I am connection to smtp.live.com but when i run program and insert email credentials it asks to use starttls command first what i can do to avoid this exception.
try {
Socket socket1= new Socket("smtp.live.com",587);
again:
while(true) {
Scanner s= new Scanner(System.in);
String from=""; String to=""; String cc=""; String bcc="";
mail[m_count]=new Message();
The problem is here:
Socket socket1 = new Socket("smtp.live.com", 587);
If the port number was 25, that would be fine. The problem is that you are trying to use SSL enabled SMTP, and that requires that you an SSL socket. But that is a bit more complicated ...
However, I think that there is a bigger issue here. The normal way to send / receive emails from Java is to use the javax.mail APIs. But if you are opening a socket (or SSL socket) to the mail server, that implies that you intend to implement the client side of the SMTP specification. Unless you have a really good reason to do that ... it sounds like a waste of effort.
The tutorial link provided by #npinti assumes that you are going to use javax.mail. If you
want to do it the hard way, this tutorial shows you how to use an SSL socket.
Related
This question already has answers here:
Java socket API: How to tell if a connection has been closed?
(9 answers)
Closed 5 years ago.
When I'm using e.g. PuTTY and my connection gets lost (or when I do a manual ipconfig /release on Windows), it responds directly and notifies my connection was lost.
I want to create a Java program which monitors my Internet connection (to some reliable server), to log the date/times when my internet fails.
I tried use the Socket.isConnected() method but that will just forever return "true". How can I do this in Java?
Well, the best way to tell if your connection is interrupted is to try to read/write from the socket. If the operation fails, then you have lost your connection sometime.
So, all you need to do is to try reading at some interval, and if the read fails try reconnecting.
The important events for you will be when a read fails - you lost connection, and when a new socket is connected - you regained connection.
That way you can keep track of up time and down time.
Even though TCP/IP is "connection oriented" protocol, normally no data is sent over an idle connection. You can have a socket open for a year without a single bit sent over it by the IP stack. In order to notice that a connection is lost, you have to send some data on the application level.(*) You can try this out by unplugging the phone cable from your ADSL modem. All connections in your PC should stay up, unless the applications have some kind of application level keepalive mechanism.
So the only way to notice lost connection is to open TCP connection to some server and read some data from it. Maybe the most simple way could be to connect to some FTP server and fetch a small file - or directory listing - once in a while. I have never seen a generic server which was really meant to be used for this case, and owners of the FTP server may not like clients doing this.
(*) There is also a mechanism called TCP keepalive but in many OS's you have to activate it for all applications, and it is not really practical to use if you want to notice loss of connection quickly
If the client disconnects properly, a read() will return -1, readLine() returns null, readXXX() for any other X throws EOFException. The only reliable way to detect a lost TCP connection is to write to it. Eventually this will throw an IOException 'connection reset', but it takes at least two writes due to buffering.
Why not use the isReachable() method of the java.net.InetAddress class?
How this works is JVM implementation specific but:
A typical implementation will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host.
If you want to keep a connection open continually so you can see when that fails you could connect to server running the ECHO protocol yourself rather than having isReachable() do it for you and read and write data and wait for it to fail.
You might want to try looking at the socket timeout interval. With a short timeout (I believe the default is 'infinite timeout') then you might be able to trap an exception or something when the host becomes unreachable.
Okay so I finally got it working with
try
{
Socket s = new Socket("stackoverflow.com",80);
DataOutputStream os = new DataOutputStream(s.getOutputStream());
DataInputStream is = new DataInputStream(s.getInputStream());
while (true)
{
os.writeBytes("GET /index.html HTTP/1.0\n\n");
is.available();
Thread.sleep(1000);
}
}
catch (IOException e)
{
System.out.println("connection probably lost");
e.printStackTrace();
}
Not as clean as I hoped but it's not working if I leave out the os.writeBytes().
You could ping a machine every number of seconds, and this would be pretty accurate. Be careful that you don't DOS it.
Another alternative would be run a small server on a remote machine and keep a connection to it.
Its probably simpler to connect to yahoo/google or somewhere like this.
URL yahoo = new URL("http://www.yahoo.com/");
URLConnection yc = yahoo.openConnection();
int dataLen = yc.getContentLength() ;
Neil
The isConnected()method inside Socket.java class is a little misleading. It does not tell you if the socket is currently connected to a remote host (like if it is unclosed). Instead, it tells you whether the socket has ever been connected to a remote host. If the socket was able to connect to the remote host at all, this method returns true, even after that socket has been closed. To tell if a socket is currently open, you need to check that isConnected() returns true and isClosed() returns false.
