Servlet--HttpServlet类

• HttpServlet类

定义
public class HttpServlet extends GenericServlet implements Serializable

这是一个抽象类，用来简化 HTTP Servlet 写作的过程。它是 GenericServlet 类的扩充，提供了一个处理 HTTP 协议的框架。在这个类中的 service 方法支持例如 GET、POST 这样的标准的 HTTP 方法。这一支持过程是通过分配他们到适当的方法（例如 doGet、doPost）来实现的。

方法
1、doDelete

protected void doDelete(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,IOException;

被这个类的 service 方法调用，用来处理一个 HTTP DELETE 操作。这个操作允许客户端请求从服务器上删除 URL。这一操作可能有负面影响，对此用户就负起责任。这一方法的默认执行结果是返回一个HTTP BAD_REQUEST 错误。 当你要处理 DELETE
请求时，你必须重载这一方法。
2、doGet

protected void doGet(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,IOException;

被这个类的 service 方法调用，用来处理一个 HTTP GET 操作。这个操作允许客户端简单地从一个 HTTP 服务器“获得”资源。对这个方法的重载将自动地支持 HEAD 方法。GET 操作应该是安全而且没有负面影响的。这个操作也应该可以安全地重复。这一方法的默认执行结果是返回一个 HTTP BAD_REQUEST 错误。
3、doHead

protected void doHead(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,IOException;

被这个类的 service 方法调用，用来处理一个 HTTP HEAD 操作。默认的情况是，这个操作会按照一个无条件的 GET 方法来执行，该操作不向客户端返回任何数据，而仅仅是返回包含内容长度的头信息。与 GET 操作一样，这个操作应该是安全而且没有负面影响的。这个操作也应该可以安全地重复。这个方法的默认执行结果是自动处理 HTTP HEAD 操作， 这个方法不需要被一个子类执行。
4、doOptions

protected void doOptions(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,IOException;

被这个类的 service 方法调用，用来处理一个 HTTP OPTION 操作。这个操作自动地决定支持哪一种 HTTP 方法。例如，一个 Servlet 写了一个 HttpServlet 的子类并重载了 doGet方法，doOption 会返回下面的头：Allow:GET,HEAD,TRACE,OPTIONS
你一般不需要重载这个方法。
5、doPost

protected void doPost(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,

IOException;
被这个类的 service 方法调用， 用来处理一个 HTTP POST 操作。 这个操作包含请求体的数据，Servlet 应该按照他行事。这个操作可能有负面影响。例如更新存储的数据或在线购物。这一方法的默认执行结果是返回一个 HTTP BAD_REQUEST 错误。当你要处理 POST操作时，你必须在 HttpServlet 的子类中重载这一方法。
6、doPut

protected void doPut(HttpServletRequest request,HttpServletResponse response) 

throws ServletException,IOException;

被这个类的 service 方法调用， 用来处理一个 HTTP PUT 操作。 这个操作类似于通过 FTP发送文件。这个操作可能有负面影响。例如更新存储的数据或在线购物。这一方法的默认执行结果是返回一个 HTTP BAD_REQUEST 错误。当你要处理 PUT 操
作时，你必须在 HttpServlet 的子类中重载这一方法。
7、doTrace

protected void doTrace(HttpServletRequest request,HttpServletResponse response)

 throws ServletException,IOException;

被这个类的 service 方法调用，用来处理一个 HTTP TRACE 操作。这个操作的默认执行结果是产生一个响应，这个响应包含一个反映 trace 请求中发送的所有头域的信息。当你开发 Servlet 时，在多数情况下你需要重载这个方法。
8、getLastModified
protected long getLastModified(HttpServletRequest request);
返回这个请求实体的最后修改时间。为了支持 GET 操作，你必须重载这一方法，以精确地反映最后修改的时间。 这将有助于浏览器和代理服务器减少装载服务器和网络资源， 从而更加有效地工作。返回的数值是自 1970-1-1 日（GMT）以来的毫秒数。默认的执行结果是返回一个负数，这标志着最后修改时间未知，它也不能被一个有条件的GET 操作使用。
9、service

protected void service(HttpServletRequest request,HttpServletResponse response)

 throws ServletException,IOException;

public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException;

这是一个 Servlet 的 HTTP-specific 方案， 它分配请求到这个类的支持这个请求的其他方法。当你开发 Servlet 时，在多数情况下你不必重载这个方法。实际开发中需要重载的就是doGet和doPost这2个方法。然后一般都是重写doPost方法，然后重写doGet的时候只需要调下doPost方法即可。下面贴出HttpServlet类的源码。

package javax.servlet.http;


import java.io.IOException;
import java.io.Serializable;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;


public abstract class HttpServlet extends GenericServlet
  implements Serializable
{
  private static final String METHOD_DELETE = "DELETE";
  private static final String METHOD_HEAD = "HEAD";
  private static final String METHOD_GET = "GET";
  private static final String METHOD_OPTIONS = "OPTIONS";
  private static final String METHOD_POST = "POST";
  private static final String METHOD_PUT = "PUT";
  private static final String METHOD_TRACE = "TRACE";
  private static final String HEADER_IFMODSINCE = "If-Modified-Since";
  private static final String HEADER_LASTMOD = "Last-Modified";
  private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
  private static ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");


