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PODeserialize JSON object/map as generic Collection with Jackson
 

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    <p>I have JSON maps like this:</p> <pre><code>"things": {"foo": {"name": "foo", ...}, "bar": {"name": "bar", ...}} </code></pre> <p>I want to deserialize them as if they were arrays:</p> <pre><code>"things": [{"name": "foo", ...}, {"name": "bar", ...}] </code></pre> <p>(to match XML/JAXB deserialization behavior):</p> <pre><code>&lt;things&gt;&lt;thing name="foo"&gt;...&lt;/thing&gt;&lt;thing name="bar"&gt;...&lt;/thing&gt;&lt;/things&gt; </code></pre> <p>into a collection such as this:</p> <pre><code>@XmlElementWrapper @XmlElement(name = "thing") @JsonDeserialize(using = MapToCollectionDeserializer.class) Collection&lt;Thing&gt; things; </code></pre> <p>Note that I have collections with various element types -- not just <code>Thing</code> -- so I need a generic mechanism.</p> <p>However, when writing a custom deserializer, what's the right way to access the type information of the context?</p> <pre><code>public class MapToCollectionDeserializer extends StdDeserializer&lt;Object&gt; { @Override public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException { Preconditions.checkState(jp.getCurrentToken() == JsonToken.START_OBJECT); final LinkedList&lt;Object&gt; result = new LinkedList&lt;&gt;(); JsonToken tok; while ((tok = jp.nextToken()) != JsonToken.END_OBJECT) { Preconditions.checkState(tok == JsonToken.FIELD_NAME); // How to get the collection element type for deserialization? result.add(...); } return result; } } </code></pre> <p>My approach so far is using <code>ContextualDeserializer</code>, which can provide a <code>BeanProperty</code> (which contains type information) to the deserializer. However, a JsonDeserializer must still have a no-arg constructor, so I end up constructing a broken object at first:</p> <pre><code>public class MapToCollectionDeserializer extends StdDeserializer&lt;Object&gt; implements ContextualDeserializer&lt;Object&gt; { private final BeanProperty property; public MapToCollectionDeserializer() { super(Collection.class); property = null; // YUCK: BROKEN!!! } private MapToCollectionDeserializer(BeanProperty property) { super(property.getType()); this.property = property; } @Override public JsonDeserializer&lt;Object&gt; createContextual(DeserializationConfig config, BeanProperty property) throws JsonMappingException { return new MapToCollectionDeserializer(property); } @Override public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException { Preconditions.checkState(jp.getCurrentToken() == JsonToken.START_OBJECT); final JavaType elementType = property.getType().containedType(0); final LinkedList&lt;Object&gt; result = new LinkedList&lt;&gt;(); JsonToken tok; while ((tok = jp.nextToken()) != JsonToken.END_OBJECT) { Preconditions.checkState(tok == JsonToken.FIELD_NAME); jp.nextToken(); final JsonDeserializer&lt;Object&gt; valueDeser = ctxt.getDeserializerProvider() .findValueDeserializer(ctxt.getConfig(), elementType, property); result.add(valueDeser.deserialize(jp, ctxt)); } return result; } } </code></pre> <p>Is there a better/simpler way to do this?</p>
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    1. USTrevor Robinson
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    1. COI know this is a few months old at this point, but did you ever find a cleaner approach to this? I find myself needing to do something similar and haven't found anything better.
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    2. COI ended up creating my own serialization framework using [Gson](http://code.google.com/p/google-gson/). Jackson was just too convoluted.
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