java - Gson 在尝试字符串化对象时抛出 UnsupportedOperationException，该对象包含 java.lang.Class 作为字段

Question

我写了 ClassTypeAdapter，它对类来说是正确的，但是当我尝试使用包含类作为字段的对象时失败了。

具有可执行 main 方法的 ClassTypeAdapter 可重现附加问题。

有什么想法吗？

    import com.google.gson.Gson;
    import com.google.gson.GsonBuilder;
    import com.google.gson.TypeAdapter;
    import com.google.gson.stream.JsonReader;
    import com.google.gson.stream.JsonWriter;

    import java.io.IOException;

    public class ClassTypeAdapter extends TypeAdapter<Class<?>> {
        private static final String PARAM_NAME = "className";

        @Override
        public void write(JsonWriter out, Class<?> value) throws IOException {
            out.beginObject();

            out.name(PARAM_NAME).value(value.getName());

            out.endObject();
        }

        @Override
        public Class<?> read(JsonReader in) throws IOException {
            Class<?> readClass = null;

            in.beginObject();

            while (in.hasNext()) {
                if (PARAM_NAME.equals(in.nextName())) {
                    try {
                        readClass = Class.forName(in.nextString());
                    } catch (ClassNotFoundException e) {
                        throw new IOException("Class not found", e);
                    }
                }
            }

            in.endObject();

            return readClass;
        }

        public static class TestClass<T> {
            private Class<T> aClass;
        }

        public static void main(String[] args) {
            final Gson gson = new GsonBuilder().registerTypeAdapter(Class.class, new ClassTypeAdapter()).create();

            final TestClass testClass = new TestClass<>();

            System.out.println(gson.toJson(testClass));    // {}
            System.out.println(gson.toJson(Object.class)); // {"className":"java.lang.Object"}

            testClass.aClass = Object.class;

            System.out.println(gson.toJson(testClass));    // UnsupportedOperationException
        }
    }

错误堆栈跟踪:

Exception in thread "main" java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: java.lang.Object. Forgot to register a type adapter?
    at com.google.gson.internal.bind.TypeAdapters$1.write(TypeAdapters.java:76)
    at com.google.gson.internal.bind.TypeAdapters$1.write(TypeAdapters.java:69)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:125)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:243)
    at com.google.gson.Gson.toJson(Gson.java:669)
    at com.google.gson.Gson.toJson(Gson.java:648)
    at com.google.gson.Gson.toJson(Gson.java:603)
    at com.google.gson.Gson.toJson(Gson.java:583)
    at ClassTypeAdapter.main(ClassTypeAdapter.java:56)

Answer 1

我找到了解决方案 here .

public class ClassTypeAdapter implements JsonSerializer<Class<?>>, JsonDeserializer<Class<?>> {
    @Override
    public JsonElement serialize(Class<?> src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(src.getName());
    }

    @Override
    public Class<?> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        try {
            return Class.forName(json.getAsString());
        } catch (ClassNotFoundException e) {
            throw new JsonParseException(e);
        }
    }
}

JUnit 测试:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class ClassTypeAdapterTest {
    @Test
    public void testReadWrite() {
        final Class<?> classToWrite = ClassTypeAdapter.class;

        final Gson gson = new GsonBuilder().registerTypeAdapter(Class.class, new ClassTypeAdapter()).create();

        final String writtenClass = gson.toJson(classToWrite);
        final Class readClass = gson.fromJson(writtenClass, Class.class);

        assertEquals(classToWrite, readClass);
    }

    @Test
    public void testInnerClassProblem() {
        final Gson gson = new GsonBuilder().registerTypeAdapter(Class.class, new ClassTypeAdapter()).create();

        final TestClass testClass = new TestClass<>();
        testClass.innerClass = Object.class;

        final String writtenClass = gson.toJson(testClass);
        final TestClass readClass = gson.fromJson(writtenClass, TestClass.class);

        assertEquals(testClass.innerClass, readClass.innerClass);
    }

    private static class TestClass<T> {
        private Class<T> innerClass;
    }
}