For example:
boolean connected = socket.isConnected() && !socket.isClosed();
If I execute the following code (NOTE: not the real IP address being used)
Socket s = new Socket("120.200.100.111", 80);
s.setSoTimeout(10 * 1000);
BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream()));
in.readLine();
After 10 seconds, I get 'SocketTimeoutException: Read timed out'
But if I telnet, I get:
telnet> open 120.200.100.111 80
Trying 120.200.100.111...
Connected to foo.bar.org.
Escape character is '^]'.
Please note I am aware that for a full application I should use some sort of existing API for socket communications, but shouldn't the above test be getting some sort of timely response from the server?
shouldn't the above test be getting some sort of timely response from the server?
No. A Telnet exchange doesn't start with the server sending you anything. You have to send something. The server is waiting for you; you are waiting for it; you're the one with the read timeout, so you get a read timeout.
The Telnet client doesn't use read timeouts by default.
freshman, both here and in Java
i know codes like:
Socket clientSocket=null;
try
{
clientSocket=new Socket(192.168.2.3,1111);
System.out.println("port open");
clientSocket.close();
}
catch(Exception e)
{
System.out.println("port closed");
}
can be used to test whether certain port is open(here is port 1111 on 192.168.2.3), just wandering is there any other way?? not using socket() maybe??
To simplyfy things a bit: yes, establishing a connection is correct way to probe for open port, and if you even find other method, it will be using socket() under the hood.
Complexity of the whole problem comes from the fact that "open port" is a number one suspect in every security book out there. System administrators use a bunch of tricks to fight so called "port scanning". This includes reporting a "false positive" (claiming that port is open while it is not) to any software that behaves like a port scanner (that is: makes a connection and disconnects without sending any data).
So in reality the best way is to establish connection, send request and process response. Every other approach can fail under certain network/host/security configurations, leaving your user puzzled as to why his application doesn't work when it claims there is no problem with connection.
For those who do not want to read a long question here is a short version:
A server has an opened socket for a client. The server gets a request to open a socket from
the same client-IP and client-port. I want to fore the server not to refuse such a request but to close the old socket and open a new one. How can I do ti?
And here is a long (original) question:
I have the following situation. There is an established connection between a server and client. Then an external software (Bonjour) says to my client the it does not see the server in the local network. Well, client does nothing about that because of the following reasons:
If Bonjour does not see the server it does not necessarily means that client cannot see the server.
Even if the client trusts the Bonjour and close the socket it does not improve the situation ("to have no open socket" is worser that "to have a potentially bad socket").
So, client do nothing if server becomes invisible to Bonjour. But than the server re-appears in the Bonjour and Bonjour notify the client about that. In this situation the following situations are possible:
The server reappears on a new IP address. So, the client needs to open a new socket to be able to communicate with the server.
The server reappears on the old IP address. In this case we have two subcases:
2.1. The server was restarted (switched off and then switched on). So, it does not remember the old socket (which is still used by the client). So, client needs to close the old socket and open a new one (on the same server-IP address and the same server-port).
2.2. We had a temporal network problem and the server was running the whole time. So, the old socket is still available for the use. In this case the client does not really need to close the old socket and reopen a new one.
But to simplify my life I decide to close and reopen the socket on the client side in any case (in spite on the fact that it is not really needed in the last described situation).
But I can have problems with that solution. If I close the socket on the client side and than try to reopen a socket from the same client-IP and client-port, server will not accept the call for a new socket. The server will think that such a socket already exists.
Can I write the server in such a way, that it does not refuse such calls. For example, if it (the server) sees that a client send a request for a socket from the same client-IP and client-port, it (server) close the available socket, associated with this client-IP and client-port and than it reopens a new socket.
You can't "reopen" a socket on your server. If the socket already exists and the client is trying to reconnect then you should get an BindException (see your previous question). The scenario that may be possible:
Client Shuts down socket
Server OS "notices" socket is dead on client side and shuts its side down
Client reconnects on the same port, but with a "new" socket
In this case you may consider it be the "same" socket, but it really isn't. That said a strategy you may wish to adopt is to have some sort of map (hash of client IP/port) to whatever mechanism you are using to service the socket or some kind of persistent state data, so that it can simulate a continuation of a previous socket (in the same vein as http sessioning). Something along the lines of:
HashMap<Client, State> sessions = ...;
public void server(){
...
while(true){
Socket socket = server.accept();
Client client = new Client(socket);
State s = sessions.get(client);
if(s == null){
s = new State();
sessions.put(client, s);
}
client.setState(s);
service(client);
}
...
}
and you can adjust the map lookup to define what a "session" means within your application (same client IP, same client IP & client port, some sessionid sent over the wire, etc).
If you are just trying to make it possible for the client to reconnect and force the server to "notice" the client is disconnected, the only real way in Java is to try and read/write data, and if it has been shutdown then it should throw an exception. Therefore as was mentioned in your other question you could add some kind of ack/nak feature to your protocol and add some type of check if you believe the client is disconnected (for example if you haven't read any data in the last N milliseconds, send a message the client must echo within M milliseconds, otherwise it is assumed to be disconnected). You can also try isConnected, isInputShutdown, isOutputShutdown, but I have found those to be unreliable in my own code to indicate the socket state, unless you have closed the socket (i.e. the one you are testing on the server).
