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 wished to doc the method.

(For context, Trello Android primarily parses JSON. We not often serialize JSON, and thus a lot of the focus right here is on deserializing.)

There have been three primary causes for the change from Gson to Moshi: security, velocity, and dangerous life decisions.

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).

Pace – 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:

Unhealthy life decisions – As an alternative of utilizing Gson to parse JSON into easy fashions, we’d write elaborate, complicated, brittle customized deserializers that had totally an excessive amount of logic in them. Refactoring gave us a chance 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 previously for tasks (each at work and at residence) and felt it labored properly. We didn’t do an in-depth examine of options.

Step one was to make sure that we may use function flags to change between utilizing our outdated Gson implementation and the brand new Moshi one.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 wished to check whether or not Moshi parsing labored in its entirety. When you’d somewhat have a mixture of Gson and Moshi on the identical time, that library could be helpful.)

Gson offers you alternatives to override the default naming of a key utilizing @SerializedName. Moshi enables you to do the identical factor with @Json. That is all properly and good, but it surely 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 may confirm that our generated Moshi adapters would have the identical consequence as Gson’s parsing. Specifically, I examined…

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

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

(I can’t embrace the supply right here, however mainly we used Guava’s ClassPath to collect all our courses, then scan by way of them for issues.)

Gson lets you parse generic JSON bushes utilizing JsonElement (and mates). We discovered this handy in some contexts like parsing socket updates (the place we wouldn’t understand how, precisely, to parse the response mannequin till after some preliminary processing).

Clearly, Moshi shouldn’t be going to be joyful about utilizing Gson’s courses, so we switched to utilizing Map (and generally Listing

) for generic bushes of knowledge. Each Gson and Moshi can parse these:

enjoyable  fromJson(map: Map?, clz: Class): 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 shouldn’t be. I discovered that utilizing BufferedSource was various, as it may be transformed to a Reader for outdated Gson code.

The best 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 simple 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 reduce 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 now have a knowledge class Foo(val depend: Int), however the precise JSON we get again is of the shape:

{
  "knowledge": { 
    "depend": 5
  }
}

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

To unravel that downside, 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 depend: Int)

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

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 simple to check.

Bear in mind how I mentioned that Gson will fortunately parse null values into non-null fashions? It seems that we have been (sadly) counting on this conduct in all kinds of locations. Specifically, Trello’s sockets usually 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 (on account of a null worth) when Gson could be joyful as a clam. That is the place function flags actually shine, because you don’t need 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 shopper.