/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet.resource;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.StringUtils;
import org.springframework.web.HttpRequestHandler;
import org.springframework.web.context.ServletContextAware;

/**
 * An {@link HttpRequestHandler} for serving static files using the Servlet container's "default" Servlet.
 *
 * <p>This handler is intended to be used with a "/*" mapping when the
 * {@link org.springframework.web.servlet.DispatcherServlet DispatcherServlet}
 * is mapped to "/", thus  overriding the Servlet container's default handling of static resources.
 * The mapping to this handler should generally be ordered as the last in the chain so that it will
 * only execute when no other more specific mappings (i.e., to controllers) can be matched.
 *
 * <p>Requests are handled by forwarding through the {@link RequestDispatcher} obtained via the
 * name specified through the {@link #setDefaultServletName "defaultServletName" property}.
 * In most cases, the {@code defaultServletName} does not need to be set explicitly, as the
 * handler checks at initialization time for the presence of the default Servlet of well-known
 * containers such as Tomcat, Jetty, Resin, WebLogic and WebSphere. However, when running in a
 * container where the default Servlet's name is not known, or where it has been customized
 * via server configuration, the  {@code defaultServletName} will need to be set explicitly.
 *
 * @author Jeremy Grelle
 * @author Juergen Hoeller
 * @since 3.0.4
 */
public class DefaultServletHttpRequestHandler implements HttpRequestHandler, ServletContextAware {

	/** Default Servlet name used by Tomcat, Jetty, JBoss, and GlassFish. */
	private static final String COMMON_DEFAULT_SERVLET_NAME = "default";

	/** Default Servlet name used by Google App Engine. */
	private static final String GAE_DEFAULT_SERVLET_NAME = "_ah_default";

	/** Default Servlet name used by Resin. */
	private static final String RESIN_DEFAULT_SERVLET_NAME = "resin-file";

	/** Default Servlet name used by WebLogic. */
	private static final String WEBLOGIC_DEFAULT_SERVLET_NAME = "FileServlet";

	/** Default Servlet name used by WebSphere. */
	private static final String WEBSPHERE_DEFAULT_SERVLET_NAME = "SimpleFileServlet";


	@Nullable
	private String defaultServletName;

	@Nullable
	private ServletContext servletContext;


	/**
	 * Set the name of the default Servlet to be forwarded to for static resource requests.
	 */
	public void setDefaultServletName(String defaultServletName) {
		this.defaultServletName = defaultServletName;
	}

	/**
	 * If the {@code defaultServletName} property has not been explicitly set,
	 * attempts to locate the default Servlet using the known common
	 * container-specific names.
	 */
	@Override
	public void setServletContext(ServletContext servletContext) {
		this.servletContext = servletContext;
		if (!StringUtils.hasText(this.defaultServletName)) {
			if (this.servletContext.getNamedDispatcher(COMMON_DEFAULT_SERVLET_NAME) != null) {
				this.defaultServletName = COMMON_DEFAULT_SERVLET_NAME;
			}
			else if (this.servletContext.getNamedDispatcher(GAE_DEFAULT_SERVLET_NAME) != null) {
				this.defaultServletName = GAE_DEFAULT_SERVLET_NAME;
			}
			else if (this.servletContext.getNamedDispatcher(RESIN_DEFAULT_SERVLET_NAME) != null) {
				this.defaultServletName = RESIN_DEFAULT_SERVLET_NAME;
			}
			else if (this.servletContext.getNamedDispatcher(WEBLOGIC_DEFAULT_SERVLET_NAME) != null) {
				this.defaultServletName = WEBLOGIC_DEFAULT_SERVLET_NAME;
			}
			else if (this.servletContext.getNamedDispatcher(WEBSPHERE_DEFAULT_SERVLET_NAME) != null) {
				this.defaultServletName = WEBSPHERE_DEFAULT_SERVLET_NAME;
			}
			else {
				throw new IllegalStateException("Unable to locate the default servlet for serving static content. " +
						"Please set the 'defaultServletName' property explicitly.");
			}
		}
	}


	@Override
	public void handleRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		Assert.state(this.servletContext != null, "No ServletContext set");
		RequestDispatcher rd = this.servletContext.getNamedDispatcher(this.defaultServletName);
		if (rd == null) {
			throw new IllegalStateException("A RequestDispatcher could not be located for the default servlet '" +
					this.defaultServletName + "'");
		}
		rd.forward(request, response);
	}

}
