Gson RuntimeTypeAdapterFactory -要序列化的空项目导致空指针异常

Question

我正在测试RuntimeTypeAdapterFactoryTest：

https://code.google.com/p/google-gson/source/browse/trunk/extras/src/test/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactoryTest.java

它在原始示例测试用例(testRuntimeTypeAdapter())中工作得很好：

https://code.google.com/p/google-gson/source/browse/trunk/extras/src/test/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactoryTest.java#27

但是如果注册的类型是空的，我会在RuntimeTypeAdapterFactory中得到一个NPE异常。扩展上面的原始示例：

static class Wallet {
    BillingInstrument payment;
}

Wallet wallet = new Wallet();
// wallet.payment = new Card("Jo", 123); // leave wallet.payment uninitialized.

gson.toJson(wallet); // throws NPE

如果我初始化wallet.payment，那么序列化就能正常工作。下面是堆栈跟踪：

Exception in thread, java.lang.NullPointerException
at com.me.test.RuntimeTypeAdapterFactory$1.write(RuntimeTypeAdapterFactory.java:218)
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:91)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:206)
at com.google.gson.Gson.toJson(Gson.java:595)
...

这一点如下：

https://code.google.com/p/google-gson/source/browse/trunk/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java#218

有没有人遇到过这种情况并找到了解决办法？默认情况下，Gson应该忽略序列化null值，所以我不确定为什么在我的示例中它会尝试序列化wallet.payment。

谢谢

Answer 1

此问题已在2.4版之前的this commit中得到修复

Answer 2

遇到了同样的问题。这个修复方法对我很有效：

(RuntimeTypeAdapterFactory.java)

           @Override public void write(JsonWriter out, R value) throws IOException {
            if(value!=null) {
                Class<?> srcType = value.getClass();
                String label = subtypeToLabel.get(srcType);
                @SuppressWarnings("unchecked") // registration requires that subtype extends T
                        TypeAdapter<R> delegate = (TypeAdapter<R>) subtypeToDelegate.get(srcType);
                if (delegate == null) {
                    throw new JsonParseException("cannot serialize " + srcType.getName()
                            + "; did you forget to register a subtype?");
                }
                JsonObject jsonObject = delegate.toJsonTree(value).getAsJsonObject();
                if (jsonObject.has(typeFieldName)) {
                    throw new JsonParseException("cannot serialize " + srcType.getName()
                            + " because it already defines a field named " + typeFieldName);
                }
                JsonObject clone = new JsonObject();
                clone.add(typeFieldName, new JsonPrimitive(label));
                for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
                    clone.add(e.getKey(), e.getValue());
                }
                Streams.write(clone, out);
            }else{
                out.nullValue();
            }