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Servlet 使用介绍(1)

说明

本篇介绍java web中比较重要的一个技术:servlet。servlet是一种对用户请求动态响应的一个技术,是java web的核心一环。对于一般服务性质的纯后台服务应用而言,或许整个应用是由若干个servlet组成
,而其他方面表现较弱。

servlet

一般的我们的应用都会发布到一个服务应用中,即一个servlet容器中,常见的servlet容器有:Tomcat,JBoss,WebLogic等。我们通过servlet容器处理用户请求,当用户向我们的某个servlet发起请求时,servlet容器会调用并转发请求结果到该servlet,同时会将servlet返回结果回馈给请求方(用户)。其实,servlet本质就是一个接口,我们可以通过注册servlet服务,实现servlet接口,用于监听各种状态先的信息交互。如下,仅仅介绍tomcat中的servlet的相关使用(主要是其他也没用过)。

Tomcat中为servlet的使用提供了一个调用的jar包:sevlet-api.jar ,该jar包位于Tomcat根目录的lib目录下(所有和Tomcat相关的Jar包均放在这里),如下为servlet-api.jar中的包信息:

servlet-api.jar
--javax.servlet           //核心
--javax.servlet.annotation
--javax.servlet.descriptor
--javax.servlet.http     //基于http的实现

可以看到,这里主要石油javax.servlet和javax.servlet.http两个包组成实现。其中javax.servlet.Servlet接口为api的核心,servlet容器通过调用其定义方法,实现网络中客户端和服务端之前通信的中转传递,Servlet提供了五个方法,其中三个方法是由servlet容器调用。如下,servlet的源码:



package javax.servlet;

import java.io.IOException;

public interface Servlet {
    /**
     *由sevler容器调用,servlet对象初始化后,servlet容器调用此方法
     *
     */
    public void init(ServletConfig config) throws ServletException;

    /**
     *提供的一个获取ServletConfig对象的方法,该方法由我们自己实现,返回的对象即是init
     *的方法传入的参数。
     *
     */
    public ServletConfig getServletConfig();

    /**
     *该方法需要在init方法成功调用后才可正常调用(即客户端请求的时候必须姚该servlet以及
     *初始化成功)
     *
     */
    public void service(ServletRequest req, ServletResponse res)
	throws ServletException, IOException;



    /**
     *返回当前servlet的创建信息包括创建者等(为一个String字串)
     *
     */
    public String getServletInfo();

    /**
     *由servlet容器调用,当一个servlet容器被销毁时调用,可以用于清理销毁servlet中的占
     *用的一些资源。
     *
     */
    public void destroy();
}

Servlet接口为最基础的Servlet组件,可供我们最大限度的对servlet进行定制.servlet-api.jar中也为我们提供了它的两个替代实现:一个是javax.servlet.GenericServlet是其实现类,完成它的最基础的一些逻辑实现,但保留其对于网络通信的处理,交由我们自行处理;另一个是javax.servlet.http.HttpServlet,该类是GenericServlet的子类,是对GenericServlet的一种扩展,其对于Servlet的servive方法进行了处理,主要用于Http请求。如下,给出这两个类的实现源码:

  1. GenericServlet

该抽象类并未对servive方法作处理。

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;



    public GenericServlet() { }


    /**
     *销毁方法
     */
    public void destroy() {
    }


    /**
     *获取servlet初始化传入参数
     *
     */
    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);
    }


   /**
    *servlet参数参数名获取
    */
    public Enumeration<String> getInitParameterNames() {
        ServletConfig sc = getServletConfig();
        if (sc == null) {
            throw new IllegalStateException(
                lStrings.getString("err.servlet_config_not_initialized"));
        }

        return sc.getInitParameterNames();
    }   


    /**
     *实现Servlet方法返回初始化获取的ServletConfig对象
     */    
    public ServletConfig getServletConfig() {
	           return config;
    }


