GSON：自定义反序列化器抛出java.lang.UnsupportedOperationException：JsonNull

Question

我有一些JSON数据，如果某些字段从未有过值，则这些字段可以为null。

我决定创建自己的反序列化器来展平一些嵌套的JSON数据。

然而，我注意到当我的deserialiser遇到一个JSON Null时，它抛出以下内容

java.lang.UnsupportedOperationException: JsonNull

这在我使用默认的GSON反序列化器之前从未发生过

public class ShopDeserializer implements JsonDeserializer<Shop> {
    @Override
    public Shop deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        JsonObject root = json.getAsJsonObject();
        String shopName = root.get("name").getAsString();
        String description = root.get("description").getAsString();
        String openingTime = root.get("opening_time").getAsString();
        String contactEmail = root.get("contact_email").getAsString();
        String telNumber = root.get("tel_number").getAsString();
        String website = root.get("website").getAsString();

基本上JSON中的一个字段值可以为null，但是我真的需要对每个字段进行空值检查吗？

默认值如何忽略它

再说一遍：我真的需要对每个字段进行空检查吗？

MCVE

此链接是一个非常小的Android程序，可以复制问题

https://www.dropbox.com/sh/b8uy179jo4yz2o7/AACbiWgLf0ZClZJpYS7KFFEBa?dl=0

这里是代码的主要部分，带有解释

  String JsonBad = "{\"shops\":{\"url\":\"www.exmaple/20932\",\"name\":\"Some shop 1\",\"description\":\"really nice\",\"opening_time\":\"every day from 9am to 6pm\",\"entry_fee\":null}}";
        String JsonGood = "{\"shops\":{\"url\":\"www.exmaple/2092\",\"name\":\"Some shop 2\",\"description\":\"quite expensive\",\"opening_time\":\"every day from 9am to 6pm\",\"entry_fee\":\"£10\"}}";
        Gson gson = new GsonBuilder().registerTypeAdapter(Shop.class, new ShopDeserializer()).create();
        Shop good = gson.fromJson(JsonGood, Shop.class);
        Shop bad = gson.fromJson(JsonBad, Shop.class);

有两个JSON字符串。 JSON bad有一个名为entry_fee的字段，该字段为null但JSON good的值为£10（非null）

  private static class ShopDeserializer implements JsonDeserializer<Shop> {

        @Override
        public Shop deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            JsonObject root = json.getAsJsonObject().get("shops").getAsJsonObject();
            String urlToShowCall = root.get("url").getAsString();
            String shopName = root.get("name").getAsString();
            String description = root.get("description").getAsString();
            String openingTime = root.get("opening_time").getAsString();
            String entryFee = root.get("entry_fee").getAsString();

            return new Shop(urlToShowCall, shopName, description, openingTime, entryFee);
        }
    }

上面是我的JsonDeserializer。当它到达JSON时它会抛出以下内容

11-03 16:33:20.049 5412-5412/? E/AndroidRuntime: FATAL EXCEPTION: main
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime: Process: com.test.gsonissue, PID: 5412
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.gsonissue/com.test.gsonissue.MainActivity}: java.lang.UnsupportedOperationException: JsonNull
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.-wrap11(ActivityThread.java)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:  Caused by: java.lang.UnsupportedOperationException: JsonNull
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.JsonElement.getAsString(JsonElement.java:191)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.test.gsonissue.MainActivity$ShopDeserializer.deserialize(MainActivity.java:57)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.test.gsonissue.MainActivity$ShopDeserializer.deserialize(MainActivity.java:48)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.TreeTypeAdapter.read(TreeTypeAdapter.java:58)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.Gson.fromJson(Gson.java:814)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.Gson.fromJson(Gson.java:779)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.Gson.fromJson(Gson.java:728)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.google.gson.Gson.fromJson(Gson.java:700)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.test.gsonissue.MainActivity.onCreate(MainActivity.java:44)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:6237)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
11-03 16:33:20.049 5412-5412/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

但是，只做JSON好工作。