servlet中Gson转换Json数据为对象，获取对象属性时报空指针问题，求解答

Question

服务端获取的json字符串如下：{"name":"王五","course":{"chinese":70,"english":90}}；
转化成student对象，代码如下：
package sourceCode;

import java.io.BufferedReader;
import java.io.IOException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.google.gson.Gson;

public class TestJson extends HttpServlet{
public void doPost(HttpServletRequest res,HttpServletResponse req) throws IOException{
req.setHeader("Content-Type", "application/json;charset=utf-8");
BufferedReader br = res.getReader();
StringBuffer sb = new StringBuffer();
String str;
while((str=br.readLine())!=null){
sb.append(str);
}
String jsonStr = sb.toString();
System.out.println("sb为："+jsonStr);
br.close();
Gson gson = new Gson();
Student stu = gson.fromJson(jsonStr, Student.class);
System.out.println(stu.getName);
System.out.println(stu.getCourse().getChinese());
}
}

student和sourse类代码如下：
public class Student {
private String name;
private Course course;

public Student() {
    super();
}

public Student(String name, Course course) {
    super();
    this.name = name;
    this.course = course;
}
//此处省略set和get函数

}

public class Course {

private int chinese;
private int english;

public Course() {
    super();
}

public Course(int chinese, int english) {
    super();
    this.chinese = chinese;
    this.english = english;
}
//此处省略set和get函数

}

当前台发起请求servlet时，控制台打印出的结果如下：

Answer 1

亲测，直接使用是没问题的，那就应该是两次提交，第二次字符串为空