java.net How to fire an HTTP request

Question

I am using java.net for sending HTTP requests in my Java client and I still can not realize/find how to actually fire the request.

For example I have this code:

Scanner sc = new Scanner(System.in);

System.out.println("Deleting subject...");
System.out.println("Subject shortcut (-1 for return):");
String shortcut = sc.next();
if( shortcut.equals("-1") )
    return ;

try
{
    URL url = new URL( "http://localhost:8080/Server/webresources/subject/delete/"+shortcut );
    HttpURLConnection con = (HttpURLConnection)url.openConnection();
    con.setDoOutput(true);
    con.setRequestMethod("DELETE");

    BufferedReader br = new BufferedReader( new InputStreamReader( con.getInputStream() ) );
    System.out.println( br.readLine() );
}catch( Exception e )
{
    System.out.println(e.getMessage());
}

In this code if I do not use these lines:

BufferedReader br = new BufferedReader( new InputStreamReader( con.getInputStream() ) );
System.out.println( br.readLine() );

the request is never sent. So in this case the request seems to be trigger by calling for InputStream from connection.

Can anyone explain me how is a HTTP request via java.net fired?

Possible duplicate of [Using java.net.URLConnection to fire and handle HTTP requests](http://stackoverflow.com/questions/2793150/using-java-net-urlconnection-to-fire-and-handle-http-requests) — Michael, Jan 06 '17 at 14:40
The first answer to that question goes into quite a lot of detail. Basically you're right that the request gets sent when you read from the input stream. — Michael, Jan 06 '17 at 14:43
@Michael URLConnection#connect() as written in the topic you have just posted does not work. — scarface, Jan 06 '17 at 14:43
@Michael What you said is written there but what if I do not want any response? Then I do not want to read and then I also do not want to create a BufferedReader — scarface, Jan 06 '17 at 14:44
A lot of this is in the javadocs for the base class: https://docs.oracle.com/javase/8/docs/api/java/net/URLConnection.html (Though it's not written _super_ clearly) — yshavit, Jan 06 '17 at 14:44

score 5 · Accepted Answer · edited May 23 '17 at 12:17

5

From the documentation, a HttpURLConnection will connect either if you call connect(), or if you call an operation that depends on being connected, like getInputStream() .

Opens a communications link to the resource referenced by this URL, if such a connection has not already been established. If the connect method is called when the connection has already been opened (indicated by the connected field having the value true), the call is ignored.

URLConnection objects go through two phases: first they are created, then they are connected. After being created, and before being connected, various options can be specified (e.g., doInput and UseCaches). After connecting, it is an error to try to set them. Operations that depend on being connected, like getContentLength, will implicitly perform the connection, if necessary.

However, several topics indicate that connect() won't commit the actual request, but getInputStream() (and most likely any method reading the server's response e.g getResponseCode() ), will :

Java URLConnection - When do I need to use the connect() method?

Why does HttpURLConnection not send the HTTP request

How to send PUT, DELETE HTTP request in HttpURLConnection?

edited May 23 '17 at 12:17

Community

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1

answered Jan 06 '17 at 14:46

Arnaud

17,229
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Unfortunately, if I call `connect()`, nohing happens. – scarface Jan 06 '17 at 15:07
@scarface : this is strange, but you must be right since the comments from the following topic state the same thing : http://stackoverflow.com/questions/1051004/how-to-send-put-delete-http-request-in-httpurlconnection – Arnaud Jan 06 '17 at 15:20
I am going to check those topics. Anyway it is weird getting InputStream when, in some cases, I do not actually get any output from server. By the way also when I ask for a response number, the request is sent. – scarface Jan 06 '17 at 15:41
1

@scarface : yes it seems that in any case, you have to read something from the server (inputstream, response code, content length...). At least it makes sense to read the response code whatever your request is . – Arnaud Jan 06 '17 at 15:46

biology.info · Answer 2 · 2017-01-06T15:46:51.350

0

A new instance of URLConnection is created every time when invoking the URLStreamHandler.openConnection(URL) method of the protocol handler for this URL.

It should be noted that a URLConnection instance does not establish the actual network connection on creation. This will happen only when calling URLConnection.connect(). see javadoc

    String url = "http://example.com";

    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();

    con.setRequestMethod("GET");

    //add request header
    con.setRequestProperty("User-Agent", USER_AGENT);

    int responseCode = con.getResponseCode();

edited Jan 06 '17 at 15:46

answered Jan 06 '17 at 14:54

biology.info

3,500
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This is cool but it did not answer my question. Instead of getting InputStream, I get responseCode and that's all. That is not an explanation. – scarface Jan 06 '17 at 15:09

java.net How to fire an HTTP request

2 Answers2