[Java知识库]JavaWeb Tomcat （三） HttpServlet

HttpServlet 继承结构

• 真正开发webapp时，创建的Servlet继承的是HttpServlet类
• 先看一下HttpServlet类的继承结构

HttpServlet类继承了GenericServlet抽象类

public abstract class HttpServlet extends GenericServlet

而GenericServlet类实现了Servlet, ServletConfig, java.io.Serializable接口

public abstract class GenericServlet implements Servlet, ServletConfig,
        java.io.Serializable 

HttpServlet执行步骤

• 从Tomcat创建一个Servlet开始看一下HttpServlet类中方法的调用步骤，为方便叙述，继承HttpServlet类的HelloHttpServlet类就是要创建的Servlet。

public class HelloHttpServlet extends HttpServlet

1. 用户通过浏览器访问webapp

2. Tomcat会分析浏览器中的资源地址。通过Web.xml配置文件查找到需要创建的Servlet类。

3. Tomcat创建HelloHttpServlet类的对象，并将Web.xml配置文件传给ServletConfig

4. 执行init方法。

作用：传入配置文件给全局变量config

	//HttpServlet类中的代码
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

5. 上述代码执行this.init()

如果HelloHttpServlet类中继承了该方法，则会执行HelloHttpServlet类中的init()

	//HttpServlet类中的代码
    public void init() throws ServletException {
        // NOOP by default
    }

6. Tomcat 继续调用service(ServletRequest req, ServletResponse res)

作用：将传入的参数强制类型转换，继续调用service方法并将转换后的变量传入

	//HttpServlet类中的代码
    @Override
    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(lStrings.getString("http.non_http"));
        }
        //调用了service(HttpServletRequest req, HttpServletResponse resp)
        service(request, response);
    }

7. 在上述代码的末尾调用了 service(request, response)

作用：获得了用户请求中的请求类型，根据类型执行对应的方法。

	//HttpServlet类中的代码
    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;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // 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);
        }
    }

8. 在HelloHttpServlet类中重写相应的方法，

在重写方法时，要明确用户发送的是GET、POST或者是被的请求。 不能把所有的响应类型都写上。
如果用户发送的请求类型不对，HttpServlet会返回405错误（请求类型错误）。

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          //Todo:
     }

     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          //Todo:
     }

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