/*
 * Copyright 2012-2020 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.boot.web.servlet.support;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

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

import org.springframework.boot.web.server.ErrorPage;
import org.springframework.boot.web.server.ErrorPageRegistrar;
import org.springframework.boot.web.server.ErrorPageRegistry;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.util.ClassUtils;
import org.springframework.web.filter.OncePerRequestFilter;
import org.springframework.web.util.NestedServletException;

A Servlet Filter that provides an ErrorPageRegistry for non-embedded applications (i.e. deployed WAR files). It registers error pages and handles application errors by filtering requests and forwarding to the error pages instead of letting the server handle them. Error pages are a feature of the servlet spec but there is no Java API for registering them in the spec. This filter works around that by accepting error page registrations from Spring Boot's ErrorPageRegistrar (any beans of that type in the context will be applied to this server). 
Author:
Dave Syer, Phillip Webb, Andy Wilkinson
Since:
2.0.0/**
 * A Servlet {@link Filter} that provides an {@link ErrorPageRegistry} for non-embedded
 * applications (i.e. deployed WAR files). It registers error pages and handles
 * application errors by filtering requests and forwarding to the error pages instead of
 * letting the server handle them. Error pages are a feature of the servlet spec but there
 * is no Java API for registering them in the spec. This filter works around that by
 * accepting error page registrations from Spring Boot's {@link ErrorPageRegistrar} (any
 * beans of that type in the context will be applied to this server).
 *
 * @author Dave Syer
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @since 2.0.0
 */
@Order(Ordered.HIGHEST_PRECEDENCE + 1)
public class ErrorPageFilter implements Filter, ErrorPageRegistry {

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

	// From RequestDispatcher but not referenced to remain compatible with Servlet 2.5

	private static final String ERROR_EXCEPTION = "javax.servlet.error.exception";

	private static final String ERROR_EXCEPTION_TYPE = "javax.servlet.error.exception_type";

	private static final String ERROR_MESSAGE = "javax.servlet.error.message";

	
The name of the servlet attribute containing request URI.
/**
	 * The name of the servlet attribute containing request URI.
	 */
	public static final String ERROR_REQUEST_URI = "javax.servlet.error.request_uri";

	private static final String ERROR_STATUS_CODE = "javax.servlet.error.status_code";

	private static final Set<Class<?>> CLIENT_ABORT_EXCEPTIONS;
	static {
		Set<Class<?>> clientAbortExceptions = new HashSet<>();
		addClassIfPresent(clientAbortExceptions, "org.apache.catalina.connector.ClientAbortException");
		CLIENT_ABORT_EXCEPTIONS = Collections.unmodifiableSet(clientAbortExceptions);
	}

	private String global;

	private final Map<Integer, String> statuses = new HashMap<>();

	private final Map<Class<?>, String> exceptions = new HashMap<>();

	private final OncePerRequestFilter delegate = new OncePerRequestFilter() {

		@Override
		protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
				throws ServletException, IOException {
			ErrorPageFilter.this.doFilter(request, response, chain);
		}

		@Override
		protected boolean shouldNotFilterAsyncDispatch() {
			return false;
		}

	};

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		this.delegate.init(filterConfig);
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		this.delegate.doFilter(request, response, chain);
	}

	private void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		ErrorWrapperResponse wrapped = new ErrorWrapperResponse(response);
		try {
			chain.doFilter(request, wrapped);
			if (wrapped.hasErrorToSend()) {
				handleErrorStatus(request, response, wrapped.getStatus(), wrapped.getMessage());
				response.flushBuffer();
			}
			else if (!request.isAsyncStarted() && !response.isCommitted()) {
				response.flushBuffer();
			}
		}
		catch (Throwable ex) {
			Throwable exceptionToHandle = ex;
			if (ex instanceof NestedServletException) {
				Throwable rootCause = ((NestedServletException) ex).getRootCause();
				if (rootCause != null) {
					exceptionToHandle = rootCause;
				}
			}
			handleException(request, response, wrapped, exceptionToHandle);
			response.flushBuffer();
		}
	}

	private void handleErrorStatus(HttpServletRequest request, HttpServletResponse response, int status, String message)
			throws ServletException, IOException {
		if (response.isCommitted()) {
			handleCommittedResponse(request, null);
			return;
		}
		String errorPath = getErrorPath(this.statuses, status);
		if (errorPath == null) {
			response.sendError(status, message);
			return;
		}
		response.setStatus(status);
		setErrorAttributes(request, status, message);
		request.getRequestDispatcher(errorPath).forward(request, response);
	}

	private void handleException(HttpServletRequest request, HttpServletResponse response, ErrorWrapperResponse wrapped,
			Throwable ex) throws IOException, ServletException {
		Class<?> type = ex.getClass();
		String errorPath = getErrorPath(type);
		if (errorPath == null) {
			rethrow(ex);
			return;
		}
		if (response.isCommitted()) {
			handleCommittedResponse(request, ex);
			return;
		}
		forwardToErrorPage(errorPath, request, wrapped, ex);
	}

	private void forwardToErrorPage(String path, HttpServletRequest request, HttpServletResponse response, Throwable ex)
			throws ServletException, IOException {
		if (logger.isErrorEnabled()) {
			String message = "Forwarding to error page from request " + getDescription(request) + " due to exception ["
					+ ex.getMessage() + "]";
			logger.error(message, ex);
		}
		setErrorAttributes(request, 500, ex.getMessage());
		request.setAttribute(ERROR_EXCEPTION, ex);
		request.setAttribute(ERROR_EXCEPTION_TYPE, ex.getClass());
		response.reset();
		response.setStatus(500);
		request.getRequestDispatcher(path).forward(request, response);
		request.removeAttribute(ERROR_EXCEPTION);
		request.removeAttribute(ERROR_EXCEPTION_TYPE);
	}

