Servlet基础总结

朴灿烈づ我的快乐病毒、 2022-04-16 01:43 203阅读 0赞

### 文章目录 ###

*  1. 引言
 *  2. Servlet接口
 *   *  2.1 一个简单的Servlet应用程序
     *  2.2 ServletRequest
     *  2.3 ServletResponse
     *  2.4 ServletConfig
     *  2.5 ServletContext
 *  3. GenericServlet
 *  4. HttpServlet
 *   *  4.1 HttpServletRequest
     *  4.2 HttpServletResponse

# 1. 引言 #

Servlet技术的核心是Servlet，Servlet API有以下4个Java包：

*  javax.servlet
 *  javax.servlet.http
 *  javax.servlet.annotation
 *  javax.servlet.descriptor

Servlet容器将Servlet类载入内存，并在Servlet实例上调用具体的方法，每个Servlet类型只能有一个实例，按照惯例Servlet类的名称要以Servlet作为后缀。

![servlet包主要结构][servlet]

javax.servlet.http中的主要类型

# 2. Servlet接口 #

package javax.servlet;
    
    import java.io.IOException;
    
    public interface Servlet {
    
      public void init(ServletConfig config) throws ServletException;
    
      public ServletConfig getServletConfig();
    
      public void service(ServletRequest req, ServletResponse res)
    	throws ServletException, IOException;
    
      public String getServletInfo();
    
      public void destroy();
    }

init,service和destroy是生命周期方法，getServletInfo和getServetConfig是非生命周期方法。

*  init: 当Servlet第一次被请求时，Servlet会调用这个方法，执行初始化动作，Servlet容器会传入一个ServletConfig，一般来说会将Servlet赋值给一个类级别变量
 *  service：每当请求Servlet时，Servlet容器会调用这个方法
 *  destroy：当要销毁Servlet时，Servlet会调用这个方法，用于回收资源并且在必要情况下同步内存的状态到持久层中
 *  getServletInfo: 返回Servlet信息，例如作者、版本等，注意此方法应该返回的是纯文本，而不是任何标记语言。
 *  getServletConfig: 返回init方法中传入的ServletConfig，ServletConfig应该被保存在servlet实现类中的类级变量中。

## 2.1 一个简单的Servlet应用程序 ##

package servlet;
 
 import javax.servlet.*;
 import javax.servlet.annotation.WebServlet;
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.util.Enumeration;
 
 /**
 * Servlet温故而知新
 * @author littlemotor
 * @date 18.11.13
 */
 @WebServlet(name = "MyServlet", urlPatterns = {"/my"})
 public class MyServlet implements Servlet {
 
 ServletConfig servletConfig;
 
 
 /**
 * Called by the servlet container to indicate to a servlet that the
 * servlet is being placed into service.
 *
 * The servlet container calls the <code>init</code>
 * method exactly once after instantiating the servlet.
 * The <code>init</code> method must complete successfully
 * before the servlet can receive any requests.
 *
 * The servlet container cannot place the servlet into service
 * if the <code>init</code> method
 * <ol>
 * <li>Throws a <code>ServletException</code>
 * <li>Does not return within a time period defined by the Web server
 * </ol>
 *
 * @param config a <code>ServletConfig</code> object
 * containing the servlet's
 * configuration and initialization parameters
 * @throws ServletException if an exception has occurred that
 * interferes with the servlet's normal
 * operation
 * @see UnavailableException
 * @see #getServletConfig
 */
 @Override
 public void init(ServletConfig config) throws ServletException {
 
 this.servletConfig = config;
 }
 
 /**
 * Returns a {@link ServletConfig} object, which contains
 * initialization and startup parameters for this servlet.
 * The <code>ServletConfig</code> object returned is the one
 * passed to the <code>init</code> method.
 *
 * Implementations of this interface are responsible for storing the
 * <code>ServletConfig</code> object so that this
 * method can return it. The {@link GenericServlet}
 * class, which implements this interface, already does this.
 *
 * @return the <code>ServletConfig</code> object
 * that initializes this servlet
 * @see #init
 */
 @Override
 public ServletConfig getServletConfig() {
 return servletConfig;
 }
 
