Gson：自定义TypeAdapter

当前项目解析json用的工具是google的gson，原因嘛，因为有GsonFormat插件，可以直接把服务端传回的json字符串转成Bean对象。不过在实际使用中出现了以下两个问题：

1. 传回的字符串或者数组为null，使用时若不加空指针判断，容易出现空指针异常。
2. 测试用的数值为0，结果用GsonFomat生成的对象默认为int类型，但可能该字段的真实类型为float，所以之后收到类型为float的数据时，就可能导致解析出错。

针对上述问题，都可以通过自定义TypeAdapter解决。

阅读过Gson的源码后发现，Gson的数据解析都是委托到各个TypeAdapter内进行处理的。在Gson的构造函数内会预先加载一部分TypeAdapter，包含String、int、long、double等类型，都存放在factories中，如下：

Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingPolicy,
      final Map<Type, InstanceCreator<?>> instanceCreators, boolean serializeNulls,
      boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe,
      boolean prettyPrinting, boolean serializeSpecialFloatingPointValues,
      LongSerializationPolicy longSerializationPolicy,
      List<TypeAdapterFactory> typeAdapterFactories) {
    this.constructorConstructor = new ConstructorConstructor(instanceCreators);
    this.serializeNulls = serializeNulls;
    this.generateNonExecutableJson = generateNonExecutableGson;
    this.htmlSafe = htmlSafe;
    this.prettyPrinting = prettyPrinting;

    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();

    // built-in type adapters that cannot be overridden
    factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
    factories.add(ObjectTypeAdapter.FACTORY);

    // the excluder must precede all adapters that handle user-defined types
    factories.add(excluder);

    // user's type adapters
    factories.addAll(typeAdapterFactories);

    // type adapters for basic platform types
    factories.add(TypeAdapters.STRING_FACTORY);
    factories.add(TypeAdapters.INTEGER_FACTORY);
    factories.add(TypeAdapters.BOOLEAN_FACTORY);
    factories.add(TypeAdapters.BYTE_FACTORY);
    factories.add(TypeAdapters.SHORT_FACTORY);
    factories.add(TypeAdapters.newFactory(long.class, Long.class,
            longAdapter(longSerializationPolicy)));
    factories.add(TypeAdapters.newFactory(double.class, Double.class,
            doubleAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.newFactory(float.class, Float.class,
            floatAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.NUMBER_FACTORY);
    factories.add(TypeAdapters.CHARACTER_FACTORY);
    factories.add(TypeAdapters.STRING_BUILDER_FACTORY);
    factories.add(TypeAdapters.STRING_BUFFER_FACTORY);
    factories.add(TypeAdapters.newFactory(BigDecimal.class, TypeAdapters.BIG_DECIMAL));
    factories.add(TypeAdapters.newFactory(BigInteger.class, TypeAdapters.BIG_INTEGER));
    factories.add(TypeAdapters.URL_FACTORY);
    factories.add(TypeAdapters.URI_FACTORY);
    factories.add(TypeAdapters.UUID_FACTORY);
    factories.add(TypeAdapters.LOCALE_FACTORY);
    factories.add(TypeAdapters.INET_ADDRESS_FACTORY);
    factories.add(TypeAdapters.BIT_SET_FACTORY);
    factories.add(DateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CALENDAR_FACTORY);
    factories.add(TimeTypeAdapter.FACTORY);
    factories.add(SqlDateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.TIMESTAMP_FACTORY);
    factories.add(ArrayTypeAdapter.FACTORY);
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(TypeAdapters.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingPolicy, excluder));

    this.factories = Collections.unmodifiableList(factories);
  }

我们可以自定义TypeAdapter，将其放入facotries中，并且gson在解析json时使用对应的TypeAdapter来的，而我们手动添加的TypeAdapter会优先于预设的TypeAdapter被使用。有兴趣的可以看看源码，还是比较简单的。

为了解决问题1，我们先定义一个StringAdapter，代码如下：

 /**
     * 自定义TypeAdapter ,null对象将被解析成空字符串
     */
    public static final TypeAdapter<String> STRING = new TypeAdapter<String>() {
        public String read(JsonReader reader) {
            try {
                if (reader.peek() == JsonToken.NULL) {
                    reader.nextNull();
                    return "";//原先是返回Null，这里改为返回空字符串
                }
                return reader.nextString();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return "";
        }

        public void write(JsonWriter writer, String value) {
            try {
                if (value == null) {
                    writer.nullValue();
                    return;
                }
                writer.value(value);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    };

这样在读取到null节点时，就自动变成返回空字符串了。

接下去定义一个Int类型的TypeAdapter:

/**
     * 自定义adapter，解决由于数据类型为Int,实际传过来的值为Float，导致解析出错的问题
     * 目前的解决方案为将所有Int类型当成Double解析，再强制转换为Int
     */
    public static final TypeAdapter<Number> INTEGER = new TypeAdapter<Number>() {
        @Override
        public Number read(JsonReader in) throws IOException {
            if (in.peek() == JsonToken.NULL) {
                in.nextNull();
                return ;
            }
            try {
                double i = in.nextDouble();//当成double来读取
                return (int) i;//强制转为int
            } catch (NumberFormatException e) {
                throw new JsonSyntaxException(e);
            }
        }

        @Override
        public void write(JsonWriter out, Number value) throws IOException {
            out.value(value);
        }
    };

