Gson的@JsonAdater注解的几种方式

总结

可以通过自定义TypeAdapter和TypeAdapterFactory的方式，自定义gson的序列化和反序列规则，TypeAdapterFactory可以拿到上下文gson、TokenType类型；

也可以通过继承JsonReader重新写一次代码，在beginArray和endArray想办法跳过array的string形式的左右 双引号", gson.fromJson(myJsonReader, Type)也可以实现 解析时自动将String变为List，但采用注解@JsonAdapter的方式更为正规一些；

问题描述

json字符串：

{
    "cityIds": "[1,2,1001,13131]",
    "types": "[{\"name\": \"biz\",\"details\": [1]}]"
}

java对象定义：

 @Data
    public static class RequestParams {
       // json字符串里对应的是String
        private List<TypeItem> types;
        private List<Integer> cityIds;
    }
    @Data
    public static class TypeItem {
        private String name;
        private List<Integer> details;
    }

可以看到json里面cityIds和types都是String，而pojo里则是List，使用gson的fromJson将json转为pojo会发生报错

方式一

  @JsonAdapter(StringCollectionTypeAdapterFactory.class)
     private List<TagItem> tags;
     @JsonAdapter(StringCollectionTypeAdapterFactory.class)
     private List<Integer> cityIds;

public static class StringCollectionTypeAdapterFactory implements TypeAdapterFactory {
        @Override
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
            // 为了write的时候能使用到elementTypeAdapter
            Type type = typeToken.getType();
            Class<? super T> rawType = typeToken.getRawType();
            if (!Collection.class.isAssignableFrom(rawType)) {
                return null;
            }
            Type elementType = $Gson$Types.getCollectionElementType(type, rawType);
            TypeAdapter<?> elementTypeAdapter = gson.getAdapter(TypeToken.get(elementType));
            
            TypeAdapter<T> result = new Adapter(gson, elementTypeAdapter, typeToken);
            return result;
        }

        private static final class Adapter<E> extends TypeAdapter<Collection<E>> {
            private final TypeAdapter<E> elementTypeAdapter;
            private final Gson gson;
            private final TypeToken listType;

            public Adapter(Gson context, TypeAdapter<E> elementTypeAdapter, TypeToken listType) {
                this.elementTypeAdapter = elementTypeAdapter;
                this.gson = context;
                this.listType = listType;
            }

            @Override public Collection<E> read(JsonReader in) throws IOException {
                if (in.peek() == JsonToken.NULL) {
                    in.nextNull();
                    return null;
                }

                List<E> list = gson.fromJson(in.nextString(), listType.getType());
                return list;
            }
 
            // write后可以将array的string格式，重新变成array
            @Override public void write(JsonWriter out, Collection<E> collection) throws IOException {
                if (collection == null) {
                    out.nullValue();
                    return;
                }

                out.beginArray();
                for (E element : collection) {
                    elementTypeAdapter.write(out, element);
                }
                out.endArray();
            }
        }
    }

方式二-write原样

public static class StringCollectionTypeAdapterFactory1 implements TypeAdapterFactory {

        @Override
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
            return new Adapter(gson, typeToken);
        }

        private static final class Adapter<E> extends TypeAdapter<Collection<E>> {
            private final Gson gson;
            private final TypeToken listType;

            public Adapter(Gson context, TypeToken listType) {
                this.gson = context;
                this.listType = listType;
            }

            @Override public Collection<E> read(JsonReader in) throws IOException {
                if (in.peek() == JsonToken.NULL) {
                    in.nextNull();
                    return null;
                }

                List<E> list = gson.fromJson(in.nextString(), listType.getType());
                return list;
            }

            @Override public void write(JsonWriter out, Collection<E> collection) throws IOException {
                if (collection == null) {
                    out.nullValue();
                    return;
                }

                out.value(gson.toJson(collection));
            }
        }
    }

方式三-简单写法

private static class CollectionStringTypeAdapterFactory implements TypeAdapterFactory {
        @Override
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            return new TypeAdapter<T>() {
                @Override
                public void write(JsonWriter out, T value) throws IOException {
                    if (value == null) {
                        out.nullValue();
                        return;
                    }
                    gson.getAdapter(type).write(out, value);
                }

                @Override
                public T read(JsonReader in) throws IOException {
                    if (in.peek() == JsonToken.NULL) {
                        return null;
                    }
                    if (in.peek() == JsonToken.BEGIN_ARRAY) {
                        return gson.getAdapter(type).read(in);
                    }
                    T collection = gson.fromJson(in.nextString(), type.getType());
                    return collection;
                }
            };
        }
    }

测试用例如下所示：

@Test
    public void testGson1() {
        Gson gson = new Gson();
        RequestParams requestParam;
        String json;
        // 1.自动将string转为属性的List
        json = "{\"id\": \"000000\",\"types\": \"[{\\\"name\\\":\\\"name1\\\",\\\"list\\\":[1,2]},{\\\"name\\\":\\\"name2\\\",\\\"list\\\":[3,4]}]\",\"keywordIds\": \"[12,13]\"}";
        System.out.println(json);
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));

        // 2.jsonArray也可以转为List
        json = "{\"id\": \"000000\",\"keywordIds\": [12,13],\"types\": [{\"name\":\"name1\",\"list\":[1,2]},{\"name\":\"name2\",\"list\":[3,4]}]}";
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));

        // 3.换行的方式呢
        json = "{\n" +
                "\t\"id\": \"000000\",\n" +
                "\t\"keywordIds\": [12,13],\n" +
                "\t\"types\": [{\"name\":\"name1\",\"list\":[1,2]},{\"name\":\"name2\",\"list\":[3,4]}]\n" +
                "}";
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));

        // 4.能否将List里面的Integer变成String呢
        json = "{\n" +
                "\t\"id\": \"000000\",\n" +
                "\t\"keywordIds\": [12,13],\n" +
                "\t\"types\": [{\"name\":\"name1\",\"list1\":[1,2]},{\"name\":\"name2\",\"list1\":[3,4]}]\n" +
                "}";
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));

        // 5.能否将List里面的Integer变成String
        json = "{\n" +
                "\t\"id\": \"000000\",\n" +
                "\t\"keywordIds1\": [12,13],\n" +
                "\t\"types\": [{\"name\":\"name1\",\"list1\":[1,2]},{\"name\":\"name2\",\"list1\":[3,4]}]\n" +
                "}";
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));

        // 6.自动将string转为属性的List, 并且list里面的Integer变为String
        json = "{\"id\": \"000000\",\"types\": \"[{\\\"name\\\":\\\"name1\\\",\\\"list1\\\":[1,2]},{\\\"name\\\":\\\"name2\\\",\\\"list1\\\":[3,4]}]\",\"keywordIds1\": \"[12,13]\"}";
        System.out.println(json);
        requestParam = gson.fromJson(json, new TypeToken<RequestParams>() {}.getType());
        Assert.assertTrue(requestParam.getTypes().get(0).getClass().getName().indexOf("TypeItem") >= 0);
        System.out.println(gson.toJson(requestParam));
    }

    @Data
    public static class RequestParams {
        private String id;
        @JsonAdapter(CollectionStringTypeAdapterFactory.class)
        private List<TypeItem> types;
        @JsonAdapter(CollectionStringTypeAdapterFactory.class)
        private List<Integer> keywordIds;
        @JsonAdapter(CollectionStringTypeAdapterFactory.class)
        private List<String> keywordIds1;
    }
    @Data
    public static class TypeItem {
        private String name;
        private List<Integer> list;
        private List<String> list1;
    }