Servlet - 会话跟踪

Servlet

标签 ： Java与Web

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会话跟踪

HTTP本身是“无状态”协议,它不保存连接交互信息,一次响应完成之后即连接断开,下一次请求需要重新建立连接,服务器不记录上次连接的内容.因此如果判断两次连接是否是同一用户, 就需要使用会话跟踪技术来解决.常见的会话跟踪技术有如下几种:

• URL重写: 在URL结尾附加会话ID标识,服务器通过会话ID识别不同用户.
• 隐藏表单域: 将会话ID埋入HTML表单隐藏域提交到服务端(会话ID不在浏览器页面显示).
• Cookie: 第一次请求时服务器主动发一小段信息给浏览器(即Cookie),下次请求时浏览器自动附带该段信息发送给服务器,服务器读取Cookie识别用户.
• Session: 服务器为每个用户创建一个Session对象保存到内存,并生成一个sessionID放入Cookie发送给浏览器,下次访问时sessionID会随Cookie传回来,服务器再根据sessionID找到对应Session对象(Java领域特有).

Session机制依赖于Cookie,如果Cookie被禁用Session也将失效.

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Cookie

Cookie是识别当前用户,实现持久会话的最好方式.最初由网景公司开发,但现在所有主流浏览器都支持.以至于HTTP协议为他定义了一些新的HTTP首部.

URL重写与隐藏表单域两种技术都有一定的局限,细节可参考博客四种会话跟踪技术

• Cookie规范
  
  • Cookie通过请求头/响应头在服务器与客户端之间传输, 大小限制为4KB;
  • 一台服务器在一个客户端最多保存20个Cookie;
  • 一个浏览器最多保存300个Cookie;

Cookie的key/value均不能保存中文,如果需要,可以在保存前对中文进行编码, 取出时再对其解码.

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Java-Cookie

在Java中使用Cookie, 必须熟悉javax.servlet.http.Cookie类, 以及HttpServletRequest/HttpServletResponse接口提供的几个方法:

Cookie	描述
Cookie(String name, String value)	Constructs a cookie with the specified name and value.
String getName()	Returns the name of the cookie.
String getValue()	Gets the current value of this Cookie.
void setValue(String newValue)	Assigns a new value to this Cookie.
void setMaxAge(int expiry)	Sets the maximum age in seconds for this Cookie.
int getMaxAge()	Gets the maximum age in seconds of this Cookie.
void setPath(String uri)	Specifies a path for the cookie to which the client should return the cookie.
void setDomain(String domain)	Specifies the domain within which this cookie should be presented.

Request	描述
Cookie[] getCookies()	Returns an array containing all of the Cookie objects the client sent with this request.

Response	描述
void addCookie(Cookie cookie)	Adds the specified cookie to the response.

• 示例: 获取上次访问时间
  从Request中获取Cookie: last_access_time, 如果没有则新建,否则显示last_access_time内容, 并更新为当前系统时间, 最后放入Response:

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Cookie[] cookies = request.getCookies();
    Cookie latCookie = null;
    if (cookies != null){
        for (Cookie cookie : cookies){
            if (cookie.getName().equals(L_A_T)){
                latCookie = cookie;
                break;
            }
        }
    }

    // 已经访问过了
    if (latCookie != null){
        printResponse("您上次访问的时间是" + latCookie.getValue(), response);
        latCookie.setValue(new Date().toString());
    } else{
        printResponse("您还是第一次访问", response);
        latCookie = new Cookie(L_A_T, new Date().toString());
    }

    response.addCookie(latCookie);
}

private void printResponse(String data, HttpServletResponse response) throws IOException {
    response.setContentType("text/html; charset=utf-8");
    response.getWriter().print("
" + data + "
");
}

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有效期

Cookie的Max-Age决定了Cookie的有效期,单位为秒.Cookie类通过getMaxAge()与setMaxAge(int maxAge)方法来读写Max-Age属性:

Max-Age	描述
0	Cookie立即作废(如果原先浏览器已经保存了该Cookie,那么可以通过设置Max-Age为0使其失效)
< 0	默认,表示只在浏览器内存中存活,一旦浏览器关闭则Cookie销毁
> 0	将Cookie持久化到硬盘上,有效期由Max-Age决定

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域属性

服务器可向Set-Cookie响应首部添加一个Domain属性来控制哪些站点可以看到该Cookie, 如

Set-Cookie: last_access_time="xxx"; Domain=.fq.com

该响应首部就是在告诉浏览器将Cookie last_access_time="xxx"发送给域”.fq.com”中的所有站点(如www.fq.com, mail.fq.com).

