聊聊spring.mvc.servlet.load-on-startup

序

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定义了loadOnStartup属性，默认为-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注册了DispatcherServletRegistrationBean，它会读取webMvcProperties.getServlet().getLoadOnStartup()然后设置其loadOnStartup属性

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;

    //......

    @Override
    protected 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定义了loadOnStartup属性，默认为-1，其configure方法会设置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
     */
    @Override
    public 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).
     */
    @Override
    public 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属性最后设置到了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 initialized
        TreeMap<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" servlets
        for (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 specified
                    if(getComputedFailCtxIfServletStartFails()) {
                        return false;
                    }
                }
            }
        }
        return true;

    }

StandardContext的loadOnStartup方法会取出所有loadOnStartup大于等于0的wrapper，按loadOnStartup值放入到TreeMap<Integer, ArrayList<Wrapper>>，然后遍历该TreeMap挨个执行wrapper.load()进行加载

小结

springboot的spring.mvc.servlet.load-on-startup属性，最后设置到tomcat的StandardWrapper；而tomcat的StandardContext的loadOnStartup方法会取出所有loadOnStartup大于等于0的wrapper，按loadOnStartup值放入到TreeMap<Integer, ArrayList<Wrapper>>，然后遍历该TreeMap挨个执行wrapper.load()进行加载。