class Inner {
        @JsonValue
        String string;
        Inner(String string) {
            this.string = string;
        }
        public String toString() {
            return "Inner(String="+this.string+")";
        }
    }
    class Outer {
        @JsonValue
        Inner inner;
        Outer(Inner inner) {
            this.inner = inner;
        }
    }
    public void test() throws Exception {
        Outer outer = new Outer(new Inner("key"));
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(outer)); // outputs "key", as expected
        System.out.println(mapper.writeValueAsString(Collections.singletonMap(outer,"value"))); // outputs {"Inner(String=key)":"value"}, expected {"key":"value"}
    }