使用Jackson將JSON解析為對象

Question

我目前正在使用Jackson來解析JSON中的對象。 當前，當我在對象上調用toString（）時，可以將該對象轉換為JSON。 但是將JSON轉換回對象變得困難。 為了測試這項工作，我正在運行以下行：

UserAppData test = UserAppData.appDataFromJSON(dummyUserAppData.toString());

這應該返回以（dummyUserAppData）開頭的同一對象，但是，運行此命令時會收到以下錯誤：

java.lang.RuntimeException: Unable to start activity ComponentInfo{appuccino.droidpacks/com.appuccino.droidpacks.activities.MainActivity}: java.lang.RuntimeException: bad input {
"boughtAppIDs" : [ 1, 2, 3, 4, 5 ],
"boughtAppPacks" : [ {
"gold" : true,
"id" : 1
}, {
"gold" : false,
"id" : 2
}, {
"gold" : true,
"id" : 6
} ]
}
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
        at android.app.ActivityThread.access$800(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5001)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.RuntimeException: bad input {
"boughtAppIDs" : [ 1, 2, 3, 4, 5 ],
"boughtAppPacks" : [ {
"gold" : true,
"id" : 1
}, {
"gold" : false,
"id" : 2
}, {
"gold" : true,
"id" : 6
} ]
}
        at com.appuccino.droidpacks.objects.UserAppData.appDataFromJSON(UserAppData.java:48)
        at com.appuccino.droidpacks.activities.MainActivity.onCreate(MainActivity.java:108)
        at android.app.Activity.performCreate(Activity.java:5231)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)
        ... 11 more

我正在使用的類如下，其中dummyAppUserData屬於UserAppData類：

public class UserAppData {

public int[] boughtAppIDs;
public UserPackData[] boughtAppPacks;

public UserAppData(int[] list, UserPackData[] packData){
    boughtAppIDs = list;
    boughtAppPacks = packData;
}

public static UserAppData appDataFromJSON(String json){
    if (json == null || json.isEmpty()) {
        return null;
    }
    ObjectMapper mapper = new ObjectMapper();
    ObjectWriter writer = mapper.writerWithDefaultPrettyPrinter();
    UserAppData myObject;
    try {
        myObject = mapper.readValue(json, UserAppData.class);
        return myObject;
    }
    catch (  JsonParseException e) {
        e.printStackTrace();
        return null;
    }
    catch (  Exception e) {
        e.printStackTrace();
        throw new RuntimeException("bad input " + json);
    }
}

@Override
public String toString(){
    //ObjectWriter ow = new ObjectMapper().writer();//.withDefaultPrettyPrinter();
    ObjectMapper mapper = new ObjectMapper();
    try {
        return mapper.writerWithDefaultPrettyPrinter().writeValueAsString(this);
        //return ow.writeValueAsString(this);
    } catch (JsonProcessingException e) {
        e.printStackTrace();
        return null;
    }
}

}

這是UserAppData具有以下實例的小類UserPackData：

public class UserPackData {
public int id;
public boolean gold;

public UserPackData(int i, boolean g){
    id = i;
    gold = g;
}

}

我做錯了什么事？

Answer 1

您尚未為Jackson正確定義構造函數以使用它們。 您需要通過使用@JsonProperty注釋其構造函數來將每個構造函數標識為要使用的構造@JsonProperty 。

public UserAppData(@JsonProperty("boughtAppIDs") int[] boughtAppIDs, @JsonProperty("boughtAppPacks") UserPackData[] boughtAppPacks) {
    this.boughtAppIDs = boughtAppIDs;
    this.boughtAppPacks = boughtAppPacks;
}
...
public UserPackData(@JsonProperty("id") int id, @JsonProperty("gold") boolean gold) {
    this.id = id;
    this.gold = gold;
}

或者，提供無參數的構造函數和getter / setter（帶有適當的@JsonProperty批注）。