Java tutorial
/* * Copyright (c) 1997-2018 Oracle and/or its affiliates. All rights reserved. * Copyright 2004 The Apache Software Foundation * * 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 * * http://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 javax.servlet.http; import java.io.IOException; import java.io.PrintWriter; import java.io.OutputStreamWriter; import java.io.UnsupportedEncodingException; import java.lang.reflect.Method; import java.text.MessageFormat; import java.util.Enumeration; import java.util.ResourceBundle; import javax.servlet.*; /** * * Provides an abstract class to be subclassed to create * an HTTP servlet suitable for a Web site. A subclass of * <code>HttpServlet</code> must override at least * one method, usually one of these: * * <ul> * <li> <code>doGet</code>, if the servlet supports HTTP GET requests * <li> <code>doPost</code>, for HTTP POST requests * <li> <code>doPut</code>, for HTTP PUT requests * <li> <code>doDelete</code>, for HTTP DELETE requests * <li> <code>init</code> and <code>destroy</code>, * to manage resources that are held for the life of the servlet * <li> <code>getServletInfo</code>, which the servlet uses to * provide information about itself * </ul> * * <p>There's almost no reason to override the <code>service</code> * method. <code>service</code> handles standard HTTP * requests by dispatching them to the handler methods * for each HTTP request type (the <code>do</code><i>XXX</i> * methods listed above). * * <p>Likewise, there's almost no reason to override the * <code>doOptions</code> and <code>doTrace</code> methods. * * <p>Servlets typically run on multithreaded servers, * so be aware that a servlet must handle concurrent * requests and be careful to synchronize access to shared resources. * Shared resources include in-memory data such as * instance or class variables and external objects * such as files, database connections, and network * connections. * See the * <a href="https://docs.oracle.com/javase/tutorial/essential/concurrency/"> * Java Tutorial on Multithreaded Programming</a> for more * information on handling multiple threads in a Java program. * * @author Various */ public abstract class HttpServlet extends GenericServlet { private static final String METHOD_DELETE = "DELETE"; private static final String METHOD_HEAD = "HEAD"; private static final String METHOD_GET = "GET"; private static final String METHOD_OPTIONS = "OPTIONS"; private static final String METHOD_POST = "POST"; private static final String METHOD_PUT = "PUT"; private static final String METHOD_TRACE = "TRACE"; private static final String HEADER_IFMODSINCE = "If-Modified-Since"; private static final String HEADER_LASTMOD = "Last-Modified"; private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings"; private static ResourceBundle lStrings = ResourceBundle.getBundle(LSTRING_FILE); /** * Does nothing, because this is an abstract class. * */ public HttpServlet() { } /** * * Called by the server (via the <code>service</code> method) to * allow a servlet to handle a GET request. * * <p>Overriding this method to support a GET request also * automatically supports an HTTP HEAD request. A HEAD * request is a GET request that returns no body in the * response, only the request header fields. * * <p>When overriding this method, read the request data, * write the response headers, get the response's writer or * output stream object, and finally, write the response data. * It's best to include content type and encoding. When using * a <code>PrintWriter</code> object to return the response, * set the content type before accessing the * <code>PrintWriter</code> object. * * <p>The servlet container must write the headers before * committing the response, because in HTTP the headers must be sent * before the response body. * * <p>Where possible, set the Content-Length header (with the * {@link javax.servlet.ServletResponse#setContentLength} method), * to allow the servlet container to use a persistent connection * to return its response to the client, improving performance. * The content length is automatically set if the entire response fits * inside the response buffer. * * <p>When using HTTP 1.1 chunked encoding (which means that the response * has a Transfer-Encoding header), do not set the Content-Length header. * * <p>The GET method should be safe, that is, without * any side effects for which users are held responsible. * For example, most form queries have no side effects. * If a client request is intended to change stored data, * the request should use some other HTTP method. * * <p>The GET method should also be idempotent, meaning * that it can be safely repeated. Sometimes making a * method safe also makes it idempotent. For example, * repeating queries is both safe and idempotent, but * buying a product online or modifying data is neither * safe nor idempotent. * * <p>If the request is incorrectly formatted, <code>doGet</code> * returns an HTTP "Bad Request" message. * * @param req an {@link HttpServletRequest} object that * contains the request the client has made * of the servlet * * @param resp an {@link HttpServletResponse} object that * contains the response the servlet sends * to the client * * @throws IOException if an input or output error is * detected when the servlet handles * the GET request * * @throws ServletException if the request for the GET * could not be handled * * @see javax.servlet.ServletResponse#setContentType */ protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_get_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } /** * * Returns the time the <code>HttpServletRequest</code> * object was last modified, * in milliseconds since midnight January 1, 1970 GMT. * If the time is unknown, this method returns a negative * number (the default). * * <p>Servlets that support HTTP GET requests and can quickly determine * their last modification time should override this method. * This makes browser and proxy caches work more effectively, * reducing the load on server and network resources. * * @param req the <code>HttpServletRequest</code> * object that is sent to the servlet * * @return a <code>long</code> integer specifying * the time the <code>HttpServletRequest</code> * object was last modified, in milliseconds * since midnight, January 1, 1970 GMT, or * -1 if the time is not known */ protected long getLastModified(HttpServletRequest req) { return -1; } /** * * * <p>Receives an HTTP HEAD request from the protected * <code>service</code> method and handles the * request. * The client sends a HEAD request when it wants * to see only the headers of a response, such as * Content-Type or Content-Length. The HTTP HEAD * method counts the output bytes in the response * to set the Content-Length header accurately. * * <p>If you override this method, you can avoid computing * the response body and just set the response headers * directly to improve performance. Make sure that the * <code>doHead</code> method you write is both safe * and idempotent (that is, protects itself from being * called multiple times for one HTTP HEAD request). * * <p>If the HTTP HEAD request is incorrectly formatted, * <code>doHead</code> returns an HTTP "Bad Request" * message. * * @param req the request object that is passed to the servlet * * @param resp the response object that the servlet * uses to return the headers to the clien * * @throws IOException if an input or output error occurs * * @throws ServletException if the request for the HEAD * could not be handled */ protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { NoBodyResponse response = new NoBodyResponse(resp); doGet(req, response); response.setContentLength(); } /** * * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a POST request. * * The HTTP POST method allows the client to send * data of unlimited length to the Web server a single time * and is useful when posting information such as * credit card numbers. * * <p>When overriding this method, read the request data, * write the response headers, get the response's writer or output * stream object, and finally, write the response data. It's best * to include content type and encoding. When using a * <code>PrintWriter</code> object to return the response, set the * content type before accessing the <code>PrintWriter</code> object. * * <p>The servlet container must write the headers before committing the * response, because in HTTP the headers must be sent before the * response body. * * <p>Where possible, set the Content-Length header (with the * {@link javax.servlet.ServletResponse#setContentLength} method), * to allow the servlet container to use a persistent connection * to return its response to the client, improving performance. * The content length is automatically set if the entire response fits * inside the response buffer. * * <p>When using HTTP 1.1 chunked encoding (which means that the response * has a Transfer-Encoding header), do not set the Content-Length header. * * <p>This method does not need to be either safe or idempotent. * Operations requested through POST can have side effects for * which the user can be held accountable, for example, * updating stored data or buying items online. * * <p>If the HTTP POST request is incorrectly formatted, * <code>doPost</code> returns an HTTP "Bad Request" message. * * * @param req an {@link HttpServletRequest} object that * contains the request the client has made * of the servlet * * @param resp an {@link HttpServletResponse} object that * contains the response the servlet sends * to the client * * @throws IOException if an input or output error is * detected when the servlet handles * the request * * @throws ServletException if the request for the POST * could not be handled * * @see javax.servlet.ServletOutputStream * @see javax.servlet.ServletResponse#setContentType */ protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_post_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } /** * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a PUT request. * * The PUT operation allows a client to * place a file on the server and is similar to * sending a file by FTP. * * <p>When overriding this method, leave intact * any content headers sent with the request (including * Content-Length, Content-Type, Content-Transfer-Encoding, * Content-Encoding, Content-Base, Content-Language, Content-Location, * Content-MD5, and Content-Range). If your method cannot * handle a content header, it must issue an error message * (HTTP 501 - Not Implemented) and discard the request. * For more information on HTTP 1.1, see RFC 2616 * <a href="http://www.ietf.org/rfc/rfc2616.txt"></a>. * * <p>This method does not need to be either safe or idempotent. * Operations that <code>doPut</code> performs can have side * effects for which the user can be held accountable. When using * this method, it may be useful to save a copy of the * affected URL in temporary storage. * * <p>If the HTTP PUT request is incorrectly formatted, * <code>doPut</code> returns an HTTP "Bad Request" message. