{"id":183,"date":"2018-07-26T11:49:25","date_gmt":"2018-07-26T11:49:25","guid":{"rendered":"http:\/\/csp12.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=183"},"modified":"2018-08-02T06:59:38","modified_gmt":"2018-08-02T06:59:38","slug":"java-networking","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/csp12\/chapter\/java-networking\/","title":{"rendered":"Java Networking"},"content":{"raw":"
\r\n\r\nURL Connections<\/strong>\r\n\r\n \r\n\r\nURL stands for Uniform Resource Locator<\/strong>\r\n\r\n \r\n\r\nIt is a scheme used for uniquely identifying all kinds of network resources.\r\n\r\nThe basic form of a URL looks like,\r\n\r\n<pr otoc ol>:<site name ><pathname >\r\n\r\nSome examples are,\r\n\r\nhttp:\/\/www.cs.ust.hk\/~liao\/comp201\/index.html\r\n\r\nftp:\/\/ftp.cs.ust.hk\/pub\/lzhang\/teach\/201\/codes\/HttpTest\/HttpTest.java file:\/MyDisk\/Letters\/Toxx2-11-15.The protocols include file, http, ftp, gopher, news, mailto, etc.\r\n\r\n<\/div>\r\n
\r\n

Generally, connecting to a URL is a hard way which means we need to manually parse out the host, protocol, path from URL and create a socket to host. Then we need to use the protocol to send the right request and interpret response. But Java makes this much easier than that.<\/p>\r\n \r\n\r\nURL Connections\/Retrieve Information<\/strong>\r\n\r\n \r\n\r\nOpen connection with java.net.URL<\/strong>\r\n\r\n \r\n

To open a connection with a URL, first we need to create an URL object using the constructor of the URL class as discussed in the previous module.<\/p>\r\n

URL url1 = new URL(\u201chttp:\/\/www.cs.ust.hk\/~liao\/comp201\/index.html<\/a>\u201d);<\/p>\r\n

Then to fetch contents of the resource, use the openStream method of URL, which returns an InputStream.<\/p>\r\n

InputStream in1 = url1.openStream();<\/p>\r\n

Now we can nest the InputStreams with other Java streams to retrieve the contents from the URL.<\/p>\r\n \r\n\r\nExample of Read stream<\/strong>\r\n\r\n \r\n

This is an example that reads a stream directly from the URL. Here we use the openStream() method of the URL class that returns an InputStream. This InputStream is wrapped with the BufferedReader to read lines of text.<\/p>\r\n \r\n\r\nimport java.net.*;\r\n\r\nimport java.io.*;\r\n\r\npublic class ReadStream\r\n\r\n{\r\n\r\npublic static void main(String args[]) throws Exception\r\n\r\n{\r\n\r\ntry{\r\n\r\nURL u = new URL(\"https:\/\/www.google.co.in \"); InputStream in = u.openStream();\r\n\r\nBufferedReader bin = new BufferedReader(new InputStreamReader(in)); String temp;\r\n\r\nwhile(( temp=bin.readLine()) != null)\r\n\r\n{\r\n\r\nSystem.out.println(temp);\r\n\r\n}\r\n\r\n}catch(IOExce ption e)\r\n\r\n{<\/span>\r\n\r\nSystem.out.println(\"Socket error\");<\/span>\r\n\r\n}<\/span>\r\n\r\n}<\/span>\r\n\r\n}<\/span>\r\n\r\n<\/div>\r\nThe output of this code is shown in Figure 21.1.\r\n\r\n \r\n

\"\"<\/p>\r\n\r\n

\r\n\r\nURLConnection<\/strong>\r\n\r\n \r\n

URLConnection is a general-purpose class for accessing the attributes of a remote resource. This URLConnection class is used to inspect the properties of the remote object before actually transporting it locally. These attributes are exposed by the HTTP protocol specification and, as such, only make sense for URL objects that are using the HTTP protocol. The instance methods defined in the URLConnection class is listed in the Table 21.1.<\/p>\r\n\r\n<\/div>\r\n

\"\"<\/p>\r\n \r\n\r\nHttpURLConnection<\/strong>\r\n\r\n \r\n

Java provides a subclass of URLConnection that provides support for HTTP connections. This class is called HttpURLConnection. We can obtain an HttpURLConnection object in the sameway, by calling the openConnection( ) method on an URL object. The instance methods defined in the HttpURLConnection class is listed in the Table 21.2.<\/p>\r\n

