我有一个由基本字段和对象字段组成的命令对象。如何将表单字段绑定到对象中的字段？

我尝试这样做，但无济于事

<form:form commandName="course" method="POST">
     <form:input path="activity.activity"/>
         .
         .
         .
</form:form>

我得到这个错误

org.springframework.beans.NotReadablePropertyException: 
      Invalid property 'course' of bean class

我的Command类是这样的

public class Course {
    private long id;
    private String owner;
    private String title;
    private List<LearningActivity> activity = new ArrayList<LearningActivity>();

    //getters and setters
}

public class LearningActivity {
private long ID;
private String activity;
    private String link;

    //getters and setters
}