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本文主要研究一下spring.mvc.servlet.load-on-startup

spring.mvc.servlet.load-on-startup

org/springframework/boot/autoconfigure/web/servlet/WebMvcProperties.java

@ConfigurationProperties(prefix = "spring.mvc")
public class WebMvcProperties {//......private final Servlet servlet = new Servlet();public static class Servlet {/*** Path of the dispatcher servlet.*/private String path = "/";/*** Load on startup priority of the dispatcher servlet.*/private int loadOnStartup = -1;public String getPath() {return this.path;}public void setPath(String path) {Assert.notNull(path, "Path must not be null");Assert.isTrue(!path.contains("*"), "Path must not contain wildcards");this.path = path;}public int getLoadOnStartup() {return this.loadOnStartup;}public void setLoadOnStartup(int loadOnStartup) {this.loadOnStartup = loadOnStartup;}public String getServletMapping() {if (this.path.equals("") || this.path.equals("/")) {return "/";}if (this.path.endsWith("/")) {return this.path + "*";}return this.path + "/*";}public String getPath(String path) {String prefix = getServletPrefix();if (!path.startsWith("/")) {path = "/" + path;}return prefix + path;}public String getServletPrefix() {String result = this.path;int index = result.indexOf('*');if (index != -1) {result = result.substring(0, index);}if (result.endsWith("/")) {result = result.substring(0, result.length() - 1);}return result;}}	
}

WebMvcProperties.Servlet定義了loadOnStartup屬性，默認為-1

DispatcherServletAutoConfiguration

org/springframework/boot/autoconfigure/web/servlet/DispatcherServletAutoConfiguration.java

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {//......@Configuration(proxyBeanMethods = false)@Conditional(DispatcherServletRegistrationCondition.class)@ConditionalOnClass(ServletRegistration.class)@EnableConfigurationProperties(WebMvcProperties.class)@Import(DispatcherServletConfiguration.class)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) {DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,webMvcProperties.getServlet().getPath());registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());multipartConfig.ifAvailable(registration::setMultipartConfig);return registration;}}//......
}	

DispatcherServletRegistrationConfiguration注冊了DispatcherServletRegistrationBean，它會讀取webMvcProperties.getServlet().getLoadOnStartup()然后設置其loadOnStartup屬性

ServletRegistrationBean

org/springframework/boot/web/servlet/ServletRegistrationBean.java

public class ServletRegistrationBean<T extends Servlet> extends DynamicRegistrationBean<ServletRegistration.Dynamic> {private static final String[] DEFAULT_MAPPINGS = { "/*" };private T servlet;private Set<String> urlMappings = new LinkedHashSet<>();private boolean alwaysMapUrl = true;private int loadOnStartup = -1;private MultipartConfigElement multipartConfig;//......@Overrideprotected void configure(ServletRegistration.Dynamic registration) {super.configure(registration);String[] urlMapping = StringUtils.toStringArray(this.urlMappings);if (urlMapping.length == 0 && this.alwaysMapUrl) {urlMapping = DEFAULT_MAPPINGS;}if (!ObjectUtils.isEmpty(urlMapping)) {registration.addMapping(urlMapping);}registration.setLoadOnStartup(this.loadOnStartup);if (this.multipartConfig != null) {registration.setMultipartConfig(this.multipartConfig);}}}	

ServletRegistrationBean定義了loadOnStartup屬性，默認為-1，其configure方法會設置loadOnStartup到ServletRegistration.Dynamic

StandardWrapper

org/apache/catalina/core/StandardWrapper.java

    /*** Set the load-on-startup order value (negative value means* load on first call).** @param value New load-on-startup value*/@Overridepublic void setLoadOnStartup(int value) {int oldLoadOnStartup = this.loadOnStartup;this.loadOnStartup = value;support.firePropertyChange("loadOnStartup",Integer.valueOf(oldLoadOnStartup),Integer.valueOf(this.loadOnStartup));}/*** @return the load-on-startup order value (negative value means* load on first call).*/@Overridepublic int getLoadOnStartup() {if (isJspServlet && loadOnStartup < 0) {/** JspServlet must always be preloaded, because its instance is* used during registerJMX (when registering the JSP* monitoring mbean)*/return Integer.MAX_VALUE;} else {return this.loadOnStartup;}}    

loadOnStartup屬性最后設置到了tomcat的StandardWrapper

StandardContext

org/apache/catalina/core/StandardContext.java

    /*** Load and initialize all servlets marked "load on startup" in the* web application deployment descriptor.** @param children Array of wrappers for all currently defined*  servlets (including those not declared load on startup)* @return <code>true</code> if load on startup was considered successful*/public boolean loadOnStartup(Container children[]) {// Collect "load on startup" servlets that need to be initializedTreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();for (Container child : children) {Wrapper wrapper = (Wrapper) child;int loadOnStartup = wrapper.getLoadOnStartup();if (loadOnStartup < 0) {continue;}Integer key = Integer.valueOf(loadOnStartup);ArrayList<Wrapper> list = map.get(key);if (list == null) {list = new ArrayList<>();map.put(key, list);}list.add(wrapper);}// Load the collected "load on startup" servletsfor (ArrayList<Wrapper> list : map.values()) {for (Wrapper wrapper : list) {try {wrapper.load();} catch (ServletException e) {getLogger().error(sm.getString("standardContext.loadOnStartup.loadException",getName(), wrapper.getName()), StandardWrapper.getRootCause(e));// NOTE: load errors (including a servlet that throws// UnavailableException from the init() method) are NOT// fatal to application startup// unless failCtxIfServletStartFails="true" is specifiedif(getComputedFailCtxIfServletStartFails()) {return false;}}}}return true;}

StandardContext的loadOnStartup方法會取出所有loadOnStartup大于等于0的wrapper，按loadOnStartup值放入到TreeMap<Integer, ArrayList<Wrapper>>，然后遍歷該TreeMap挨個執行wrapper.load()進行加載

小結

springboot的spring.mvc.servlet.load-on-startup屬性，最后設置到tomcat的StandardWrapper；而tomcat的StandardContext的loadOnStartup方法會取出所有loadOnStartup大于等于0的wrapper，按loadOnStartup值放入到TreeMap<Integer, ArrayList<Wrapper>>，然后遍歷該TreeMap挨個執行wrapper.load()進行加載。