來寫個AJAX版的聊天室吧！先看看直接使用AJAX要如何做到，首先需要一個簡單的聊天室Servlet…
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package onlyfun.caterpillar;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ChatRoomServlet extends javax.servlet.http.HttpServlet implements javax.servlet.Servlet {
private static LinkedList<Message> messages = new LinkedList<Message>();
public ChatRoomServlet() {
super();
}
private List<Message> addMessage(String text) {
if (text != null && text.trim().length() > 0) {
messages.addFirst(new Message(text));
while (messages.size() > 10) {
messages.removeLast();
}
}
return messages;
}
private List<Message> getMessages() {
return messages;
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Message> list = null;
if("send".equals(request.getParameter("task"))) {
list = addMessage(request.getParameter("msg"));
}
else if("query".equals(request.getParameter("task"))){
list = getMessages();
}
PrintWriter out = response.getWriter();
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
out.println("<response>");
for(int i = 0; i < list.size(); i++) {
String msg = list.get(i).getText();
out.println("<message>" + msg + "</message>");
}
out.println("</response>");
}
}
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Message物件如下…
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package onlyfun.caterpillar;
public class Message {
private long id = System.currentTimeMillis();
private String text;
public Message(String newtext) {
text = newtext;
if (text.length() > 256) {
text = text.substring(0, 256);
}
text = text.replace('<', '[');
text = text.replace('&', '_');
}
public long getId() {
return id;
}
public String getText() {
return text;
}
}
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Servlet接受訊息新增與查詢，判斷的方式是檢查請求參數task是send或query。
Servlet會以XML傳回目前List當中的訊息，客戶端可以查詢或插入新訊息時，取得目前List中的訊息，接著在web.xml中設定一下…
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<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<description>
</description>
<display-name>
ChatRoomServlet</display-name>
<servlet-name>ChatRoomServlet</servlet-name>
<servlet-class>
onlyfun.caterpillar.ChatRoomServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ChatRoomServlet</servlet-name>
<url-pattern>/ChatRoomServlet</url-pattern>
</servlet-mapping>
<session-config>
<session-timeout>
30
</session-timeout>
</session-config>
</web-app>
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在網頁中，使用者可以在輸入訊息後按下按鈕送出資訊，並在XML回應取得時，將訊息以一列一列的表格方式顯示出來，另外還設定了週期性的輪詢，即使不輸入新訊息，也可以週期性的取得新的聊天訊息…
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=BIG5">
<title>Chat Room</title>
<script type="text/javascript">
var xmlHttp;
function createXMLHttpRequest() {
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}
function sendMessage() {
var msg = document.getElementById("text").value;
if(msg == "") {
refreshMessage();
return;
}
var param = "task=send&msg=" + msg;
ajaxRequest(param);
document.getElementById("text").value = "";
}
function queryMessage() {
var param = "task=query";
ajaxRequest(param);
}
function ajaxRequest(param) {
var url = "ChatRoomServlet?timestamp" + new Date().getTime();
createXMLHttpRequest();
xmlHttp.onreadystatechange = refreshMessage;
xmlHttp.open("POST", url, true);
xmlHttp.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded;");
xmlHttp.send(param);
}
function refreshMessage() {
if(xmlHttp.readyState == 4) {
if(xmlHttp.status == 200) {
var messages = xmlHttp.responseXML.getElementsByTagName("message");
var table_body = document.getElementById("dynamicUpdateArea");
var length = table_body.childNodes.length;
for (var i = 0; i < length; i++) {
table_body.removeChild(table_body.childNodes[0]);
}
var length = messages.length;
for(var i = length - 1; i >= 0 ; i--) {
var message = messages[i].firstChild.data;
var row = createRow(message);
table_body.appendChild(row);
}
setTimeout("queryMessage()", 2000);
}
}
}
function createRow(message) {
var row = document.createElement("tr");
var cell = document.createElement("td");
var cell_data = document.createTextNode(message);
cell.appendChild(cell_data);
row.appendChild(cell);
return row;
}
</script>
</head>
<body>
<p>
Your Message:
<input id="text"/>
<input type="button" value="Send"
onclick="sendMessage()"/>
</p>
<p>Messages:</p>
<table align="left">
<tbody id="dynamicUpdateArea"></tbody>
</table>
</body>
</html>
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簡單抓個畫面…
直接用AJAX，後端用JSP/Servlet，您要對請求參數做些判斷，看看是新增訊息或查詢，並要自行輸出XML，有的沒的…
改成DWR的話，就很簡單了，寫個簡單的Java物件…
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package onlyfun.caterpillar;
import java.util.LinkedList;
import java.util.List;
public class Chat {
private static LinkedList<Message> messages = new LinkedList<Message>();
public List addMessage(String text) {
if (text != null && text.trim().length() > 0) {
messages.addFirst(new Message(text));
while (messages.size() > 10) {
messages.removeLast();
}
}
return messages;
}
public List getMessages() {
return messages;
}
}
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接著…在dwr.xml中開放一下…
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dwr PUBLIC "-//GetAhead Limited//DTD Direct Web Remoting 1.0//EN" "http://www.getahead.ltd.uk/dwr/dwr10.dtd">
<dwr>
<allow>
<create creator="new" javascript="Chat">
<param name="class" value="onlyfun.caterpillar.Chat"/>
</create>
<convert converter="bean" match="onlyfun.caterpillar.Message"/>
</allow>
</dwr>
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使用者取得訊息時，是直接傳回List物件，而List中裝的是Message物件，Message物件是自訂物件，conterver設定為 bean，表示DWR會自動將Message的getter名稱轉換為傳回客戶端的JavaScript物件中的屬性，例如Message中有 getText()，則在客戶端可以用message.text這樣的方式來存取。
接著是簡單的客戶端網頁…
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=BIG5">
<title>Insert title here</title>
<script src="dwr/interface/Chat.js" type="text/javascript"></script>
<script src="dwr/engine.js" type="text/javascript"></script>
<script src="dwr/util.js" type="text/javascript"></script>
<script type="text/javascript">
function sendMessage() {
var text = DWRUtil.getValue("text");
DWRUtil.setValue("text", "");
Chat.addMessage(text, gotMessages);
}
function gotMessages(messages) {
var chatlog = "";
for (var data in messages) {
chatlog = "<div>" + messages[data].text +
"</div>" + chatlog;
}
DWRUtil.setValue("chatlog", chatlog);
setTimeout("queryMessage()", 2000);
}
function queryMessage() {
Chat.getMessages(gotMessages);
}
</script>
</head>
<body>
<p>
Your Message:
<input id="text"/>
<input type="button" value="Send"
onclick="sendMessage()"/>
</p>
<p>Messages:</p>
<div id="chatlog"></div>
</body>
</html>
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當List物件傳回時，它成為gotMessages(messages)中的messages物件，而messages物件中包括 Message物件轉換後對應的JavaScript物件，由於我們已經設定了Converter，所以可以用messages[data].text來 取得傳回的訊息…
簡單抓個畫面…
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