Http客戶端程序已集成在Java語言中,可以通過URLConnection類調用。遺憾的是,由於SUN沒有公佈Http客戶程序的源碼,它實現的細節仍是一個謎。本文根據HTTP協議規範,用Java.net.Socket類實現一個HTTP協議客戶端程序。

  1.Socket類:



  瞭解TCP/IP協議集通信的讀者知道,協議間的通信是通過Socket完成的。在Java.net包中,Socket類就是對Socket的具體實現。它通過連接到主機後,返回一個I/O流,實現協議間的信息交換。



  2 . HTTP協議



  HTTP協議同其它TCP/IP協議集中的協議一樣,是遵循客戶/服務器模型工作的。客戶端發往服務端的信息格式如下:



請求方法 URL HTTP協議的版本號

提交的元信息

**空行**

實體



  請求方法是對這次連接工作的說明,目前HTTP協議已經發展到1.1版,它包括GET、HEAD、POST、DELETE、OPTIONS、TRACE、PUT七種。元信息是關於當前請求的信息。通過分析元信息,可以檢查實體數據是否完整,接收過程是否出錯,類型是否匹配等。元信息的引入使HTTP協議通信更加穩妥可靠。實體是請求的具體內容。



  將上述報文發往Web服務器,如果成功,應答格式如下:



HTTP協議的版本號 應答狀態碼 應答狀態碼說明

接收的元信息

**空行**

實體



  以上報文發向客戶端,並且接收成功,彼此間關閉連接,完成一次握手。下面用最常用的GET方法,來說明具體的報文應用:





GET http://www.youhost.com HTTP/1.0

accept: www/source; text/html; image/gif; image/jpeg; */*

User_Agent: myAgent

**空行**



  這個報文是向www.youhost.com主機請求一個缺省HTML文檔。客戶端HTTP協議版本號是1.0版,元信息包括可接收的文件格式,用戶代理,每一段之間用回車換行符分隔,最後以一個空行結束。發向服務器後,如果執行過程正常,服務器返回以下代碼:



HTTP/1.1 200 OK

Date: Tue, 14 Sep 1999 02:19:57 GMT

Server: Apache/1.2.6

Connection: close

Content-Type: text/html

**空行**

<html><head>...</head><body>...</body></html>



  HTTP/1.1表示這個HTTP服務器是1.1版,200是服務器對客戶請求的應答狀態碼,OK是對應答狀態碼的解釋,之後是這個文檔的元信息和文檔正文。(相關應答狀態碼和元信息的解釋請參閱Inetrnet標準草案:RFC2616)。



  3. HTTP客戶端程序:



import java.net.*;

import java.io.*;

import java.util.Properties;

import java.util.Enumeration;

public class Http {

protected Socket client;

protected BufferedOutputStream sender;

protected BufferedInputStream receiver;

protected ByteArrayInputStream byteStream;

protected URL target;

private int responseCode=-1;

private String responseMessage="";

private String serverVersion="";

private Properties header = new Properties();

public Http() { }

public Http(String url) {

GET(url) ;

}

/* GET方法根據URL,會請求文件、數據庫查詢結果、程序運行結果等多種內容 */

public void GET(String url) {

try {

checkHTTP(url);

openServer(target.getHost(),target.getPort() );

String cmd = "GET "+ getURLFormat(target) +" HTTP/1.0\r\n"

+ getBaseHeads()+"\r\n";

sendMessage(cmd);

receiveMessage();

}catch(ProtocolException p) {

p.printStackTrace();

return;

}catch(UnknownHostException e) {

e.printStackTrace();

return;

}catch(IOException i)



i.printStackTrace();

return;

}

}



/*

* HEAD方法只請求URL的元信息,不包括URL本身。若懷疑本機和服務器上的

* 文件相同,用這個方法檢查最快捷有效。

*/

public void HEAD(String url) {

try {

checkHTTP(url);

openServer(target.getHost(),target.getPort() );

String cmd = "HEAD "+getURLFormat(target)+" HTTP/1.0\r\n"

+getBaseHeads()+"\r\n";

sendMessage(cmd);

receiveMessage();

}catch(ProtocolException p) {

p.printStackTrace();

return;

}catch(UnknownHostException e) {

e.printStackTrace();

return;

}catch(IOException i)



i.printStackTrace();

return;

}

}

/*

* POST方法是向服務器傳送數據,以便服務器做出相應的處理。例如網頁上常用的

* 提交表格。

*/

public void POST(String url,String content) {

try {

checkHTTP(url);

openServer(target.getHost(),target.getPort() );

String cmd = "POST "+ getURLFormat(target) +"

HTTP/1.0\r\n"+getBaseHeads();

cmd += "Content-type: application/x-www-form-urlencoded\r\n";

cmd += "Content-length: " + content.length() + "\r\n\r\n";

cmd += content+"\r\n";

sendMessage(cmd);

receiveMessage();

}catch(ProtocolException p) {

p.printStackTrace();

return;

}catch(UnknownHostException e) {

e.printStackTrace();

return;