The situation you describe is impossible. You can't get a new connect request from the same remote IP:port as an existing connection. The client will not permit it to occur.
Based on the comments:
You cannot write the server in a way that it will close a socket it still thinks is connected and automatically accept the new connection, as application code does not have that kind of control over the TCP stack, nor is there a way to reopen a connection.
The chance of the port numbers being the same between your client restarts is very small.
But still, if that happens, the server will note that that you're trying to set up an already connected socket, and refuse your new connection. There's not much else your client can do in this case besides close your socket, create a new one and try to connect again - and another random port will be selected.
additional note, your server should take some form of action to detect and close dead sockets, if all your server does is read incoming data, the "dead" sockets will never be
closed as they will never be detected as dead.(enabling tcp keepalive is one cheap measure to take against dead sockets staying up for months, though it will take a couple of hours to detect them as such by default.)
This question already has answers here:
Java socket API: How to tell if a connection has been closed?
(9 answers)
Closed 5 years ago.
When I'm using e.g. PuTTY and my connection gets lost (or when I do a manual ipconfig /release on Windows), it responds directly and notifies my connection was lost.
I want to create a Java program which monitors my Internet connection (to some reliable server), to log the date/times when my internet fails.
I tried use the Socket.isConnected() method but that will just forever return "true". How can I do this in Java?
Well, the best way to tell if your connection is interrupted is to try to read/write from the socket. If the operation fails, then you have lost your connection sometime.
So, all you need to do is to try reading at some interval, and if the read fails try reconnecting.
The important events for you will be when a read fails - you lost connection, and when a new socket is connected - you regained connection.
That way you can keep track of up time and down time.
Even though TCP/IP is "connection oriented" protocol, normally no data is sent over an idle connection. You can have a socket open for a year without a single bit sent over it by the IP stack. In order to notice that a connection is lost, you have to send some data on the application level.(*) You can try this out by unplugging the phone cable from your ADSL modem. All connections in your PC should stay up, unless the applications have some kind of application level keepalive mechanism.
So the only way to notice lost connection is to open TCP connection to some server and read some data from it. Maybe the most simple way could be to connect to some FTP server and fetch a small file - or directory listing - once in a while. I have never seen a generic server which was really meant to be used for this case, and owners of the FTP server may not like clients doing this.
(*) There is also a mechanism called TCP keepalive but in many OS's you have to activate it for all applications, and it is not really practical to use if you want to notice loss of connection quickly
If the client disconnects properly, a read() will return -1, readLine() returns null, readXXX() for any other X throws EOFException. The only reliable way to detect a lost TCP connection is to write to it. Eventually this will throw an IOException 'connection reset', but it takes at least two writes due to buffering.
Why not use the isReachable() method of the java.net.InetAddress class?
How this works is JVM implementation specific but:
A typical implementation will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host.
If you want to keep a connection open continually so you can see when that fails you could connect to server running the ECHO protocol yourself rather than having isReachable() do it for you and read and write data and wait for it to fail.
You might want to try looking at the socket timeout interval. With a short timeout (I believe the default is 'infinite timeout') then you might be able to trap an exception or something when the host becomes unreachable.
Okay so I finally got it working with
try
{
Socket s = new Socket("stackoverflow.com",80);
DataOutputStream os = new DataOutputStream(s.getOutputStream());
DataInputStream is = new DataInputStream(s.getInputStream());
while (true)
{
os.writeBytes("GET /index.html HTTP/1.0\n\n");
is.available();
Thread.sleep(1000);
}
}
catch (IOException e)
{
System.out.println("connection probably lost");
e.printStackTrace();
}
Not as clean as I hoped but it's not working if I leave out the os.writeBytes().
You could ping a machine every number of seconds, and this would be pretty accurate. Be careful that you don't DOS it.
Another alternative would be run a small server on a remote machine and keep a connection to it.
Its probably simpler to connect to yahoo/google or somewhere like this.
URL yahoo = new URL("http://www.yahoo.com/");
URLConnection yc = yahoo.openConnection();
int dataLen = yc.getContentLength() ;
Neil
The isConnected()method inside Socket.java class is a little misleading. It does not tell you if the socket is currently connected to a remote host (like if it is unclosed). Instead, it tells you whether the socket has ever been connected to a remote host. If the socket was able to connect to the remote host at all, this method returns true, even after that socket has been closed. To tell if a socket is currently open, you need to check that isConnected() returns true and isClosed() returns false.
For example:
boolean connected = socket.isConnected() && !socket.isClosed();