How Trello Android transformed from Gson to Moshi

Trello Android lately transformed from utilizing Gson to Moshi for dealing with JSON. It was a bit tough so I needed to doc the method.

(For context, Trello Android primarily parses JSON. We hardly ever serialize JSON, and thus many of the focus right here is on deserializing.)

There have been three predominant causes for the change from Gson to Moshi: security, pace, and unhealthy life selections.

Security – Gson doesn’t perceive Kotlin’s null security and can fortunately place null values into non-null properties. Additionally, default values solely generally work (relying on the constructor setup).

Velocity – Loads of benchmarks (1, 2, 3) have demonstrated that Moshi is normally quicker than Gson. After we transformed, we arrange some benchmarks to see how real-world parsing in contrast in our app, and we noticed a 2x-3.5x speedup:

Dangerous life selections – As a substitute of utilizing Gson to parse JSON into easy fashions, we might write elaborate, complicated, brittle customized deserializers that had fully an excessive amount of logic in them. Refactoring gave us a possibility to appropriate this architectural snafu.

As for why we picked Moshi over opponents (e.g. Kotlin serialization), we typically belief Sq.’s libraries, we have used Moshi prior to now for initiatives (each at work and at residence) and felt it labored effectively. We didn’t do an in-depth examine of options.

Step one was to make sure that we might use characteristic flags to modify between utilizing our previous Gson implementation and the brand new Moshi one. I wrote a JsonInterop class which, based mostly on the flag, would both parse all JSON responses utilizing Gson or Moshi.

(I opted to keep away from utilizing instruments like moshi-gson-interop as a result of I needed to check whether or not Moshi parsing labored in its entirety. In the event you’d fairly have a mixture of Gson and Moshi on the similar time, that library can be helpful.)

Gson provides you alternatives to override the default naming of a key utilizing @SerializedName. Moshi allows you to do the identical factor with @Json. That is all effectively and good, nevertheless it appeared very easy to me to make a mistake right here, the place a property is parsed underneath totally different names in Gson vs. Moshi.

Thus, I wrote some unit assessments that will confirm that our generated Moshi adapters would have the identical consequence as Gson’s parsing. Specifically, I examined…

• …that Moshi might generate an adapter (not essentially an accurate one!) for every class we needed to deserialize. (If it could not, Moshi would throw an exception.)
• …that every discipline annotated with @SerializedName was additionally annotated with @Json (utilizing the identical key).

Between these two checks, it was straightforward to seek out once I’d made a mistake updating our courses in later steps.

(I can’t embody the supply right here, however principally we used Guava’s ClassPath to assemble all our courses, then scan via them for issues.)

Gson means that you can parse generic JSON timber utilizing JsonElement (and pals). We discovered this convenient in some contexts like parsing socket updates (the place we wouldn’t know the way, precisely, to parse the response mannequin till after some preliminary processing).

Clearly, Moshi isn’t going to be comfortable about utilizing Gson’s courses, so we switched to utilizing Map<String, Any?> (and generally Listing<Map<String, Any?>>) for generic timber of knowledge. Each Gson and Moshi can parse these:

enjoyable <T> fromJson(map: Map<String, Any?>?, clz: Class<T>): T? {
  return if (USE_MOSHI) {
    moshi.adapter(clz).fromJsonValue(map)
  }
  else {
    gson.fromJson(gson.toJsonTree(map), clz)
  }
}

As well as, Gson is pleasant in the direction of parsing through Readers, however Moshi isn’t. I discovered that utilizing BufferedSource was a superb various, as it may be transformed to a Reader for previous Gson code.

The simplest adapters for Moshi are those the place you simply slap @JsonClass on them and name it a day. Sadly, as I discussed earlier, we had a variety of unlucky customized deserialization logic in our Gson parser.

It’s fairly straightforward to write a customized Moshi adapter, however as a result of there was a lot customized logic in our deserializers, simply writing a single adapter wouldn’t minimize it. We ended up having to create interstitial fashions to parse the uncooked JSON, then adapt from that to the fashions we’re used to utilizing.

To present a concrete instance, think about we have now a knowledge class Foo(val rely: Int), however the precise JSON we get again is of the shape:

{
  "knowledge": { 
    "rely": 5
  }
}

With Gson, we might simply manually take a look at the tree and seize the rely out of the knowledge object, however we have now found that approach lies insanity. We might fairly simply parse utilizing easy POJOs, however we nonetheless wish to output a Foo ultimately (so we do not have to alter our entire codebase).

To unravel that drawback, we’d create new fashions and use these in customized adapter, like so:

@JsonClass(generateAdapter = true) knowledge class JsonFoo(val knowledge: JsonData)

@JsonClass(generateAdapter = true) knowledge class JsonData(val rely: Int)

object FooAdapter {
  @FromJson
  enjoyable fromJson(json: JsonFoo): Foo {
    return Foo(rely = json.knowledge.rely)
  }
}

Voila! Now the parser can nonetheless output Foo, however we’re utilizing easy POJOs to mannequin our knowledge. It’s each simpler to interpret and straightforward to check.

Bear in mind how I stated that Gson will fortunately parse null values into non-null fashions? It seems that we had been (sadly) counting on this conduct in all types of locations. Specifically, Trello’s sockets typically return partial fashions – so whereas we’d usually anticipate, say, a card to return again with a reputation, in some instances it received’t.

That meant having to watch our crashes for instances the place the Moshi would blow up (because of a null worth) when Gson can be comfortable as a clam. That is the place characteristic flags actually shine, because you don’t wish to should push a buggy parser on unsuspecting manufacturing customers!

After fixing a dozen of those bugs, I really feel like I’ve gained a hearty appreciation for non-JSON applied sciences with well-defined schemas like protocol buffers. There are a variety of bugs I bumped into that merely wouldn’t have occurred if we had a contract between the server and the consumer.