Spring MVC之Servlet2.x与Servlet3.x的区别
一、基于Servlet 2.x的Web应用
1.定义Servlet
public class EchoServlet extends HttpServlet {
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter writer = response.getWriter();
writer.println("Hello world!");
writer.flush();
}
}
2.定义Filter
public class EchoFilter implements Filter {
public void init(FilterConfig filterConfig) throws ServletException {
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
System.out.println("before...");
chain.doFilter(request, response);
System.out.println("after...");
}
public void destroy() {
}
}
3.部署描述文件web.xml
<?xml version="1.0" encoding="UTF-8"?>
<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_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>EchoServlet</servlet-name>
<servlet-class>servlet2.EchoServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>EchoServlet</servlet-name>
<url-pattern>/echo</url-pattern>
</servlet-mapping>
<filter>
<filter-name>EchoFilter</filter-name>
<filter-class>servlet2.EchoFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>EchoFilter</filter-name>
<url-pattern>/echo/*</url-pattern>
</filter-mapping>
</web-app>
在Servlet2.x中，Servlet、Filter的定义必须通过部署描述文件web.xml进行描述。
二、Servlet3.x特性
1.使用注解定义Servlet、Filter、Listener(无需定义web.xml)
Servlet:
// Servlet
@WebServlet(name = "echoServlet", urlPatterns = "/echo")
public class EchoServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
PrintWriter writer = response.getWriter();
writer.println("Hello world!+1");
writer.flush();
}
}
@WebServlet(urlPatterns = "/hello", loadOnStartup = 1,
initParams = {
@WebInitParam(name = "db", value = "jdbc://..."),
@WebInitParam(name = "dbuser", value = "fred"),
@WebInitParam(name = "dbpasswd", value = "fred")
})
public class HelloServlet extends HttpServlet {
@Override
public void init(ServletConfig config) throws ServletException {
String jdbc = config.getInitParameter("db"); // 获取Servlet初始化参数
System.out.println("db: " + jdbc);
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.getWriter().println("hello+2");
resp.getWriter().flush();
}
}
Filter:
@WebFilter(urlPatterns = "/hello", dispatcherTypes = DispatcherType.REQUEST)
public class HelloServletFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
// ...
chain.doFilter(request, response);
System.out.println("filtered...");
}
@Override
public void destroy() {
}
}
Listener:
@WebListener("Auto configure Struts")
public class MyListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
ServletContext context = sce.getServletContext();
// register the servlet
ServletRegistration.Dynamic registration =
context.addServlet("echo", "servlet30.EchoServlet");
// set init param
registration.setInitParameter("com.jay.anno.config", "/WEB-INF/struts-com.jay.anno.config.xml");
registration.setLoadOnStartup(2);
// add mapping
registration.addMapping("/echo");
// Annotate with @MultipartConfig 处理multipart/form-data
registration.setMultipartConfig(new MultipartConfigElement("/tmp"));
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
}
}
2.Multipart支持
@WebServlet("/multi")
@MultipartConfig(location = "/tmp")
public class MultiPartServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
Part part = req.getPart("myFile"); // 读取文件
InputStream in = part.getInputStream();
// read file...
byte[] data = new byte[1024];
int len;
while((len=in.read(data)) != -1) {
System.out.println(new String(data, 0, len));
}
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
3.动态配置Servlet、Filter、Listener
(1) 使用ServletContextListener
ServletContextListener由应用程序开发人员提供实现，其contextInitialized方法在应用程序启动时调用。
// 编程式配置
@WebListener("Auto configure Struts")
public class MyListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
ServletContext context = sce.getServletContext();
// register the servlet 注册Servlet
ServletRegistration.Dynamic registration =
context.addServlet("echo", "servlet30.EchoServlet");
// set init param 设置初始化参数
registration.setInitParameter("com.jay.anno.config", "/WEB-INF/struts-com.jay.anno.config.xml");
registration.setLoadOnStartup(2);
// add mapping 添加映射
registration.addMapping("/echo");
// Annotate with @MultipartConfig 处理multipart/form-data
registration.setMultipartConfig(new MultipartConfigElement("/tmp"));
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
}
}
以上代码通过定义ServletContextListener实现，在其contextInitialized回调的时候动态配置EchoServlet。
(2) 使用ServletContainerInitializer
ServletContainerInitializer主要由框架实现，在应用程序启动时调用其onStartUp()方法，负责框架的初始化动作，如初始化控制器Servlet(对应Spring MVC，DispatcherServlet)、连接数据库等。下面给出简单示例：
@HandlesTypes(StrategyHandler.class) // 感兴趣的类型
public class MyServletContainerInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
// 在这里动态配置Servlet、Filter、Listener
System.out.println("scanned clazz:");
c.forEach(clazz -> {
System.out.println("\t" + clazz.getCanonicalName());
});
}
}
然后在META-INF/services目录下定义该实现，文件名为javax.servlet.ServletContainerInitializer，文件内容为initializer.MyServletContainerInitializer，Web容器通过SPI机制发现该实现。
输出：
scanned clazz:
servlet30.handler.impl.BizStrategyHandler
4.异步Servlet
非常适用于以下情况的 web 应用程序：
- 长处理时间或伪长处理时间(长轮询)
- 等待资源释放 — 如数据库连接
- 等待事件发生 — 如聊天消息
- 等待缓慢服务的响应 — 如 web 服务
@WebServlet(urlPatterns = "/async", asyncSupported = true) // Servlet声明支持异步
public class AsyncServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// Put the request in async mode 1.将请求置于异步模式，释放Tomcat请求线程
AsyncContext asyncContext = req.startAsync();
// Create an instance of handler
SlowResourceHandler handler = new SlowResourceHandler(asyncContext);
// Starts the process 2.启动工作线程处理该请求
asyncContext.start(handler);
// Request will not commit on return 对于请求的客户端，请求不会返回
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
public class SlowResourceHandler implements Runnable {
private AsyncContext asyncContext;
public SlowResourceHandler(AsyncContext asyncContext) {
this.asyncContext = asyncContext;
}
public void run() {
try {
int i = ThreadLocalRandom.current().nextInt(3) + 1;
TimeUnit.SECONDS.sleep(i); // 模拟长时间的处理
asyncContext.getResponse().getWriter().println("finished..." + i);
} catch (Exception e) {
//
}
// 3.只有工作线程处理完毕，调用AsyncContext.complete()方法，客户端请求才返回
asyncContext.complete();
}
}
