ServletException在jsp mvc中从db中检索数据

Question

我想知道网络在我的 JSP 项目中使用 MVC 并找到了一个很好的讨论 here，现在我有以下内容：

public class Subject {
private String subjectId = null;
private String subjectName = null;

public String getSubjectId() {
    return subjectId;
}

public void setSubjectId(String subjectId) {
    this.subjectId = subjectId;
}

public String getSubjectName() {
    return subjectName;
}

public void setSubjectName(String subjectName) {
    this.subjectName = subjectName;
}
}

我的SubjectDAO 班级是：

public class SubjectDAO {

public List<Subject> subjectList() throws SQLException{
    Connection connection = null;
    PreparedStatement statement = null;
    ResultSet resultSet = null;
    List<Subject> subjects = new ArrayList<>();

    try {
        connection = DatabaseManager.initConnection();
        statement = connection.prepareStatement(Constant.SUBJECT_QUERY);
        resultSet = statement.executeQuery();

        while (resultSet.next()) {
            Subject subject = new Subject();
            subject.setSubjectId(resultSet.getString("_id"));
            subject.setSubjectName(resultSet.getString("subj_name"));               
            subjects.add(subject);
        }
    } finally {
        if (resultSet != null) try { resultSet.close(); } catch (SQLException ignore) {}
        if (statement != null) try { statement.close(); } catch (SQLException ignore) {}
        if (connection != null) try { connection.close(); } catch (SQLException ignore) {}
    }

    return subjects;
}
}

在我的ControllerServlet 我有：

protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
try {
        List<Subject> subjects = subjectDAO.subjectList();          
        request.setAttribute("subjects", subjects);
        getServletContext().getRequestDispatcher("/content.jsp").forward(
                request, response);
    } catch (Exception e) {
        throw new ServletException("Cannot obtain subjects from DB", e.getCause());
    }
}

我的 JSP 作为视图是：

    <c:forEach items="${subjects}" var="subject">                       
    <li
        <c:catch>
            <c:choose>
                <c:when test="${subject.subjectId == param.subj_id}">
                <c:out value="class=\"selected\""/>
                </c:when>
            </c:choose> 
        </c:catch>                      
    ><a href='/programming-iqs/admin-controller?action=manage-content&amp;subj_id=
    <c:out value="${subject.subjectId}"/>'>
    <c:out value="${subject.subjectId}"/>
    </a> 
        </li>
   </c:forEach>

使用上面的代码我得到：

javax.servlet.ServletException: Cannot obtain subjects from DB

我无法解决此问题，非常感谢任何帮助。

注意

在使用/实现 MVC 之前，我的 JSP 代码是：

<%
ResultSet sujectResult = DatabaseManager.getSubjects();

    while (sujectResult.next()) { %>
        <li <%
            String selectedSubject = request.getParameter("subj_id");
            try {
                if (selectedSubject.equals(sujectResult.getString("_id"))) {%>
                                class="selected" <%}%>><a
                        href='/programming-iqs/admin-controller?action=manage-content&amp;subj_id=
<%= sujectResult.getString("_id")%>'>
<%= sujectResult.getString("subj_name")%>
</a> <%
} catch (NullPointerException e) {

} 


} %></li>

当然，该代码运行良好，现在请帮助我实现 MVC。 谢谢

Answer 1

java.lang.IllegalStateException: Cannot forward after response has been committed 异常的一般原因是以下任何一种：

1. 您已经从您的 servlet/JSp 重定向/转发了控件，并且仍在尝试使用 outStream out.write("something") 编写一些响应来响应
2. 您已经从您的 Servlet/JSP 重定向控制，并再次尝试在没有适当条件的情况下从相同的 servlet/JSP 将控制发送到低于先前重定向/转发的某个位置（尽管根据条件您可以重定向/转发到不同的资源，但有两个重定向/转发不应相互矛盾/冲突）

例如： 这是正确的--

if(condition1)
 redirect to resource 1
else if(condition 2)
 redirect to resource 2
else
 redirect to resource 3

But this is wrong --

if(condition 1)
 redirect to resource 1
again
 redirect to resource 2

有关更多信息，请参阅以下链接：Link1Link2Link3

Answer 2

你可以加e.printStackTrace();

    protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
try {
        List<Subject> subjects = subjectDAO.subjectList();          
        request.setAttribute("subjects", subjects);
        getServletContext().getRequestDispatcher("/content.jsp").forward(
                request, response);
    } catch (Exception e) {
        e.printStackTrace();
        throw new ServletException("Cannot obtain subjects from DB", e.getCause());
    }
}

然后只需检查您的应用服务器控制台。它应该会告诉你问题出在哪里。