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.

There is an an open issue in the Jackson project to allow support for @JsonUnwrapped on maps. It is currently tagged as targeted for the 2.13 release, so it looks to be in the near-term feature roadmap.

Until this feature is supported, the workaround about using @JsonAnySetter/@JsonAnyGetter proposed in another answer appears to be the way to go, and is in fact a suggested approach by the Jackson project author in the open issue:

as per this:

http://www.cowtowncoder.com/blog/archives/2011/07/entry_458.html

one can use @JsonAnyGetter/setter to do something possibly similar. One missing pieces is that currently one must have getter (can't use it on Map filed), but that should be easy enough to address.

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

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