本文详细介绍了Servlet的概念、生命周期及其在Java Web开发中的应用。包括Servlet接口分析、实现方式、以及请求与响应对象的使用。
2.利用Servlet实现web浏览器与服务器之间的动态交互
3.Servlet只能运行在Servlet容器中，常用的比如有:Tomcat
A.分析Servlet接口：
public void init(ServletConfig config) throws ServletException;
public ServletConfig getServletConfig();
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
public String getServletInfo();
public void destroy();
这些方法是Servlet接口仅有的方法，init,service,destroy描述了一个完整Servlet的生命周期，在容器运行后对应用进行初始化，用户访问那么首先执行的是init方法，然后是service方法，以后用户访问都只是运行service方法，init方法只执行一次。destroy方法当容器关闭后执行，一般这种方法用于资源的清理
package Day01;
import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
@WebServlet(
name="firstServlet",urlPatterns={"/firstServlet"},
initParams={
@WebInitParam(name="username",value="admin"),
@WebInitParam(name="password",value="admin")
}
)
public class firstServlet implements Servlet{
private transient ServletConfig config;
public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
this.config = config;
System.out.println("init运行");
}
public ServletConfig getServletConfig() {
// TODO Auto-generated method stub
return this.config;
}
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("service运行");
System.out.println(config.getInitParameter("username"));
System.out.println(config.getInitParameter("password"));
}
public String getServletInfo() {
// TODO Auto-generated method stub
return null;
}
public void destroy() {
// TODO Auto-generated method stub
System.out.println("destory执行");
}
}
B.如何使用Servlet接口
▲init方法会传进一个servletConfig，我们可以利用它获取配置的信息
，当然最重要的是我们可以从它获得一个代表上下文context的ServletContext，整个应用只有一个ServletContext（存放在应用中的javabean最好实现序列化接口）。
（本文所有的servlet例子都是使用注解的方式，使用注解比较方便，但是缺陷也比较多，比如如果要修改，那么得重新编译，所以一般都配置在web.xml文件里，在配置servlet的时候，可以配置一个load-on-start这个变量，当配置的值大于0，那么当容器运行后马上对servlet进行初始化，即运行init，当init方法运行比较费时的时候，建议在容器运行时马上初始化，数字大小决定初始化顺序，数字越大初始化顺序越后，反之，当数字小于0，那么当servlet被访问时才初始化）
C.现实中如何快速便捷实用Servlet接口
package Day01;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(name="secondServlet",
urlPatterns={"/secondServlet"})
public class secondServlet extends HttpServlet{
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("get访问");
System.out.println(req.getRequestURI());
}
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("post访问");
}
}
这个类是Servlet接口的便捷类，抽象实现类，它让我们更方便实用它，直接继承即可，但是GenericServlet又有一个实现类，也是最常用的类：
public abstract class HttpServlet extends GenericServlet
1.public abstract class GenericServlet
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
*
* Copyright (c) 1997-2010 Oracle and/or its affiliates. All rights reserved.
*
* The contents of this file are subject to the terms of either the GNU
* General Public License Version 2 only ("GPL") or the Common Development
* and Distribution License("CDDL") (collectively, the "License"). You
* may not use this file except in compliance with the License. You can
* obtain a copy of the License at
* https://glassfish.dev.java.net/public/CDDL+GPL_1_1.html
* or packager/legal/LICENSE.txt. See the License for the specific
* language governing permissions and limitations under the License.
*
* When distributing the software, include this License Header Notice in each
* file and include the License file at packager/legal/LICENSE.txt.
*
* GPL Classpath Exception:
* Oracle designates this particular file as subject to the "Classpath"
* exception as provided by Oracle in the GPL Version 2 section of the License
* file that accompanied this code.
*
* Modifications:
* If applicable, add the following below the License Header, with the fields
* enclosed by brackets [] replaced by your own identifying information:
* "Portions Copyright [year] [name of copyright owner]"
*
* Contributor(s):
* If you wish your version of this file to be governed by only the CDDL or
* only the GPL Version 2, indicate your decision by adding "[Contributor]
* elects to include this software in this distribution under the [CDDL or GPL
* Version 2] license." If you don't indicate a single choice of license, a
* recipient has the option to distribute your version of this file under
* either the CDDL, the GPL Version 2 or to extend the choice of license to
* its licensees as provided above. However, if you add GPL Version 2 code
* and therefore, elected the GPL Version 2 license, then the option applies
* only if the new code is made subject to such option by the copyright
* holder.
*
*
* This file incorporates work covered by the following copyright and
* permission notice:
*
* 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;
import java.io.IOException;
import java.util.Enumeration;
import java.util.ResourceBundle;
/**
*
* Defines a generic, protocol-independent
* servlet. To write an HTTP servlet for use on the
* Web, extend {@link javax.servlet.http.HttpServlet} instead.
