CODEWITHSUNDEEP
javajacksongsondeserialization
sjngm
sjngm

Reputation: 12861

Jackson/GSON: Apply Map to POJO

Let's say I have a POJO with quite a few fields. I also have a map with a bunch of properties that would map nicely to fields in the POJO. Now I want to apply the properties in the map to my POJO. How can I do this?

Jackson provides method new ObjectMapper().convertValue(), but that creates a fresh instance of the POJO. Do I really have to do something like this?

om = new ObjectMapper();
pojoMap = om.convertValue(pojo, Map.class);
pojoMap.putAll(properties);
pojo = om.convertValue(pojoMap, Pojo.class);

Isn't there an easier way?

As I have no experience with GSON and we also have it lying around here, how would I do that with GSON?

Upvotes: 1

Views: 1236

Answers (1)

araqnid
araqnid

Reputation: 133552

Yes, you can create an ObjectReader that will update an existing instance from the root JSON object rather than instantiating a new one, using the readerForUpdating method of ObjectMapper:

@Test
public void apply_json_to_existing_object() throws Exception {
    ExampleRecord record = new ExampleRecord();
    ObjectReader reader = mapper.readerForUpdating(record)
            .with(JsonParser.Feature.ALLOW_SINGLE_QUOTES)
            .with(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES);
    reader.readValue("{ firstProperty: 'foo' }");
    reader.readValue("{ secondProperty: 'bar' }");
    assertThat(record.firstProperty, equalTo("foo"));
    assertThat(record.secondProperty, equalTo("bar"));
}

public static class ExampleRecord {
    public String firstProperty;
    public String secondProperty;
}

You can also create a value-updating reader from an existing ObjectReader. The following declaration seems equivalent:

    ObjectReader reader = mapper.reader(ExampleRecord.class)
            .withValueToUpdate(record)
            .with(/* features etc */);

Addition

The above didn't actually answer your question, though.

Since you don't have the changes you want to make to the record as JSON, but rather as a map, you have to finagle things so that Jackson will read your Map. Which you can't do directly, but you can write the "JSON" out to a token buffer and then read it back:

@Test
public void apply_map_to_existing_object_via_json() throws Exception {
    ExampleRecord record = new ExampleRecord();
    Map<String, Object> properties = ImmutableMap.of("firstProperty", "foo", "secondProperty", "bar");

    TokenBuffer buffer = new TokenBuffer(mapper, false);
    mapper.writeValue(buffer, properties);
    mapper.readerForUpdating(record).readValue(buffer.asParser());

    assertThat(record.firstProperty, equalTo("foo"));
    assertThat(record.secondProperty, equalTo("bar"));
}

(btw if this seems laborious, serializing to a token buffer and deserializing again is in fact how ObjectMapper.convertValue is implemented, so it's not a big change in functionality)

Upvotes: 4

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