QUESTIONS

I have an endpoint comsuming data in application/x-www-form-urlencoded format

If I send a PUT request like this:

curl --request PUT \
  --url http://localhost:18080/user \
  --header 'Content-Type: application/x-www-form-urlencoded;charset=UTF-8' \
  --data name=ken%

As you can see the data name=ken% is in illegal url format so I get 500 response and the server side error log:

{
  "timestamp": "2021-05-27T02:25:36.849+00:00",
  "status": 500,
  "error": "Internal Server Error",
  "path": "/user"
}

java.lang.IllegalArgumentException: URLDecoder: Incomplete trailing escape (%) pattern
	at java.net.URLDecoder.decode(URLDecoder.java:187) ~[na:1.8.0_291]
	at org.springframework.http.converter.FormHttpMessageConverter.read(FormHttpMessageConverter.java:356) ~[spring-web-5.3.7.jar:5.3.7]
	at org.springframework.web.filter.FormContentFilter.parseIfNecessary(FormContentFilter.java:109) ~[spring-web-5.3.7.jar:5.3.7]
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:88) ~[spring-web-5.3.7.jar:5.3.7]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.7.jar:5.3.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:96) ~[spring-boot-actuator-2.5.0-20210520.224402-561.jar:2.5.0-SNAPSHOT]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.7.jar:5.3.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.7.jar:5.3.7]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.7.jar:5.3.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:893) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1707) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_291]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_291]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.46.jar:9.0.46]
	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_291]

My goal is to let it return 400 status instead of 500, since this error is actually the client's fault not my server's fault

So I extended the org.springframework.http.converter.FormHttpMessageConverter and overrided the read() method

public class CustomizedFormHttpMsgConverter extends FormHttpMessageConverter {
    @Override
    public MultiValueMap<String, String> read(@Nullable Class<? extends MultiValueMap<String, ?>> clazz,
            HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {

        MediaType contentType = inputMessage.getHeaders().getContentType();
        Charset charset = (contentType != null && contentType.getCharset() != null ?
                contentType.getCharset() : StandardCharsets.UTF_8);
        String body = StreamUtils.copyToString(inputMessage.getBody(), charset);

        String[] pairs = StringUtils.tokenizeToStringArray(body, "&");
        MultiValueMap<String, String> result = new LinkedMultiValueMap<>(pairs.length);
        for (String pair : pairs) {
            int idx = pair.indexOf('=');
            if (idx == -1) {
                result.add(URLDecoder.decode(pair, charset.name()), null);
            }
            else {
                try {
                    String name = URLDecoder.decode(pair.substring(0, idx), charset.name());
                    String value = URLDecoder.decode(pair.substring(idx + 1), charset.name());
                    result.add(name, value);
                } catch (Throwable th) {
                    System.out.println("HI KEN I GOT HERE!");
                    throw new HttpMessageNotReadableException(th.getMessage());
                }
            }
        }
        return result;
    }
}

Then I registered this customized converter in my configuration class:

@Configuration
public class CoreConfig {
    @Bean("customizedFormHttpMsgConverter")
    public FormHttpMessageConverter createMyConverter() {
        return new CustomizedFormHttpMsgConverter();
    }
}

Now I can see my converter is successfully registered through my debug endpoint /myConfig:

{
  "requestMappingHandlerAdapter-converters": [
    "com.ken.demo.CustomizedFormHttpMsgConverter@576c5536",
    "org.springframework.http.converter.ByteArrayHttpMessageConverter@45d20f3d",
    "org.springframework.http.converter.StringHttpMessageConverter@55ecbafe",
    "org.springframework.http.converter.StringHttpMessageConverter@3b567dad",
    "org.springframework.http.converter.ResourceHttpMessageConverter@60ecdde8",
    "org.springframework.http.converter.ResourceRegionHttpMessageConverter@54fac191",
    "org.springframework.http.converter.xml.SourceHttpMessageConverter@10497d4",
    "org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter@9b7294c",
    "org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@5a8ba37c",
    "org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@f32a60f",
    "org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter@dc61831"
  ],
  "configurers": [
    "org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter@257cc1fc",
    "org.springframework.boot.actuate.autoconfigure.metrics.web.servlet.WebMvcMetricsAutoConfiguration$MetricsWebMvcConfigurer@42e22a53"
  ],
  "converters": [
    "com.ken.demo.CustomizedFormHttpMsgConverter@576c5536",
    "org.springframework.http.converter.StringHttpMessageConverter@55ecbafe",
    "org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@5a8ba37c"
  ]
}

However, it looks like the CustomizedFormHttpMsgConverter is never used because the illegal request will still result in the same error:

java.lang.IllegalArgumentException: URLDecoder: Incomplete trailing escape (%) pattern
	at java.net.URLDecoder.decode(URLDecoder.java:187) ~[na:1.8.0_291]
	at org.springframework.http.converter.FormHttpMessageConverter.read(FormHttpMessageConverter.java:356) ~[spring-web-5.3.7.jar:5.3.7]

Of course my debug msg "HI KEN I GOT THERE" is never printed out

My questions are:

1. Why my registered CustomizedFormHttpMsgConverter never get used?

2. Is my approach (to extend FormHttpMessageConverter) the right way of doing things?

Again my goal is just to change the 500 response to 400 upon URL decoding failures