Select Tag with Object - Thymeleaf and Spring MVC

Question

I\'m trying to change the code of this example thymeleafexamples-stsm, so I changed enum type for class type:

Type.java

public class Type { 

    p         


        

Answer 1

I know this question is old but below answer may help someone as I could not find it easily.

To solve this issue, Thymeleaf uses Formatters to convert between Object and String.

• In display phase (GET), Formatter Service will convert Object to String.

• In submitting phase (POST), Formatter Service will convert String back to

  object.

First implement Formatter service for your Class to be used in tag:

@Service
public class TypeFormatter implements Formatter<Type> {

    @Autowired
    TypeService typeService;//Service -> DB

    @Override
    public String print(Type object, Locale locale) {
        return (object != null ? object.getId().toString() : "");
    }

    @Override
    public Type parse(String text, Locale locale) throws ParseException {
        Integer id = Integer.valueOf(text);
        return this.typeService.get(id);//return Type object form DB
    }
}

It is very simple class with two methods:

• print: converts object to string.
• parse: converts string to object.

Now, we've to tell Spring-Thymeleaf abbout our formatter or we may call it converter. To do that we've to register this formatter in our WebConfig (Configuration class whic extends WebMvcConfigurerAdapter):

@Configuration
@EnableWebMvc
@ComponentScan(value = { "your package" })
public class WebConfig extends WebMvcConfigurerAdapter {

....
    //Formatters

    @Autowired //Without autowire, this solution may not work
    private TypeFormatter typeFormatter;

    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addFormatter(typeFormatter);
    }
}

Now our solution is ready to be implemented in html file but how to tell Thymeleaf to apply conversion? Answer is by using th:field="*{type}" attribute and using Double-bracket syntax th:value="${{type}}":

<select th:field="*{type}">
    <option th:value="NULL" th:text="---Select Type---"></option>
    <option th:each="type : ${allTypes}" th:value="${{type}}" th:text="${type.type}">Wireframe</option>
</select>

• th:field="*{type}" is applying registered Formatter service by default. It will convert type to String (here the string will be Type Id)
• th:value="${{type}}" is converting type to string as well.
• In submit, Spring will use the Formatter service to convert Id back to object.

Last thing to tell, sometimes we want to add a header to the dropdown list like "-----Select Type-----" in order to prevent default selection as well as explain to user. In this case you must set th:value="NULL" unless you'll get conversion error.

Answer 2

That error message basically says Spring don't know how to convert the string thymeleafexamples.stsm.business.entities.Type@2c08cec0 into an instance of Type. This is a bug on your code because it doesn't make any sense trying to do so.

You're not supposed to use the toString() value of Object as a form dropdown identifier. You need to have a (much) better strategy for the code to identify the Type selected by user.

Common approach is to use the id attribute:

<option th:each="type : ${allTypes}" th:value="${type.id}" th:text="${type.type}">Wireframe</option>

When the form is submitted, you then need to retireve an instance of Type based on its id name on your controller