Cookie类通过setDomain()方法设置域属性.

如果没有指定域, 则Domain默认为产生Set-Cookie响应的服务器主机名.

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路径属性

Cookie规范允许用户将Cookie与部分Web站点关联起来.该功能可通过向Set-Cookie响应首部添加Path属性来实现:

Set-Cookie:last_access_time="Tue Apr 26 19:35:16 CST 2016"; Path=/servlet/

这样如果访问http://www.example.com/hello_http_servlet.do就不会获得last_access_time,但如果访问http://www.example.com/servlet/index.html, 就会带上这个Cookie.

Cookie类中通过setPath()方法设置路径属性.

如果没有指定路径, Path默认为产生Set-Cookie响应的URL的路径.

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Session

在所有的会话跟踪技术中, Session是功能最强大,最多的. 每个用户可以没有或者有一个HttpSession对象, 并且只能访问他自己的Session对象.

与URL重写, 隐藏表单域和Cookie不同, Session是保存在服务器内存中的数据,在达到一定的阈值后, Servlet容器会将Session持久化到辅助存储器中, 因此最好将使保存到Session内的对象实现java.io.Serializable接口.

使用Session, 必须熟悉javax.servlet.http.HttpSession接口, 以及HttpServletRequest接口中提供的几个方法:

HttpSession	描述
void setAttribute(String name, Object value)	Binds an object to this session, using the name specified.
Object getAttribute(String name)	Returns the object bound with the specified name in this session, or null if no object is bound under the name.
void invalidate()	Invalidates this session then unbinds any objects bound to it.
Enumeration getAttributeNames()	Returns an Enumeration of String objects containing the names of all the objects bound to this session.
void removeAttribute(String name)	Removes the object bound with the specified name from this session.
String getId()	Returns a string containing the unique identifier assigned to this session.
boolean isNew()	Returns true if the client does not yet know about the session or if the client chooses not to join the session.

Request	描述
HttpSession getSession()	Returns the current session associated with this request, or if the request does not have a session, creates one.
HttpSession getSession(boolean create)	Returns the current HttpSession associated with this request or, if there is no current session and create is true, returns a new session.
String getRequestedSessionId()	Returns the session ID specified by the client.

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示例-购物车

• domain

/**
 * @author jifang.
 * @since 2016/5/1 20:14.
 */
public class Product implements Serializable {

    private int id;
    private String name;
    private String description;
    private double price;

    public Product(int id, String name, String description, double price) {
        this.id = id;
        this.name = name;
        this.description = description;
        this.price = price;
    }

    // ...
}

public class ShoppingItem implements Serializable {
    private Product product;
    private int quantity;

    public ShoppingItem(Product product, int quantity) {
        this.product = product;
        this.quantity = quantity;
    }

    // ...
}

• 商品列表页面(/jsp/products.jsp)

<%@ page import="com.fq.web.domain.Product" %>
<%@ page import="com.fq.web.util.ProductContainer" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Productstitle>
head>
<body>
<h2>Productsh2>
<ul>
    <%
        for (Product product : ProductContainer.products) {
    %>
    <li><%=product.getName()%>
        ($<%=product.getPrice()%>)
        (<a href="${pageContext.request.contextPath}/jsp/product_details.jsp?id=<%=product.getId()%>">Detailsa>)
    li>
    <%
        }
    %>
ul>
<a href="${pageContext.request.contextPath}/jsp/shopping_cart.jsp">Shopping Carta>
body>
html>

• 商品详情(/jsp/product_details.jsp)

