java - 如何在servlet中使用json对象返回项目数组以及如何使用jquery显示

Question

我需要将一些项目从我的数据库传递到网页。为此，我使用 json 对象将项目从 servlet 传递到 jquery。但我遇到的问题是，

• 如果数据库包含更多元素
• 那么我显然需要使用ArrayList。
• 当我使用 ArrayList 时，我不知道如何通过 JSON 传递元素。

我的代码是:

    PrintWriter out = response.getWriter();
    response.setContentType("text/html");
    response.setHeader("Cache-control", "no-cache, no-store");
    response.setHeader("Pragma", "no-cache");
    response.setHeader("Expires", "-1");

    response.setHeader("Access-Control-Allow-Origin", "*");
    response.setHeader("Access-Control-Allow-Methods", "GET");
    response.setHeader("Access-Control-Allow-Headers", "Content-Type");
    response.setHeader("Access-Control-Max-Age", "86400");

    Gson gson = new Gson();
    JsonObject myObj = new JsonObject();

    ArrayList<Commenter> commentInfo;
    try {
        commentInfo = commenting(uname,message,itemId,albumId);
        JsonElement commentObj = gson.toJsonTree(commentInfo);
        boolean nonNullElemExist= true;
        for (Commenter s: commentInfo) {
            if (s == null) {
                nonNullElemExist = false;
                break;
            }
        }
    if(nonNullElemExist == false){
        myObj.addProperty("success", false);
    }
    else {
        myObj.addProperty("success", true);
    }
    myObj.add("commentInfo", commentObj);
    out.println(myObj.toString());                   // I think problem is in this statement

    out.close();
    } catch (ClassNotFoundException | SQLException e) {
        System.out.println( "Error --> " + displayErrorForWeb(e));
    }

方法是:

   private ArrayList<Commenter> commenting(String uname,String message,int itemId,int albumId) throws ClassNotFoundException, SQLException {
   ArrayList<Commenter> commentList = new ArrayList<Commenter>();
   Connection conn = null; 
   conn=prepareConnection();
   PreparedStatement stmt = null;    
   String sql = null;

    try {     
        StringBuilder sb1=new StringBuilder(1024);
        sb1.append("insert into ").append(uname.trim()).append("comments values(?,?,?)");
        String sql1=sb1.toString();
        PreparedStatement stmt1 = conn.prepareStatement(sql1);
        stmt1.setString(1,uname);
        stmt1.setString(2,message);
        stmt1.setInt(3,itemId);

        StringBuilder sb=new StringBuilder(1024);
        sb.append("select * from ").append(uname.trim()).append("comments");
        sql=sb.toString();
        stmt = conn.prepareStatement(sql);
       ResultSet rs = stmt.executeQuery();

        while(rs.next()){
            Commenter comment = new Commenter();
            comment.setUname(rs.getString("uname").trim());
            comment.setComment(rs.getString("comment").trim());
            commentList.add(comment);
           }                                                                        

        rs.close();                                                              
        stmt.close();                                                            
        stmt = null;                                                             


        conn.close();                                                            
        conn = null;                                                  

    }                                                              
    catch(Exception e){System.out.println( "Error --> " + displayErrorForWeb(e));}                     

    finally {                                                      

        if (stmt != null) {                                           
            try {                                                        
                stmt.close();                                               
            } catch (SQLException sqlex) {                               
                System.out.println( "Error --> " + displayErrorForWeb(sqlex));         
            }                                                            

            stmt = null;                                           
        }                                                       

        if (conn != null) {                                     
            try {                                                  
                conn.close();                                         
            } catch (SQLException sqlex) {                         
                System.out.println( "Error --> " + displayErrorForWeb(sqlex));
            }                                                      

            conn = null;                                           
        }                                                       
    }             

    return commentList;

} 

jquery 是:

  $.ajax({
       type: "GET",
        url: "Comments",
        data:'comm='+encodeURIComponent(comm)+'&'+'data-id='+encodeURIComponent(dataid)+'&'+'data-alid='+encodeURIComponent(dataalid),
        dataType: "json",
        success: function( data, textStatus, jqXHR) 
        {
            if(data.success)
            {
       /*   $.each(data, function(i, item)
                     {*/

                    var newcommhtml = '<div id="c0'+thecid+'" class="cnew clearfix"> <section class="c-author">';
                    newcommhtml = newcommhtml + '<h3>Anonymous</h3>';
                    newcommhtml = newcommhtml + '<span class="pubdate">'+month+' '+day+', '+year+'</span> </section>';
                    newcommhtml = newcommhtml + '<section class="c-content">';
                    newcommhtml = newcommhtml + '<img src="images/green-avatar.png" alt="avatar" width="80" height="80" class="ava">';
                    newcommhtml = newcommhtml + '<p>'+nl2br(data.commentInfo.comment)+'   '+nl2br(data.commentInfo.itemId)+'</p> </section></div>';
                    /*newcommhtml = newcommhtml + '<p>'+nl2br(item.commentInfo.comment)+'   '+nl2br(item.commentInfo.itemId)+'</p> </section></div>';
                    });
                    */                  
                    var thelm = "#c0"+thecid;
                    commwrap.append(newcommhtml);
                    $(thelm).hide().fadeIn('slow');

                    setTimeout(function() { $(thelm).addClass('green'); }, 800);

                    $("#comm").val("");
                    thecid++;

                    if(errorspan.html() != null) {
                        errorspan.remove();
                    }
            }

          },
     error: function(jqXHR, textStatus, errorThrown)
      {
         alert("error"+errorThrown);
         console.log("Something really bad happened " + textStatus);
      },
});

如果我使用 $each() success block 根本不起作用。

请任何人告诉我如何为这个案例做这些事情......谢谢......

Answer 1

使用 new Gson().toJson(object) 方法将 ArrayList 转换为 json。现在你将得到一个 jsonobjects 的 jsonarray，然后你可以将它像 json 一样传递到服务器，反之亦然。

数组列表是一个集合，所以遵循这个

Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();
Collection<Integer> ints2 = gson.fromJson(json, collectionType);

gson user guide

要使用 jquery 显示 json，请点击此链接。 Best way to display data via JSON using jQuery