Develop Reference

Write to remote file java

How can I write to remote file in server? I try this but I can´t
try {
BufferedWriter out = new BufferedWriter(new FileWriter("http://myserver/file.txt"));
out.write("Hello");
out.close();
} catch (IOException e) {
//handle exception
}

You can use the FTP library from Apache to get the file in to your local, modify it, then ftp it back.
2nd option, if you don't want to do heavy text modifying is to use sshExec. It lets you run commands on the server you connect to.
3rd option, execute the class on the server that the txt file exists.

Start and Stop Tomcat from java code

Based in a code I saw in Stackoverflow and other pages on Internet, I've created a method to stop and start tomcat at the moment I'll run a process in my system because I need to clean memory in my OS, I use System.gc() but still not enough to free memory, this is the code:
Global declaration:
private String server = "localhost";
Method to stop-start tomcat:
public void tomcat(){
try{
Socket s = new Socket(server,8005);
if(s.isConnected()){
PrintWriter print = new PrintWriter(s.getOutputStream(),true);
print.println("SHUTDOWN"); /*Command to stop tomcat according to the line "<Server port="8005" shutdown="SHUTDOWN">" in catalina_home/conf/server.xml*/
print.close();
s.close();
}
Runtime.getRuntime().exec(System.getProperty("catalina.home")+"\\bin\\startup.bat"); /*Instruction to run tomcat after it gets stopped*/
}catch (Exception ex){
ex.printStackTrace();
}
}
The code line to start tomcat works perfectly, but no the instructions to stop it because, when I instance the socket, gives me the following message: Connection refused: connect.
How can I solve this? or, is there another way to stop tomcat?
Thanks in advance.

public static void shut_server(String lien)
{
try {
Process child = Runtime.getRuntime().exec(lien+"/shutdown.sh");
System.out.println("Serveur est atteins");
} catch (IOException ex) {
Logger.getLogger(Installation.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("erreur de demarrage");
}
}
lien = path to your tomcat bin file
for example - /home/zakaria/Téléchargements/apache-tomcat-8.0.21/bin

I had similar issue. I was getting the "Connection refused: connect" error message on creating the socket.
However, my use case is different from the one posted by Vlad. When the Tomcat server is starting up, my app is checking availability of some resources and if they are not, it needs to shutdown the server.
I added a 30 seconds sleep just before the line creating socket:
try {
Thread.sleep(30000);
}
catch (Exception excp) {}
Socket socket = new Socket("localhost", port);
and it started working.
I think when Tomcat is starting up it needs some time to make the shutdown port ready to work.
The selection of 30 seconds is arbitrary, it could be probably shorter.
FYI, my Tomcat is running as Windows service.

Downloading an XML file via FTP

I have a list of feeds in a database that I use to download a XML file from a FTP server and then parse it. The scrpt is bundled up into a jar file which is run daily using Windows Task Scheduler. Occasionally the request get haulted at grabbing a certain xml file. So far it has happened about 3 times in 2 weeks with no real pattern that I can see.
When it does mess up, I go to the computer it is being run from, I see the command window open and it is stopped before the xml has been fully downloaded. If I close the command window and run the task manually everything will work fine.
The code that I am using to download the xml file is:
private void loadFTPFile(String host, String username, String password, String filename, String localFilename){
System.out.println(localFilename);
FTPClient client = new FTPClient();
FileOutputStream fos = null;
try {
client.connect(host);
client.login(username, password);
String localFilenameOutput = createFile(assetsPath + localFilename);
fos = new FileOutputStream(localFilenameOutput);
client.retrieveFile(filename, fos);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (fos != null)
fos.close();
client.disconnect();
} catch (IOException e) {
e.printStackTrace();
}
}
}
This function is being called in a loop and when it fails, everything stops and the script doesn't go onto the next feed.
I'm not sure what is happening, possibly the connection being lost, but I would think that the try/catch would catch if that is happening. I'm not sure if a timeout would do the trick or threads need to be used (but I've never worked with threads)
Could anyone point me in the right direction onto why this is happening and what I can do to fix the problem

