1.2Servlet学习——Servlet和HttpServlet源码解析及Servlet生命周期

Servlet抽象类：

package javax.servlet;

import java.io.IOException;

public interface Servlet {

    public void init(ServletConfig config) throws ServletException;//初始化

    public ServletConfig getServletConfig();//获取ServletConfig对象

    public void service(ServletRequest req, ServletResponse res)//调用service方法
	throws ServletException, IOException;
	
    public String getServletInfo();//返回servlet的信息
	
    public void destroy();//销毁
}

HttpServlet类：

package javax.servlet.http;

import java.io.IOException;
import java.io.PrintWriter;
import java.io.OutputStreamWriter;
import java.io.UnsupportedEncodingException;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.Locale;
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
{
    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(LSTRING_FILE);
   

    public HttpServlet() { }
    
    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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }

    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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }

    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 {
            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);
        }
    }
    //将ServleRerquest和ServletResponse 转化
 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);
    }
}

Servlet的生命周期：

1.加载和实例化：

在服务器运行中，客户机首次向Servlet发送请求或是重新装入Servlet时（如服务器重新启动、Servlet被修改）时Servlet容器会加载和实例化一个Servlet。当Servlet配置了自动装入选项时（load-on-startup）时，服务器在启动时会自动装入Servlet。

2.初始化

Servlet实例化后，Servlet容器会调用Servlet的init(ServletConfig config)方法来进行初始化(Servlet对象由Servlet容器创建并传递给Servlet，并在初始化完成后一直在内存中存在，直达Servlet被销毁)。

若初始化成功，Servlet在Web容器中会处于服务可用状态，若失败则被Servlet容器清理掉。

当Servlet运行出现异常的时候，Servlet容器会使该实例变为服务不可用状态。人为的也可也设置Servlet是否可用。

3.处理请求

服务器接受到客户端请求时，会为该请求创建一个“请求”和“响应”对象，并调用service（）方法，service调用其他方法来处理请求。

在Servlet的生命周期内，service()方法可以被多次调用。采用多线程的方式为每个请求创建一个线程。

4.销毁

当Servlet容器需要终止Servlet（如web服务器关掉），它会调用Servlet的destroy()方法释放正在使用的资源。在调用destroy()之前，会让service()方法的任何线程完成执行或超过服务器定义的时间限制。destroy()方法完成后，Servlet容器必须释放Servlet实例以便被垃圾回收。