Servlet

标签： Java与Web

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.

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.

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("<H1>" + data + "</H1>");
}

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

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.

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.

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<String> 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.

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>Products</title>
</head>
<body>
<h2>Products</h2>
<ul>
    <%
        for (Product product : ProductContainer.products) {
    %>
    <li><%=product.getName()%>
        ($<%=product.getPrice()%>)
        (<a href="${pageContext.request.contextPath}/jsp/product_details.jsp?id=<%=product.getId()%>">Details</a>)
    </li>
    <%
        }
    %>
</ul>
<a href="${pageContext.request.contextPath}/jsp/shopping_cart.jsp">Shopping Cart</a>
</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 Details</title>
</head>
<body>
<h2>Product Details</h2>
<%
    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">Products</a></td>
            <td><a href="${pageContext.request.contextPath}/jsp/shopping_cart.jsp">Shopping Cart</a></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<ShoppingItem> items = (List<ShoppingItem>) session.getAttribute(SessionConstant.CART_ATTRIBUTE);
        if (items == null) {
            items = new ArrayList<ShoppingItem>();
            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 Cart</title>
</head>
<body>
<h2>Shopping Cart</h2>
<a href="${pageContext.request.contextPath}/jsp/products.jsp">Products</a>
<table>
    <tr>
        <td style="width: 150px">Quantity</td>
        <td style="width: 150px">Product</td>
        <td style="width: 150px">Price</td>
        <td>Amount</td>
    </tr>
    <%
        List<ShoppingItem> items = (List<ShoppingItem>) 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>

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>30</session-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.

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:

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");

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;
}

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: