Exploring Common Java Annotations

In this blog post, we will delve into several essential Java annotations used for various purposes, including validation and code organization.

@NotNull

The @NotNull annotation is used to indicate that a field or parameter must not be null. It is commonly employed in validation to ensure that a value is provided.

@NotNull String userId;

@NotBlank

The @NotBlank annotation is typically applied to String fields or parameters and is used to validate that the value is not null and not just whitespace characters.

@NotBlank String userId;

@Size

The @Size annotation allows you to specify the minimum and maximum size constraints for fields or parameters, applicable to Strings, Collections, Maps, and arrays.

@Size(max = 10) String userId

@Min and @Max

The @Min and @Max annotations are used for numeric validation. They specify the minimum and maximum values that a numeric field or parameter can have.

@Email

The @Email annotation is used to validate that a String field or parameter contains a valid email address format.

Optional<@Email String> email;
//without optional
@Email String email;

@AssertTrue and @AssertFalse

These annotations are used to validate that a boolean field or parameter is true or false, respectively.

@Positive, @Negative, @PositiveOrZero, and @NegativeOrZero

These annotations validate numeric fields or parameters to ensure they are positive, negative, or zero, depending on the specific annotation used.

@Past and @Future

The @Past annotation validates that a date field or parameter represents a date in the past, while @Future validates that it represents a date in the future.

@NotNull @Future LocalDate fromDate;

@NotNull @Past LocalDate fromDate;

@Pattern

The @Pattern annotation allows you to specify a regular expression pattern that a String field or parameter must match.

Note: It is important to use regular expressions carefully when using the @Pattern annotation to avoid over-validation or under-validation.

@Size(max = 16) @Pattern(regexp = "^[0-9]+$") @NotBlank String userId

@JsonFormat

The @JsonFormat annotation, while not a validation annotation, is often used in conjunction with validation to specify the format for date serialization and deserialization when working with JSON data.

//we can change pattern and it will work on LocalDateTime as well.
@NotNull @JsonFormat(pattern = "dd-MMM-yyyy") LocalDate fromDate;

To learn more about each annotation and see practical examples, refer to the official Java documentation and relevant libraries or frameworks' documentation.

Happy coding ❤️!