【Gson四】范型POJO的反序列化
编程技术 / houtizong 发布于 2年前 190
在下面这个例子中，POJO(Data类)是一个范型类，在Tests中，指定范型类为PieceData，POJO初始化完成后，通过
String str = new Gson().toJson(data);
得到范型化的POJO序列化得到的JSON串，然后将这个JSON串反序列化为POJO
import com.google.gson.Gson;import java.util.ArrayList;import java.util.List;class PieceData { private String name; private int weight; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getWeight() { return weight; } public void setWeight(int weight) { this.weight = weight; }}class Data<T> { List<T> pieces; public List<T> getPieces() { return pieces; } public void setPieces(List<T> pieces) { this.pieces = pieces; }}public class Tests { public static void main(String[] args) { PieceData data1 = new PieceData(); data1.setName("Cat"); data1.setWeight(6); PieceData data2 = new PieceData(); data2.setName("Dog"); data2.setWeight(36); List<PieceData> pieces = new ArrayList<PieceData>(); pieces.add(data1); pieces.add(data2); Data<PieceData> data = new Data<PieceData>(); data.setPieces(pieces); String str = new Gson().toJson(data); System.out.println(str); Data<PieceData> result = new Gson().fromJson(str, Data.class); for (int i = 0; i < result.getPieces().size(); i++) { Object o = result.getPieces().get(i); System.out.println(o.getClass() + "\t" + o); // System.out.println(piece.getName() + "\t" + piece.getWeight()); } }}
针对上面的代码，有如下分析：
1. POJO序列化的JSON串是{"pieces":[{"name":"Cat","weight":6},{"name":"Dog","weight":36}]}
2. Data<PieceData> result = new Gson().fromJson(str, Data.class);为什么可以编译通过，直观的感觉是因为参数是Data.class,因此返回值只能是Data result = new Gson().fromJson(str, Data.class):通过察看Gson的fromJson方法 public <T> T fromJson(String json, Class<T> classOfT)，看不出来上面的代码为什么可以编译通过
3. 上面的代码输出是
{"pieces":[{"name":"Cat","weight":6},{"name":"Dog","weight":36}]}
class com.google.gson.internal.LinkedTreeMap {name=Cat, weight=6.0}
class com.google.gson.internal.LinkedTreeMap {name=Dog, weight=36.0}
为什么result.getPieces()返回的List，其中的数据类型为TreeMap,明明是PieceData的，原因我猜想，虽然代码中，result类型由于是Data<PieceData>，即result.getPieces()应该是PieceData的集合，可实际运行时，范型有个特性称为Generics Erasure,也就是说实际运行中result.getPieces()未必是PieceData的集合，在这里是TreeMap的集合，TreeMap保存的是PieceData数据的key-value对。
4.可否把
Data<PieceData> result = new Gson().fromJson(str, Data.class);
改为
Data<PieceData> result = new Gson().fromJson(str, Data<PieceData>.class);
不可以！编译不通过，为什么Data<PieceData>编译出错
5. 使用Type包装Data<PieceData>,代码如下
package generic;import com.google.gson.Gson;import com.google.gson.reflect.TypeToken;import java.lang.reflect.Type;import java.util.ArrayList;import java.util.List;class PieceData { private String name; private int weight; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getWeight() { return weight; } public void setWeight(int weight) { this.weight = weight; }}class Data<T> { List<T> pieces; public List<T> getPieces() { return pieces; } public void setPieces(List<T> pieces) { this.pieces = pieces; }}public class Tests { public static void main(String[] args) { PieceData data1 = new PieceData(); data1.setName("Cat"); data1.setWeight(6); PieceData data2 = new PieceData(); data2.setName("Dog"); data2.setWeight(36); List<PieceData> pieces = new ArrayList<PieceData>(); pieces.add(data1); pieces.add(data2); Data<PieceData> data = new Data<PieceData>(); data.setPieces(pieces); String str = new Gson().toJson(data); System.out.println(str); Type type = new TypeToken<Data<PieceData>>() { }.getType(); Data<PieceData> result = new Gson().fromJson(str, type); for (int i = 0; i < result.getPieces().size(); i++) { Object o = result.getPieces().get(i); System.out.println(o.getClass() + "\t" + o); PieceData p = result.getPieces().get(i); System.out.println(p.getName() + "\t" + p.getWeight()); } }}
此时结果符合预期
{"pieces":[{"name":"Cat","weight":6},{"name":"Dog","weight":36}]}
class generic.PieceData generic.PieceData@674a93a6
Cat 6
class generic.PieceData generic.PieceData@5123ac44
Dog 36
总结
1.当要反序列化的POJO是范型POJO时，如例子中的Data，那么必须使用TypeToken对范型进行包装，即调用public <T> T fromJson(String json, Type typeOfT)
2.如果POJO本身不是范型类，但是POJO中包含范型集合时，可以不使用TypeToken进行包装，即调用 public <T> T fromJson(String json, Class<T> classOfT)即可完成反序列化，参见http://bit1129.iteye.com/blog/2101301
3.public <T> T fromJson(String json, Type typeOfT)和public <T> T fromJson(String json, Class<T> classOfT)的javadoc明确说明了它们的应用场景
public <T> T fromJson(String json, Class<T> classOfT)
/* public <T> T fromJson(String json, Class<T> classOfT) deserializes the specified Json into an object of the specified class. It is not * suitable to use if the specified class is a generic type since it will not have the generictype information because of the Type Erasure feature of Java. Therefore, this method should not * be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a * generic type. For the cases when the object is of generic type, invoke {@link #fromJson(String, Type)}. If you have the Json in a {@link Reader} instead of * a String, use {@link #fromJson(Reader, Class)} instead. */ public <T> T fromJson(String json, Type typeOfT)
/** * This method deserializes the specified Json into an object of the specified type. This method * is useful if the specified object is a generic type. For non-generic objects, use * {@link #fromJson(String, Class)} instead. If you have the Json in a {@link Reader} instead of * a String, use {@link #fromJson(Reader, Type)} instead. * * @param <T> the type of the desired object * @param json the string from which the object is to be deserialized * @param typeOfT The specific genericized type of src. You can obtain this type by using the * {@link com.google.gson.reflect.TypeToken} class. For example, to get the type for * {@code Collection<Foo>}, you should use: * <pre> * Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType(); * </pre> * @return an object of type T from the string * @throws JsonParseException if json is not a valid representation for an object of type typeOfT * @throws JsonSyntaxException if json is not a valid representation for an object of type */
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