SpringBoot Request DispatchServlet 注册
2023-03-24, by zhaosiyuan
总在好奇发出的 Http 请求， Spring 是如何精确的定位到具体的方法的，请求的参数又如何解析的？
Spring Framework文档版本5.3.26Spring Boot版本2.3.4.RELEASE
1.DispatchServlet 何时注册
早期写 JavaWeb 的时候，每一个 Servlet 都需要在 web.xml 中注册，而在 SpringBoot 中找了很久却没有找到 web.xml ，调试发现 SpringBoot 是
采用代码注册的。后面查看文档，发现 Spring Framework>Web Servlet>Spring MVC>DispatcherServlet 文档中有写
The DispatcherServlet, as any Servlet, needs to be declared and mapped according to the Servlet specification by using Java configuration or in web.xml. In turn, the DispatcherServlet uses Spring configuration to discover the delegate components it needs for request mapping, view resolution, exception handling
1.1 代码跟踪
在 DispatcherServletAutoConfiguration 中向 Spring 容器注入 DispatcherServletRegistrationBean
public class DispatcherServletAutoConfiguration {
protected static class DispatcherServletConfiguration {
@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public DispatcherServlet dispatcherServlet(WebMvcProperties webMvcProperties) {
...
return dispatcherServlet;
}
}
protected static class DispatcherServletRegistrationConfiguration {
@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,
WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {
...
return registration;
}
}
}
DispatcherServletRegistrationBean 实现了 ServletContextInitializer 接口
public interface ServletContextInitializer {
void onStartup(ServletContext servletContext) throws ServletException;
}
可以这么理解， Spring 容器会在合适的时机，来通知 DispatcherServletRegistrationBean 可以向 Servlet 容器中注册 Servlet 了
DispatcherServletRegistrationBean 的继承关系如下
public class DispatcherServletRegistrationBean extends ServletRegistrationBean<DispatcherServlet>
implements DispatcherServletPath {
}
public class ServletRegistrationBean<T extends Servlet> extends DynamicRegistrationBean<ServletRegistration.Dynamic> {
@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
String name = getServletName();
// Web 容器中注册 Servlet
return servletContext.addServlet(name, this.servlet);
}
}
public abstract class DynamicRegistrationBean<D extends Registration.Dynamic> extends RegistrationBean {
@Override
protected final void register(String description, ServletContext servletContext) {
//开始注册
D registration = addRegistration(description, servletContext);
if (registration == null) {
logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");
return;
}
configure(registration);
}
}
public abstract class RegistrationBean implements ServletContextInitializer, Ordered {
/*回调注册*/
@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
String description = getDescription();
if (!isEnabled()) {
logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
return;
}
//注册
register(description, servletContext);
}
protected abstract void register(String description, ServletContext servletContext);
}
public interface ServletContextInitializer {
void onStartup(ServletContext servletContext) throws ServletException;
}
大概向容器中注册 Servlet 的流程是这样的
那么何时注册呢？具体又是怎么注册的呢？
在 run 启动类后
SpringApplication.run(MainApplication.class, args);
public class SpringApplication {
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
return new SpringApplication(primarySources).run(args);
}
public ConfigurableApplicationContext run(String... args) {
...
refreshContext(context);
...
}
private void refreshContext(ConfigurableApplicationContext context) {
refresh((ApplicationContext) context);
...
}
}
经过一系列的 refresh 后最终会来到父类的 onRefresh
public class ServletWebServerApplicationContext extends GenericWebApplicationContext
implements ConfigurableWebServerApplicationContext {
protected void onRefresh() {
...
createWebServer();
...
}
private void createWebServer() {
...
//这里会获取 WebServerFactory，因为工程引入的是 Tomcat，所以这里为 TomcatServletWebServerFactory
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
...
}
}
public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory
implements ConfigurableTomcatWebServerFactory, ResourceLoaderAware {
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
...
return getTomcatWebServer(tomcat);
}
protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
}
}
再来到 TomcatWebServer
public class TomcatWebServer implements WebServer {
public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
...
initialize();
}
private void initialize() throws WebServerException {
...
this.tomcat.start();
...
}
}
public class Tomcat {
public void start() throws LifecycleException {
...
server.start();
}
}
经过一系列的方法最终回调到 TomcatStarter 的 onStartup 方法
/**
* {@link ServletContainerInitializer} used to trigger {@link ServletContextInitializer
* ServletContextInitializers} and track startup errors.
*
* @author Phillip Webb
* @author Andy Wilkinson
*/
class TomcatStarter implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
...
for (ServletContextInitializer initializer : this.initializers) {
initializer.onStartup(servletContext);
}
...
}
}
public class ServletWebServerApplicationContext extends GenericWebApplicationContext
implements ConfigurableWebServerApplicationContext {
private void selfInitialize(ServletContext servletContext) throws ServletException {
...
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
//查找类型为 ServletContextInitializer 的 JavaBean
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
beans.onStartup(servletContext);
}
}
}
可以看到最终会去 Spring 容器中查找类型为 ServletContextInitializer 的 JavaBean，然后调用其 onStartup 方法， 而我们的开头提到的 DispatcherServletRegistrationBean 正是实现了 ServletContextInitializer 接口
1.2 注册自己的 Servlet
查看源码，可以知道，要注册自己的 Servlet 除了使用原生的注入外，还可以在容器中注入一个 继承 ServletRegistrationBean 的 Java Bean
public class MyServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.getWriter().write("hallo ya");
}
}
@Configuration
public class MyConfig {
@Bean
public ServletRegistrationBean<MyServlet> myServlet(){
MyServlet myServlet = new MyServlet();
return new ServletRegistrationBean<>(myServlet, "/my");
}
}
/**************************原生方式**************************/
@WebServlet(urlPatterns = "/raw")
public class RawServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.getWriter().write("raw servlet");
}
}
@RestController
@ServletComponentScan(basePackages = "com.zhaosiyuan.springboot.servlet")//不要忘记这个注解
@SpringBootApplication(scanBasePackages = "com.zhaosiyuan")
public class MainApplication {
}
1.3 小结
-
SpringBoot经过一系列的初始化，上下文环境准备完成之后，会发出ServletContext的初始化通知（即开始回调实现ServletContextInitializer接口的类）， 之后开始向Web Server中注册Servlet -
如果想修改
DispatchServlet的访问路径，可以通过在application.yml中修改spring.mvc.servlet.path来实现 -
注册自己的
Servlet除了使用原生的注入外，还可以在容器中注入一个 继承ServletRegistrationBean的Java Bean