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Networking FAQ Home Page

FAQ Manager is guru Brandon Rohlfs.

The java.net package provides an essential mechanism for implementing network applications. [FAQ Previously managed by Tim Rohaly].

What's New

How do I maintain cookies across separate network connections?
Java:API:Networking
John Zukowski PREMIUM, Jun 30, 2006
Java 5 introduced the abstract CookieHandler class to help with the process, though Java 5 lacks a usable implementation. You must implement yourself....

How do I use Java to ping a host?
Java:API:Networking
John Zukowski PREMIUM, Dec 5, 2005
Starting with Java 5, there is an isReachable() method in the InetAddress class. You can specify either a timeout or the NetworkInterface to use. For more...

How can I encode a URL String?
Java:API:Networking:HTTP
Brandon Rohlfs, Sep 2, 2005
A URL String can be simply encoded using the static encode(String s, String enc) method within the URLEncoder class. encode(String s) has been deprecated...

When using a URLConnection how can I get a specific header field returned by the server or all of them?
Java:API:Networking:HTTP
Brandon Rohlfs, Aug 25, 2005
The URLConnection class contains various methods, which return specific header fields or the whole lot. Specific header fields can be obtained either...

How can I load a class from a remote server?
Java:API:Networking
Brandon Rohlfs, Aug 21, 2005
The URLClassLoader class was created specifically to facilitate the downloading of classes over a network. Three constructors are provided, the first...

While I can certainly show the contents of a web page in a JEditorPane, how do I launch the desktop's browser with the new URL?
Java:API:Networking
John Zukowski PREMIUM, Jul 31, 2005
The JDesktop Integration Components (JDIC), available from https://jdic.dev.java.net, offers support for this: import java.net.*; import org.jde...

How can I parse a URL?
Java:API:Networking:HTTP
Brandon Rohlfs, Jul 24, 2005
The URL class contains a variety of accessor methods, which return the values of a URL's components. URL's use of accesors makes having to parse...

How can I obtain an IP address from a host name?
Java:API:Networking
Brandon Rohlfs, Jun 22, 2005
The InetAddress class gives the programmer a solution to this problem through a process called resolution. When an IP address is obtained from an host...

What is a URL and how can I construct one?
Java:API:Networking
Brandon Rohlfs, Jun 16, 2005
A URL (Uniform Resource Locator) represents a reference to resource on a network. The resource may take many forms. Maybe it is an image stored on a...

How can I access an NNTP newsgroup using JSP or Servlets?
Java:API:Servlets, Java:API:JSP, Java:API:Networking
Alex Chaffee PREMIUM, Dec 4, 2002
Your servlet must know how to speak NNTP, or call a library that does. There are NNTP implementation in the following packages: JavaMail: See...

I'm looking for information on how to write a java program that uses a http url connection object to request data from a servlet... however, I don't want to construct a new tcp/ip connection every time...
Java:API:Networking:HTTP
Krzysztof Raciniewski, Mar 16, 2002
You can set the "http.keepAlive" propety t o true. You can do it running java -Dhttp.keepAlive=true or from code System.getProperties ().put ("http.keepAlive",...

Java application can't estabilish network connection, if the connection came up after application start. First I'm running a the java program below with no network connection. While the program running,...
Java:API:Networking
Karl Logan, Mar 16, 2002
I've found the solution for this problem. If I use the ip address instead of the name of the server then I'm able to reconnect after network break.

HttpUrlConnection headache and the TIMEOUT I have many links(urls) to be checked for their activeness. I want to set a timeout and check whether urls response within the timeout. My final goal...
Java:API:Networking:HTTP
Sean Sullivan, Mar 16, 2002
Instead of using java.net.URLConnection, I recommend that you try the Apache Jakarta HttpClient http://jakarta.apache.org/commons/httpclient/

I am using URLConnection to simulate a browser HTTP POST. However, the website (ebay) claims I cannot accept cookies? I have tried setting the "User-Agent", "Referer" fields etc...What am I doing wrong?...
Java:API:Networking
Stephen Ostermiller PREMIUM, Mar 10, 2002
Cookies are persistant data that the server wants you to store. The cookie is given to you by the server in a header like: Set-Cookie: foo=bar; path=/;...

I want to interrupt an accept()-operation on a ServerSocketChannel with jdk14. But issueing an interrupt() blocks. here is a test program demonstrating this: import java.net.*; import java.nio.*; import...
Java:API:Networking
Simon Ablett PREMIUM, Mar 10, 2002
Interrupting blocked IO operations is dangerous as it could leave the supplier/receiver in an indeterminate state (imagine interrupting a database operation)....

A SocketChannel that has registered with the Selector showing interest in both OP_READ|OP_WRITE I would expect the select() method to continously return showing that the channel isWritable() until the...
Java:API:Networking
Jim Morris, Mar 10, 2002
Actually now I understand how the windows semantics works, due to the way the underlying windows WSAAsyncSelect works.... The FD_WRITE event is handled...

I'm doing this: try { InetAddress address = InetAddress.getLocalHost(); String IP = new String (address.getHostAddress()); } catch (Exception e) { e.printStackTrace(); } on...
Java:API:Networking
Burke Cox, Mar 10, 2002
I've been all over this problem today. It seems to vary across platforms -- and has real trouble working with DHCP. Apparently, the NetworkInterface...

This doubt has been there in my mind regarding socket-port relationship that whether a port is associated with only one unique socket object or multiple socket objects can be associated with a single...
Java:API:Networking
Simon Ablett PREMIUM, Mar 10, 2002
As I understand it, you can have any number of Sockets associated with a host:port combination and only a single ServerSocket. At least this is how it...

In Java, is there any way of getting all the IP addresses associated with a multi-homed system?
Java:Language, Java:API:Networking
Davanum Srinivas PREMIUM, Feb 28, 2002
Support has been added in JDK1.4. import java.net.InetAddress; import java.net.NetworkInterface; import java.net.SocketException; import...

Here is my question: I have a java serversocket application staying on a windows machine, and listening a certain port say 1111. Can I using other window's application (written in Delphi, for example)...
Java:API:Networking
Simon Ablett PREMIUM, Feb 28, 2002
It shouldn't make a difference what language you write your server/clients in so long as they use TCP/IP sockets. Are you sure that you are not, for example,...

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