Deserializing into the general Object tree

[bdnettleton]

I just wrote a gson deserializer class for deserializing json into a
general object-tree (I think someone else was looking for this on the
list). It takes json and turns it into a String, Integer, Boolean,
Object[], Map<String, Object> from the corresponding JsonElement
objects.

-Brian

Here is the source (I don't see a way to attach the file directly, so
you'll have to cut and paste).

/*
* GeneralObjectDeserializer.java
* Created on Sep 24, 2008
*
* Copyright 2008 Wind River Systems.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.windriver.gson.extension;

import java.io.Reader;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
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;
import com.google.gson.JsonPrimitive;

/**
* Use this class to create a general object-tree using the Gson
parser
* (see http://code.google.com/p/google-gson/). This class
deserializes
* Gson output such that all objects are one of the following types:
*
* <ul>
* <li>String
* <li>Boolean
* <li>Number (may be Integer, Double, BigInteger, or BigDecimal per
gson)
* <li>Object[] (where each element is also one of these 5 types)
* <li>Map<String, Object> (where each value is one of these 5 types)
* </ul>
*
* This is often useful when creating a complete new specific
* deserializer is too much work because just a specific subset
* of the output is desired, or for when it isn't known exactly
* in advance which object maps to any given json input.
*
* @author Brian Nettleton
*
*/
public class GeneralObjectDeserializer implements
JsonDeserializer<Object> {

/**
* <code>generalGson</code> is a Gson instance which provides
* for general deserializing.
*/
public static final Gson generalGson =
new GsonBuilder().registerTypeAdapter(Object.class, new
GeneralObjectDeserializer())
.create();

/**
* This is a convenience routine which deserializes a
* string into one of the five general Java types as
* described above.
*
* @param json the json string to be deserialized
* @return a general Java object
* @throws JsonParseException if json is not a valid representation
for an object of type
*
* @see com.google.gson.Gson#fromJson(String, Class)
*/
public static Object fromJson(String json) throws JsonParseException
{
return generalGson.fromJson(json, Object.class);
}

/**
* This is a convenience routine deserializes the Json read from the
* specified reader into one of the five general Java types as
* described above.
*
* @param json the reader producing the Json from which the object is
to be deserialized.
* @return a general Java object
* @throws JsonParseException if json is not a valid representation
for an object of type
*
* @see com.google.gson.Gson#fromJson(Reader , Class)
*/
public static Object fromJson(Reader json) throws JsonParseException
{
return generalGson.fromJson(json, Object.class);
}

/* (non-Javadoc)
* @see
com.google.gson.JsonDeserializer#deserialize(com.google.gson.JsonElement,
java.lang.reflect.Type, com.google.gson.JsonDeserializationContext)
*/
public Object deserialize(JsonElement json, Type typeOfT,
JsonDeserializationContext context) throws JsonParseException {
if( json.isJsonNull() ) {
return null;
}
else if ( json.isJsonPrimitive() ) {
JsonPrimitive primitive = json.getAsJsonPrimitive();
if ( primitive.isString() ) {
return primitive.getAsString();
}
else if ( primitive.isNumber() ) {
return primitive.getAsNumber();
}
else if ( primitive.isBoolean() ) {
return primitive.getAsBoolean();
}
}
else if ( json.isJsonArray() ) {
JsonArray array = json.getAsJsonArray();
Object[] result = new Object[array.size()];
int i = 0;
for( JsonElement element : array ) {
result[i] = deserialize(element, null, context);
++i;
}
return result;
}
else if ( json.isJsonObject() ) {
JsonObject object = json.getAsJsonObject();
Map<String, Object> result = new HashMap<String,Object>();
for( Map.Entry<String, JsonElement> entry : object.entrySet() ) {
Object value = deserialize(entry.getValue(), null, context);
result.put(entry.getKey(), value);
}
return result;
}
else {
throw new JsonParseException("Unknown JSON type for JsonElement " +
json.toString());
}
return null;
}

}

[inde...@gmail.com]

Hi Brian,

Thanks for this class, it certainly is useful for many users.

We have had several discussions on providing this functionality in
Gson itself, (and we actually supported it at one point in time, but
then removed it). Your mail will certainly open up that discussion
again.

Will you be interested in posting this on Gson wiki page at:
http://sites.google.com/site/gson/

We can create a section there on User-contributed content, and provide
links to it from the user guide.

Thanks
Inder

On Sep 24, 3:09 pm, bdnettleton <brian.nettle...@windriver.com> wrote:
> I just wrote a gson deserializer class for deserializing json into a
> general object-tree (I think someone else was looking for this on the
> list).  It takes json and turns it into a String, Integer, Boolean,
> Object[], Map<String, Object> from the corresponding JsonElement
> objects.
>
> -Brian
>
> Here is the source (I don't see a way to attach the file directly, so
> you'll have to cut and paste).
>
> /*
>  * GeneralObjectDeserializer.java
>  * Created on Sep 24, 2008
>  *
>  * Copyright 2008 Wind River Systems.
>  *
>  * Licensed under the Apache License, Version 2.0 (the "License");
>  * you may not use this file except in compliance with the License.
>  * You may obtain a copy of the License at
>  *

>  *http://www.apache.org/licenses/LICENSE-2.0

>  * (seehttp://code.google.com/p/google-gson/).  This class