, @NotEmpty和@NotBlank

在Byte类型上使用了@NotEmpty，运行到这里的时候报错，报错信息大致如下：

No validator could be found for constraint 'javax.validation.constraints.Size' validating type 'java.lang.Byte'. Check configuration for 'licenseType'

遂查看了@NotNull, @NotEmpty, 和@NotBlank三种注解类型的适用场景：

1. @NotNull

/**
* The annotated element must not be {@code null}.
* Accepts any type.
*
* @author Emmanuel Bernard
*/
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
@Retention(RUNTIME)
@Documented
@Constraint(validatedBy = { })
public @interface NotNull

@NotNull适用于任何类型，被注解的元素必须不能是null

2. @NotEmpty

/**
* Asserts that the annotated string, collection, map or array is not {@code null} or empty.
*
* @author Emmanuel Bernard
* @author Hardy Ferentschik
*/
@Documented
@Constraint(validatedBy = { })
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
@Retention(RUNTIME)
@ReportAsSingleViolation
@NotNull
@Size(min = 1)
public @interface NotEmpty

@NotEmpty适用于String，Collection，Map或者Array，不能为null或empty（从最短长度为1也能看出）

@NotEmpty的元素首先肯定是@NotNull的

3. @NotBlank

/**
* Validate that the annotated string is not {@code null} or empty.
* The difference to {@code NotEmpty} is that trailing whitespaces are getting ignored.
*
* @author Hardy Ferentschik
*/
@Documented
@Constraint(validatedBy = { })
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
@Retention(RUNTIME)
@ReportAsSingleViolation
@NotNull
public @interface NotBlank

@NotBlank与@NotEmpty的区别是：

@NotBlank只能用在String上

@NotBlank会忽略尾部的空格（如果只有空格，@NotEmpty认为不是empty的，而@NotBlank会认为是empty的）

@NotBlank的String首先肯定是@NotEmpty的