Gson fromJson 拋出 ClassCastException

Gson API Reference

• 使用環境 : jdk1.8.0_11 & gson-2.2.4
• Gson API Documentation

JsonParseException

• gson.fromJson(jsonString, Map.class) 方法時拋出 JsonParseException 訊息

Caused by: com.google.gson.JsonParseException: The JsonDeserializer MapTypeAdapter failed to deserialize json object given the type interface java.util.Map

Gson Api Documentation

• 根據文件說明

If the object that your are serializing/deserializing is a ParameterizedType (i.e. contains at least one type parameter and may be an array) then you must use the toJson(Object, Type) or fromJson(String, Type) method.

• 因此在使用 gson.fromJson() 時第二個參數要帶入 TypeToken，也就是要告知 Gson 如何將 String 轉型。以下是範例 :

	import com.google.gson.Gson;
	import com.google.gson.GsonBuilder;
	import com.google.gson.reflect.TypeToken;
			 
	public class MyDomainEntity<T>{
		int age;
		String name;
		T vehicle;
	}
		 
	 Type type = new TypeToken< MyDomainEntity<Car>>(){}.getType();
	 MyDomainEntity map = gson.fromJson(jsonString, type);

• 下面是 Pastgresql Jsonb 取出後直接將欄位值轉 String 的範例：

	import com.google.gson.Gson;
    import com.google.gson.GsonBuilder;		 
			 
    private Map<String, String> jsonbToMap(String jsonb) {
        Map<String, String> varMap = new LinkedHashMap<>();
        if (jsonb != null) {
            Gson gson2 = new GsonBuilder().serializeNulls().create();
            @SuppressWarnings("unchecked")
            Map<String, String> fromJson = (Map<String, String>) gson2
                    .fromJson(jsonb, Map.class);
            varMap.putAll(fromJson);
        }
        return varMap;
    }