Conversions in spring


The story is really simple. I wanted to accept my class as rest controller method param. I decided that I don’t want to convert a simple string to object every time and it would be faster if I do the conversion in the single place. After a quick research I’ve found Converter interface which looked like perfect for the job. After some digging and investigation it turns out that there is a lot of automagic in the Spring conversion service.

Let’s start from the beginning and the controller I want to have:

@WebMvcTest(controllers = Controller.class)
public class ControllerTest {
  private MockMvc mockMvc;

  public void should_parse_id_from_plain_string() throws Exception {
    final String emptyId = "00000000-0000-0000-0000-000000000000";

    mockMvc.perform(MockMvcRequestBuilders.get("/find-id/" + emptyId))

        .andExpect(jsonPath("$.id", equalTo(emptyId)));

In order to make this test pass I need the controller:

class Controller {
  public ObjectToFind findId(@PathVariable Id id) {
    return new ObjectToFind(id);

  static class ObjectToFind {
    private final String id;

    private ObjectToFind(Id id) { = id.toString();

    public String getId() {
      return id;

Note that I expect custom type as an input parameter of the findOne method. If I try to run this test it will fail which makes sense because spring has no idea how to handle my Id class. My first approach, which failed, was to implement Converter interface and register it as a spring bean. Something like this:

class IdConverter implements Converter<String, Id> {
  public Id convert(String source) {
    return new Id(UUID.fromString(source));

It looks clean. It has single responsibility, doesn’t have any state, and is easy to test. It will work if you start your application using bootRun or if your tests set up full spring context. The problem is I am too lazy to start the whole application to manually check if my controllers work, or wait until application context boots up (this requires setting up the DB, configuring tons of other beans, in general, it takes time).

The problem and the solution are hidden deep in the @WebMvcTest. To be more specific problem is in the @TypeExcludeFilter registered in the @WebMvcTest If you look into the filter implementation you’ll notice classes that are considered to be part of the web environment. Converter nor @Component are not considered to be the part of it. So now we know why it is not picked up while running tests.

To be honest, my first approach was @InitBinder and ancient PropertyEditor which is not as nice as stateless Converter but it will work:

class ArgumentConverterBinder {
  void setupBinder(WebDataBinder binder) {
    binder.registerCustomEditor(Id.class, new IdPropertyEditor());

  private class IdPropertyEditor extends PropertyEditorSupport {
    public String getAsText() {
      return getValue() != null
          ? getValue().toString()
          : "";

    public void setAsText(String text) throws IllegalArgumentException {
      setValue(new Id(UUID.fromString(text)));

Luckily at some point, it clicked that @ControllerAdvice is basically @Component. Turns out all I had to do was to change from @Component to @ControllerAdvice and that’s all. No more property editors and no more WebDatabinders :)

First I created the solution with ProeprtyEditor, then after finding out a bit hacky @ControlerAdvice I’ve discovered yet another solution to the problem. It is the simplest one from the developer perspective but requires a lot of "magic" from the spring. All you have to do to have automatic conversion of raw string to your class in a @Controller method is to create public (that’s important) constructor which accepts single String as parameter. If you are interested in how this works you should take a look at ObjectToObjectConverter.

It is also worth pointing out that spring does support a lot of types out of the box. Here is the list of some of the interesting converters registered by default in spring:

And a lot of converters for jsr310 (new date time API) not to mention joda date time converts if you have joda on the classpath

You can find examples usage converters for some of those converts on my GitHub.

When I started writing this post I thought I will end it with hacky @ControllerAdvice. Turns out there is so much more to automagic types conversion available in spring. I’m sure that during my career I’ve written at least one or two converters for the types that are now handled by the spring out of the box. To feel better I keep telling myself that a lot of those converts must’ve been added recently, but I’m not sure if I want to check when they’ve been created ;)

Samples can be found on my GitHub.

Tested with spring-boot 1.5.7

See Also

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10 Nov 2017 #java #spring