Can not construct Map key of type java.lang.Class from String: unable to parse key as Class

Question

I am having problems deserializing an object containing map with Class as key:

public class TestSerializeMap {

    public static class TestClass {
        private Map<Class<? extends Object>, String> map = new HashMap<>();

        public TestClass() {
        }

        public Map<Class<? extends Object>, String> getMap() {
            return map;
        }
    }

    @Test
    public void testPropertyMapWithClassAsKey() throws Exception {
        TestClass testClass = new TestClass();
        testClass.getMap().put(ArrayList.class, "ArrayList");
        testClass.getMap().put(HashMap.class, "HashMap");

        ObjectMapper mapper = new ObjectMapper();

        String json = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(testClass);
        System.out.println(json);
        mapper.readValue(json, TestClass.class);
    }
}

Throws this exception:

com.fasterxml.jackson.databind.exc.InvalidFormatException: Can not construct     Map key of type java.lang.Class from String "class java.util.ArrayList": not a valid representation: Can not construct Map key of type java.lang.Class from String "class java.util.ArrayList": unable to parse key as Class
 at [Source: {
  "map" : {
    "class java.util.ArrayList" : "ArrayList",
    "class java.util.HashMap" : "HashMap"
  }
}; line: 3, column: 5]
 at [Source: {
  "map" : {
    "class java.util.ArrayList" : "ArrayList",
    "class java.util.HashMap" : "HashMap"
  }
}; line: 3, column: 5] (through reference chain: org.test.TestClass["map"])
    at com.fasterxml.jackson.databind.exc.InvalidFormatException.from(InvalidFormatException.java:55)
    at com.fasterxml.jackson.databind.DeserializationContext.weirdKeyException(DeserializationContext.java:913)
    at com.fasterxml.jackson.databind.deser.std.StdKeyDeserializer.deserializeKey(StdKeyDeserializer.java:131)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer._readAndBind(MapDeserializer.java:404)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:333)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:25)
    ...

When I am trying to serialize the map directly:

@Test
public void testMapWithClassAsKey() throws Exception {
    Map<Class<? extends Object>, String> map = new HashMap<>();
    map.put(ArrayList.class, "ArrayList");
    map.put(HashMap.class, "HashMap");

    ObjectMapper mapper = new ObjectMapper();

    String json = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(map);
    System.out.println(json);
    mapper.readValue(json, Map.class);
}

Then it works correctly. What am I missing in my first test?

EDIT:

Found out that the second test was incorrect. It should be:

@Test
public void testMapWithClassAsKey() throws Exception {
    Map<Class<? extends Object>, String> map = new HashMap<>();
    map.put(ArrayList.class, "ArrayList");
    map.put(HashMap.class, "HashMap");

    ObjectMapper mapper = new ObjectMapper();

    String json = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(map);
    System.out.println(json);
    mapper.readValue(json, new TypeReference<Map<Class<? extends Object>, String>>(){});
}

Now it fails with same exception as the first one.

Answer 1

I submitted a finding on github . Tatu from FasterXML looked into it and fixed the issue. Problem was serialization of Class<?> keys in map.

It should be fixed in jackson-databind version 2.5.1