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * PUT request * * @throws ServletException if the request for the PUT * cannot be handled */ protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_put_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } /** * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a DELETE request. * * The DELETE operation allows a client to remove a document * or Web page from the server. * * <p>This method does not need to be either safe * or idempotent. Operations requested through * DELETE can have side effects for which users * can be held accountable. When using * this method, it may be useful to save a copy of the * affected URL in temporary storage. * * <p>If the HTTP DELETE request is incorrectly formatted, * <code>doDelete</code> returns an HTTP "Bad Request" * message. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * DELETE request * * @throws ServletException if the request for the * DELETE cannot be handled */ protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_delete_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } private Method[] getAllDeclaredMethods(Class<? extends HttpServlet> c) { Class<?> clazz = c; Method[] allMethods = null; while (!clazz.equals(HttpServlet.class)) { Method[] thisMethods = clazz.getDeclaredMethods(); if (allMethods != null && allMethods.length > 0) { Method[] subClassMethods = allMethods; allMethods = new Method[thisMethods.length + subClassMethods.length]; System.arraycopy(thisMethods, 0, allMethods, 0, thisMethods.length); System.arraycopy(subClassMethods, 0, allMethods, thisMethods.length, subClassMethods.length); } else { allMethods = thisMethods; } clazz = clazz.getSuperclass(); } return ((allMethods != null) ? allMethods : new Method[0]); } /** * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a OPTIONS request. * * The OPTIONS request determines which HTTP methods * the server supports and * returns an appropriate header. For example, if a servlet * overrides <code>doGet</code>, this method returns the * following header: * * <p><code>Allow: GET, HEAD, TRACE, OPTIONS</code> * * <p>There's no need to override this method unless the * servlet implements new HTTP methods, beyond those * implemented by HTTP 1.1. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * OPTIONS request * * @throws ServletException if the request for the * OPTIONS cannot be handled */ protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { Method[] methods = getAllDeclaredMethods(this.getClass()); boolean ALLOW_GET = false; boolean ALLOW_HEAD = false; boolean ALLOW_POST = false; boolean ALLOW_PUT = false; boolean ALLOW_DELETE = false; boolean ALLOW_TRACE = true; boolean ALLOW_OPTIONS = true; for (int i = 0; i < methods.length; i++) { String methodName = methods[i].getName(); if (methodName.equals("doGet")) { ALLOW_GET = true; ALLOW_HEAD = true; } else if (methodName.equals("doPost")) { ALLOW_POST = true; } else if (methodName.equals("doPut")) { ALLOW_PUT = true; } else if (methodName.equals("doDelete")) { ALLOW_DELETE = true; } } // we know "allow" is not null as ALLOW_OPTIONS = true // when this method is invoked StringBuilder allow = new StringBuilder(); if (ALLOW_GET) { allow.append(METHOD_GET); } if (ALLOW_HEAD) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_HEAD); } if (ALLOW_POST) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_POST); } if (ALLOW_PUT) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_PUT); } if (ALLOW_DELETE) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_DELETE); } if (ALLOW_TRACE) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_TRACE); } if (ALLOW_OPTIONS) { if (allow.length() > 0) { allow.append(", "); } allow.append(METHOD_OPTIONS); } resp.setHeader("Allow", allow.toString()); } /** * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a TRACE request. * * A TRACE returns the headers sent with the TRACE * request to the client, so that they can be used in * debugging. There's no need to override this method. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * TRACE request * * @throws ServletException if the request for the * TRACE cannot be handled */ protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { int responseLength; String CRLF = "\r\n"; StringBuilder buffer = new StringBuilder("TRACE ").append(req.getRequestURI()).append(" ") .append(req.getProtocol()); Enumeration<String> reqHeaderEnum = req.getHeaderNames(); while (reqHeaderEnum.hasMoreElements()) { String headerName = reqHeaderEnum.nextElement(); buffer.append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName)); } buffer.append(CRLF); responseLength = buffer.length(); resp.setContentType("message/http"); resp.setContentLength(responseLength); ServletOutputStream out = resp.getOutputStream(); out.print(buffer.toString()); } /** * Receives standard HTTP requests from the public * <code>service</code> method and dispatches * them to the <code>do</code><i>XXX</i> methods defined in * this class. This method is an HTTP-specific version of the * {@link javax.servlet.Servlet#service} method. There's no * need to override this method. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * HTTP request * * @throws ServletException if the HTTP request * cannot be handled * * @see javax.servlet.Servlet#service */ protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn't support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); if (ifModifiedSince < lastModified) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp, lastModified); doHead(req, resp); } else if (method.equals(METHOD_POST)) { doPost(req, resp); } else if (method.equals(METHOD_PUT)) { doPut(req, resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req, resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req, resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req, resp); } else { // // Note that this means NO servlet supports whatever // method was requested, anywhere on this server. // String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg); } } /* * Sets the Last-Modified entity header field, if it has not * already been set and if the value is meaningful. Called before * doGet, to ensure that headers are set before response data is * written. A subclass might have set this header already, so we * check. */ private void maybeSetLastModified(HttpServletResponse resp, long lastModified) { if (resp.containsHeader(HEADER_LASTMOD)) return; if (lastModified >= 0) resp.setDateHeader(HEADER_LASTMOD, lastModified); } /** * Dispatches client requests to the protected * <code>service</code> method. There's no need to * override this method. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param res the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @throws IOException if an input or output error occurs * while the servlet is handling the * HTTP request * * @throws ServletException if the HTTP request cannot * be handled or if either parameter is not * an instance of its respective {@link HttpServletRequest} * or {@link HttpServletResponse} counterparts. * * @see javax.servlet.Servlet#service */ @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { HttpServletRequest request; HttpServletResponse response; if (!(req instanceof HttpServletRequest && res instanceof HttpServletResponse)) { throw new ServletException("non-HTTP request or response"); } request = (HttpServletRequest) req; response = (HttpServletResponse) res; service(request, response); } } /* * A response that includes no body, for use in (dumb) "HEAD" support. * This just swallows that body, counting the bytes in order to set * the content length appropriately. All other methods delegate directly * to the wrapped HTTP Servlet Response object. */ // file private class NoBodyResponse extends HttpServletResponseWrapper { private static final ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings"); private NoBodyOutputStream noBody; private PrintWriter writer; private boolean didSetContentLength; private boolean usingOutputStream; // file private NoBodyResponse(HttpServletResponse r) { super(r); noBody = new NoBodyOutputStream(); } // file private void setContentLength() { if (!didSetContentLength) { if (writer != null) { writer.flush(); } setContentLength(noBody.getContentLength()); } } @Override public void setContentLength(int len) { super.setContentLength(len); didSetContentLength = true; } @Override public void setContentLengthLong(long len) { super.setContentLengthLong(len); didSetContentLength = true; } @Override public void setHeader(String name, String value) { super.setHeader(name, value); checkHeader(name); } @Override public void addHeader(String name, String value) { super.addHeader(name, value); checkHeader(name); } @Override public void setIntHeader(String name, int value) { super.setIntHeader(name, value); checkHeader(name); } @Override public void addIntHeader(String name, int value) { super.addIntHeader(name, value); checkHeader(name); } private void checkHeader(String name) { if ("content-length".equalsIgnoreCase(name)) { didSetContentLength = true; } } @Override public ServletOutputStream getOutputStream() throws IOException { if (writer != null) { throw new IllegalStateException(lStrings.getString("err.ise.getOutputStream")); } usingOutputStream = true; return noBody; } @Override public PrintWriter getWriter() throws UnsupportedEncodingException { if (usingOutputStream) { throw new IllegalStateException(lStrings.getString("err.ise.getWriter")); } if (writer == null) { OutputStreamWriter w = new OutputStreamWriter(noBody, getCharacterEncoding()); writer = new PrintWriter(w); } return writer; } } /* * Servlet output stream that gobbles up all its data. */ // file private class NoBodyOutputStream extends ServletOutputStream { private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings"; private static ResourceBundle lStrings = ResourceBundle.getBundle(LSTRING_FILE); private int contentLength = 0; // file private NoBodyOutputStream() { } // file private int getContentLength() { return contentLength; } @Override public void write(int b) { contentLength++; } @Override public void write(byte buf[], int offset, int len) throws IOException { if (buf == null) { throw new NullPointerException(lStrings.getString("err.io.nullArray")); } if (offset < 0 || len < 0 || offset + len > buf.length) { String msg = lStrings.getString("err.io.indexOutOfBounds"); Object[] msgArgs = new Object[3]; msgArgs[0] = Integer.valueOf(offset); msgArgs[1] = Integer.valueOf(len); msgArgs[2] = Integer.valueOf(buf.length); msg = MessageFormat.format(msg, msgArgs); throw new IndexOutOfBoundsException(msg); } contentLength += len; } public boolean isReady() { return false; } public void setWriteListener(WriteListener writeListener) { } }