UPDATE - Set a timeout for the data connection
Since the last file is only partially downloaded, and given the source of FTPClient.retrieveFile(), I think it may be a problem on the server side (something that make it hang, or even die - who knows). Obviously one can't repair the server or even know what's going on there, anyway I suggest to add a timeout with setDataTimeout(int) and catch the possible SocketTimeoutException separately to be logged in a different place and maybe sent to the FTP server admins (along with the time information when it happened) so they can merge the logs and see what's the issue.
OLD ANSWER
I didn't notice that you connect and login for each and every file, so the following is just an optimization not to close the control connection and succesfully logout, but it should not address the problem.
You could start the JVM in debug mode and attach a debugger when it hangs, anyway according to this answer and this thread it can be a timeout problem on the network equipment devices (routers). From the FTPClient Javadoc
During file transfers, the data connection is busy, but the control
connection is idle. FTP servers know that the control connection is in
use, so won't close it through lack of activity, but it's a lot harder
for network routers to know that the control and data connections are
associated with each other. Some routers may treat the control
connection as idle, and disconnect it if the transfer over the data
connection takes longer than the allowable idle time for the router.
One solution to this is to send a safe command (i.e. NOOP) over the control connection to reset the router's idle timer. This is enabled as follows:
ftpClient.setControlKeepAliveTimeout(300); // set timeout to 5 minutes

Do you check the return status of any of the calls or is that the code?
There is a call completePendingCommand() that has to be used on occassion. That may be something to look into.
Also, you won't see an IO exception, I belive it gets repackaged as a CopyStreamException
You might want to also change the return value to a boolean since you trap the exceptions, at least the calling loop will know whether the tranfer happened or not.
private boolean loadFTPFile(String host, String username, String password, String filename, String localFilename){
System.out.println(localFilename);
FTPClient client = new FTPClient();
FileOutputStream fos = null;
try {
client.connect(host);
int reply = client.getReplyCode();
if (!FTPReply.isPositiveCompletion(reply)){
client.disconnect();
System.err.println("FTP server refused connection.");
return false;
}
if (!client.login(username, password)){
client.logout();
return false;
}
String localFilenameOutput = createFile(assetsPath + localFilename);
fos = new FileOutputStream(localFilenameOutput);
boolean result = client.retrieveFile(filename, fos);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
if (result){
System.out.println("\tFile Transfer Completed Successfully at: " + sdf.format(Calendar.getInstance().getTime()));
// ftp.completePendingCommand();
}
else {
System.out.println("\tFile Transfer Failed at: " + sdf.format(Calendar.getInstance().getTime()));
}
return result;
}catch (CopyStreamException cse){
System.err.println("\n\tFile Transfer Failed at: " + sdf.format(Calendar.getInstance().getTime()));
System.err.println("Error Occurred Retrieving File from Remote System, aborting...\n");
cse.printStackTrace(System.err);
System.err.println("\n\nIOException Stack Trace that Caused the Error:\n");
cse.getIOException().printStackTrace(System.err);
return false;
}catch (Exception e){
System.err.println("\tFile Transfer Failed at: " + sdf.format(Calendar.getInstance().getTime()));
System.out.println("Error Occurred Retrieving File from Remote System, aborting...");
e.printStackTrace(System.err);
return false;
} finally {
try {
if (fos != null)
fos.close();
client.disconnect();
} catch (IOException e) {
e.printStackTrace();
}
}
}

It's not a threading issue. Chances are it is caused by something in the loop since that code looks like it should clean up just fine. That said, for testing you will probably want to add
catch (Exception e) {
e.printStackTrace();
}
after the IOException catch clause. It's possible that another exception is being thrown.
Another thing, if you are pulling results from the database result set one at a time and doing the FTP gets, that might be a problem. Unless the results are all brought back by the JDBC call at once, that too could time out. Not all database queries actually return the entire result set to the client at once.

Java app throws ClosedByInterruptException immediately when opening a socket, cause?

I have a java app that holds open many connections to an address, probably in the ballpark of 2,000 at once, with hardly any activity, mostly open for monitoring purposes passing a few bytes every now and then. When new connections need to be opened up, it automatically opens them and adds them to its pool. Sometimes though, for an unknown reason, the application receives a ClosedByInterruptException immediately during/after creating the socket to the remote address. To the best of my knowledge, this only occurs on the client side as a result of an interrupt signal to the thread. I have checked and rechecked the source code surrounding the problem area and it seems ok. I was hoping I could get someone's expertise as to if there could be an alternate cause, besides source code, for instance, is there a system reason that causes this? Is there a hardware cause? Server level/router level? My network knowledge I would consider amateur, but is 2K connections too many for a router, or no?
INFO [08 Sep 2011 23:11:45,982]: Reconnecting id 20831
ERROR [08 Sep 2011 23:11:45,990]: IOException while creating plain socket channel
java.nio.channels.ClosedByInterruptException
at java.nio.channels.spi.AbstractInterruptibleChannel.end(AbstractInterruptibleChannel.java:184)
at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:518)
at com.*.createSocketChannelPlain(MyTask.java:441)
at com.*._executeTask(MyTask.java:176)
at com.*.executeTask(MyTask.java:90)
at com.*.ThreadPool$WorkerThread.run(ThreadPool.java:55)
ERROR [08 Sep 2011 23:11:45,990]: Could not open socket
WARN [08 Sep 2011 23:11:45,990]: WorkerThread_24 received interrupted exception in ThreadPool
java.lang.InterruptedException
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:485)
at com.*.TaskQueue.getTask(TaskQueue.java:39)
at com.*.ThreadPool$WorkerThread.run(ThreadPool.java:48)
Update: I would like to try and offer all I can to help others contribute to a diagnosis. So here is the actual function where the exception occurs, only difference being the line marking I added to line 441.
private SocketChannel createSocketChannelPlain() throws TaskFailedException {
SocketChannel socketChannel = null;
try {
// Create a non-blocking socket channel to use to communicate for imap connection
socketChannel = SocketChannel.open();
socketChannel.configureBlocking(false);
try {socketChannel.socket().setSoLinger(true, 0);} catch (Exception e) {}
try {socketChannel.socket().setKeepAlive(true);} catch (Exception e) {}
/*Line 441*/ socketChannel.connect(new InetSocketAddress(_HOSTNAME, _PORT));
//System.out.println("Started connection");
// Complete connection
while (!socketChannel.finishConnect()) {
// do something until connect completed
try {
//do what you want to do before sleeping
Thread.sleep(500);//sleep for 500 ms
//do what you want to do after sleeping
} catch(InterruptedException ie){
//If this thread was interrupted by another thread
try { socketChannel.close(); } catch (Exception e) {}
finally { socketChannel = null; }
break;
}
}
//System.out.println("Finished connecting");
return socketChannel;
} catch (IOException e) {
logger.error("IOException while creating plain socket channel to gmail", e);
try { socketChannel.close(); } catch (Exception e1) {}
finally { socketChannel = null; }
//throw new TaskFailedException("IOException occurred in createSocketChannel");
}
return socketChannel;
}

What OS are you running this on? I don't know about Windows, but on Linux (and presumably other Unix-like OSes), you can run out of file handles by having large numbers of sockets. You can work around this by doing ulimit -n 8192 or similar before running the Java app. Alternatively, edit /etc/security/limits.conf and set nofile. All of that said, ClosedByInterruptedException would be an odd way to notice this.
If the above isn't the issue, the next thing I'd try would be to run tcpdump (if we're talking about a GUI-less machine) or Wireshark (if we aren't) and capture the traffic your program's generating, looking for weird things happening at the time that connection starts.

How do I have to configure a RMI environment so that I'm able to use it in a "real" network?