*
* GenericServlet implements the Servlet
* and ServletConfig interfaces. GenericServlet
* may be directly extended by a servlet, although it's more common to extend
* a protocol-specific subclass such as HttpServlet.
*
*
GenericServlet makes writing servlets
* easier. It provides simple versions of the lifecycle methods
* init and destroy and of the methods
* in the ServletConfig interface. GenericServlet
* also implements the log method, declared in the
* ServletContext interface.
*
*
To write a generic servlet, you need only
* override the abstract service method.
*
*
* @author Various
*/
public abstract class GenericServlet
implements Servlet, ServletConfig, java.io.Serializable
{
private static final String LSTRING_FILE = "javax.servlet.LocalStrings";
private static ResourceBundle lStrings =
ResourceBundle.getBundle(LSTRING_FILE);
private transient ServletConfig config;
/**
*
* Does nothing. All of the servlet initialization
* is done by one of the init methods.
*
*/
public GenericServlet() { }
/**
* Called by the servlet container to indicate to a servlet that the
* servlet is being taken out of service. See {@link Servlet#destroy}.
*
*
*/
public void destroy() {
}
/**
* Returns a String containing the value of the named
* initialization parameter, or null if the parameter does
* not exist. See {@link ServletConfig#getInitParameter}.
*
*
This method is supplied for convenience. It gets the
* value of the named parameter from the servlet's
* ServletConfig object.
*
* @param name a String specifying the name
* of the initialization parameter
*
* @return String a String containing the value
* of the initialization parameter
*
*/
public String getInitParameter(String name) {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameter(name);
}
/**
* Returns the names of the servlet's initialization parameters
* as an Enumeration of String objects,
* or an empty Enumeration if the servlet has no
* initialization parameters. See {@link
* ServletConfig#getInitParameterNames}.
*
*
This method is supplied for convenience. It gets the
* parameter names from the servlet's ServletConfig object.
*
*
* @return Enumeration an enumeration of String
* objects containing the names of
* the servlet's initialization parameters
*/
public Enumeration
getInitParameterNames() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameterNames();
}
/**
* Returns this servlet's {@link ServletConfig} object.
*
* @return ServletConfig the
ServletConfig object
* that initialized this servlet
*/
public ServletConfig getServletConfig() {
return config;
}
/**
* Returns a reference to the {@link ServletContext} in which this servlet
* is running. See {@link ServletConfig#getServletContext}.
*
*
This method is supplied for convenience. It gets the
* context from the servlet's ServletConfig object.
*
*
* @return ServletContext the ServletContext object
* passed to this servlet by the init
* method
*/
public ServletContext getServletContext() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletContext();
}
/**
* Returns information about the servlet, such as
* author, version, and copyright.
* By default, this method returns an empty string. Override this method
* to have it return a meaningful value. See {@link
* Servlet#getServletInfo}.
*
*
* @return String information about this servlet, by default an
* empty string
*/
public String getServletInfo() {
return "";
}
/**
* Called by the servlet container to indicate to a servlet that the
* servlet is being placed into service. See {@link Servlet#init}.
*
*
This implementation stores the {@link ServletConfig}
* object it receives from the servlet container for later use.
* When overriding this form of the method, call
* super.init(config).
*
* @param config the ServletConfig object
* that contains configutation
* information for this servlet
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's normal
* operation
*
* @see UnavailableException
*/
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
/**
* A convenience method which can be overridden so that there's no need
* to call super.init(config).
*
*
Instead of overriding {@link #init(ServletConfig)}, simply override
* this method and it will be called by
* GenericServlet.init(ServletConfig config).
* The ServletConfig object can still be retrieved via {@link
* #getServletConfig}.
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's
* normal operation
*/
public void init() throws ServletException {
}
/**
* Writes the specified message to a servlet log file, prepended by the
* servlet's name. See {@link ServletContext#log(String)}.
*
* @param msg a String specifying
* the message to be written to the log file
*/
public void log(String msg) {
getServletContext().log(getServletName() + ": "+ msg);
}
/**
* Writes an explanatory message and a stack trace
* for a given Throwable exception
* to the servlet log file, prepended by the servlet's name.
* See {@link ServletContext#log(String, Throwable)}.
*
*
* @param message a String that describes
* the error or exception
*
* @param t the java.lang.Throwable error
* or exception
*/
public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
/**
* Called by the servlet container to allow the servlet to respond to
* a request. See {@link Servlet#service}.
*
*
This method is declared abstract so subclasses, such as
* HttpServlet, must override it.
*
* @param req the ServletRequest object
* that contains the client's request
*
* @param res the ServletResponse object
* that will contain the servlet's response
*
* @exception ServletException if an exception occurs that
* interferes with the servlet's
* normal operation occurred
*
* @exception IOException if an input or output
* exception occurs
*/
public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
/**
* Returns the name of this servlet instance.
