Spring MVC: How to read and change a @PathVariable value

Question

This question is very similar to this one, but I dont know where to start.

Suppose I have an action like this:

@GetMapping("/foo/{id}")
public Collection<Foo> listById(@PathVariable("id") string id) {
    return null;
}

How could one intercept the listById method and change the value of id (Eg.: concat a string, pad with zeros etc)?

My scenario is that mostly of the IDs are left-padded with zeros (lengths differ) and I dont want to leave this to my ajax calls.

Expected solution:

@GetMapping("/foo/{id}")
public Collection<Foo> listById(@PathVariablePad("id", 4) string id) {
    // id would be "0004" on "/foo/4" calls
    return null;
}

Answer 1

Ok, here is how I've done it.

Since we can't inherit annotations and thus @PathVariable's target are only parameters, we have to create a new annotation, as follows:

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface PathVariablePad {

    int zeros() default 0;

    @AliasFor("name")
    String value() default "";

    @AliasFor("value")
    String name() default "";

    boolean required() default true;

}

Now we need to create a HandlerMethodArgumentResolver. In this case, since all I want is to left-pad a @PathVariable with zeros, we're going to inherit PathVariableMethodArgumentResolver, like this:

public class PathVariablePadderMethodArgumentResolver extends PathVariableMethodArgumentResolver {

    private String leftPadWithZeros(Object target, int zeros) {
        return String.format("%1$" + zeros + "s", target.toString()).replace(' ', '0'); // Eeeewwwwwwwwwwww!
    }

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(PathVariablePad.class);
    }

    @Override
    protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
        PathVariablePad pvp = parameter.getParameterAnnotation(PathVariablePad.class);

        return new NamedValueInfo(pvp.name(), pvp.required(), leftPadWithZeros("", pvp.zeros()));
    }

    @Override
    protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
        PathVariablePad pvp = parameter.getParameterAnnotation(PathVariablePad.class);

        return leftPadWithZeros(super.resolveName(name, parameter, request), pvp.zeros());
    }

}

Finally, let's register our method argument resolver (xml):

<mvc:annotation-driven>
    <mvc:argument-resolvers>
        <bean class="my.package.PathVariablePadderMethodArgumentResolver" />
    </mvc:argument-resolvers>
</mvc:annotation-driven>

The usage is pretty simple and heres how to do this:

@GetMapping("/ten/{id}")
public void ten(@PathVariablePad(zeros = 10) String id) {
    // id would be "0000000001" on "/ten/1" calls
}

@GetMapping("/five/{id}")
public void five(@PathVariablePad(zeros = 5) String id) {
    // id would be "00001" on "/five/1" calls
}