使用 GSON 反序列化泛型类型时遇到问题

Question

I have a set of 4 classes which represent my database objects (database is currently only an ArrayList).
I am using GSON to store these objects in a.txt file.
A single class named Storage is in charge of reading and writing these objects to those.txt files.
Here is my problem: I have a generic object T extends MyInterface . That object represents the 4 database classes. I put T in the Storage method, so I could use just this one class for any database object. Here is the code:

public class Storage<T extends MyInterface> {

     Gson GSON = new Gson();

     public ArrayList<T> readAll() {
         String objectsJSON = TextFileHandler.readFromFile(fileStorageLocation); // This works
         System.out.println(objectsJSON);   // This also works  (prints out correctly)
         Type collectionType = new TypeToken<ArrayList<T>>() {}.getType();  
         return GSON.fromJson(objectsJSON, collectionType);     // This fails
     }

}

I get the following exception:

java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to 
interfaces.MyInterface

Do you guys have any idea what it could be?

Answer 1

The whole point of anonymous TypeToken subtypes is that they're defined at compile time so that generic type information is saved. You'll need to do something to make the token information available dynamically. This approach might work:

private final TypeToken<ArrayList<T>> targetType;

public Storage(Class<T> dataType) {
    targetType = new TypeToken<ArrayList<T>>(){}
        .where(new TypeParameter<T>(){}, dataType);
}

public List<T> readAll() { // avoid using concrete ArrayList in public API;
                           // not a bad idea to change it in the other locations
    ...
    return GSON.fromJson(objectsJson, targetType);
}

Answer 2

You should use interface List , not that implementation as ArrayList

public List<T> readAll() {
     String objectsJSON = TextFileHandler.readFromFile(fileStorageLocation);
     System.out.println(objectsJSON);
     Type collectionType = new TypeToken<List<T>>() {}.getType();  
     return GSON.fromJson(objectsJSON, collectionType);
 }

If that doesn't work, check objectJSON if is correct with JSON format.

Answer 3

For anybody who might be looking for an answer to this, here's what I exactly did to fix it:

The problem was that GSON did not know which class to instantiate since my Storage<T> was used for a couple of different entities. I had to somehow tell it, to which particular entity class was each instance of Storage<T> used. The error I was getting was something like:

Cannot cast LinkedTreeMap to MyInterface.
Because at runtime, the Storage had no idea which class to instantiate.

I'm doing a regular Java project, so I downloaded the Guava.jar and added it to my project: http://search.maven.org/remotecontent?filepath=com/google/guava/guava/23.0/guava-23.0.jar

I modified my generic class Storage<T> constructor, to take in the Class<T> dataType parameter so it can know at runtime, which class it's being used for. I also added a private field TypeToken<List<T>> so it could store information about the class being deserialized.

Essentially, all I did was this:

import com.google.common.reflect.TypeParameter;
import com.google.common.reflect.TypeToken;
import com.google.gson.*;

public class Storage<T extends MyInterface> {

     // This was used so the Storage objects knows which dataType it's associated to 
     // Because T is deleted at runtime, so you can't use that.
     private TypeToken<List<T>> targetDataType;

     // I changed the constructor to take in 1 bonus argument
     public Storage(Class<T> dataType) {
          targetDataType = new TypeToken<List<T>>() {}
                        .where(new TypeParameter<T>() {}, dataType);            
     }

     public List<T> deserialize() {
         // Read JSON data into a <jsonString> variable
    
         Gson GSON = new Gson();
         return GSON.fromJson(jsonString, targetDataType.getType());
     }
}