环境
- JDK 8
- Spring Tool Suite 4.6.1
- Servlet 3.1
- Tomcat 8.5
- Maven 3.6.3
HttpServlet 说明
HttpServlet 继承于 GenericServlet，关于 GenericServlet 的说明参考：JavaWeb：GenericServlet
METHOD 属性
在 HttpServlet 中定义了七种 http 请求的方法，分别是：
- OPTIONS
- GET
- HEAD
- POST
- PUT
- DELETE
- TRACE
我们最常用的是 GET 和 POST。
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";
HEADER 属性
网页过期和缓存相关的两个消息头。
private static final String HEADER_IFMODSINCE = "If-Modified-Since";
private static final String HEADER_LASTMOD = "Last-Modified";
国际化属性
还有两个和 GenericServlet 类似的国际化属性。
private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
private static ResourceBundle lStrings = ResourceBundle.getBundle(LSTRING_FILE);
doXXX 系列方法
根据不同的请求类型，会调用对应的方法。
如果请求的方式没有实现，则会给客户端返回一个错误页面。
service
除了之前的 service 方法，新增了一个 service(HttpServletRequest req, HttpServletResponse resp) 方法，
可以避免对 req 和 resp 的转换，HttpServletRequest、HttpServletResponse、ServletResponse、ServletRequest 之后学习。
getLastModified
页面缓存相关，获取最后的修改时间，-1 表示时间未知，不缓存。
protected long getLastModified(HttpServletRequest req) {
return -1;
}
HttpServlet 示例
pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>jiangbo.javaweb</groupId>
<artifactId>08java-web-http-servlet</artifactId>
<version>1.0.0</version>
<packaging>war</packaging>
<description>Http Servlet 的示例</description>
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version>
<scope>provided</scope>
</dependency>
</dependencies>
</project>
方法示例
package jiangbo.javaweb;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@SuppressWarnings("serial")
public class Servlet01 extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("service...");
resp.getWriter().write("Hello World!");
}
}
配置 web.xml
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
version="3.1">
<servlet>
<servlet-name>servlet01</servlet-name>
<servlet-class>jiangbo.javaweb.Servlet01</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>servlet01</servlet-name>
<url-pattern>/servlet01</url-pattern>
</servlet-mapping>
</web-app>
浏览器访问
将该项目部署到服务器并启动，在客户端访问 http://localhost:8080/08java-web-http-servlet/servlet01 项目地址，然后关闭 tomcat。
控制台输出
service...
Hello World!
HttpServlet 源码分析
请求的转化
HttpServlet 将 ServletRequest 转为了 HttpServletRequest，将 ServletResponse 转为了 HttpServletResponse。
@Override
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);
}
请求类型的分发
HttpServlet 将不同的请求类型分发到不同的方法上。可以看到，用的只是普通的 if-else 判断
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 {
//
// 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);
}
}