GSON不反序列化对外部类的引用 [英] GSON does not deserialize reference to outer class
问题描述
在我的Java应用程序中，我定义了两个类，分别称为 A 和 B ，其中 B 是 A 的内部类。两者都被定义为可序列化的
public class A implements Serializable {
int attrParent;
列表< B> items = new ArrayList< B>（）;
public void setAttrParent（int attrParent）{
this.attrParent = attrParent;
}
public int getAttrParent（）{
return attrParent;
}
public class B实现了Serializable {
private int attr;
public void setAttr（int attr）{
this.attr = attr;
}
public int getAttr（）{
return attr;
}
public int getSomeCalculationValue（）{
return this.attr * A.this.attrParent; //出现问题
}
}
}
在使用GSON序列化此对象之前， getSomeCalculationValue 可以正常工作。但是，在序列化和反序列化之后， getSomeCalculationValue 会因 NullPointerException 而失败。
发生这种情况是因为内部类 B 没有引用外部类 A 了，所以 A.this 失败。
有谁知道怎么可能我解决了这个问题，那就是在反序列化这个对象时恢复内部到外部的引用吗？
正如Gson文档所说： >
Gson可以很容易地序列化静态嵌套类。
Gson还可以反序列化静态嵌套类。然而， Gson可以
不会自动反序列化纯内部类，因为它们的
no-args构造函数也需要对包含在反序列化时不可用的对象
的引用即可。您可以通过使内部类为静态或
为其提供自定义InstanceCreator来解决此问题。
将B更改为静态内部类是不可能的，因为您的方法需要对 getSomeCalculationValue 中的外部类的引用，所以，我尝试用 InstanceCreator 但解决方案有点难看，所以我建议您使用自定义的反序列化。我稍微改变了你的 A 类，使公开的项目更容易创建我展示给你的例子。
public class ADeserializer实现了JsonDeserializer< A> {
public反序列化（JsonElement json，类型typeOfT，
JsonDeserializationContext上下文）抛出JsonParseException {
A a = new A（）;
a.attrParent = json.getAsJsonObject（）。get（attrParent）。getAsInt（）;
JsonArray ja = json.getAsJsonObject（）。get（items）。getAsJsonArray（）;
for（JsonElement e：ja）{
B b = a.new B（）;
b.setAttr（e.getAsJsonObject（）。get（attr）。getAsInt（））;
.items.add（b）;
}
return a;
}
}
这就是我使用它的方式：
public class Q19449761 {
public static void main（String [] args）{
A a = new A（）;
a.setAttrParent（3）;
B b = a.new B（）;
b.setAttr（10）;
.items.add（b）;
System.out.println（序列化之前：+ a.items.get（0）.getSomeCalculationValue（））;
Gson g = new Gson（）;
字符串json = g.toJson（a，A.class）;
System.out.println（JSON string：+ json）;
test2 = g.fromJson（json，A.class）;
尝试{
System.out.println（After standard deserialization：+ test2.items.get（0）.getSomeCalculationValue（））;
} catch（Exception e）{
e.printStackTrace（）;
}
GsonBuilder builder = new GsonBuilder（）;
builder.registerTypeAdapter（A.class，new ADeserializer（））;
test3 = builder.create（）。fromJson（json，A.class）;
System.out.println（自定义反序列化之后：+ test3.items.get（0）.getSomeCalculationValue（））;
}
}
这是我的执行：
序列化前：30
JSON字符串：{attrParent：3，items：[ {attr：10}]}
java.lang.NullPointerException
at stackoverflow.questions.q19449761.A $ B.getSomeCalculationValue（A.java:32）
at stackoverflow.questions。 q19449761.Q19449761.main（Q19449761.java:26）
自定义反序列化后：30
两个最后的注释：
- 您不需要实现
Serializable接口，JSON有与Java序列化没有什么共同之处。
- 我的反序列化器缺少空案例管理，您应该填写空JSON或空字段 。
On my Java application, I defined two classes, called A and B where B is inner class of A. Both are defined as serializable
public class A implements Serializable {
int attrParent;
List<B> items = new ArrayList<B>();
public void setAttrParent(int attrParent) {
this.attrParent = attrParent;
}
public int getAttrParent() {
return attrParent;
}
public class B implements Serializable {
private int attr;
public void setAttr(int attr) {
this.attr = attr;
}
public int getAttr() {
return attr;
}
public int getSomeCalculationValue() {
return this.attr * A.this.attrParent; // Problems occurs here
}
}
}
Before serializing this object with GSON, getSomeCalculationValue works fine. But, after serializing and deserializing, getSomeCalculationValue fails with a NullPointerException.
This happens because the inner class B doesn´t have a reference to outer class A anymore, so, A.this fails.
Does anybody knows how could I solve this, that is restoring the inner to outer reference while deserializing this object?
As Gson documentation says:
Gson can serialize static nested classes quite easily.
Gson can also deserialize static nested classes. However, Gson can not automatically deserialize the pure inner classes since their no-args constructor also need a reference to the containing Object which is not available at the time of deserialization. You can address this problem by either making the inner class static or by providing a custom InstanceCreator for it.
Changing B to a static inner class is not possible since your method needs a reference to the outer class in getSomeCalculationValue, so, I've tried to solve your problem with an InstanceCreator but solution was a bit ugly, so I propose you to use a custom deserialized. I changed your A class a little, making items public to make easier to create the example I show you.
public class ADeserializer implements JsonDeserializer<A> {
public A deserialize(JsonElement json, Type typeOfT,
JsonDeserializationContext context) throws JsonParseException {
A a = new A();
a.attrParent = json.getAsJsonObject().get("attrParent").getAsInt();
JsonArray ja = json.getAsJsonObject().get("items").getAsJsonArray();
for(JsonElement e: ja){
B b = a.new B();
b.setAttr(e.getAsJsonObject().get("attr").getAsInt());
a.items.add(b);
}
return a;
}
}
And this is how I use it:
public class Q19449761 {
public static void main(String[] args) {
A a = new A();
a.setAttrParent(3);
B b = a.new B();
b.setAttr(10);
a.items.add(b);
System.out.println("Before serializing: " + a.items.get(0).getSomeCalculationValue());
Gson g = new Gson();
String json = g.toJson(a, A.class);
System.out.println("JSON string: " + json);
A test2 = g.fromJson(json, A.class);
try {
System.out.println("After standard deserialization: " +test2.items.get(0).getSomeCalculationValue());
} catch (Exception e) {
e.printStackTrace();
}
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(A.class, new ADeserializer());
A test3 = builder.create().fromJson(json, A.class);
System.out.println("After custom deserialization: " + test3.items.get(0).getSomeCalculationValue());
}
}
And this is my execution:
Before serializing: 30
JSON string: {"attrParent":3,"items":[{"attr":10}]}
java.lang.NullPointerException
at stackoverflow.questions.q19449761.A$B.getSomeCalculationValue(A.java:32)
at stackoverflow.questions.q19449761.Q19449761.main(Q19449761.java:26)
After custom deserialization: 30
Two final notes:
- You do not need to implement
Serializableinterface, JSON has nothing in common with Java serialization - My deserializer lacks of null cases management, you should complete for null JSON or null fields.
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