* See {@link ServletConfig#getServletName}.
*
* @return the name of this servlet instance
*/
public String getServletName() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletName();
}
}
我们从源码很容易的发现GenericServlet 把ServletConfig给写成了一个类变量，并且赋予了transient关键字，防止在网络中传输。
我们可以发现GenericServlet，不仅实现了Servlet接口，还实现了ServletConfig接口，为其方法提供了实现。不仅如此，GenericServlet 还额外提供了一个init方法，这样可以让我们覆盖实现init，又不需要操作servletConfig。但是这样还不够遍历，因为我们每次访问都要访问service访问，如果我们自己去实现它，还得分析http协议标头的访问方法，常见的有get，post，put
2.public abstract class HttpServlet extends GenericServlet
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
*
* Copyright (c) 1997-2011 Oracle and/or its affiliates. All rights reserved.
*
* The contents of this file are subject to the terms of either the GNU
* General Public License Version 2 only ("GPL") or the Common Development
* and Distribution License("CDDL") (collectively, the "License"). You
* may not use this file except in compliance with the License. You can
* obtain a copy of the License at
* https://glassfish.dev.java.net/public/CDDL+GPL_1_1.html
* or packager/legal/LICENSE.txt. See the License for the specific
* language governing permissions and limitations under the License.
*
* When distributing the software, include this License Header Notice in each
* file and include the License file at packager/legal/LICENSE.txt.
*
* GPL Classpath Exception:
* Oracle designates this particular file as subject to the "Classpath"
* exception as provided by Oracle in the GPL Version 2 section of the License
* file that accompanied this code.
*
* Modifications:
* If applicable, add the following below the License Header, with the fields
* enclosed by brackets [] replaced by your own identifying information:
* "Portions Copyright [year] [name of copyright owner]"
*
* Contributor(s):
* If you wish your version of this file to be governed by only the CDDL or
* only the GPL Version 2, indicate your decision by adding "[Contributor]
* elects to include this software in this distribution under the [CDDL or GPL
* Version 2] license." If you don't indicate a single choice of license, a
* recipient has the option to distribute your version of this file under
* either the CDDL, the GPL Version 2 or to extend the choice of license to
* its licensees as provided above. However, if you add GPL Version 2 code
* and therefore, elected the GPL Version 2 license, then the option applies
* only if the new code is made subject to such option by the copyright
* holder.
*
*
* This file incorporates work covered by the following copyright and
* permission notice:
*
* 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.Locale;
import java.util.ResourceBundle;
import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
/**
*
* Provides an abstract class to be subclassed to create
* an HTTP servlet suitable for a Web site. A subclass of
* HttpServlet must override at least
* one method, usually one of these:
*
*
-
*
-
doGet, if the servlet supports HTTP GET requests
* -
doPost, for HTTP POST requests
* -
doPut, for HTTP PUT requests
* -
doDelete, for HTTP DELETE requests
* -
init and destroy,
* to manage resources that are held for the life of the servlet
* -
getServletInfo, which the servlet uses to
* provide information about itself
*
*
* There's almost no reason to override the service
* method. service handles standard HTTP
* requests by dispatching them to the handler methods
* for each HTTP request type (the doXXX
* methods listed above).
*
*
Likewise, there's almost no reason to override the
* doOptions and doTrace methods.
*
*
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
*
* Java Tutorial on Multithreaded Programming 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 service method) to
* allow a servlet to handle a GET request.
*
*
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.
*
*
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 PrintWriter object to return the response,
* set the content type before accessing the
* PrintWriter object.
*
*
The servlet container must write the headers before
* committing the response, because in HTTP the headers must be sent
* before the response body.
*
*
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.
*
*
When using HTTP 1.1 chunked encoding (which means that the response
* has a Transfer-Encoding header), do not set the Content-Length header.
*
*
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.
*
*
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.
*
*
If the request is incorrectly formatted, doGet
* 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
*
* @exception IOException if an input or output error is
* detected when the servlet handles
* the GET request
*
* @exception 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 HttpServletRequest
* 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).
*
*
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 HttpServletRequest
* object that is sent to the servlet
*
* @return a long integer specifying
* the time the HttpServletRequest
* 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;
}
/**
*
*
*
Receives an HTTP HEAD request from the protected
* service 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.
*
*
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
* doHead method you write is both safe
* and idempotent (that is, protects itself from being
* called multiple times for one HTTP HEAD request).
*
*
If the HTTP HEAD request is incorrectly formatted,
* doHead 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
*
* @exception IOException if an input or output error occurs
*
* @exception 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 service 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.
*
*
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
* PrintWriter object to return the response, set the
* content type before accessing the PrintWriter object.
