How to use @ConfigurationProperties with Records?

Question

Java 16 introduced Records, which help to reduce boilerplate code when writing classes that carry immutable data. When I try to use a Record as @ConfigurationProperties bean as follows I get the following error message:

@ConfigurationProperties("demo")
public record MyConfigurationProperties(
        String myProperty
) {
}

***************************
APPLICATION FAILED TO START
***************************

Description:

Parameter 0 of constructor in com.example.demo.MyConfigurationProperties required a bean of type 'java.lang.String' that could not be found.

How can I use Records as @ConfigurationProperties?

Answer 1

Answering my own question.

The above error raises from Spring Boot not being able to construct the bean because of the lack of a no-argument constructor. Records implicitly declare a constructor with a parameter for every member.

Spring Boot allows us to use the @ConstructorBinding annotation to enable property binding by constructor instead of setter methods (as stated in the docs and the answer to this question). This also works for records, so this works:

@ConfigurationProperties("demo")
@ConstructorBinding
public record MyConfigurationProperties(
        String myProperty
) {
}

Update: As of Spring Boot 2.6, using records works out of the box and @ConstructorBinding is not required anymore when the record has a single constructor. See the release notes.

Answer 2

If you need to declare default values programatically:

@ConfigurationProperties("demo")
public record MyConfigurationProperties(String myProperty) { 
    
    @ConstructorBinding
    public MyConfigurationProperties(String myProperty) {
        this.myProperty = Optional.ofNullable(myProperty).orElse("default");
    }
}

java.util.Optional properties:

@ConfigurationProperties("demo")
public record MyConfigurationProperties(Optional<String> myProperty) {

    @ConstructorBinding
    public MyConfigurationProperties(String myProperty) {
        this(Optional.ofNullable(myProperty));
    }
}

@Validated and java.util.Optional in combination:

@Validated
@ConfigurationProperties("demo")
public record MyConfigurationProperties(@NotBlank String myRequiredProperty,
                                        Optional<String> myProperty) {

    @ConstructorBinding
    public MyConfigurationProperties(String myRequiredProperty, 
                                     String myProperty) {
        this(myRequiredProperty, Optional.ofNullable(myProperty));
    }
}

Based on this Spring Boot issue.