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POContextualDeserializer for mapping JSON to different types of maps with Jackson
 

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    <p>This JSON snippet should be mapped to a Java-Objects that contains a <code>cars</code> field of type <code>Map&lt;String, Car&gt;</code> and a <code>bikes</code> field of type <code>Map&lt;String, Bike&gt;</code>. Because bikes and cars can be empty strings in the JSON file, i need a custom deserializer (<a href="https://stackoverflow.com/questions/6684358/map-a-json-field-that-can-have-different-types-with-jackson">See this question</a>).</p> <pre><code>{ "id" : "1234", "name" : "John Doe", "cars" : { "Tesla Model S" : { "color" : "silver", "buying_date" : "2012-06-01" }, "Toyota Yaris" : { "color" : "blue", "buying_date" : "2005-01-01" } }, "bikes" : { "Bike 1" : { "color" : "black" }, "Bike 2" : { "color" : "red" } } } </code></pre> <p>I thought about having instances of a generic custom deserializer that can be returned by the <code>createContextual(DeserializationConfig cfg, BeanProperty property)</code> method of a <code>ContextualDeserializer</code> based on the BeanProperty parameter. The generic custom deserializer looks like this:</p> <pre><code>public class MapsGenericDeserializer&lt;T&gt; extends JsonDeserializer&lt;Map&lt;String, T&gt;&gt; { private ObjectMapper mapper; // ObjectMapper without special map deserializer public MapsGenericDeserializer(ObjectMapper mapper) { this.mapper = mapper; } @Override public Map&lt;String, T&gt; deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException { ObjectCodec codec = jp.getCodec(); JsonNode node = codec.readTree(jp); if (!"".equals(node.getTextValue())) { return mapper.readValue(node, new TypeReference&lt;Map&lt;String, T&gt;&gt;() {}); } return null; // Node was an empty string } } </code></pre> <p>The contextual serializer below does not work because casting from <code>MapsGenericDeserializer&lt;Car&gt;</code> to <code>JsonDeserializer&lt;Map&lt;String,?&gt;&gt;</code> is not possible. Maybe this is possible in newer Versions of Java, but it does not work on the Version of Android I am coding for. So how can I implement the desired behaviour?</p> <pre><code>public class MapsDeserializer extends JsonDeserializer&lt;Map&lt;String, ?&gt;&gt; implements ContextualDeserializer&lt;Map&lt;String, ?&gt;&gt; { private ObjectMapper mapper; MapsGenericDeserializer&lt;Car&gt; carDeserializer = new MapsGenericDeserializer&lt;Car&gt;(mapper); MapsGenericDeserializer&lt;Bike&gt; bikeDeserializer = new MapsGenericDeserializer&lt;Bike&gt;(mapper); public MapsDeserializer(ObjectMapper mapper) { this.mapper = mapper; } @Override public JsonDeserializer&lt;Map&lt;String, ?&gt;&gt; createContextual(DeserializationConfig cfg, BeanProperty property) throws JsonMappingException { Class&lt;?&gt; targetClass = property.getType().containedType(1).getRawClass(); if(targetClass.equals(Car.class) { return carDeserializer; // Type mismatch! } else if (targetClass.equals(Bike.class)) { return bikeDeserializer; // Type mismatch! } else { return this; } } // ... } </code></pre>
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