Gson ignore null when deserializing object

Question

I want to deserialize a json string that containts a null value in Java. I want to deserialize the object to a Properties object. The json string is something l

Answer 1

The problem is indeed that Gson's default adapter tries to put null into the Properties, which is forbidden.

To solve this, you could write your own TypeAdapter for Properties. You would then have to create Gson instances using a GsonBuilder on which you registered that type adapter.

The following shows how such an adapter could look. It is slightly more strict in that it prevents non-String keys and values during serialization (which Gson's default adapter does not) since they would cause issues during deserialization. You could however replace that and delegate serialization to Gson's adapter by using Gson.getDelegateAdapter​.

private static final TypeAdapter PROPERTIES_ADAPTER = new TypeAdapter() {
    @Override
    public Properties read(JsonReader in) throws IOException {
        in.beginObject();

        Properties properties = new Properties();
        while (in.hasNext()) {
            String name = in.nextName();
            JsonToken peeked = in.peek();

            // Ignore null values
            if (peeked == JsonToken.NULL) {
                in.nextNull();
                continue;
            }
            // Allow Json boolean
            else if (peeked == JsonToken.BOOLEAN) {
                properties.setProperty(name, Boolean.toString(in.nextBoolean()));
            }
            // Expect string or number
            else {
                properties.setProperty(name, in.nextString());
            }
        }

        in.endObject();
        return properties;
    }

    private String asString(Object obj) {
        if (obj.getClass() != String.class) {
            throw new IllegalArgumentException("Properties contains non-String object " + obj);
        }
        return (String) obj;
    }

    /*
     * Could also delegate to Gson's implementation for serialization.
     * However, that would not fail if the Properties contains non-String values,
     * which would then cause issues when deserializing the Json again. 
     */
    @Override
    public void write(JsonWriter out, Properties properties) throws IOException {
        out.beginObject();

        for (Map.Entry entry : properties.entrySet()) {
            // Make sure that key is a String, otherwise properties
            // cannot be deserialized again
            out.name(asString(entry.getKey()));

            Object value = entry.getValue();
            // Be lenient and allow Numbers and Booleans as values
            if (value instanceof Number) {
                out.value((Number) value);
            } else if (value instanceof Boolean) {
                out.value((Boolean) value);
            } else {
                // Require that value is a String
                out.value(asString(value));
            }
        }

        out.endObject();
    }

}.nullSafe(); // Handle null Properties, e.g. `Properties props = null`

public static void main(String[] args) throws IOException {
    Gson gson = new GsonBuilder()
        // Register the custom type adapter
        .registerTypeAdapter(Properties.class, PROPERTIES_ADAPTER)
        .create();

    String json = "{\"prop1\":true, \"prop2\":\"text\", \"prop3\":null}";
    Properties deserialized = gson.fromJson(json, Properties.class); 
    System.out.println("Deserialized: " + deserialized);

    Properties properties = new Properties();
    properties.setProperty("prop", "text");
    // Discouraged to put non-Strings, but type adapter supports these
    properties.put("boolean", true);
    properties.put("number", 1234);
    System.out.println("Serialized: " + gson.toJson(properties));
}