使用Gson从JSON文件中检索嵌套对象

Question

I'm trying to create a cache for a game using JSON files. Writing and retrievng this data would be done by using GSON. I have a JSON file containing multiple objects, within these objects other objects are stored as well.

My question is: how do I retrieve this data the best way?

Object 1:

public class GameItem {

    private final int id;
    private final String name;
    private final String description;
    private final int shopValue;
    private final ItemStats stats;

    public GameItem(int id, String name, String description, int shopValue, ItemStats stats) {
        this.id = id;
        this.name = name;
        this.description = description;
        this.shopValue = shopValue;
        this.stats = stats;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    public int getShopValue() {
        return shopValue;
    }

    public ItemStats getStats() {
        return stats;
    }

}

Object 2:

public class ItemStats {

    private final int stabAttack;
    private final int slashAttack;
    private final int crushAttack;
    private final int magicAttack;
    private final int rangeAttack;
    private final int stabDefence;
    private final int slashDefence;
    private final int crushDefence;
    private final int magicDefence;
    private final int rangeDefence;
    private final int strengthBonus;
    private final int prayerBonus;

    public ItemStats(int stabAttack, int slashAttack, int crushAttack, int magicAttack, int rangeAttack, int stabDefence, int slashDefence, int crushDefence, int magicDefence, int rangeDefence, int strengthBonus, int prayerBonus) {
        this.stabAttack = stabAttack;
        this.slashAttack = slashAttack;
        this.crushAttack = crushAttack;
        this.magicAttack = magicAttack;
        this.rangeAttack = rangeAttack;
        this.stabDefence = stabDefence;
        this.slashDefence = slashDefence;
        this.crushDefence = crushDefence;
        this.magicDefence = magicDefence;
        this.rangeDefence = rangeDefence;
        this.strengthBonus = strengthBonus;
        this.prayerBonus = prayerBonus;
    }

    public int getStabAttack() {
        return stabAttack;
    }

    public int getSlashAttack() {
        return slashAttack;
    }

    public int getCrushAttack() {
        return crushAttack;
    }

    public int getMagicAttack() {
        return magicAttack;
    }

    public int getRangeAttack() {
        return rangeAttack;
    }

    public int getStabDefence() {
        return stabDefence;
    }

    public int getSlashDefence() {
        return slashDefence;
    }

    public int getCrushDefence() {
        return crushDefence;
    }

    public int getMagicDefence() {
        return magicDefence;
    }

    public int getRangeDefence() {
        return rangeDefence;
    }

    public int getStrengthBonus() {
        return strengthBonus;
    }

    public int getPrayerBonus() {
        return prayerBonus;
    }
}

JSON data:

[
    {
        "id": 0,
        "name": "Dwarf_remains",
        "description": "The_body_of_a_Dwarf_savaged_by_Goblins.",
        "shopValue": 1,
        "stats": [
            "stabAttack": 0,
            "slashAttack": 0,
            "crushAttack": 0,
            "magicAttack": 0,
            "rangeAttack": 0,
            "stabDefence": 0,
            "slashDefence": 0,
            "crushDefence": 0,
            "magicDefence": 0,
            "rangeDefence": 0,
            "strengthBonus": 0,
            "prayerBonus": 0
        ]
    },
    {
        "id": 1,
        "name": "Toolkit",
        "description": "Good_for_repairing_a_broken_cannon.",
        "shopValue": 1,
        "stats": [
            "stabAttack": 0,
            "slashAttack": 0,
            "crushAttack": 0,
            "magicAttack": 0,
            "rangeAttack": 0,
            "stabDefence": 0,
            "slashDefence": 0,
            "crushDefence": 0,
            "magicDefence": 0,
            "rangeDefence": 0,
            "strengthBonus": 0,
            "prayerBonus": 0
        ]
    },
    {
        "id": 2,
        "name": "Cannonball",
        "description": "Ammo_for_the_Dwarf_Cannon.",
        "shopValue": 3000,
        "stats": [
            "stabAttack": 0,
            "slashAttack": 0,
            "crushAttack": 0,
            "magicAttack": 0,
            "rangeAttack": 0,
            "stabDefence": 0,
            "slashDefence": 0,
            "crushDefence": 0,
            "magicDefence": 0,
            "rangeDefence": 0,
            "strengthBonus": 0,
            "prayerBonus": 0
        ]
    }
]

Answer 1

There can be no "the best" way since it may depend on your real case. But in order to deserialize and serialize back this object efficiently, you have to:

• First off, fix your JSON file

Because it's syntactically illegal: the stats property should be declared as an object enclosed with { and } (not as an array with [ and ] ):

"stats": {
    "stabAttack": 0,
    "slashAttack": 0,
    "crushAttack": 0,
    "magicAttack": 0,
    "rangeAttack": 0,
    "stabDefence": 0,
    "slashDefence": 0,
    "crushDefence": 0,
    "magicDefence": 0,
    "rangeDefence": 0,
    "strengthBonus": 0,
    "prayerBonus": 0
}

• Tell Gson the real type of the array elements using a proper TypeToken

This is a special Gson mechanism to specify the target type (various libraries use similar techniques). Specifying List.class as the type of the source won't work and essentially results in List<Map<String, Object>> due to how Java generics work (type parameterization can only be written to classes, fields, or methods, but not directly to instances unless they have enough type information themselves or use custom deserializers that are aware of where such information can be gained from [say, a special synthetic field]).

private static final TypeToken<List<GameItem>> listOfGamesTypeToken = new TypeToken<List<GameItem>>() {
};

Note that type tokens are thread-safe (as well as Gson instances are) and can be easily shared globally (but wisely).

• Apply buffered reading and writing from and to I/O streams

The best way to deserialize using Gson instances is reading from Reader or JsonReader (both work with streams perfectly). The same goes to Writer and JsonWriter .

private static List<GameItem> readGameItemsFromFile(final File file)
        throws IOException {
    try ( final InputStream inputStream = new FileInputStream(file);
          final Reader reader = new BufferedReader(new InputStreamReader(inputStream)) ) {
        // Note that this .fromJson overload:
        // - does not require an intermediate string to be collected and then parsed (a common misuse of Gson parsing facilities)
        // - specifies the target list element type because List.class is just a class, 
        return gson.fromJson(reader, listOfGamesTypeToken.getType());
    }
}

private static void writeGameItemsToFile(final List<GameItem> gameItems, final File file)
        throws IOException {
    try ( final OutputStream outputStream = new FileOutputStream(file);
          final Writer writer = new BufferedWriter(new OutputStreamWriter(outputStream)) ) {
        // Note that this .toJson overload:
        // - allows to "calibrate" the source list type more precisely
        // - does not require an intermediate JSON string collected into memory and then flushed to a file (another common misuse)
        gson.toJson(gameItems, listOfGamesTypeToken.getType(), writer);
    }
}

public static void main(final String... args)
        throws IOException {
    final File file = new File("gameItems.json");
    final List<GameItem> gameItems = readGameItemsFromFile(file);
    // ... process the game items here ...
    writeGameItemsToFile(gameItems, file);
}