第三章 HttpServlet
- HttpServlet
- HttpServlet执行步骤
- 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。
- 5. 上述代码执行`this.init()`
- 6. Tomcat 继续调用`service(ServletRequest req, ServletResponse res)`
- 7. 在上述代码的末尾调用了 service(request, response)
- 8. 在HelloHttpServlet类中重写相应的方法,
HttpServlet
- 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,
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//Todo:
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//Todo:
}