我已经使用 Gson 几个星期了,我发现了运行时类型适配器工厂类,它允许“调整运行时类型可能与其声明类型不同的值”。
这是我当前使用 Gson 的代码:
public class Database {
private final Gson gson;
private Database() {
// Initialize Gson
RuntimeTypeAdapterFactory<Base> runtimeTypeAdapterFactory = RuntimeTypeAdapterFactory
.of(Base.class, "table")
.registerSubtype(AdminsTbl.class, "admins");
this.gson = new GsonBuilder().registerTypeAdapterFactory(runtimeTypeAdapterFactory).create();
}
}
我的 JSON 中有一个“表”字段,它告诉 Gson 使用哪个类(在本例中:“admins”-> AdminsTbl.class)。 Moshi 有类似的东西吗?
最佳答案
事实上,Gson 在其标准包中本身并未提供 RuntimeTypeAdapterFactory
。快速谷歌搜索 Moshi 实现会将您的问题放在搜索结果的顶部,至少对我来说是这样。 :) 我在com.squareup.moshi.recipes
中也找不到类似的东西。包裹。但您可以轻松地自己实现:
public final class MoshiRuntimeTypeJsonAdapterFactory
implements JsonAdapter.Factory {
private static final String DEFAULT_CLASS_NAME_PROPERTY = "type";
private final Class<?> baseClass;
private final String classNameProperty;
private final Map<String, Class<?>> classNameToClass = new HashMap<>();
private final Map<Class<?>, String> classToClassName = new HashMap<>();
private MoshiRuntimeTypeJsonAdapterFactory(final Class<?> baseClass, final String classNameProperty) {
this.baseClass = baseClass;
this.classNameProperty = classNameProperty;
}
public static MoshiRuntimeTypeJsonAdapterFactory of(final Class<?> expectedClass) {
return new MoshiRuntimeTypeJsonAdapterFactory(expectedClass, DEFAULT_CLASS_NAME_PROPERTY);
}
public static MoshiRuntimeTypeJsonAdapterFactory of(final Class<?> expectedClass, final String classNameProperty) {
return new MoshiRuntimeTypeJsonAdapterFactory(expectedClass, classNameProperty);
}
public MoshiRuntimeTypeJsonAdapterFactory with(final Class<?> concreteClass) {
return with(concreteClass, concreteClass.getSimpleName());
}
public MoshiRuntimeTypeJsonAdapterFactory with(final Class<?> concreteClass, final String className)
throws IllegalArgumentException {
if ( classNameToClass.containsKey(className) ) {
throw new IllegalArgumentException(className + " is already registered for " + concreteClass);
}
if ( classToClassName.containsKey(concreteClass) ) {
throw new IllegalArgumentException(concreteClass + " is already registered for " + className);
}
classNameToClass.put(className, concreteClass);
classToClassName.put(concreteClass, className);
return this;
}
@Nullable
@Override
public JsonAdapter<?> create(final Type type, final Set<? extends Annotation> annotations, final Moshi moshi) {
if ( !(type instanceof Class) ) {
return null;
}
final Class<?> typeAsClass = (Class<?>) type;
if ( !baseClass.isAssignableFrom(typeAsClass) ) {
return null;
}
final JsonAdapter<Object> jsonObjectJsonAdapter = moshi.nextAdapter(this, Map.class, ImmutableSet.of());
final LoadingCache<Class<?>, JsonAdapter<Object>> jsonAdaptersCache = CacheBuilder.newBuilder()
.build(new CacheLoader<Class<?>, JsonAdapter<Object>>() {
@Override
public JsonAdapter<Object> load(final Class<?> clazz) {
return moshi.nextAdapter(MoshiRuntimeTypeJsonAdapterFactory.this, clazz, ImmutableSet.copyOf(clazz.getAnnotations()));
}
});
return new JsonAdapter<Object>() {
@Nullable
@Override
public Object fromJson(final JsonReader jsonReader)
throws IOException {
try {
@SuppressWarnings("unchecked")
final Map<String, Object> jsonObject = (Map<String, Object>) jsonReader.readJsonValue();
assert jsonObject != null;
final Object rawClassName = jsonObject.get(classNameProperty);
if ( !(rawClassName instanceof String) ) {
throw new IOException("Type name: expected a string in " + classNameProperty + ", but got " + rawClassName);
}
final String className = (String) rawClassName;
final Class<?> concreteClass = classNameToClass.get(className);
if ( concreteClass == null ) {
throw new IOException("No mapping registered for " + className);
}
final JsonAdapter<Object> jsonAdapter = jsonAdaptersCache.get(concreteClass);
return jsonAdapter.fromJsonValue(jsonObject);
} catch ( final ExecutionException ex ) {
throw new RuntimeException(ex);
}
}
@Override
public void toJson(final JsonWriter jsonWriter, @Nullable final Object value)
throws IOException {
try {
assert value != null;
final Class<?> concreteClass = value.getClass();
final String className = classToClassName.get(concreteClass);
if ( className == null ) {
throw new IOException("No mapping registered for " + concreteClass);
}
final JsonAdapter<Object> valueJsonAdapter = jsonAdaptersCache.get(concreteClass);
@SuppressWarnings("unchecked")
final Map<String, Object> jsonObject = (Map<String, Object>) valueJsonAdapter.toJsonValue(value);
assert jsonObject != null;
jsonObject.put(classNameProperty, className);
jsonObjectJsonAdapter.toJson(jsonWriter, jsonObject);
} catch ( final ExecutionException ex ) {
throw new RuntimeException(ex);
}
}
};
}
}
此实现在不可变集合 (ImmutableSet
) 和缓存 (LoadingCache
) 中使用了 Google Guava,但您可以轻松地自行替换它们。我还相信这种实现也可以针对潜在的 Moshi 相关性能问题进行改进。
来自RuntimeTypeAdapterFactory
的简单示例如here所示适应:
private static final Moshi moshi = new Moshi.Builder()
.add(MoshiRuntimeTypeJsonAdapterFactory.of(Shape.class)
.with(Shape.Circle.class)
.with(Shape.Diamond.class)
.with(Shape.Rectangle.class)
)
.build();
关于java - Moshi 有像 Gson 这样的运行时类型适配器工厂吗?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/49781604/