400 Bad Request when uploading byte[] with Spring RestTemplate to SpringMVC rest endpoint

Question 1

I found my problem. I needed to add an AbstractResource (in this case a ByteArrayResource) to my MultiValueMap instead of the raw byte array. Here's the code that fixed it:

public void scaleImage(byte[] image, float percent) throws Exception {
    String url = "http://localhost:8080/scale/" + percent;

    final byte[] rawBytes = image.clone();

    MultiValueMap<String, Object> mvm = new LinkedMultiValueMap<>();
    ByteArrayResource bar = new ByteArrayResource(rawBytes) {
        @Override
        public String getFilename() {
            return "Test-"+rawBytes.length + ".jpg";
        }
    };

    mvm.add("file", bar);
    TransformedResponse r = doPost(url, mvm);
}

Question 2

First of all, when using Spring, make sure that you have proper MultiPartFile resolver defined in your servlet context:

<bean id="multipartResolver"
      class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
    <property name="maxUploadSize" value="52428800"/>
    <property name="maxInMemorySize" value="52428800"/>
</bean>

If you're using maven, this resolver is located in spring-web artifact:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>${your.spring.version}</version>
</dependency>

Then, create form and make sure you're using proper enctype:

<form method="post" action="upload.form" enctype="multipart/form-data">
    <input type="file" name="file"/>
    <input type="submit"/>
</form>

Finally, handle file upload in your controller

@RequestMapping(value="/path", method= RequestMethod.POST)
public StringscaleImage(@RequestParam("file") MultipartFile file) {
    //....
}

Remember that asynch file upload is supported only with HTML5, with others you'd need to use some workarounds (like flash or iframes).

If you're still facing 400 Error, add to your logging service this logger (or similar, depending on logging framework):

<appender name="console" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out"/>
    <param name="threshold" value="TRACE"/>
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%d{HH:mm:ss,SSS} %-5p [%c] %m%n"/>
    </layout>
</appender>

<logger name="org.springframework.web.method.HandlerMethod">
    <level value="TRACE"/>
</logger>

<root>
    <priority value="info"/>
    <appender-ref ref="console"/>
</root>

It should output exception thrown during request handling

Question 3

Instead of trying to pass the MultipartFile in as a request parameter, try the following:

@RequestMapping(value="/scale/{percent}", method= RequestMethod.POST)
public ResponseEntity scaleImage(@PathVariable("percent") float percent,
        MultipartHttpServletRequest request) {
    Iterator<String> fileNames = request.getFileNames();
    MultipartFile uploadedFile = request.getFile(fileNames.next());
    String fileName = uploadedFile.getName();

This is the only way I could actually get my multipart file to be accepted.

If you are still getting an empty file from this process, it must be something to do with how you are POST'ing the data.