Jackson deserialization of YAML file into Map (with no custom deserializer)

Question

I'm trying to load a YAML file into an instance of a Map . Is there a way to load it without defining the custom deserializer (eg using annotations?)?

This is an example of my YAML file:

- site: First Site
  url: some_url
  username: some_name
  password: some_password
- site: Second Site
  url: its_url
  username: its_name
  password: its_password

This is the java bean class to deserialize one "site configuration" into (generated by http://www.jsonschema2pojo.org/ ):

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import org.apache.commons.lang3.builder.ToStringBuilder;

@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonPropertyOrder({
        "site",
        "url",
        "username",
        "password"
})
public class SiteConfiguration {

    @JsonProperty("site")
    private String site;
    @JsonProperty("url")
    private String url;
    @JsonProperty("username")
    private String username;
    @JsonProperty("password")
    private String password;

    /**
     * No args constructor for use in serialization
     *
     */
    public SiteConfiguration() {
    }

    /**
     *
     * @param site
     * @param username
     * @param password
     * @param url
     */
    public SiteConfiguration(String site, String url, String username, String password) {
        super();
        this.site = site;
        this.url = url;
        this.username = username;
        this.password = password;
    }

    @JsonProperty("site")
    public String getSite() {
        return site;
    }

    @JsonProperty("site")
    public void setSite(String site) {
        this.site = site;
    }

    @JsonProperty("url")
    public String getUrl() {
        return url;
    }

    @JsonProperty("url")
    public void setUrl(String url) {
        this.url = url;
    }

    @JsonProperty("username")
    public String getUsername() {
        return username;
    }

    @JsonProperty("username")
    public void setUsername(String username) {
        this.username = username;
    }

    @JsonProperty("password")
    public String getPassword() {
        return password;
    }

    @JsonProperty("password")
    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return new ToStringBuilder(this).append("site", site).append("url", url).append("username", username).append("password", password).toString();
    }

}

And finally, this is the code that deserializes the YAML above into Map .

    private static Map<String,SiteConfiguration> sites;
    public static SiteConfiguration getSiteConfiguration(String key) {
        if (sites == null) {
            ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
            try {
//          todo this finishes on: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.LinkedHashMap` out of START_ARRAY token
//                sites = mapper.readValue(YamlReader.class.getClass().getResource("/site-configuration.yaml"), new TypeReference<Map<String,SiteConfiguration>>() {});
                SiteConfiguration[] sitesArray = mapper.readValue(YamlReader.class.getClass().getResource("/site-configuration.yaml"), SiteConfiguration[].class);
                sites = new HashMap<>();
                for (SiteConfiguration site : sitesArray) {  //todo is there Jackson built-in deserialization?
                    sites.put(site.getSite(), site);
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
        return sites.get(key);
    }

As you can see, I'm doing it in two steps. First I deserialize the file into an array of SiteConfiguration instances, then I put those into a Map where site field represents the map key.

Is there a way to do the same while omitting the array of SiteConfiguration instances? Is the only way to do it using a custom deserializer?

Answer 1

Change the structure of your YAML. If you want to deserialize your data as a map, it needs to start as a map in the YAML file. You have a sequence of mappings in your example, but you want a mapping of mappings.

https://yaml.org/spec/1.2/spec.html#id2759963

Example 2.4. Sequence of Mappings (players' statistics)

 - name: Mark McGwire hr: 65 avg: 0.278 - name: Sammy Sosa hr: 63 avg: 0.288 

Example 10.1. !!map Examples

 Block style: !!map Clark : Evans Ingy : döt Net Oren : Ben-Kiki 

Try this instead:

First Site:
  site: First Site
  url: some_url
  username: some_name
  password: some_password
Second Site:
  site: Second Site
  url: its_url
  username: its_name
  password: its_password

This structure works with Jackson 2.9.9 with no custom deserializers.