求教json的有关问题,google库gson的bug有关问题

package toonyoo.ums.test;import com.google.gson.annotations.Expose;import com.google.gson.annotations.SerializedName;import com.google.gson.annotations.Since;public class User {           public User(String string, String string2, String string3, String string4) {        // TODO Auto-generated constructor stub        this.username=string;        this.password=string2;        this.gender=string3;        this.sex=string4;    }    @SerializedName("pwd")      private String password;       @Expose      @SerializedName("uname")      private String username;       @Expose      @Since(1.1)      private String gender;       @Expose      @Since(1.0)      private String sex;    public String getPassword() {        return password;    }    public void setPassword(String password) {        this.password = password;    }    public String getUsername() {        return username;    }    public void setUsername(String username) {        this.username = username;    }    public String getGender() {        return gender;    }    public void setGender(String gender) {        this.gender = gender;    }    public String getSex() {        return sex;    }    public void setSex(String sex) {        this.sex = sex;    }  }  调用：        List<User> userList = new ArrayList<User>();          User jack = new User("123456","Jack", "Male","Female");           User marry = new User("888888","Marry", "Female","Male");           userList.add(jack);           userList.add(marry);           Gson son=new Gson();        Type targetType = new TypeToken<List<User>>() {}.getType();          String str=son.toJson(userList, targetType);        /*[{"pwd":"Jack","uname":"123456","gender":"Male","sex":"Female"},{"pwd":"Marry","uname":"888888","gender":"Female","sex":"Male"}]*/        users =son.fromJson(str,targetType);///转换失败，提示下面的错误