<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.39</version>
</dependency>

public class OutputT<T> {
    private int code;   //返回码
    private String msg; //消息
    private T data;     //数据

    //get/set method
}

public class User {
    private Long id;
    private String username;
    
    //get/set method
}

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.TypeReference;

public class Demo {
    public static void main(String[] args) {
        String jsonStr = "{\"code\":0,\"msg\":\"ok\",\"data\":{\"id\":1,\"username\":\"tom\"}}";
        OutputT<User> output = JSON.parseObject(jsonStr, new TypeReference<OutputT<User>>(){});
        System.out.println(output);
    }
}