Salient Soft

I was using this to add CORS headers dynamically based on application.properties.

However initially Spring stubbornly refused to add the headers.

My initial (incorrect) code was as follows:-

@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
 
    @Autowired
    private ConfigProperties config;
   
    @Override
    public void addCorsMappings(CorsRegistry registry) {
        config.getCorsOriginsAllowed().forEach(origin -> registry.addMapping(origin).allowedOrigins(origin));
    }
}

The error was that I had used origin both as the registry mapping key and the resulting allowed origin.

The correct code should have used a URL path as the mapping key. The correct replacement line was as follows:-

        config.getCorsOriginsAllowed().forEach(origin -> registry.addMapping("/**").allowedOrigins(origin));

This fixed version mapped every URL path to the given set of origins, using “/**” as above.

This fixed the problem.

This post here by Baeldung and this Spring guide post detail the various ways to add CORS headers, both in a fixed static way using annotations, and in dynamic ways such as the above.

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STS tries to warn you when editing application.properties, if any properties do not match your configuration classes.

Unfortunately it sometimes gets out of step and does not give correct warnings.

I had a few intermittent instances of this which were cleared by refreshing/rebuilding/reloading.

However one persistent one was that whilst Spring Boot was happy with the map[key] syntax for specifying a property for a map entry, the IDE continued to give a warning on this.

In the end I could not resolve this, but using the map.key syntax instead was fine and the IDE was happy with this and did not give warnings.

The only real need for the [] syntax would be to support keys with special characters in like “.”, but in my case this was not an issue and I would normally avoid this anyway if at all possible.

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I was using @Configuration on a configuration class in order to load its properties from application.properties.

This is detailed in this post here.

As per the post, Spring Boot supports lists and maps in the way you would expect, and handles nested objects including Collections such as maps and lists.

It can therefore handle a complete object graph of configuration properties in a clean manner, and also happily supports generics too.

Map keys can be specified either using the “.key” syntax, or the bracketed [key] syntax (which is also used with numeric values to create lists).

The problem I hit was that a map property group, used to create mock response objects keyed on a token string, was being completely ignored even though correctly specified in application.properties.

There were no errors or messages about this the log, even when debug logging was turned on – Spring appeared to be just ignoring the properties completely.

After some significant head banging the problem was trivial.

I was using Lombok to eliminate boiler plate, and both the map class and its descendants  had Lombok @AllArgsContructors and static factory methods for my own object creation.

However this meant that they did not have a no args constructor as I had not specified one. I did not need one, but Spring Boot absolutely relies on this to work!

Once I created the required constructor via @NoArgsConstructor, everything worked fine.

Hats off to Spring for this excellent way to manage config as a nice object graph, but it would be useful to have some kind of warning when a target class does not have a no args constructor!

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