文章目錄

1. 引言
2. Servlet接口
- 2.1 一個簡單的Servlet應用程式
- 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技術的核心是Servlet，Servlet API有以下4個Java包：

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

Servlet容器将Servlet類載入記憶體，并在Servlet執行個體上調用具體的方法，每個Servlet類型隻能有一個執行個體，按照慣例Servlet類的名稱要以Servlet作為字尾。

Servlet基礎總結1. 引言2. Servlet接口3. GenericServlet4. HttpServlet

javax.servlet.http中的主要類型

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: 當Servlet第一次被請求時，Servlet會調用這個方法，執行初始化動作，Servlet容器會傳入一個ServletConfig，一般來說會将Servlet指派給一個類級别變量
service：每當請求Servlet時，Servlet容器會調用這個方法
destroy：當要銷毀Servlet時，Servlet會調用這個方法，用于回收資源并且在必要情況下同步記憶體的狀态到持久層中
getServletInfo: 傳回Servlet資訊，例如作者、版本等，注意此方法應該傳回的是純文字，而不是任何标記語言。
getServletConfig: 傳回init方法中傳入的ServletConfig，ServletConfig應該被儲存在servlet實作類中的類級變量中。

2.1 一個簡單的Servlet應用程式

package servlet;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

/**
 * Servlet溫故而知新
 * @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.
     *
     * <p>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.
     *
     * <p>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.
     *
     * <p>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.
     *
     * <p>This method is only called after the servlet's <code>init()</code>
     * method has completed successfully.
     *
     * <p>  The status code of the response always should be set for a servlet
     * that throws or sends an error.
     *
     *
     * <p>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" target="_blank" rel="external nofollow" >
     * 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.
     *
     * <p>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.
     *
     * <p>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

對于每一個HTTP請求，Servlet容器都會建立一個ServletRequest執行個體，封裝了這個請求的資訊，并将他傳給Service方法

public String getParameter(String name);  //傳回指定參數名的值
public int getContentLength()             //請求主體的位元組數
public String getContentType()            //傳回主體的MIME類型
public String getProtocol()               //傳回HTTP請求的協定名稱和版本

2.3 ServletResponse

表示一個Servlet響應，在調用Servlet的Service方法之前，Servlet容器首先建立一個ServletResponse，并将它作為第二個參數傳給Service方法，ServletResponse隐藏了向浏覽器發送響應的複雜過程。

首先應該調用setContentType方法，設定響應内容類型為"text/html",調用getWriter方法傳回一個可以向用戶端發送文本的PrintWriter，還有getOutputStream方法，可以發送二進制資料。

2.4 ServletConfig

Servlet容器初始化Servlet時會向init方法傳入一個ServletConfig執行個體封裝了@WebServlet或者xml檔案傳遞給Servlet的配置資訊，初始參數為key-vealue形式存儲在HashMap中。

String getInitParameter(String name)  //擷取初始參數值
Enumeration<String> getInitParameterNames()  //傳回所有參數名稱枚舉類型

2.5 ServletContext

負責定義一系列和容器通訊的方法，例如擷取MIMI類型，排程器請求，寫日志。每個虛拟機的每個web應用隻有唯一的context，對于分布式web應用來說每個虛拟機都會有一個上下文，是以他們不能用來存儲全局資訊，可以用外部資料庫代替，ServletContext對象包含于ServletConfig當中。

3. GenericServlet

GenericServlet是Servlet接口的抽象類，完成了部分功能，通過繼承他隻需要重寫service方法即可。值得注意的是他有兩個init方法的實作

/**
     
     * <p>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>.
     *
     * <p>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 {
      //通過覆寫此方法，可以避免寫super.init(config)
    }

4. HttpServlet

HttpServlet類繼承自GenericServlet，并覆寫了原有的service方法，首先将原來的ServletRequest和ServletResponse轉換為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);
    }
}

并通過重載service(HttpServletRequest,HttpServletResponse)方法，并調用以下方法之一，每一種方法都表示一個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><i>XXX</i> 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);
        }
    }

是以繼承HttpServlet類後隻要根據具體需要覆寫這七種方法之一就可以。

4.1 HttpServletRequest

定義的部分方法如下

Cookie[]  getCookies()  //傳回Cookie對象數組
String getHeader(String name)  //傳回頭檔案對應key name的value
HttpSession getSession()  //傳回與這個請求相關的會話對象，如果沒有就建立一個新的
String getMethod()  //傳回HTTP Method名稱
String getQueryString()  //傳回URL中查詢字元串

4.2 HttpServletResponse

定義的部分方法如下：

void addCookie(Cookie cookie)  //給響應對象添加一個cookie
void addHeader(String name,String value)  //給響應對象添加一個header
void sendRedirect(String location)  //給浏覽器發送一個重定向位址

Servlet基礎總結1. 引言2. Servlet接口3. GenericServlet4. HttpServlet

文章目錄

1. 引言

2. Servlet接口

2.1 一個簡單的Servlet應用程式

2.2 ServletRequest

2.3 ServletResponse

2.4 ServletConfig

2.5 ServletContext

3. GenericServlet

4. HttpServlet

4.1 HttpServletRequest

4.2 HttpServletResponse

繼續閱讀

SpringMVC開發技術~1~MVC模式中Servlet控制器的簡介和應用模型一視圖一控制器(MVC)模式

Spring——SpringMVC 核心技術請求轉發和重定向1.請求轉發和重定向的概念介紹 2.代碼實作

Spring MVC之全局異常統一處理一、SpringMVC異常處理二、全局異常統一處理

Servlet 基礎入門②

servlet 基礎入門

Servlet的生命周期Servlet的生命周期

Servlet（八）ServletContext對象

Spring MVC初始化過程源碼分析1. 引言2. HttpServletBean3. FrameworkServlet4. DispatcherServlet5. 小結

Spring MVC RESTful CRUD

從源碼角度解析SpringMVC執行流程先看流程再看源碼總結

SpringMVC——SSM整合小項目1.整合思路與開發步驟 2.SSM整合開發代碼部分2.8.啟動Tomcat檢驗結果

Spring MVC 資料轉換 & 資料格式化 & 資料校驗資料綁定流程

【Spring IOC容器】Bean管理（工廠Bean）Bean管理（工廠Bean）

SpringMVC——SpringMVC架構基本概述1.SpringMVC架構概述2.第一個注解的SpringMVC程式 3.使用SpringMVC架構處理web請求的順序

請求轉發和響應重定向

Spring MVC參數傳遞各種方式彙總@RequestParam @RequestBody時間參數等一、基本資料類型二、普通對象三、數組四、集合List五、對象+List同時接收六、時間參數傳遞