Servlet解析

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Servlet继承图：

HttpServlet是用Http协议实现的Servlet基类，写Servlet时可以直接继承它；接下来首先看看ServletConfig接口。

public interface ServletConfig {

public String getServletName();

public ServletContext getServletContext();

public String getInitParameter(String name);

public Enumeration getInitParameterNames();

}

ServletConfig 中封装了我们在web.xml中定义的Servlet的一些配置信息；例如：

<servlet>
<servlet-name>demo</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>demo.servlet</param-value>
</init-param>
</servlet>

在ServletConfig中：

public String getServletName(); 获得Servlet的名字即：<servlet-name>demo</servlet-name>；

public ServletContext getServletContext(); 获得ServletContext代表这个应用本身；

public String getInitParameter(String name);获得init-param配置的参数；

public Enumeration getInitParameterNames();获取init-param中的名字集合，<param-name>contextConfigLocation</param-name>；

接下来看看Servlet的接口：

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();

}

init方法会在容器启动时调用一次；这时这个init方法会接受一个来自容器的ServletConfig类型的参数，这是由容器实现ServletConfig接口的类；

而这个参数GenericServlet 中有很大的作用。

getServletConfig()方法返回ServletConfig ，也就是init中传入的ServletConfig类型的参数；

service用于集体处理一个请求；有容器的Wrapper管道调用；

getServletInfo获取Servlet的一些相关信息；

destroy销毁Servlet；

接下来看看Servlet的默认实现类GenericServlet：

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

private transient ServletConfig config;

public GenericServlet() { }

public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}

public void init() throws ServletException {
}

public abstract void service(ServletRequest req, ServletResponse res) throws ServletException, IOException;

public String getServletInfo() {
return "";
}

public void destroy() {
}

// GenericServlet 实现了ServletConfig接口，实际上是在底层调用了init方法传进来的ServletConfig；

public String getServletName() {

ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletName(); //调用init传来的ServletConfig的getServletName();方法；
}

//调用init传来的ServletConfig的getServletContext();方法；

public ServletContext getServletContext() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletContext();
}

//调用init传来的ServletConfig的getInitParameterNames();方法；

public Enumeration getInitParameterNames() {

ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameterNames();
}

//调用init传来的ServletConfig的getInitParameter(name);方法；

public String getInitParameter(String name) {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}

return sc.getInitParameter(name);
}

}

接下来看看HttpServlet，它是用来处理请求的主要重写了service方法，它是用来将不同的请求路由到不同的处理方法，

所以其子类只需要实现doGet,doPost等就行了：

public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequestrequest;
HttpServletResponseresponse;

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

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) {
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
  if (ifModifiedSince < (lastModified / 1000 * 1000)) {
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);
}
}

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