*
*
The servlet container must write the headers before committing the
* response, because in HTTP the headers must be sent before the
* response body.
*
*
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.
*
*
When using HTTP 1.1 chunked encoding (which means that the response
* has a Transfer-Encoding header), do not set the Content-Length header.
*
*
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.
*
*
If the HTTP POST request is incorrectly formatted,
* doPost 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
*
* @exception IOException if an input or output error is
* detected when the servlet handles
* the request
*
* @exception 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 service 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.
*
*
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
* .
*
*
This method does not need to be either safe or idempotent.
* Operations that doPut 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.
*
*
If the HTTP PUT request is incorrectly formatted,
* doPut 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
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* PUT request
*
* @exception 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 service 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.
*
*
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.
*
*
If the HTTP DELETE request is incorrectly formatted,
* doDelete 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
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* DELETE request
*
* @exception 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
c) {
if (c.equals(javax.servlet.http.HttpServlet.class)) {
return null;
}
Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass());
Method[] thisMethods = c.getDeclaredMethods();
if ((parentMethods != null) && (parentMethods.length > 0)) {
Method[] allMethods =
new Method[parentMethods.length + thisMethods.length];
System.arraycopy(parentMethods, 0, allMethods, 0,
parentMethods.length);
System.arraycopy(thisMethods, 0, allMethods, parentMethods.length,
thisMethods.length);
thisMethods = allMethods;
}
return thisMethods;
}
/**
* Called by the server (via the service 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 doGet, this method returns the
* following header:
*
*
Allow: GET, HEAD, TRACE, OPTIONS
*
*
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
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* OPTIONS request
*
* @exception 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
service 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
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* TRACE request
*
* @exception 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
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
*
service method and dispatches
* them to the
do
XXX 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
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* HTTP request
*
* @exception 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
*
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 res the {@link HttpServletResponse} object that
* contains the response the servlet returns
* to the client
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* HTTP request
*
* @exception ServletException if the HTTP request cannot
* be handled
*
* @see javax.servlet.Servlet#service
*/
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());
}
}
public void setContentLength(int len) {
super.setContentLength(len);
didSetContentLength = true;
}
public ServletOutputStream getOutputStream() throws IOException {
if (writer != null) {
throw new IllegalStateException(
lStrings.getString("err.ise.getOutputStream"));
}
usingOutputStream = true;
return noBody;
}
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;
}
public void write(int b) {
contentLength++;
}
public void write(byte buf[], int offset, int len)
throws IOException
{
if (len >= 0) {
contentLength += len;
} else {
// This should have thrown an IllegalArgumentException, but
// changing this would break backwards compatibility
throw new IOException(lStrings.getString("err.io.negativelength"));
}
}
}
观察源码，我们不难发现这个类实现了2个service，道理类似上面的init方法，但是不同的是，这个额外的service方法，它已经帮我们实现了，帮我们判断http方法头然后跳转到对应的方法，比如doGet doPost。
没错，当我们要使用servlet的时候，就是直接继承HttpServlet，然后相对应实现doGet doPost方法即可
这是可能会出现的问题，所以不要直接利用@override 直接重载这个方法
D.分析请求和响应
ServletRequest HttpServletRequest
ServletResponse HttpServletResponse
常用的是HttpServletRequest HttpServletResponse，分别代表请求和响应，顾名思义。我们可以看看这2个接口里面的方法：
HttpServletRequest
public String getHeader(String name);//放回请求头，百度一下可以自己查阅一下http请求头有多少种，然后就明了了
public String getMethod();//获取请求方法，比如get post
public String getContextPath();//获取上下文路径
public HttpSession getSession();//获取session，下节讲解
public String getRequestURI();//获取请求路径，比如刚才的get连接，返回"/servletApp/secondServlet",与之相类似的还可以根据请求头refer来获取。
public RequestDispatcher getRequestDispatcher(String path);//转发，可以带着request和response，利用forward方法
public String getRealPath(String path);//获取指定文件的绝对路径
public Object setAttribute(String name);//设置属性，转发后可以从JSP，servlet中获取
public void setAttribute(String name, Object o);同上
public String getParameter(String name);//获取属性，比如表单的属性，根据表单name来获取，也可以使连接带属性方式比如url？username='admin'&password='password'
HttpServletResponse
public void addCookie(Cookie cookie);//加入cookie，下节讲解
public void sendRedirect(String location) throws IOException;//重定向，指定跳转
public void setStatus(int sc);//设置状态码，比如200即成功状态吗
public void setHeader(String name, String value);//设置响应头
public void setContentType(String type);//设置内容类型，比如如果是普通页面则 'text/html' 如果是图片 'image/jpeg'，具体图片类型映射可以查看tomcat里面的配置文件
public PrintWriter getWriter() throws IOException;
public ServletOutputStream getOutputStream() throws IOException;
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