数组部分略麻烦，由于gson用以数组解析的Adapter是不可重写的，只好拷贝出来，重新写了个类，如下

import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.internal.$Gson$Types;
import com.google.gson.internal.ConstructorConstructor;
import com.google.gson.internal.ObjectConstructor;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

import java.io.IOException;
import java.lang.reflect.Type;
import java.util.Collection;

/**
 * 自定义CollectionTypeAdapterFactory，使json内的数组为null时，返回空数组而不是null对象
 */
public final class CollectionTypeAdapterFactory implements TypeAdapterFactory {
    private final ConstructorConstructor constructorConstructor;

    public CollectionTypeAdapterFactory(ConstructorConstructor constructorConstructor) {
        this.constructorConstructor = constructorConstructor;
    }

    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
        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));
        ObjectConstructor<T> constructor = constructorConstructor.get(typeToken);

        @SuppressWarnings({"unchecked", "rawtypes"}) // create() doesn't define a type parameter
                TypeAdapter<T> result = new Adapter(gson, elementType, elementTypeAdapter, constructor);
        return result;
    }

    private static final class Adapter<E> extends TypeAdapter<Collection<E>> {
        private final TypeAdapter<E> elementTypeAdapter;
        private final ObjectConstructor<? extends Collection<E>> constructor;

        public Adapter(Gson context, Type elementType,
                       TypeAdapter<E> elementTypeAdapter,
                       ObjectConstructor<? extends Collection<E>> constructor) {
            this.elementTypeAdapter =
                    new TypeAdapterRuntimeTypeWrapper<E>(context, elementTypeAdapter, elementType);
            this.constructor = constructor;
        }

        public Collection<E> read(JsonReader in) throws IOException {
            if (in.peek() == JsonToken.NULL) {
                in.nextNull();
                //这里做了修改，原先是返回null，改为返回空数组
                return constructor.construct();
            }

            Collection<E> collection = constructor.construct();
            in.beginArray();
            while (in.hasNext()) {
                E instance = elementTypeAdapter.read(in);
                collection.add(instance);
            }
            in.endArray();
            return collection;
        }

        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();
        }
    }
}

注意上面的TypeAdapterRuntimeTypeWrapper类不是public的，所以也得拷贝出来写一个到本地。

接下去是使用部分：

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.InstanceCreator;
import com.google.gson.JsonElement;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import com.google.gson.JsonSyntaxException;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.internal.ConstructorConstructor;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;
import java.lang.reflect.Field;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Objects;

/**
 * Created by linjizong on 15/7/20.
 */
public class GsonUtils {

    public static Gson gson;

    /**
     * 自定义TypeAdapter ,null对象将被解析成空字符串
     */
    public static final TypeAdapter<String> STRING = new TypeAdapter<String>() {
        public String read(JsonReader reader) {
            try {
                if (reader.peek() == JsonToken.NULL) {
                    reader.nextNull();
                    return "";//原先是返回Null，这里改为返回空字符串
                }
                return reader.nextString();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return "";
        }

        public void write(JsonWriter writer, String value) {
            try {
                if (value == null) {
                    writer.nullValue();
                    return;
                }
                writer.value(value);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    };

    /**
     * 自定义adapter，解决由于数据类型为Int,实际传过来的值为Float，导致解析出错的问题
     * 目前的解决方案为将所有Int类型当成Double解析，再强制转换为Int
     */
    public static final TypeAdapter<Number> INTEGER = new TypeAdapter<Number>() {
        @Override
        public Number read(JsonReader in) throws IOException {
            if (in.peek() == JsonToken.NULL) {
                in.nextNull();
                return ;
            }
            try {
                double i = in.nextDouble();
                return (int) i;
            } catch (NumberFormatException e) {
                throw new JsonSyntaxException(e);
            }
        }

        @Override
        public void write(JsonWriter out, Number value) throws IOException {
            out.value(value);
        }
    };

    static {
        GsonBuilder gsonBulder = new GsonBuilder();
        gsonBulder.registerTypeAdapter(String.class, STRING);   //所有String类型null替换为字符串“”
        gsonBulder.registerTypeAdapter(int.class, INTEGER); //int类型对float做兼容

        //通过反射获取instanceCreators属性
        try {
            Class builder = (Class) gsonBulder.getClass();
            Field f = builder.getDeclaredField("instanceCreators");
            f.setAccessible(true);
            Map<Type, InstanceCreator<?>> val = (Map<Type, InstanceCreator<?>>) f.get(gsonBulder);//得到此属性的值
            //注册数组的处理器
            gsonBulder.registerTypeAdapterFactory(new CollectionTypeAdapterFactory(new ConstructorConstructor(val)));
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

        gson = gsonBulder.create();
    }

 /**
     * Json字符串 转为指定对象
     *
     * @param json json字符串
     * @param type 对象类型
     * @param <T>  对象类型
     * @return
     * @throws JsonSyntaxException
     */
    public static <T> T toBean(String json, Class<T> type) throws JsonSyntaxException {
        T obj = gson.fromJson(json, type);
        return obj;
    }

}

通过GsonBuilder的registerTypeAdapter方法可以直接注册TypeAdapter。而CollectionTypeAdapterFactory方法需要使用到GsonBuidler的instanceCreators字段，只好通过反射来获取了。接下去只要使用GsonUtils.toBean()就行了。