}catch(IOException i)



i.printStackTrace();

return;

}



}

protected void checkHTTP(String url) throws ProtocolException {

try {

URL target = new URL(url);

if(target==null || !target.getProtocol().toUpperCase().equals("HTTP") )

throw new ProtocolException("這不是HTTP協議");

this.target = target;

}catch(MalformedURLException m) {

throw new ProtocolException("協議格式錯誤");

}

}

/*

* 與Web服務器連接。若找不到Web服務器,InetAddress會引發UnknownHostException

* 異常。若Socket連接失敗,會引發IOException異常。

*/

protected void openServer(String host,int port) throws

UnknownHostException,IOException {

header.clear();

responseMessage=""; responseCode=-1;

try {

if(client!=null) closeServer();

if(byteStream != null) {

byteStream.close(); byteStream=null;

}

InetAddress address = InetAddress.getByName(host);

client = new Socket(address,port==-1?80:port);

sender = new BufferedOutputStream(client.getOutputStream());

receiver = new BufferedInputStream(client.getInputStream());

}catch(UnknownHostException u) {

throw u;

}catch(IOException i) {

throw i;

}

}

/* 關閉與Web服務器的連接 */

protected void closeServer() throws IOException {

if(client==null) return;

try {

client.close(); sender.close(); receiver.close();

}catch(IOException i) {

throw i;

}

client=null; sender=null; receiver=null;

}

protected String getURLFormat(URL target) {

String spec = "http://"+target.getHost();

if(target.getPort()!=-1)

spec+=":"+target.getPort();

return spec+=target.getFile();

}



/* 向Web服務器傳送數據 */

protected void sendMessage(String data) throws IOException{

sender.write(data.getBytes(),0,data.length());

sender.flush();

}

/* 接收來自Web服務器的數據 */

protected void receiveMessage() throws IOException{

byte data[] = new byte[1024];

int count=0;

int word=-1;

// 解析第一行

while( (word=receiver.read())!=-1 ) {

if(word=='\r'||word=='\n') {

word=receiver.read();

if(word=='\n') word=receiver.read();

break;

}

if(count == data.length) data = addCapacity(data);

data[count++]=(byte)word;

}

String message = new String(data,0,count);

int mark = message.indexOf(32);

serverVersion = message.substring(0,mark);

while( mark responseCode = Integer.parseInt(message.substring(mark+1,mark+=4));

responseMessage = message.substring(mark,message.length()).trim();



// 應答狀態碼和處理請讀者添加

switch(responseCode) {

case 400:

throw new IOException("錯誤請求");

case 404:

throw new FileNotFoundException( getURLFormat(target) );

case 503:

throw new IOException("服務器不可用" );

}

if(word==-1) throw new ProtocolException("信息接收異常終止");

int symbol=-1;

count=0;

// 解析元信息

while( word!='\r' && word!='\n' && word>-1) {

if(word=='\t') word=32;

if(count==data.length) data = addCapacity(data);

data[count++] = (byte)word;

parseLine: {

while( (symbol=receiver.read()) >-1 ) {

switch(symbol) {

case '\t':

symbol=32; break;

case '\r':

case '\n':

word = receiver.read();

if( symbol=='\r' && word=='\n') {

word=receiver.read();

if(word=='\r') word=receiver.read();

}

if( word=='\r' || word=='\n' || word>32) break parseLine;

symbol=32; break;

}

if(count==data.length) data = addCapacity(data);

data[count++] = (byte)symbol;

}

word=-1;

}

message = new String(data,0,count);

mark = message.indexOf(':');

String key = null;

if(mark>0) key = message.substring(0,mark);

mark++;

while( mark String value = message.substring(mark,message.length() );

header.put(key,value);

count=0;

}

// 獲得正文數據

while( (word=receiver.read())!=-1) {

if(count == data.length) data = addCapacity(data);

data[count++] = (byte)word;

}

if(count>0) byteStream = new ByteArrayInputStream(data,0,count);

data=null;

closeServer();

}

public String getResponseMessage() {

return responseMessage;

}

public int getResponseCode() {

return responseCode;

}

public String getServerVersion() {

return serverVersion;

}

public InputStream getInputStream() {

return byteStream;

}

public synchronized String getHeaderKey(int i) {

if(i>=header.size()) return null;

Enumeration enum = header.propertyNames();

String key = null;

for(int j=0; j<=i; j++)

key = (String)enum.nextElement();

return key;

}

public synchronized String getHeaderValue(int i) {

if(i>=header.size()) return null;

return header.getProperty(getHeaderKey(i));

}

public synchronized String getHeaderValue(String key) {

return header.getProperty(key);

}

protected String getBaseHeads() {

String inf = "User-Agent: myselfHttp/1.0\r\n"+

"Accept: www/source; text/html; image/gif; */*\r\n";

return inf;

}

private byte[] addCapacity(byte rece[]){

byte temp[] = new byte[rece.length+1024];

System.arraycopy(rece,0,temp,0,rece.length);

return temp;

}

}



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