 /**
 * Called by the servlet container to allow the servlet to respond to
 * a request.
 *
 * This method is only called after the servlet's <code>init()</code>
 * method has completed successfully.
 *
 * The status code of the response always should be set for a servlet
 * that throws or sends an error.
 *
 *
 * Servlets typically run inside multithreaded servlet containers
 * that can handle multiple requests concurrently. Developers must
 * be aware to synchronize access to any shared resources such as files,
 * network connections, and as well as the servlet's class and instance
 * variables.
 * More information on multithreaded programming in Java is available in
 * <a href="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">
 * the Java tutorial on multi-threaded programming</a>.
 *
 * @param request the <code>ServletRequest</code> object that contains
 * the client's request
 * @param response the <code>ServletResponse</code> object that contains
 * the servlet's response
 * @throws ServletException if an exception occurs that interferes
 * with the servlet's normal operation
 * @throws IOException if an input or output exception occurs
 */
 @Override
 public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
 response.setContentType("text/html");
 PrintWriter writer = response.getWriter();
 writer.println("<html>" +
 "<head></head>" +
 "<body>hello,world</body>");
 }
 
 /**
 * Returns information about the servlet, such
 * as author, version, and copyright.
 *
 * The string that this method returns should
 * be plain text and not markup of any kind (such as HTML, XML,
 * etc.).
 *
 * @return a <code>String</code> containing servlet information
 */
 @Override
 public String getServletInfo() {
 return null;
 }
 
 /**
 * Called by the servlet container to indicate to a servlet that the
 * servlet is being taken out of service. This method is
 * only called once all threads within the servlet's
 * <code>service</code> method have exited or after a timeout
 * period has passed. After the servlet container calls this
 * method, it will not call the <code>service</code> method again
 * on this servlet.
 *
 * This method gives the servlet an opportunity
 * to clean up any resources that are being held (for example, memory,
 * file handles, threads) and make sure that any persistent state is
 * synchronized with the servlet's current state in memory.
 */
 @Override
 public void destroy() {
 
 }
 }

## 2.2 ServletRequest ##

对于每一个HTTP请求，Servlet容器都会创建一个ServletRequest实例，封装了这个请求的信息，并将他传给Service方法

public String getParameter(String name);  //返回指定参数名的值
    public int getContentLength()             //请求主体的字节数
    public String getContentType()            //返回主体的MIME类型
    public String getProtocol()               //返回HTTP请求的协议名称和版本

## 2.3 ServletResponse ##

表示一个Servlet响应，在调用Servlet的Service方法之前，Servlet容器首先创建一个ServletResponse，并将它作为第二个参数传给Service方法，ServletResponse隐藏了向浏览器发送响应的复杂过程。  
首先应该调用setContentType方法，设置响应内容类型为"text/html",调用getWriter方法返回一个可以向客户端发送文本的PrintWriter，还有getOutputStream方法，可以发送二进制数据。

## 2.4 ServletConfig ##

Servlet容器初始化Servlet时会向init方法传入一个ServletConfig实例封装了@WebServlet或者xml文件传递给Servlet的配置信息，初始参数为key-vealue形式存储在HashMap中。

String getInitParameter(String name) //获取初始参数值
 Enumeration<String> getInitParameterNames() //返回所有参数名称枚举类型

## 2.5 ServletContext ##

负责定义一系列和容器通讯的方法，例如获取MIMI类型，调度器请求，写日志。每个虚拟机的每个web应用只有唯一的context，对于分布式web应用来说每个虚拟机都会有一个上下文，所以他们不能用来存储全局信息，可以用外部数据库代替，ServletContext对象包含于ServletConfig当中。

# 3. GenericServlet #

GenericServlet是Servlet接口的抽象类，完成了部分功能，通过继承他只需要重写service方法即可。值得注意的是他有两个init方法的实现

/**
 
 * This implementation stores the {@link ServletConfig}
 * object it receives from the servlet container for later use.
 * When overriding this form of the method, call 
 * <code>super.init(config)</code>.
 *
 public void init(ServletConfig config) throws ServletException {
 	this.config = config;
 	this.init();
 }
 
