@NotNull、@NotEmpty、@NotBlank的区别
大致区别如下：
@NotEmpty用在集合类上面 @NotBlank 用在String上面 @NotNull 用在基本类型上
只有简单的结果，但是再更具体一点的内容就搜不到了，所以去看了看源码，发现了如下的注释：
1. @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 {
String message() default "{org.hibernate.validator.constraints.NotEmpty.message}";
Class<?>[] groups() default { };
也就是说，加了@NotEmpty的String类、Collection、Map、数组，是不能为null并且长度必须大于0的（String、Collection、Map的isEmpty()方法）。
2. @NotBlank
/** Validate that the annotated string isnot {@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 {
String message() default "{org.hibernate.validator.constraints.NotBlank.message}";
注意：@NotBlank用于String类型
“The difference to {@code NotEmpty} is that trailingwhitespaces are getting ignored.” –> 和{@code NotEmpty}不同的是，尾部空格被忽略，也就是说，纯空格的String也是不符合规则的。所以才会说@NotBlank用于String，只能作用在String上，不能为null，而且调用trim()后，长度必须大于0。
("test") 即：必须有实际字符
3. @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 {
String message() default "{javax.validation.constraints.NotNull.message}";
Class<?>[] groups() default { };
这个就很好理解了，不能为null，但可以为empty。
examples:
1.String name = null;
@NotNull: false
@NotEmpty:false
@NotBlank:false
2.String name = "";
@NotNull:true
@NotEmpty: false
@NotBlank: false
3.String name = " ";
@NotNull: true
@NotEmpty: true
@NotBlank: false
4.String name = "Great answer!";
@NotNull: true
@NotEmpty:true
@NotBlank:true
附上一个使用例子：
@NotBlank(message = "startTime must not be null")
private String startTime;
@NotBlank(message = "endTime must not be null")
private String endTime;
@NotNull(message = "areaType must not be null")
private Integer areaType;
@NotBlank(message = "userId must not be null")
private String userId;
