Provide fine-grained configuration support for HTTP message converters created in WebMvcConfigurationSupport#addDefaultHttpMessageConverters [SPR-17218]

[spring-projects-issues]

lgoldstein opened SPR-17218 and commented

The current code creates and initializes said converters without providing the user any way to intervene and modify the defaults. E.g., provide some external ObjectMapper to the MappingJackson2HttpMessageConverter, modify the default charset of StringHttpMessageConverter etc... The only recourse currently available is to override extendMessageConverters, somehow detect the converter(s) of interest and re-initialize them - potentially repeating already existing code (a.k.a. the D.R.Y. principle). I propose the following (IMO simple) feature - add a preConfigureDefaultHttpMessageConverter method that allows users to re-configure the initial converter - e.g.:

// if returns null then discard the converter
protected <T, C extends HttpMessageConverter<T>> C preConfigureDefaultHttpMessageConverter(C converter, ApplicationContext context) {
    return converter;    // by default do nothing
}

protected <T, C extends HttpMessageConverter<T>> C addDefaultHttpMessageConverter(
        List<HttpMessageConverter<?>> messageConverters, C converter, ApplicationContext context) {
    converter = preConfigureDefaultHttpMessageConverter(converter, context);
    if (converter != null) {
        messageConverters.add(converter);
    }
    return converter;    // echo the effective converter
}

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
    StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
    stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316
    addDefaultHttpMessageConverter(messageConverters, new ByteArrayHttpMessageConverter(), applicationContext);
    addDefaultHttpMessageConverter(messageConverters, stringHttpMessageConverter, applicationContext);
    ...etc...
}

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Affects: 5.0.8

[rstoyanchev]

Fine idea but it's yet another option on top of multiple existing options, not to mention the
extensive further options in Spring Boot. Using Java 8 Stream API with a filter on the converter type should provide a pretty good compromise.