`@NotNull` is added by default for fields that are not required

Given the following step:

Swagger Definition:

address_start_date:
        type: string
        format: 'date-time'
        example: "2011-12-03 10:15:30"
        description: |
          Start Date for address

swagger-codegen-maven-plugin 3.0.56 generates the following model:

   /**
   * Start Date for address 
   * @return addressStartDate
  **/
  @Valid
  @Schema(description = "Start Date for address ")
  public LocalDateTime getAddressStartDate() {
    return addressStartDate;
  }

While swagger-codegen-maven-plugin 3.0.57 generates this:

   /**
   * Start Date for address 
   * @return addressStartDate
  **/
  @NotNull // <--- This is added by default
  @Valid
  @Schema(description = "Start Date for address ")
  public LocalDateTime getAddressStartDate() {
    return addressStartDate;
  }

It seems that this is what introduced with https://github.com/swagger-api/swagger-codegen-generators/pull/1291

Compare the generated model output from swagger-codegen-maven-plugin 3.0.56 and 3.0.57, then inspect the change referenced in swagger-api/swagger-codegen-generators pull request 1291. The fix is complete when fields without a required declaration no longer receive @NotNull in the generated Java model.