\"\"<\/p>\r\n \r\n

Communicating with Web Servers<\/strong><\/p>\r\n\r\n

\r\n\r\n \r\n

Class java.net.URL represents a Uniform Resource Locator. Create an Java object that represents an URL using the constructor of the URL class.<\/p>\r\n \r\n\r\nURL url = new URL(\u201chttp:\/\/www.cs.ust.hk\/~lzhang\/comp201\/index.html<\/a>\u201d);\r\n\r\n \r\n\r\nThe instance methods of the URL class are etc.\r\n\r\n \r\n\r\nThis java.net.URLConnection<\/em> represents a communication link between the application and a URL.\r\n\r\n \r\n\r\nWe can use the Constructor of this class to create an URLConnection object.\r\n\r\n \r\n\r\nURLConnection con = new URLConnection( url).\r\n\r\n \r\n

The connection can also be obtainable from the URL class. But here we can create an URLConnection object by using the openConnection() method of the URL class to create an object.<\/span><\/p>\r\n \r\n

URLConnection con = url.openConnection();<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\u00a0 \u00a0 <\/strong>\r\n\r\nURLConnection<\/strong>\r\n\r\n \r\n

We can directly open a stream for reading using the URL class. The signature of the openStream() method is given below,public final<\/p>\r\n \r\n

InputStream <\/a>openStream() throws IOException<\/a><\/p>\r\nInputStream in = u.openStream();\r\n\r\n \r\n

This opens a connection to this URL and returns an InputStream for reading from that connection. This method is a shorthand for,<\/p>\r\nopenConnection().getInputStream()\r\n\r\n \r\n\r\nConnecting to a URL using URLConnection<\/strong>\r\n\r\n \r\n

We can connect to URL using the URL\u2019s openConnection()<\/em> method that returns an URLConnection object. The signature is given below,URLConnection openConnection() throws IOExce ption ;<\/p>\r\n \r\n\r\nExample:<\/strong>\r\n\r\n \r\n\r\ntry {\r\n\r\nURL url = new URL(\"http:\/\/www.cs.biu.ac.il\/~freskom1\/\") ; URLConnection uc = url.openConnection() ;\r\n\r\n}\r\n\r\ncatch (MalformedURLException e)\r\n\r\n{ \u2026 }\r\n\r\ncatch (IOException e)\r\n\r\n{ \u2026 }\r\n\r\n<\/div>\r\n\u00a0 Reading from a URL connection<\/strong>\r\n\r\n \r\n

To read from the URL connection, we first create an URL object using the constrcutor of the URL class. Then we can use the openConnection()<\/em> method of the URL class that returns an URLConnection object. The method getInputStream()<\/em> of this URLConnection object returns an InputStream that can be wrapped with other streams to read data from the URL.<\/p>\r\n \r\n\r\nimport java.net.* ;\r\n\r\nimport java.io.* ;\r\n\r\npublic class URLConnectionReader1\r\n\r\n{\r\n\r\npublic static void main(String[] args) throws Exception\r\n\r\n{\r\n\r\nURL url = new URL(\"http:\/\/www.google.com\") ; URLConnection uc = url.openConnection() ; BufferedReader br=new BufferedReader(new InputStreamReader(uc.getInputStream()));\r\n\r\nString line ;\r\n\r\nwhile ((line=br.readLine())!=null)\r\n\r\nSystem.out.println(line) ;\r\n\r\nbr.close() ;\r\n\r\n}\r\n\r\n}\r\n

The output of reading from URL using URLConnection is given in the following Figure 21.2.<\/p>\r\n \r\n\r\nOutput of Reading from a URL connection<\/strong>\r\n\r\n \r\n

