Spring Boot Servlet : ServletContextInitializer

概述

功能介绍

Spring Boot提供的在Servlet 3.0+环境中用于程序化配置ServletContext的接口。该接口ServletContextInitializer主要被RegistrationBean实现用于往ServletContext容器中注册Servlet,Filter或者EventListener。这些ServletContextInitializer的设计目的主要是用于这些实例被Spring IoC容器管理。这些ServletContextInitializer实例不会被SpringServletContainerInitializer检测，因此不会被Servlet容器自动启动。

该接口ServletContextInitializer和Spring Web的另外一个接口WebApplicationInitializer看起来几乎一模一样。但二者使用目的不同。Spring Web中,WebApplicationInitializer也是针对Servlet 3.0+环境，设计用于程序化配置ServletContext，跟传统的web.xml相对或者配合使用。WebApplicationInitializer实现类会被SpringServletContainerInitializer自动检测和启动。

继承关系

使用

关于ServletContextInitializer的应用可以参考下面的例子。TomcatStarter是Spring Boot Web Servlet应用结合Tomcat使用时的一个ServletContainerInitializer实现。从下面代码不难看出，一组ServletContextInitializer会被设置到ServletContainerInitializer TomcatStarter上，而TomcatStarter在Servlet容器启动过程中调用自己的方法#onStartup,会逐一调用这些ServletContextInitializer的方法#onStartup初始化ServletContext。

package org.springframework.boot.web.embedded.tomcat;

import java.util.Set;

import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.boot.web.servlet.ServletContextInitializer;


class TomcatStarter implements ServletContainerInitializer {

	private static final Log logger = LogFactory.getLog(TomcatStarter.class);

	// 使用者会指定一组 ServletContextInitializer 给 TomcatStarter
	private final ServletContextInitializer[] initializers;

	private volatile Exception startUpException;

	TomcatStarter(ServletContextInitializer[] initializers) {
		this.initializers = initializers;
	}

	// Servlet 容器启动时回会用该方法，该方法会逐一调用每个 ServletContextInitializer 的方法
	// #onStartup 会指定 ServletContext 进行初始化。这些 ServletContextInitializer 的目的
	// 通常会是 注册 Servlet, Filter 或者 EventListener 。
	@Override
	public void onStartup(Set<Class<?>> classes, ServletContext servletContext)
			throws ServletException {
		try {
			for (ServletContextInitializer initializer : this.initializers) {
				initializer.onStartup(servletContext);
			}
		}
		catch (Exception ex) {
			this.startUpException = ex;
			// Prevent Tomcat from logging and re-throwing when we know we can
			// deal with it in the main thread, but log for information here.
			if (logger.isErrorEnabled()) {
				logger.error("Error starting Tomcat context. Exception: "
						+ ex.getClass().getName() + ". Message: " + ex.getMessage());
			}
		}
	}

	public Exception getStartUpException() {
		return this.startUpException;
	}

}

源代码

源代码版本 : 2.1.3.RELEASE

package org.springframework.boot.web.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import org.springframework.web.SpringServletContainerInitializer;
import org.springframework.web.WebApplicationInitializer;


@FunctionalInterface
public interface ServletContextInitializer {

	/**
	 * Configure the given ServletContext with any servlets, filters, listeners
	 * context-params and attributes necessary for initialization.
	 * @param servletContext the ServletContext to initialize
	 * @throws ServletException if any call against the given ServletContext
	 * throws a ServletException
	 */
	void onStartup(ServletContext servletContext) throws ServletException;

}

参考文章

• Spring Boot Servlet : RegistrationBean