Unravel › Java concepts › I/O and files

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At a glance

1. 1gson.toJson(obj) writes JSON; gson.fromJson(json, Order.class) reads itFor generics use new TypeToken<List<Order>>(){}.getType() because the type is erased at run time.
2. 2@SerializedName("order_id") maps a different JSON name onto a field; @Expose with excludeFieldsWithoutExposeAnnotation() whitelists fields; transient fields are skipped
3. 3Tree model: JsonParser.parseString(text).getAsJsonObject().get("id").getAsInt() reads without a class; JsonObject and JsonArray can also build JSON by hand
4. 4org.json offers new JSONObject(text).getString("name"), which throws on missing keys, while optString returns a default

• Gson uses fields directly through reflection and skips constructors (it uses Unsafe if there is no no-arg constructor), so initialisers and invariants may not run
• Numbers parsed into Object or Map become Double, so a 64-bit id loses precision
• Gson builds a registry of TypeAdapters per type (reflective adapters for classes, built-ins for collections and dates) and caches them in the Gson instance, which is why creating it per call is wasteful

import com.google.gson.*;import com.google.gson.reflect.TypeToken;import java.util.List;public class Json {  private static final Gson GSON = new GsonBuilder().setPrettyPrinting().create();  static class Order { @com.google.gson.annotations.SerializedName("order_id") long id; String customer; }  static String write(Order o) { return GSON.toJson(o); }  static List<Order> read(String json) { return GSON.fromJson(json, new TypeToken<List<Order>>() {}.getType()); }  static int idOf(String json) { return JsonParser.parseString(json).getAsJsonObject().get("id").getAsInt(); }}

1. line 2 uses TypeToken, the core of Gson and other JSON APIs
2. line 5 uses Gson, the core of Gson and other JSON APIs
3. line 6 uses SerializedName, the core of Gson and other JSON APIs
4. line 7 uses toJson, the core of Gson and other JSON APIs
5. line 8 uses fromJson, the core of Gson and other JSON APIs
6. line 9 uses JsonParser, the core of Gson and other JSON APIs

How Unravel recognises it

When you paste code, Unravel looks for these signs of gson and other json apis. Spotting them yourself is a good habit when you read unfamiliar code:

• Annotations: @SerializedName @Expose
• Types: Gson GsonBuilder JsonParser JsonObject JsonArray JsonElement TypeToken JSONObject JSONArray JsonReader JsonWriter Jsonb JsonbBuilder JSONTokener
• Method calls: toJson() fromJson()

Part of a learning path

• I/O, networking and time · previous: XML parsing: DOM, SAX, StAX · next: Properties files, Locale and resource bundles

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