• 1.@NotNull：不能为null，但可以为empty

  (""," ","   ")      
  

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• 2.@NotEmpty：不能为null，而且长度必须大于0

  (" ","  ")
  

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• 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
  

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*

        @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.

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• 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