Why I’m not able to unwrap and serialize a Java map using the Jackson Java library?

@JsonUnwrapped doesn’t work for maps, only for proper POJOs with getters and setters. For maps, You should use @JsonAnyGetter and @JsonAnySetter (available in jackson version >= 1.6).

In your case, try this:

@JsonAnySetter 
public void add(String key, String value) {
    map.put(key, value);
}

@JsonAnyGetter
public Map<String,String> getMap() {
    return map;
}

That way, you can also directly add properties to the map, like add('abc','xyz') will add a new key abc to the map with value xyz.