java FetchURLConnection url in Java

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java FetchURLConnection url
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The application will issue an HTTP request, the display the MIME content type and length of the resource and then the resource itself
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926 Modifying and Examining Header Fields with URLConnection
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In the previous example, you learned how to use the URLConnection to fetch HTTP resources, and how to determine the length and MIME content type of a resource However, there are many more HTTP response header fields to examine, and you can also modify HTTP request header fields to make it possible for server-side applications (such as CGI scripts, Active Server Pages, or Java servlets) to customize their output In this next example, you'll learn how to modify request properties, and how to get response header fields Code for HTTPHeaders
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import javanet*; import javaio*;
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// 9, Listing 4 public class HTTPHeaders { public static void main(String args[]) throws Exception { int argc = argslength; // Check for valid number of parameters if (argc != 1) { Systemoutprintln ("Syntax :"); Systemoutprintln ("java HTTPHeaders url"); return; } // Catch any thrown exceptions try { // Create an instance of javanetURL javanetURL myURL = new URL ( args[0] ); // Create a URLConnection object, for this URL // NOTE : no connection has yet been established URLConnection connection = myURLopenConnection(); // Set some basic request fields // Set user agent, to identify the application // as Netscape compatible connectionsetRequestProperty ("User-Agent", "Mozilla/40 (compatible; JavaApp)"); // Set our referer field - set to any URL you'd // like connectionsetRequestProperty ("Referer", "http://wwwdavidreillycom/"); // Set use-caches field, to prevent caching connectionsetUseCaches(false); // Now open a connection connectionconnect(); // Examine request properties, to verify their // settings Systemoutprintln ("Request properties"); Systemoutprintln(); Systemoutprintln ("User-Agent: " + connectiongetRequestProperty("User-Agent")); Systemoutprintln ("Referer: " + connectiongetRequestProperty("Referer")); Systemoutprintln (); Systemoutprintln (); // Examine response properties, to see their // settings Systemoutprintln ("Response properties"); Systemoutprintln(); int i = 1; // Search through each header field, until no // more exist
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while ( connectiongetHeaderField ( i ) != null ) { // Get the name of this header field String headerName = connectiongetHeaderFieldKey(i); // Get the value of this header field String headerValue = connectiongetHeaderField(i); // Output header field key, and header //field value Systemoutprintln ( headerName + ": " + headerValue); // Goto the next element in the set of // header fields i++; } // Pause for user Systemoutprintln ("Hit enter to continue"); Systeminread(); } // MalformedURLException indicates parsing error catch (MalformedURLException mue) { Systemerrprintln ("Unable to parse URL!"); return; } // IOException indicates network or I/O error catch (IOException ioe) { Systemerrprintln ("I/O Error : " + ioe); return; } } }
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How HTTPHeaders Works Like previous examples, this example uses an instance of the URLConnection class to issue HTTP requests The chief difference here is that, before any request is sent, custom HTTP request fields are added These header fields provide additional information to server-side applications, which can then be used to customize the HTTP response When a Web browser sends a request, it identifies itself by sending a "User-Agent" field in the request Well-behaved HTTP clients do the same, and it is often advantageous to pose as a Web browser by including the Mozilla keyword in the identification string and then appending a legitimate-sounding application name, since CGI scripts and servlets sometimes offer different output depending on whether it's an HTTP agent like a search engine or an actual browser Other request fields can also be set, such as the referring URL and the cache flag, which determines whether or not a unique request will be sent each time to the server
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// Create a URLConnection object, for this URL // NOTE : no connection has yet been established URLConnection connection = myURLopenConnection(); // Set some basic request fields // Set user agent, to identify the application as Netscape compatible connectionsetRequestProperty ("User-Agent",
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"Mozilla/40 (compatible; JavaApp)"); // Set our referer field connectionsetRequestProperty ("Referer", "http://wwwdavidreillycom/"); // Set use-caches field, to prevent caching connectionsetUseCaches(false);
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Once the request settings are made, the URLConnection object can send the request If a call to the connect() method is made before assigning request properties, then the server will not receive them and they will not take effect After connecting and sending the request, the application displays the request fields to the user
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// Now open a connection connectionconnect(); // Examine request properties, to verify their settings Systemoutprintln ("Request properties"); Systemoutprintln(); Systemoutprintln ("User-Agent: " + connectiongetRequestProperty("User-Agent")); Systemoutprintln ("Referer: " + connectiongetRequestProperty("Referer")); Systemoutprintln (); Systemoutprintln ();
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The next set of header fields displayed by the application is from the server response In the previous section, the names of the request fields were known It would be impossible, however, to know the name of every field that might be sent back by a server Not all servers support the same fields, and some server-side applications may send back custom fields that a client has never before encountered The URLConnection offers several methods that provide access to request fields, two of which support a numerical index value rather than a key name This allows us to read the nth key, and to iterate through every element in the set of header fields The program prints out both the name of the field and its contents
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// Examine response properties, to see their settings Systemoutprintln ("Response properties"); Systemoutprintln(); int i = 1; // Search through each header field, until no more exist while ( connectiongetHeaderField ( i ) != null ) { // Get the name of this header field String headerName = connectiongetHeaderFieldKey(i); // Get the name of this header field String headerValue = connectiongetHeaderField(i); // Output header field key, and header field value Systemoutprintln ( headerName + ": " + headerValue); // Goto the next element in the set of header fields i++; }
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Finally, the application pauses to allow the user to read the header fields, and then terminates The actual contents of the resource are not displayed, although code for this exists in the previous example
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Running HTTPHeaders This application takes the same command-line parameter as previous examples You should pass a valid URL as the only command-line parameter, either of a local machine on your network, or a Web site, if you are connected to the Internet Try URLs pointing to different Web servers, to see a variation in the type of headers returned
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