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    <p>Gson currently has a mechanism to <a href="http://groups.google.com/group/google-gson/browse_thread/thread/6272c9be58676e47">register a Type Hierarchy Adapter</a> that reportedly can be configured for simple polymorphic deserialization, but I don't see how that's the case, as a Type Hierarchy Adapter appears to just be a combined serializer/deserializer/instance creator, leaving the details of instance creation up to the coder, without providing any actual polymorphic type registration.</p> <p>It looks like Gson will soon have the <code>RuntimeTypeAdapter</code> for simpler polymorphic deserialization. See <a href="http://code.google.com/p/google-gson/issues/detail?id=231">http://code.google.com/p/google-gson/issues/detail?id=231</a> for more info.</p> <p>If use of the new <code>RuntimeTypeAdapter</code> isn't possible, and you gotta use Gson, then I think you'll have to roll your own solution, registering a custom deserializer either as a Type Hierarchy Adapter or as Type Adapter. Following is one such example.</p> <pre><code>// output: // Starting machine1 // Stopping machine2 import java.lang.reflect.Type; import java.util.HashMap; import java.util.Map; import com.google.gson.FieldNamingPolicy; import com.google.gson.Gson; import com.google.gson.GsonBuilder; import com.google.gson.JsonDeserializationContext; import com.google.gson.JsonDeserializer; import com.google.gson.JsonElement; import com.google.gson.JsonObject; import com.google.gson.JsonParseException; public class Foo { // [{"machine_name":"machine1","command":"start"},{"machine_name":"machine2","command":"stop"}] static String jsonInput = "[{\"machine_name\":\"machine1\",\"command\":\"start\"},{\"machine_name\":\"machine2\",\"command\":\"stop\"}]"; public static void main(String[] args) { GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES); CommandDeserializer deserializer = new CommandDeserializer("command"); deserializer.registerCommand("start", Start.class); deserializer.registerCommand("stop", Stop.class); gsonBuilder.registerTypeAdapter(Command.class, deserializer); Gson gson = gsonBuilder.create(); Command[] commands = gson.fromJson(jsonInput, Command[].class); for (Command command : commands) { command.execute(); } } } class CommandDeserializer implements JsonDeserializer&lt;Command&gt; { String commandElementName; Gson gson; Map&lt;String, Class&lt;? extends Command&gt;&gt; commandRegistry; CommandDeserializer(String commandElementName) { this.commandElementName = commandElementName; GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES); gson = gsonBuilder.create(); commandRegistry = new HashMap&lt;String, Class&lt;? extends Command&gt;&gt;(); } void registerCommand(String command, Class&lt;? extends Command&gt; commandInstanceClass) { commandRegistry.put(command, commandInstanceClass); } @Override public Command deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException { try { JsonObject commandObject = json.getAsJsonObject(); JsonElement commandTypeElement = commandObject.get(commandElementName); Class&lt;? extends Command&gt; commandInstanceClass = commandRegistry.get(commandTypeElement.getAsString()); Command command = gson.fromJson(json, commandInstanceClass); return command; } catch (Exception e) { throw new RuntimeException(e); } } } abstract class Command { String machineName; Command(String machineName) { this.machineName = machineName; } abstract void execute(); } class Stop extends Command { Stop(String machineName) { super(machineName); } void execute() { System.out.println("Stopping " + machineName); } } class Start extends Command { Start(String machineName) { super(machineName); } void execute() { System.out.println("Starting " + machineName); } } </code></pre>
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    1. COIf you can change APIs, then note that Jackson currently has a mechanism for relatively simple polymorphic deserialization. I posted some examples at http://programmerbruce.blogspot.com/2011/05/deserialize-json-with-jackson-into.html
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    2. CO[`RuntimeTypeAdapter`](http://code.google.com/p/google-gson/source/browse/trunk/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java) is now complete, unfortunately it doesn't look like it's in Gson core yet. :-(
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