    /**
     *获取ServletContext对象(Servlet上下文),是在ServletConfig中获取
     */
    public ServletContext getServletContext() {
        ServletConfig sc = getServletConfig();
        if (sc == null) {
            throw new IllegalStateException(
                lStrings.getString("err.servlet_config_not_initialized"));
        }

        return sc.getServletContext();
    }



    public String getServletInfo() {
         	return "";
    }


    /**
     *实现Servlet初始化方法,并回调init()无参方法,可供实现者监听状态.
     *
     *
     */
    public void init(ServletConfig config) throws ServletException {
	      this.config = config;
	       this.init();
    }


    /**
     * 子类可通过重写监听Servlet对象初始化状态.
     */
    public void init() throws ServletException {

    }


    public void log(String msg) {
	       getServletContext().log(getServletName() + ": "+ msg);
    }



    public void log(String message, Throwable t) {
	     getServletContext().log(getServletName() + ": " + message, t);
    }


    public abstract void service(ServletRequest req, ServletResponse res)
	throws ServletException, IOException;

    public String getServletName() {
        ServletConfig sc = getServletConfig();
        if (sc == null) {
            throw new IllegalStateException(
                lStrings.getString("err.servlet_config_not_initialized"));
        }

        return sc.getServletName();
    }
}

  1. HttpServlet

是GenericServlet的子类,对service添加了Http的处理,一般适用于Http通信的Servlet,是较为常用的一种Servlet.

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;
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);
public HttpServlet() { }
/**
*Http协议Get方式请求回调
*
*/
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);
}
}
protected long getLastModified(HttpServletRequest req) {
return -1;
}
/**
*Http协议HHEAD(只请求页面的头部)方式请求回调
*/
protected void doHead(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
NoBodyResponse response = new NoBodyResponse(resp);
doGet(req, response);
response.setContentLength();
}
/**
*Http协议POST方式请求回调
*/
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);
}
}
/**
*Http协议Put方式请求回调
*/
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);
}
}
/**
*Http协议Delete方式请求回调
*/
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;
}
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++) {
Method m = methods[i];
if (m.getName().equals("doGet")) {
ALLOW_GET = true;
ALLOW_HEAD = true;
}
if (m.getName().equals("doPost"))
ALLOW_POST = true;
if (m.getName().equals("doPut"))
ALLOW_PUT = true;
if (m.getName().equals("doDelete"))
ALLOW_DELETE = true;
}
String allow = null;
if (ALLOW_GET)
allow=METHOD_GET;
if (ALLOW_HEAD)
if (allow==null) allow=METHOD_HEAD;
else allow += ", " + METHOD_HEAD;
if (ALLOW_POST)
if (allow==null) allow=METHOD_POST;
else allow += ", " + METHOD_POST;
if (ALLOW_PUT)
if (allow==null) allow=METHOD_PUT;
else allow += ", " + METHOD_PUT;
if (ALLOW_DELETE)
if (allow==null) allow=METHOD_DELETE;
else allow += ", " + METHOD_DELETE;
if (ALLOW_TRACE)
if (allow==null) allow=METHOD_TRACE;
else allow += ", " + METHOD_TRACE;
if (ALLOW_OPTIONS)
if (allow==null) allow=METHOD_OPTIONS;
else allow += ", " + METHOD_OPTIONS;
resp.setHeader("Allow", allow);
}
/**
*处理TRACE请求
*/
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());
}
/**
*http协议处理
*/
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);
}
}
private void maybeSetLastModified(HttpServletResponse resp,
long lastModified) {
if (resp.containsHeader(HEADER_LASTMOD))
return;
if (lastModified >= 0)
resp.setDateHeader(HEADER_LASTMOD, lastModified);
}
/**
*重写父类方法,转化为http协议处理,调用自有http处理
*/
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);
}
}
// 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"));
}
}
}

如上,Servlet对于用户的请求获取和返回主要是通过方法:

public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;

实现的。这里,servlet容器在调用时传入两个参数:一个ServletRequest对象,用于接收用户请求信息;一个ServletResponse对象,用于返回请求用户信息。而HttpServlet对于对象则更改为HttpServletRequest和HttpServletResponse对象。其实这两个接口是同样在对应的ServletRequest和ServletResponse接口的基础上扩展的。

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