 
 /**
 * A convenience method which can be overridden so that there's no need
 * to call <code>super.init(config)</code>.
 *
 * Instead of overriding {@link #init(ServletConfig)}, simply override
 * this method and it will be called by
 * <code>GenericServlet.init(ServletConfig config)</code>.
 * The <code>ServletConfig</code> object can still be retrieved via {@link
 * #getServletConfig}. 
 
 public void init() throws ServletException {
 //通过覆盖此方法，可以避免写super.init(config)
 }

# 4. HttpServlet #

HttpServlet类继承自GenericServlet，并覆盖了原有的service方法，首先将原来的ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse

/**
 * Dispatches client requests to the protected
 * <code>service</code> method. There's no need to
 * override this method.
 */
 @Override
 public void service(ServletRequest req, ServletResponse res)
 throws ServletException, IOException
 {
 HttpServletRequest request;
 HttpServletResponse response;
 
 if (!(req instanceof HttpServletRequest &&
 res instanceof HttpServletResponse)) {
 throw new ServletException("non-HTTP request or response");
 }
 
 request = (HttpServletRequest) req;
 response = (HttpServletResponse) res;
 
 service(request, response);
 }
 }

并通过重载service(HttpServletRequest,HttpServletResponse)方法，并调用以下方法之一，每一种方法都表示一个HTTP方法：

*  doGet
 *  doPost
 *  doHead
 *  doPut
 *  doTrace
 *  doOptions
 *  doDelete

/**
 * Receives standard HTTP requests from the public
 * <code>service</code> method and dispatches
 * them to the <code>do</code>XXX methods defined in 
 * this class. This method is an HTTP-specific version of the 
 * {@link javax.servlet.Servlet#service} method. There's no
 * need to override this method.
 */
 protected void service(HttpServletRequest req, HttpServletResponse resp)
 throws ServletException, IOException
 {
 String method = req.getMethod();
 
 if (method.equals(METHOD_GET)) {
 long lastModified = getLastModified(req);
 if (lastModified == -1) {
 // servlet doesn't support if-modified-since, no reason
 // to go through further expensive logic
 doGet(req, resp);
 } else {
 long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
 if (ifModifiedSince < lastModified) {
 // If the servlet mod time is later, call doGet()
 // Round down to the nearest second for a proper compare
 // A ifModifiedSince of -1 will always be less
 maybeSetLastModified(resp, lastModified);
 doGet(req, resp);
 } else {
 resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
 }
 }
 
 } else if (method.equals(METHOD_HEAD)) {
 long lastModified = getLastModified(req);
 maybeSetLastModified(resp, lastModified);
 doHead(req, resp);
 
 } else if (method.equals(METHOD_POST)) {
 doPost(req, resp);
 
 } else if (method.equals(METHOD_PUT)) {
 doPut(req, resp);
 
 } else if (method.equals(METHOD_DELETE)) {
 doDelete(req, resp);
 
 } else if (method.equals(METHOD_OPTIONS)) {
 doOptions(req,resp);
 
 } else if (method.equals(METHOD_TRACE)) {
 doTrace(req,resp);
 
 } else {
 //
 // Note that this means NO servlet supports whatever
 // method was requested, anywhere on this server.
 //
 
 String errMsg = lStrings.getString("http.method_not_implemented");
 Object[] errArgs = new Object[1];
 errArgs[0] = method;
 errMsg = MessageFormat.format(errMsg, errArgs);
 
 resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
 }
 }

因此继承HttpServlet类后只要根据具体需要覆盖这七种方法之一就可以。

## 4.1 HttpServletRequest ##

定义的部分方法如下

Cookie[]  getCookies()  //返回Cookie对象数组
    String getHeader(String name)  //返回头文件对应key name的value
    HttpSession getSession()  //返回与这个请求相关的会话对象，如果没有就创建一个新的
    String getMethod()  //返回HTTP Method名称
    String getQueryString()  //返回URL中查询字符串

## 4.2 HttpServletResponse ##

定义的部分方法如下：

void addCookie(Cookie cookie)  //给响应对象添加一个cookie
    void addHeader(String name,String value)  //给响应对象添加一个header
    void sendRedirect(String location)  //给浏览器发送一个重定向地址

[servlet]: https://github.com/little-motor/uml/raw/master/javax.servletPackage.png