<%@ page import="com.fq.web.domain.Product" %>
<%@ page import="com.fq.web.util.ProductContainer" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Product Detailstitle>
head>
<body>
<h2>Product Detailsh2>
<%
    int id = Integer.parseInt(request.getParameter("id"));
    Product product = ProductContainer.getProduct(id);
    assert product != null;
%>
<form action="${pageContext.request.contextPath}/session/add_to_card.do" method="post">
    <input type="hidden" name="id" value="<%=id%>"/>
    <table>
        <tr>
            <td>Name:td>
            <td><%=product.getName()%>
            td>
        tr>
        <tr>
            <td>Price:td>
            <td><%=product.getPrice()%>
            td>
        tr>
        <tr>
            <td>Description:td>
            <td><%=product.getDescription()%>
            td>
        tr>
        <tr>
            <td><input type="text" name="quantity">td>
            <td><input type="submit" value="Buy">td>
        tr>
        <tr>
            <td><a href="${pageContext.request.contextPath}/jsp/products.jsp">Productsa>td>
            <td><a href="${pageContext.request.contextPath}/jsp/shopping_cart.jsp">Shopping Carta>td>
        tr>
    table>
form>
body>
html>

• 加入购物车(AddCardServlet)

@WebServlet(name = "AddCardServlet", urlPatterns = "/session/add_to_card.do")
public class AddCardServlet extends HttpServlet {

    @SuppressWarnings("All")
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        Product product = ProductContainer.getProduct(id);
        int quantity = Integer.parseInt(request.getParameter("quantity"));

        HttpSession session = request.getSession();
        List items = (List) session.getAttribute(SessionConstant.CART_ATTRIBUTE);
        if (items == null) {
            items = new ArrayList();
            session.setAttribute(SessionConstant.CART_ATTRIBUTE, items);
        }
        items.add(new ShoppingItem(product, quantity));

        request.getRequestDispatcher("/jsp/products.jsp").forward(request, response);
    }

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

• 购物车(/jsp/shopping_card.jsp)

<%@ page import="com.fq.web.constant.SessionConstant" %>
<%@ page import="com.fq.web.domain.ShoppingItem" %>
<%@ page import="java.util.List" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Shopping Carttitle>
head>
<body>
<h2>Shopping Carth2>
<a href="${pageContext.request.contextPath}/jsp/products.jsp">Productsa>
<table>
    <tr>
        <td style="width: 150px">Quantitytd>
        <td style="width: 150px">Producttd>
        <td style="width: 150px">Pricetd>
        <td>Amounttd>
    tr>
    <%
        List items = (List) session.getAttribute(SessionConstant.CART_ATTRIBUTE);
        if (items != null) {
            double total = 0.0;
            for (ShoppingItem item : items) {
                double subtotal = item.getQuantity() * item.getProduct().getPrice();
    %>
    <tr>
        <td><%=item.getQuantity()%>
        td>
        <td><%=item.getProduct().getName()%>
        td>
        <td><%=item.getProduct().getPrice()%>
        td>
        <td><%=subtotal%>
        td>
    tr>
    <%
            total += subtotal;
        }%>
    <tr>
        <td>Total: <%=total%>
        td>
    tr>
    <%
        }
    %>

table>
body>
html>

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有效期

Session有一定的过期时间: 当用户长时间不去访问该Session,就会超时失效,虽然此时sessionID可能还在Cookie中, 只是服务器根据该sessionID已经找不到Session对象了.
Session的超时时间可以在web.xml中配置, 单位为分钟:

<session-config>
    <session-timeout>30session-timeout>
session-config>

另外一种情况: 由于sessionID保存在Cookie中且Max-Age为-1,因此当用户重新打开浏览器时已经没有sessionID了, 此时服务器会再创建一个Session,此时新的会话又开始了.而原先的Session会因为超时时间到达而被销毁.

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字符编码

字符编码就是以二进制的数字来对应字符集的字符,常见字符编码方式有:ISO-8859-1(不支持中文),GB2312,GBK,UTF-8等.在JavaWeb中, 经常遇到的需要编码/解码的场景有响应编码/请求编码/URL编码:

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响应编码

服务器发送数据给客户端由Response对象完成,如果响应数据是二进制流,就无需考虑编码问题.如果响应数据为字符流,那么就一定要考虑编码问题:

response.getWriter()默认使用ISO-889-1发送数据,而该字符集不支持中文,因此遇到中文就一定会乱码.