  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_get_not_supported");
    if (protocol.endsWith("1.1"))
      resp.sendError(405, msg);
    else
      resp.sendError(400, msg);
  }


  protected long getLastModified(HttpServletRequest req)
  {
    return -1L;
  }


  protected void doHead(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    NoBodyResponse response = new NoBodyResponse(resp);


    doGet(req, response);
    response.setContentLength();
  }


  protected void doPost(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_post_not_supported");
    if (protocol.endsWith("1.1"))
      resp.sendError(405, msg);
    else
      resp.sendError(400, msg);
  }


  protected void doPut(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_put_not_supported");
    if (protocol.endsWith("1.1"))
      resp.sendError(405, msg);
    else
      resp.sendError(400, msg);
  }


  protected void doDelete(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_delete_not_supported");
    if (protocol.endsWith("1.1"))
      resp.sendError(405, msg);
    else
      resp.sendError(400, msg);
  }


  private static Method[] getAllDeclaredMethods(Class c)
  {
    if (c.equals(HttpServlet.class)) {
      return null;
    }


    Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass());
    Method[] thisMethods = c.getDeclaredMethods();


    if ((parentMethods != null) && (parentMethods.length > 0)) {
      Method[] allMethods = new Method[parentMethods.length + thisMethods.length];


      System.arraycopy(parentMethods, 0, allMethods, 0, parentMethods.length);


      System.arraycopy(thisMethods, 0, allMethods, parentMethods.length, thisMethods.length);


      thisMethods = allMethods;
    }


    return thisMethods;
  }


  protected void doOptions(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    Method[] methods = getAllDeclaredMethods(getClass());


    boolean ALLOW_GET = false;
    boolean ALLOW_HEAD = false;
    boolean ALLOW_POST = false;
    boolean ALLOW_PUT = false;
    boolean ALLOW_DELETE = false;
    boolean ALLOW_TRACE = true;
    boolean ALLOW_OPTIONS = true;


    for (int i = 0; i < methods.length; i++) {
      Method m = methods[i];


      if (m.getName().equals("doGet")) {
        ALLOW_GET = true;
        ALLOW_HEAD = true;
      }
      if (m.getName().equals("doPost"))
        ALLOW_POST = true;
      if (m.getName().equals("doPut"))
        ALLOW_PUT = true;
      if (m.getName().equals("doDelete")) {
        ALLOW_DELETE = true;
      }
    }
    String allow = null;
    if ((ALLOW_GET) && 
      (allow == null)) allow = "GET";
    if (ALLOW_HEAD)
      if (allow == null) allow = "HEAD"; else
        allow = allow + ", HEAD";
    if (ALLOW_POST)
      if (allow == null) allow = "POST"; else
        allow = allow + ", POST";
    if (ALLOW_PUT)
      if (allow == null) allow = "PUT"; else
        allow = allow + ", PUT";
    if (ALLOW_DELETE)
      if (allow == null) allow = "DELETE"; else
        allow = allow + ", DELETE";
    if (ALLOW_TRACE)
      if (allow == null) allow = "TRACE"; else
        allow = allow + ", TRACE";
    if (ALLOW_OPTIONS) {
      if (allow == null) allow = "OPTIONS"; else
        allow = allow + ", OPTIONS";
    }
    resp.setHeader("Allow", allow);
  }


  protected void doTrace(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String CRLF = "
";
    String responseString = "TRACE " + req.getRequestURI() + " " + req.getProtocol();


    Enumeration reqHeaderEnum = req.getHeaderNames();


    while (reqHeaderEnum.hasMoreElements()) {
      String headerName = (String)reqHeaderEnum.nextElement();
      responseString = responseString + CRLF + headerName + ": " + req.getHeader(headerName);
    }


    responseString = responseString + CRLF;


    int responseLength = responseString.length();


    resp.setContentType("message/http");
    resp.setContentLength(responseLength);
    ServletOutputStream out = resp.getOutputStream();
    out.print(responseString);
    out.close();
  }


  protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String method = req.getMethod();


    if (method.equals("GET")) {
      long lastModified = getLastModified(req);
      if (lastModified == -1L)
      {
        doGet(req, resp);
      } else {
        long ifModifiedSince = req.getDateHeader("If-Modified-Since");
        if (ifModifiedSince < lastModified / 1000L * 1000L)
        {
          maybeSetLastModified(resp, lastModified);
          doGet(req, resp);
        } else {
          resp.setStatus(304);
        }
      }
    }
    else if (method.equals("HEAD")) {
      long lastModified = getLastModified(req);
      maybeSetLastModified(resp, lastModified);
      doHead(req, resp);
    }
    else if (method.equals("POST")) {
      doPost(req, resp);
    }
    else if (method.equals("PUT")) {
      doPut(req, resp);
    }
    else if (method.equals("DELETE")) {
      doDelete(req, resp);
    }
    else if (method.equals("OPTIONS")) {
      doOptions(req, resp);
    }
    else if (method.equals("TRACE")) {
      doTrace(req, resp);
    }
    else
    {
      String errMsg = lStrings.getString("http.method_not_implemented");
      Object[] errArgs = new Object[1];
      errArgs[0] = method;
      errMsg = MessageFormat.format(errMsg, errArgs);


      resp.sendError(501, errMsg);
    }
  }


  private void maybeSetLastModified(HttpServletResponse resp, long lastModified)
  {
    if (resp.containsHeader("Last-Modified"))
      return;
    if (lastModified >= 0L)
      resp.setDateHeader("Last-Modified", lastModified);
  }


  public void service(ServletRequest req, ServletResponse res)
    throws ServletException, IOException
  {
    HttpServletRequest request;
    HttpServletResponse response;
    try
    {
      request = (HttpServletRequest)req;
      response = (HttpServletResponse)res;
    } catch (ClassCastException e) {
      throw new ServletException("non-HTTP request or response");
    }
    service(request, response);
  }
}