问题描述
给定一个这样的 JSON 映射：
{
"category1": [],"category2": ["item1"],"category3": ["item1","item2"]
}
我想把它转换成一个 Java ArrayList of Categories（不是地图），像这样：
class Category {
final String categoryName;
final ArrayList<String> items;
public Category(String categoryName,ArrayList<String> items) {
this.categoryName = categoryName;
this.items = items;
}
}
ArrayList<Category> data = new ArrayList<>();
所以我希望 data 看起来像这样：
for (Category c: data) {
System.out.println(c.categoryName + "," + c.items);
}
/**
Expected data =
category1,empty list [] or null
category2,[item1]
category2,[item1,item2]
**/
我尝试过使用 Gson 库：
app/build.gradle：
dependencies {
...
implementation 'com.google.code.gson:gson:2.8.6'
}
JsonConverterTest.java
import com.google.gson.Gson;
import com.google.gson.JsonArray;
import com.google.gson.JsonParser;
import com.google.gson.internal.LinkedTreeMap;
import com.google.gson.reflect.Typetoken;
import org.junit.Test;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Arrays;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertthat;
public class JsonConverterTest {
@Test
public void gsonMapToListtest() {
String json = "{\"category1\": [],\"category2\": [\"item1\"],\"category3\": [\"item1\",\"item2\"]}";
class Category {
final String categoryName;
final ArrayList<String> items;
public Category(String categoryName,ArrayList<String> items) {
this.categoryName = categoryName;
this.items = items;
}
}
ArrayList<Category> expectedData = new ArrayList<>();
expectedData.add(new Category("category1",new ArrayList<String>()));
expectedData.add(new Category("category2",new ArrayList<>(Arrays.asList("item1"))));
expectedData.add(new Category("category2",new ArrayList<>(Arrays.asList("item1","item2"))));
System.out.println("Expected Data:");
for (Category c: expectedData) {
System.out.println(c.categoryName + "," + c.items);
}
// This works,but it returns a Map instead of a List
LinkedTreeMap<String,ArrayList<String>> dataMap = new Gson().fromJson(json,LinkedTreeMap.class);
System.out.println("\n data as Map = " + dataMap);
// This causes "IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $"
Type listofCategoriesType = new Typetoken<ArrayList<Category>>() {}.getType();
ArrayList<Category> data = new Gson().fromJson(json,listofCategoriesType); // IllegalStateException here
assertthat(data,is(expectedData));
// This causes "java.lang.IllegalStateException: Not a JSON Array: {...}"
JsonArray jsonArray = JsonParser.parseString(json).getAsJsonArray(); // IllegalStateException here
data = new Gson().fromJson(jsonArray,listofCategoriesType);
assertthat(data,is(expectedData));
}
}
使用指南 https://www.baeldung.com/gson-list 和 https://futurestud.io/tutorials/gson-mapping-of-maps ，我只能将 JSON 映射转换为 Java 映射。但是，如果我尝试将 JSON 映射转换为 Java 列表，则会收到 IllegalStateException：
Type listofCategoriesType = new Typetoken<ArrayList<Category>>() {}.getType();
ArrayList<Category> data = new Gson().fromJson(json,listofCategoriesType); // IllegalStateException
或
JsonArray jsonArray = JsonParser.parseString(json).getAsJsonArray(); // IllegalStateException
ArrayList<Category> data2 = new Gson().fromJson(jsonArray,listofCategoriesType);
那么根据上面的单元测试 gsonMapToListtest()，Gson 中将 Json Map 转换为 Java 列表的正确方法是什么？
解决方法
最简单的方法是简单地解析为Map，然后将Map转换为List<Category>。
LinkedTreeMap<String,ArrayList<String>> dataMap = new Gson().fromJson(json,new TypeToken<LinkedTreeMap<String,ArrayList<String>>>() {}.getType());
ArrayList<Category> dataList = new ArrayList<>();
for (Entry<String,ArrayList<String>> entry : dataMap.entrySet())
dataList.add(new Category(entry.getKey(),entry.getValue()));
另一种方法是编写您自己的 TypeAdapter，或使用其他 JSON 库。
这不是最简单的方法，但 Gson 允许实现这样的类型适配器，因此您可以在没有中间集合的情况下反序列化结果对象中的 JSON。
public final class CombinerTypeAdapterFactory<V,C>
implements TypeAdapterFactory {
// used to create a collection to populate
private final TypeToken<? extends Collection<? super C>> collectionTypeToken;
// represents a type of each map entry value
private final TypeToken<V> valueTypeToken;
// a strategy to map an entry to an element
private final BiFunction<? super String,? super V,? extends C> combine;
private CombinerTypeAdapterFactory(final TypeToken<? extends Collection<? super C>> collectionTypeToken,final TypeToken<V> valueTypeToken,final BiFunction<? super String,? extends C> combine) {
this.collectionTypeToken = collectionTypeToken;
this.valueTypeToken = valueTypeToken;
this.combine = combine;
}
public static <V,C> TypeAdapterFactory create(final TypeToken<List<C>> collectionTypeToken,? extends C> combine) {
return new CombinerTypeAdapterFactory<>(collectionTypeToken,valueTypeToken,combine);
}
@Override
@Nullable
public <T> TypeAdapter<T> create(final Gson gson,final TypeToken<T> typeToken) {
// check if supported
if ( !typeToken.equals(collectionTypeToken) ) {
return null;
}
// grab the value type adapter
final TypeAdapter<V> valueTypeAdapter = gson.getAdapter(valueTypeToken);
// and the type adapter the collection to be populated
final TypeAdapter<? extends Collection<? super C>> collectionTypeAdapter = gson.getDelegateAdapter(this,collectionTypeToken);
return new TypeAdapter<T>() {
@Override
public void write(final JsonWriter out,final T value) {
throw new UnsupportedOperationException(); // TODO
}
@Override
public T read(final JsonReader in)
throws IOException {
// assuming all of those are always { e1,e2,e3,... }
in.beginObject();
// a hack to create an empty collection (it's modifiable right?)
final Collection<? super C> collection = collectionTypeAdapter.fromJson("[]");
// for each entry in the map that's being read
while ( in.hasNext() ) {
// get is name
final String name = in.nextName();
// and its value
final V value = valueTypeAdapter.read(in);
// combine
final C combined = combine.apply(name,value);
// and add to the list
collection.add(combined);
}
in.endObject();
// we know better than javac
@SuppressWarnings("unchecked")
final T result = (T) collection;
return result;
}
}
.nullSafe();
}
}
下面的测试是绿色的：
public final class CombinerTypeAdapterFactoryTest {
@AllArgsConstructor
@EqualsAndHashCode
@ToString
private static final class Category {
@Nullable
final String categoryName;
@Nullable
final List<String> items;
}
private static final Gson gson = new GsonBuilder()
.disableInnerClassSerialization()
.disableHtmlEscaping()
.registerTypeAdapterFactory(CombinerTypeAdapterFactory.create(new TypeToken<List<Category>>() {},new TypeToken<List<String>>() {},Category::new))
.create();
@Test
public void test()
throws IOException {
try ( final JsonReader jsonReader = new JsonReader(new InputStreamReader(CombinerTypeAdapterFactoryTest.class.getResourceAsStream("input.json"))) ) {
final List<Category> actualCategories = gson.fromJson(jsonReader,new TypeToken<List<Category>>() {}.getType());
Assertions.assertIterableEquals(
ImmutableList.of(
new Category("category1",ImmutableList.of()),new Category("category2",ImmutableList.of("item1")),new Category("category3",ImmutableList.of("item1","item2"))
),actualCategories
);
}
}
}