在需要发送中文时, 需要使用:

response.setCharacterEncoding("UTF-8");
// getWriter() ...

设置编码方式,由于在getWriter()输出前已经设置了UTF-8编码,因此输出字符均为UTF-8编码,但我们并未告诉客户端使用什么编码来读取响应数据,因此我们需要在响应头中设置编码信息(使用Content-Type):

response.setContentType("text/html;charset=UTF-8");
// getWriter() ...

注意: 这句代码不只在响应头中添加了编码信息,还相当于调用了一次response.setCharacterEncoding("UTF-8");

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请求编码

1. 浏览器地址栏编码

在浏览器地址栏书写字符数据,由浏览器编码后发送给服务器,因此如果在地址栏输入中文,则其编码方式由浏览器决定:

浏览器	编码
IE/FireFox	GB2312
Chrome	UTF-8

2. 页面请求

如果通过页面的超链接/表单向服务器发送数据,那么其编码方式由当前页面的编码方式确定:

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

3. GET

当客户端发送GET请求时,无论客户端发送的数据编码方式为何,服务端均已ISO-8859-1解码(Tomcat8.x之后改用UTF-8),这就需要我们在request.getParameter()获取数据后再转换成正确的编码:

private Map<String, String> convertToParameterMap(HttpServletRequest request) throws UnsupportedEncodingException {
    Enumeration<String> names = request.getParameterNames();
    Map<String, String> parameters = new HashMap<String, String>();
    if (names != null) {
        while (names.hasMoreElements()) {
            String name = names.nextElement();
            String value = request.getParameter(name);
            parameters.put(name, new String(value.getBytes("ISO-8859-1"), "UTF-8"));
        }
    }
    return parameters;
}

4. POST

当客户端发送POST请求时,服务端也是默认使用IOS-8859-1解码,但POST的数据是通过请求体传送过来,因此POST请求可以通过request.setCharacterEncoding()来指定请求体编码方式:

private Map<String, String> convertToParameterMap(HttpServletRequest request) throws IOException {
    Map<String, String> parameters = new HashMap<String, String>();
    if (request.getMethod().equals("POST")) {
        request.setCharacterEncoding("UTF-8");
        Enumeration<String> names = request.getParameterNames();
        while (names.hasMoreElements()) {
            String key = names.nextElement();
            parameters.put(key, request.getParameter(key));
        }
    } else {
        Enumeration<String> names = request.getParameterNames();
        while (names.hasMoreElements()) {
            String key = names.nextElement();
            String value = request.getParameter(key);
            parameters.put(key, new String(value.getBytes("ISO-8859-1"), "UTF-8"));
        }
    }

    return parameters;
}

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URL编码

网络标准RFC 1738规定：

“…Only alphanumerics [0-9a-zA-Z], the special characters "$-_.+!*'()," [not including the quotes - ed], and reserved characters used for their reserved purposes may be used unencoded within a URL.”
“只有字母和数字[0-9a-zA-Z]、一些特殊符号"$-_.+!*'(),"[不包括双引号]、以及某些保留字，才可以不经过编码直接用于URL。”

如果URL中有汉字,就必须编码后使用, 而URL编码过程其实很简单:

首先需要指定一种字符编码,把字符串解码后得到byte[],然后把小于0的字节+256,再将其转换成16进制,最后前面再添加一个%.

这个编码过程在Java中已经封装成了现成的库, 可直接使用:

URLEncoder	描述
static String encode(String s, String enc)	Translates a string into application/x-www-form-urlencoded format using a specific encoding scheme.

URLDecoder	描述
static String decode(String s, String enc)	Decodes a application/x-www-form-urlencoded string using a specific encoding scheme.

注: 在Web中Tomcat容器会自动识别URL是否已经编码并自动解码.

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参考

更多有关编码知识, 可以参考:
1. 阮一峰: 关于URL编码
2. Web开发者应知的URL编码知识
3. 字符集和字符编码（Charset & Encoding）

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转载于:https://www.cnblogs.com/itrena/p/5926897.html