Servlet 使用介绍(1)

时间：2022-09-12 23:20:21

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。

Enjoytoday,EnjoyCoding
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