Question

我正在尝试编写一个将使用3个不同servlet的应用程序。一个将显示信息，另一个将允许用户输入数据并显示它，最后一个servlet将允许用户编辑现有数据。我无法找到一种方法将数据存储到一个servlet中的ArrayList中，并显示在显示信息的servlet上的表中。

这是我到目前为止所做的：

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/DisplayItems")
public class DisplayItems extends HttpServlet {
private static final long serialVersionUID = 1L;
public static ArrayList list = new ArrayList();

public DisplayItems() {
    super();
    // TODO Auto-generated constructor stub
}

public void init(ServletConfig config) throws ServletException {
    ServletContext context = config.getServletContext();
    if (context.getAttribute("data_list") == null) {
        context.setAttribute("data_list", new ArrayList<String>());
    }

}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<String> list = (List<String>) getServletContext().getAttribute("data_list");
        list.add("1");
    PrintWriter out = response.getWriter();
    response.setContentType("text/html");
    out.println("<html>");
    out.println("<head><title>Department Library</title>");
    //out.println("<style>h1,h2,form,p{text-align:center;color:white}body{background-color:black;}</style></head>");
    out.println("<body>");
    out.println("<h1>Welcome to the Department Library</h1>");
    out.println("<table border='1' cellpadding='2' cellspacing='2'>");
    out.println("<thead><tr><th>ID</th><th>Type</th><th>Name</th><th>Addition Info</th><th>Available</th><th>Operation</th></tr><thead>");
    out.println("<tr><td>"+list.get(1)+"</td><td>"+list.get(0)+"</td><td>"+list.get(0)+"</td><td>"+list.get(0)+"</td><td>"+list.get(0)+"</td><td><a href='./EditItem'>Edit</a></td></tr>");
    out.println("<p><a href='./AddItem'>Add Item</a></p>");
    out.println("<p><a href='./EditItem'>Edit Item</a></p>");
    //out.println("<form method='post'><input id='inputtext' name='inputtext'></inputText><input type='submit' value='Translate' name='submit'></input></form>");
    out.println("</body></html>");
    out.close();

}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

}

public  void row(String id, String type, String name, String info ){

}
}

的AddItem

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Array;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/AddItem")
public class AddItem extends HttpServlet {
private static final long serialVersionUID = 1L;

public AddItem() {
    super();
    // TODO Auto-generated constructor stub
}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    PrintWriter out = response.getWriter();
    response.setContentType("text/html");
    out.println("<html>");
    out.println("<head><title>Add Items</title>");
    out.println("<body>");
    out.println("<h1>Add Items</h1>");
    out.println("<form method='post'");
    //name=type
    //name=name
    //name=info
    //name=copies
    out.println("<table><tbody><tr><td>Type:</td><td><select name='type'><option>Book</option><option>Tablet</option> </select></td><tr>");
    out.println("<tr><td>Name:</td><td><input name='name' size='60'/></td></tr>");
    out.println("<tr><td>Additional Info:</td><td><input name='info' size='60' /></td></tr>");
    out.println("<tr><td># of Copies:</td><td><input name='copies' size='8' /></td></tr>");
    out.println("<tr><td colspan='2' rowspan='1'><input name='add' type='submit' value='Add' /></td></tr></tbody></table>");
    out.println("</form>");
    out.println("<p><a href='./DisplayItems'>Display Items</a></p>");
    //out.println("<p><a href='./EditItem'>Edit Item</a></p>");
    //out.println("<form method='post'><input id='inputtext' name='inputtext'></inputText><input type='submit' value='Translate' name='submit'></input></form>");
    out.println("</body></html>");
    out.close();    }

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    PrintWriter out = response.getWriter();
    request.getParameter("type");
    String name = request.getParameter("name");
    String info = request.getParameter("info");
    String copies = request.getParameter("copies");
    //type.add(value);

    out.println(name);
    out.println(info);
    out.println(copies);
}

}

Answer 1

修改：将演员从getAttribute()添加到List

您可以使用在servlet上下文属性中存储ArrayList来在所有servlet之间共享它。但是如果它比单个String更复杂，你应该考虑使用类来存储数据。

您可以尝试在servlet的init方法中添加类似的内容：

public void init(ServletConfig config) throws ServletException {

    ServletContext context = config.getServletContext();
    if (context.getAttribute("data_list") == null) {
        context.setAttribute("data_list", new ArrayList<String>());
    }

}

然后在get或post方法中，您只需访问共享列表：

List<String> list = (List<String>) getServletContext().getAttribute("data_list");

根据Vighanesh Gursale的建议，如果您的数据具有自然键，您也可以使用HashMap。

Edit2：

为了确定发生了什么，您应该使用调试器来查看什么是null或添加如下所示的日志：创建属性后每个init方法中有一个

    if (context.getAttribute("data_list") == null) {
        context.setAttribute("data_list", new ArrayList<String>());
        log("Creating attribute data_list");
    }
    else {
        log("Attribute data_list already created");
    }

然后在doGet中：

ServletContext context = getServletContext();
log ((context == null) ? "Context null!?" : "Context ok");
Object attribute = context.getAttribute("data_list");
log((attribute == null) ? "Attribute null", attribute.toString());

通过servlet将数据存储到ArrayList中

1 个答案: