在网上搜索的内容，大致如下：

@NotEmpty 用在集合类上面 
@NotBlank 用在String上面 
@NotNull 用在基本类型上

1. @NotEmpty

/** 
* Asserts that the annotated string, collection, map or array is not {@code null} or empty. 
* 
* @author Emmanuel Bernard 
* @author Hardy Ferentschik 
*/

也就是说，加了@NotEmpty的String类、Collection、Map、数组，是不能为null或者长度为0的（String、Collection、Map的isEmpty()方法）。

2. @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 
*/

“The difference to {@code NotEmpty} is that trailing whitespaces 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。

3. @NotNull

/** 
* The annotated element must not be {@code null}. 
* Accepts any type. 
* 
* @author Emmanuel Bernard 
*/

这个就很好理解了，不能为null。
 

• 1.@NotNull：不能为null，但可以为empty

  (""," ","   ")      
  

• 2.@NotEmpty：不能为null，而且长度必须大于0

  (" ","  ")
  

• 3.@NotBlank：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
  

*

        @NotNull: The CharSequence, Collection, Map or Array object is not null, 
        but can be empty.

        @NotEmpty: The CharSequence, Collection, Map or Array object is not null 
        and size > 0.

        @NotBlank: The string is not null and the trimmed length is greater than
         zero.

• 4.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