java gson在需要对象时遇到空数组问题

2 年，1 月 Questions & Answers 1730

我试图使用gson处理一个json文件，但遇到了一个奇怪的错误。我从中读取的json（无法修改）有一种处理空字段的奇怪方式。它将[]放在没有数据的地方，使gson在需要对象时认为它是一个数组

gson的一个例子：

//non-empty field "prizes":[ { "year":"1902", "category":"physics", "share":"2", "motivation":"\"in recognition of the extraordinary service they rendered by their researches into the influence of magnetism upon radiation phenomena\"", "affiliations":[ { "name":"Leiden University", "city":"Leiden", "country":"the Netherlands" } ] } ] //empty field "prizes":[ { "year":"1903", "category":"physics", "share":"4", "motivation":"\"in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel\"", "affiliations":[ [] ] } ]

这是我处理json的代码：

public static void main(String[] args) throws IOException { // Get Gson object Gson gson = new Gson(); // read JSON file data as String String fileData = new String(Files.readAllBytes(Paths.get("laureates.json"))); // parse json string to object Example laur = gson.fromJson(fileData, Example.class); // print object data System.out.println("\n\nLaureates Object\n\n" + laur); }

我已经安排好了所有的课程，我相信一旦这个问题得到解决，它就会起作用。我得到的错误是“预期的BEGIN_对象，但在第1行第3401列是BEGIN_数组”（第3401列是第一个[]的确切位置）

"prizes":[ { "year":"1903", "category":"physics", "share":"4", "motivation":"\"in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel\"", "affiliations":[ ] } ]

# 3 楼答案

您总是可以使用类型适配器来适应设计糟糕但格式良好的JSON文档。例如，以下类型适配器修复了您的案例：

final class EmptyListFixTypeAdapterFactory
        implements TypeAdapterFactory {

    private static final TypeAdapterFactory instance = new EmptyListFixTypeAdapterFactory();

    private EmptyListFixTypeAdapterFactory() {
    }

    static TypeAdapterFactory get() {
        return instance;
    }

    @Override
    public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
        // If it's not a list, then just let Gson pass through the rest of the type adapters chain
        if ( !List.class.isAssignableFrom(typeToken.getRawType()) ) {
            return null;
        }
        // Get the original List adapter - we'll use it below
        @SuppressWarnings("unchecked")
        final TypeAdapter<List<Object>> delegateTypeAdapter = (TypeAdapter<List<Object>>) gson.getDelegateAdapter(this, typeToken);
        // Wrap it
        @SuppressWarnings("unchecked")
        final TypeAdapter<T> typeAdapter = (TypeAdapter<T>) EmptyListFixTypeAdapter.get(delegateTypeAdapter);
        return typeAdapter;
    }

    private static final class EmptyListFixTypeAdapter<E>
            extends TypeAdapter<List<E>> {

        // JsonParser as of Gson 2.8.2 holds no state
        private static final JsonParser jsonParser = new JsonParser();

        private final TypeAdapter<List<E>> delegateTypeAdapter;

        private EmptyListFixTypeAdapter(final TypeAdapter<List<E>> delegateTypeAdapter) {
            this.delegateTypeAdapter = delegateTypeAdapter;
        }

        private static <E> TypeAdapter<List<E>> get(final TypeAdapter<List<E>> delegateTypeAdapter) {
            return new EmptyListFixTypeAdapter<>(delegateTypeAdapter)
                    .nullSafe(); // A convenient method to add null-checking automatically
        }

        @Override
        public void write(final JsonWriter out, final List<E> value)
                throws IOException {
            // In case if you need to produce document with this quirks
            if ( value.isEmpty() ) {
                out.beginArray();
                out.beginArray();
                out.endArray();
                out.endArray();
                return;
            }
            delegateTypeAdapter.write(out, value);
        }

        @Override
        public List<E> read(final JsonReader in) {
            final JsonElement jsonElement = jsonParser.parse(in);
            final JsonArray array = jsonElement.getAsJsonArray();
            // Is it [[]]?
            if ( array.size() == 1 ) {
                final JsonElement element = array.get(0);
                if ( element.isJsonArray() && ((JsonArray) element).size() == 0 ) {
                    // Yes, detected
                    return new ArrayList<>();
                }
            }
            // No, proceed with the delegate type adapter
            return delegateTypeAdapter.fromJsonTree(array);
        }

    }

}

现在假设您有以下映射：

final class Laureate {
    final List<Prize> prizes = new ArrayList<>();
}

final class Prize {
    final int year = Integer.valueOf(0);
    final String category = null;
    final List<Affiliation> affiliations = new ArrayList<>();
}

final class Affiliation {
    final String name = null;
    final String city = null;
    final String country = null;
}

然后：

private static final Gson gson = new GsonBuilder()
        .registerTypeAdapterFactory(EmptyListFixTypeAdapterFactory.get())
        .create();

private static final Type laureatesType = new TypeToken<List<Laureate>>() {
}.getType();

public static void main(final String... args)
        throws IOException {
    try ( final JsonReader jsonReader = Resources.getPackageResourceJsonReader(Q49603826.class, "laureates.json") ) {
        gson.<List<Laureate>>fromJson(jsonReader, laureatesType)
                .stream()
                .flatMap(laureate -> laureate.prizes.stream())
                .peek(prize -> System.out.println("Prize: " + prize.year + " " + prize.category))
                .flatMap(prize -> prize.affiliations.stream())
                .peek(affiliation -> System.out.println("\tAffiliation: " + affiliation.name + " " + affiliation.city + " " + affiliation.country))
                .forEach(affiliation -> {
                });
    }
}

输出：

Prize: 1902 physics
........Affiliation: Leiden University Leiden the Netherlands
Prize: 1903 physics

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java gson在需要对象时遇到空数组问题

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