\"\"<\/p>\r\n\r\n

\r\n\r\nInstance Methods of URLConnection<\/strong>\r\n\r\n \r\n

Some of the instance methods of the URLConnection class is demonstrated in this example code. The method getDate()<\/em> prints the time and date of the response in milliseconds. The method getContentT ype() <\/em>prints the type of content found in the resource. The method getLastModified() <\/em>prints the time and date when the resource was last modified in milliseconds. The method getExpiration() <\/em>prints the expiration time and date of the resource in milliseconds. The method getContentLength() <\/em>prints the size in bytes of the content associated with the resource if it is available. Finally, if the content is available it opens an input stream to the resource and reads the content.<\/p>\r\n \r\n\r\nimport java.net.*;\r\n\r\nimport java.io.*;\r\n\r\nimport java.util.Date;\r\n\r\npublic class URLConnectionReade r\r\n\r\n{\r\n\r\npublic static void main(String[] args) throws Exception {\r\n\r\nint c;<\/span>\r\n\r\n\/\/URL yahoo = new URL(\"http:\/\/java.sun.com\/docs\/books\/tutorial\/\"); URL yahoo = new URL(\"https:\/\/www.google.co.in\/\");<\/span>\r\n\r\nURLConnection yc = yahoo.openConnection();<\/span>\r\n\r\nlong d = yc.getDate();<\/span>\r\n\r\nif(d==0)<\/span>\r\n\r\nSystem.out.println(\"no date info\");<\/span>\r\n\r\nelse<\/span>\r\n\r\nSystem.out.println(\"Date:\" + new Date(d)); System.out.println(\"content type\" + yc.getContentType()); d = yc.getExpiration();<\/span>\r\n\r\nif(d==0)<\/span>\r\n\r\nSystem.out.println(\"no expiration info\");<\/span>\r\n\r\nelse<\/span>\r\n\r\nSystem.out.println(\"Expires\" + new Date(d));<\/span>\r\n\r\nd = yc.getLastModified();<\/span>\r\n\r\nif(d==0)<\/span>\r\n\r\nSystem.out.println(\"no last modified info\");<\/span>\r\n\r\nelse<\/span>\r\n\r\nSystem.out.println(\"last modified\"+ new Date(d))<\/span>\r\n\r\nint len = yc.getContentLength();<\/span>\r\n\r\nif(len == -1)<\/span>\r\n\r\nSystem.out.println(\"content length unavailable\"); else<\/span>\r\n\r\ns<\/span>ystem.out.println(\"content length\" + len);<\/span>\r\n\r\nif(len!=0)<\/span>\r\n\r\n{<\/span>\r\n\r\nSystem.out.println(\"===content===\");<\/span>\r\n\r\nInputStream input = yc.getInputStream();<\/span>\r\n\r\nwhile((( c= input.read()) != -1))<\/span>\r\n\r\n{<\/span>\r\n\r\nSystem.out.print((char) c);<\/span>\r\n\r\n}\r\n\r\ninput.close();<\/span>\r\n\r\n}<\/span>\r\n\r\nelse<\/span>\r\n\r\n{<\/span>\r\n\r\nSystem.out.println(\"no content available\");<\/span>\r\n\r\n}<\/span>\r\n\r\n}<\/span>\r\n\r\n}<\/span>\r\n\r\n<\/div>\r\n

The output of demonstrating the instance methods of URLConnection is shown in Figure 21.3<\/p>\r\n \r\n\r\nOutput of URL Connection<\/strong>\r\n\r\n \r\n

\"\"<\/p>\r\n \r\n

\r\n\r\n\u00a0 \u00a0 \u00a0 \u00a0URL Connections\/Sending Information<\/strong>\r\n\r\n \r\n

The browser allows the user to enter data in text fields and other GUI components. Like processing HTML forms, the browser writes the data to the URL. On the server a CGI-BIN script processes it and returns a response.<\/p>\r\n \r\n

The Web servers receive information from clients using either GET or POST. The request method is specified in the form as below,<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\u00a0 \u00a0 <form method=\u201cpost\u201d action=\"\/cgi-bin\/ipc\/idbsprd\">\r\n\r\n \r\n

Appropriate CGI (common gateway interface) script is called to process information received and produce an HTML page to send back to client. CGI scripts are usually written in C, Perl, shellscript etc.<\/p>\r\n \r\n\r\nSend information to CGI script using GET<\/strong>\r\n\r\n \r\n

The \"GET\" request attaches the parameters to the end of URL as shown below,http:\/\/host\/script?parameters.<\/a><\/p>\r\n

The parameters are seperated using \u201c&\u201d and parameters are encoded as follows to avoid misinterpretation which is called as URL encoding.<\/p>\r\n \r\n\r\nThe URL encoding is done as follows,\r\n