	
Return the description for the given request. By default this method will return a description based on the request servletPath and pathInfo. 
Params:
request – the source request
Returns:
the description
Since:
1.5.0/**
	 * Return the description for the given request. By default this method will return a
	 * description based on the request {@code servletPath} and {@code pathInfo}.
	 * @param request the source request
	 * @return the description
	 * @since 1.5.0
	 */
	protected String getDescription(HttpServletRequest request) {
		String pathInfo = (request.getPathInfo() != null) ? request.getPathInfo() : "";
		return "[" + request.getServletPath() + pathInfo + "]";
	}

	private void handleCommittedResponse(HttpServletRequest request, Throwable ex) {
		if (isClientAbortException(ex)) {
			return;
		}
		String message = "Cannot forward to error page for request " + getDescription(request)
				+ " as the response has already been"
				+ " committed. As a result, the response may have the wrong status"
				+ " code. If your application is running on WebSphere Application"
				+ " Server you may be able to resolve this problem by setting"
				+ " com.ibm.ws.webcontainer.invokeFlushAfterService to false";
		if (ex == null) {
			logger.error(message);
		}
		else {
			// User might see the error page without all the data here but throwing the
			// exception isn't going to help anyone (we'll log it to be on the safe side)
			logger.error(message, ex);
		}
	}

	private boolean isClientAbortException(Throwable ex) {
		if (ex == null) {
			return false;
		}
		for (Class<?> candidate : CLIENT_ABORT_EXCEPTIONS) {
			if (candidate.isInstance(ex)) {
				return true;
			}
		}
		return isClientAbortException(ex.getCause());
	}

	private String getErrorPath(Map<Integer, String> map, Integer status) {
		if (map.containsKey(status)) {
			return map.get(status);
		}
		return this.global;
	}

	private String getErrorPath(Class<?> type) {
		while (type != Object.class) {
			String path = this.exceptions.get(type);
			if (path != null) {
				return path;
			}
			type = type.getSuperclass();
		}
		return this.global;
	}

	private void setErrorAttributes(HttpServletRequest request, int status, String message) {
		request.setAttribute(ERROR_STATUS_CODE, status);
		request.setAttribute(ERROR_MESSAGE, message);
		request.setAttribute(ERROR_REQUEST_URI, request.getRequestURI());
	}

	private void rethrow(Throwable ex) throws IOException, ServletException {
		if (ex instanceof RuntimeException) {
			throw (RuntimeException) ex;
		}
		if (ex instanceof Error) {
			throw (Error) ex;
		}
		if (ex instanceof IOException) {
			throw (IOException) ex;
		}
		if (ex instanceof ServletException) {
			throw (ServletException) ex;
		}
		throw new IllegalStateException(ex);
	}

	@Override
	public void addErrorPages(ErrorPage... errorPages) {
		for (ErrorPage errorPage : errorPages) {
			if (errorPage.isGlobal()) {
				this.global = errorPage.getPath();
			}
			else if (errorPage.getStatus() != null) {
				this.statuses.put(errorPage.getStatus().value(), errorPage.getPath());
			}
			else {
				this.exceptions.put(errorPage.getException(), errorPage.getPath());
			}
		}
	}

	@Override
	public void destroy() {
	}

	private static void addClassIfPresent(Collection<Class<?>> collection, String className) {
		try {
			collection.add(ClassUtils.forName(className, null));
		}
		catch (Throwable ex) {
		}
	}

	private static class ErrorWrapperResponse extends HttpServletResponseWrapper {

		private int status;

		private String message;

		private boolean hasErrorToSend = false;

		ErrorWrapperResponse(HttpServletResponse response) {
			super(response);
		}

		@Override
		public void sendError(int status) throws IOException {
			sendError(status, null);
		}

		@Override
		public void sendError(int status, String message) throws IOException {
			this.status = status;
			this.message = message;
			this.hasErrorToSend = true;
			// Do not call super because the container may prevent us from handling the
			// error ourselves
		}

		@Override
		public int getStatus() {
			if (this.hasErrorToSend) {
				return this.status;
			}
			// If there was no error we need to trust the wrapped response
			return super.getStatus();
		}

		@Override
		public void flushBuffer() throws IOException {
			sendErrorIfNecessary();
			super.flushBuffer();
		}

		private void sendErrorIfNecessary() throws IOException {
			if (this.hasErrorToSend && !isCommitted()) {
				((HttpServletResponse) getResponse()).sendError(this.status, this.message);
			}
		}

		String getMessage() {
			return this.message;
		}

		boolean hasErrorToSend() {
			return this.hasErrorToSend;
		}

		@Override
		public PrintWriter getWriter() throws IOException {
			sendErrorIfNecessary();
			return super.getWriter();
		}

		@Override
		public ServletOutputStream getOutputStream() throws IOException {
			sendErrorIfNecessary();
			return super.getOutputStream();
		}

	}

}