Because I didn't want to implement a communication protocol for my client-server based application, I implemented a RMI client and a RMI server on both sides for the information exchange between the two components.
If I try to use my application by starting the two components on the same machine, everything is working fine. But if I split the components to two different computers (Kubuntu 9.04 within as a virtual machine within an Windows 7 RC environment with disabled firewall and a native Ubuntu 9.04 environment), it seems like the RMI client is not able to execute the methods which are defined on the server side. (Every functions call leads to a RMI exception.)
Currently I only set the system property "java.rmi.server.hostname" on both sides to the network interface which should be used for the data exchange and registered the default port for the communication with rmi daemon (?) rmid.
Does somebody has an idea what might be going wrong? Do I have to set some other parameters like "java.rmi.server.codebase" (http://java.sun.com/j2se/1.4.2/docs/guide/rmi/javarmiproperties.html) to be able to use the RMI functionality within my application?
Edit: Okay, here is some additional information for you:
In the initialization phase my client tries to establish a connection to the RMI server of server component, which was initialized using the following two methods:
private void initialize()
{
// set ip address of rmi server
System.setProperty("java.rmi.server.hostname", ipAddress);
// try to register rmi server
try
{
LocateRegistry.createRegistry(Registry.REGISTRY_PORT);
}
catch (Exception e)
{
// ignore
}
}
public void start()
{
System.out.print("starting master control RMI server ...");
try
{
Naming.rebind("MasterControl", this);
}
catch (Exception e)
{
System.out.println("error: could not initialize master control RMI server");
System.exit(1);
}
// set running flag
isRunning = true;
System.out.println(" done");
}
"ipAddress" is here the ip address of the network interface of the server component.
The method which is used by the client component to establish the connection looks like this:
public void connect()
{
// build connection url
String url = "rmi://" + masterControlIpAddress + "/MasterControl";
System.out.println(url);
System.out.print("connecting to master control ...");
// try to connect to master control server
while (connection == null)
{
try
{
connection = (MasterControlInterface) Naming.lookup(url);
id = connection.register(localIpAddress);
}
catch (Exception e)
{
// ignore
}
if (connection == null)
{
try
{
Thread.sleep(100);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
System.out.println(" done");
}
As you can see my client calls a function to register the connection at the server:
#Override
public int register(String ipAddress) throws RemoteException
{
// add connection to registrationHandler
masterControl.registrationHandler.addConnection(ipAddress);
// log
int connectionCount = masterControl.registrationHandler.getConnectionCount();
System.out.println("slave control (" + ipAddress + ") instance has been registered at the master control server under the following id: " + connectionCount);
return connectionCount;
}
If I run my program using a real network connection, the text "slave control ..." is not displayed on the server side. Therefore I'm not sure, if the function is really called by the client component.
After the client component is intialized it tries to notify the server component by calling the following method using it's RMI connection to the server:
public void sendInitializationDone()
{
try
{
connection.initializationDone();
}
catch (RemoteException e)
{
System.out.println("error: could not send 'initializationDone' message to master control");
System.out.println(e);
System.exit(1);
}
}
to set a flag on the server side.
The error occures inside this function on the client side:
java.rmi.ConnectException: Connection refused to host 127.0.1.1; nested exception is: java.net.ConnectException: Connection refused.
I have no idea why the host is here 127.0.1.1 ...
#nos
Of course, I disabled the windows firewall and the protection mechanismn of Kaspersky Internet Security. I don't think that there is a running firewall in my Kubuntu. In generell it is possible to establish a connection, because I already used scp to copy my program to the other machine.
Edit2:
Mhhh, after setting the entry in /etc/hosts which refers to the machine to the ip address of the machine it seems to work, but don't really understand why it does ...
BR,
Markus

You need to add an entry to the hosts file of the machines containing an entry of the form
machinename privateip
e.g.
virtualmachine 192.168.1.16
This will prevent RMI from sending the localhost host name as a 'call me back' address.
To test this approach, run the following code before and after performing the change.
System.out.println(java.net.InetAddress.getLocalHost());
It should output a local address before the changes and a non-local address after the changes.

Using different versions of the JDK on each server could cause this problem.
Use the
java -version
command to make sure that you are using the same version of the jre.