定义
servlet是service applet的缩写,即表示运行在服务端的应用。狭义的servlet是jdk中的一个接口,广义的解释实现了该接口的类。
继承体系
显而易见,这个体系比之前分析Spring的创建bean涉及的体系要简单的多。
接下来我们单独分析下这个体系涉及的接口
Servlet
package javax.servlet;
import java.io.IOException;
/**
* Defines methods that all servlets must implement.
*
* <p>
* A servlet is a small Java program that runs within a Web server. Servlets
* receive and respond to requests from Web clients, usually across HTTP, the
* HyperText Transfer Protocol.
*
* <p>
* To implement this interface, you can write a generic servlet that extends
* <code>javax.servlet.GenericServlet</code> or an HTTP servlet that extends
* <code>javax.servlet.http.HttpServlet</code>.
*
* <p>
* This interface defines methods to initialize a servlet, to service requests,
* and to remove a servlet from the server. These are known as life-cycle
* methods and are called in the following sequence:
* <ol>
* <li>The servlet is constructed, then initialized with the <code>init</code>
* method.
* <li>Any calls from clients to the <code>service</code> method are handled.
* <li>The servlet is taken out of service, then destroyed with the
* <code>destroy</code> method, then garbage collected and finalized.
* </ol>
*
* <p>
* In addition to the life-cycle methods, this interface provides the
* <code>getServletConfig</code> method, which the servlet can use to get any
* startup information, and the <code>getServletInfo</code> method, which allows
* the servlet to return basic information about itself, such as author,
* version, and copyright.
*
* @see GenericServlet
* @see javax.servlet.http.HttpServlet
*/
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();
}
其实jdk的介绍说的很清楚,我们现在对它总结下
1.所有的Servlets都必须实现这个接口,一个Servlets就是一个运行在web服务器中的小应用,这个应用大概干了什么勒?处理从web客户端来的Request 然后返回一个Response。
2.这个接口定义了生命周期函数,如如何初始化这个Servlets,这个Servlets如何处理请求,这个Servlets如何从web服务器中被移除,此外,他还提供了获取这个Servlets基本信息的方法,和获取初始化信息的方法
ServletConfig
首先应该清楚,这个是对servlet级别的配置信息,这也是一种设计思路,许多框架都会对配置信息进行抽象。
package javax.servlet;
import java.util.Enumeration;
/**
* A servlet configuration object used by a servlet container to pass
* information to a servlet during initialization.
*/
public interface ServletConfig {
//获取名称
public String getServletName();
//获取容器
public ServletContext getServletContext();
//根据初始化参数名称获取参数值
public String getInitParameter(String name);
//获取初始化参数名称
public Enumeration<String> getInitParameterNames();
}
如我们在没有SpringBoot前要写Web.xml,假如用到了Spring Mvc 那么我们要用如下的配置
<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/springmvc-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
那么在在该Servlet在加载的时候就会把contextConfigLocation对应的值封装到servlet config中去。
GenericServlet
这是一个与具体协议无关的抽象实现,它实现了ServletConfig接口中定义的方法,留下Service给具体的协议相关Servlet去实现
HttpServlet
HttpServlet是对GenericServlet的http协议的拓展
它核心干了什么事情勒?在运行器,它通过Service处理ServletRequest,它首先会把它转换成HttpServletRequest,然后根据请求的方法类型,如post,get等来把请求路由到具体的doPost,doGet等方法中去。代码如下
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 / 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);
}
}
我们在开发自己的Servlet时,我们直接继承这个类就好了。
这个类也有意思,它是抽象的,但是却没有抽象方法,因为doGet,doPost,doHead等等,我们通常不需要全都去实现,假如把它们都设置成抽象方法,那我们就不得不全都实现。现在我们只需要覆盖我们自己像覆盖的。