[FIXED] Spring property name casing by example

Issue

Spring will automatically bind properties defined in application.properties or application.yml to fields defined in @ConfigurationProperties-annotated classes. For instance, in my application.properties I can have:

fizz.buzz=35
fizz.foo=hello

And in my Java code I can have:

@ConfigurationProperties("fizz")
public class FizzProperties {
    private Integer buzz;
    private String foo;
    // ...
}

And at runtime FizzProperties#buzz will get a value of 35 injected into it and FizzProperties#foo will have a value of "hello" injected into it.

I'm wondering what the naming convention is for camel-cased Java fields, and also for hyphens ("-") and periods (".") used in the properties files. For instance, if I had:

fizz.whistle-feather=true
fizz.baz.boo=always

What would their corresponding Java fields need to look like in order for Spring to map and inject them properly?

public class Baz {
    private String boo;
}

@ConfigurationProperties("fizz")
public class FizzProperties {
    private Integer whistleFeather; // correct?
    private Baz baz; // correct?
    // ...
}

Are my assumptions correct here or misled (and if misled, how so)? I can't find this explained in the Spring docs.

Solution

As stated in spring-boot docs, it uses "relaxed binding", so both properties "whistle-feather" and "whistleFeather" will be mapped to your private Integer whistleFeather, but it's recommended, when possible, to store properties in lower-case kebab format, such as fizz.whistle-feather=10.

So your first case is correct.

The second case is also correct, because dots are used as delimiters in application.properties, while YAML file uses as delimiters both dots and colon.

You also may define nested properties as nested classes to store them in one place like this:

@ConfigurationProperties("fizz")
public class FizzProperties {
    private Integer whistleFeather;
    private Baz baz;
    // getters, setters
    
    public static class Baz {
        private String boo;
        // getters, setters
    }
}

Take a look here for more info about spring-boot properties binding and examples.

Answered By - AndrewThomas
Answer Checked By - David Goodson (JavaFixing Volunteer)