How to check if TCP socket is connected in android

I'm writing a short test app to practice connecting to a server. All the app does is take an IP from an editText box, and then connect to the server. It looks like I can connect to the server because I am able to send data to the server and have the server print it.

I wanted to add some error checking to confirm that I am connected before attempting to send anything to the server. However, the problem is, whenever I use the Socket.isConnected() or isBound() methods my app crashes.

So how do I check if I'm connected if these methods doesn't seem to work. As I said, I know I'm connected because I can send stuff to the server.

Below is the code, I connect on the press of a button. What I want to do is confirm that I'm connected, and then kick off a thread that will work in the background sending and receiving data from the server. In the section that say's s.isBound() is where the program crashes. I can even put in s.isConnected() and it will crash also.

Last, what is the difference between isBound, and isConnected?

private OnClickListener connectListener = new OnClickListener() {            
    @Override
    public void onClick(View v) {
        if (!connected) {
            serverIpAddress = serverIp.getText().toString();
            if (!serverIpAddress.equals("")) {
                try{
                    InetAddress serverAddr = InetAddress.getByName(serverIpAddress);
                    Log.d("ClientActivity", "Trying to Connect");
                    s = new Socket(serverAddr, SERVERPORT);
                    Log.d("ClientActivity", "Connected");

                    output = s.getOutputStream();

                    input = s.getInputStream();

                } 
                catch (UnknownHostException e) {
                    e.printStackTrace();
                } 
                catch (IOException e) {
                    e.printStackTrace();
                }

                if(s.isBound()){
                    connected = true;
                    cThread = new Thread(new ClientThread());
                    cThread.setName("Client Connection Thread");
                    cThread.start();
                }

            }
        }
    }
};

This is what the log outputs.

11-13 17:03:56.718: D/ClientActivity(2039): Trying to Connect
11-13 17:03:56.757: W/System.err(2039): java.net.ConnectException: /192.168.16.1:6340 - Connection refused
11-13 17:03:56.757: W/System.err(2039):     at org.apache.harmony.luni.net.PlainSocketImpl.connect(PlainSocketImpl.java:207)
11-13 17:03:56.757: W/System.err(2039):     at org.apache.harmony.luni.net.PlainSocketImpl.connect(PlainSocketImpl.java:183)
11-13 17:03:56.757: W/System.err(2039):     at java.net.Socket.startupSocket(Socket.java:705)
11-13 17:03:56.757: W/System.err(2039):     at java.net.Socket.<init>(Socket.java:263)
11-13 17:03:56.757: W/System.err(2039):     at com.AUIEE.client_test.Client_TestActivity$1.onClick(Client_TestActivity.java:88)
11-13 17:03:56.757: W/System.err(2039):     at android.view.View.performClick(View.java:2485)
11-13 17:03:56.765: W/System.err(2039):     at android.view.View$PerformClick.run(View.java:9089)
11-13 17:03:56.765: W/System.err(2039):     at android.os.Handler.handleCallback(Handler.java:587)
11-13 17:03:56.765: W/System.err(2039):     at android.os.Handler.dispatchMessage(Handler.java:92)
11-13 17:03:56.773: W/System.err(2039):     at android.os.Looper.loop(Looper.java:130)
11-13 17:03:56.773: W/System.err(2039):     at android.app.ActivityThread.main(ActivityThread.java:3859)
11-13 17:03:56.773: W/System.err(2039):     at java.lang.reflect.Method.invokeNative(Native Method)
11-13 17:03:56.773: W/System.err(2039):     at java.lang.reflect.Method.invoke(Method.java:507)
11-13 17:03:56.773: W/System.err(2039):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:840)
11-13 17:03:56.773: W/System.err(2039):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:598)
11-13 17:03:56.773: W/System.err(2039):     at dalvik.system.NativeStart.main(Native Method)
11-13 17:03:56.781: D/AndroidRuntime(2039): Shutting down VM
11-13 17:03:56.781: W/dalvikvm(2039): threadid=1: thread exiting with uncaught exception (group=0x4001e560)
11-13 17:03:56.789: E/AndroidRuntime(2039): FATAL EXCEPTION: main
11-13 17:03:56.789: E/AndroidRuntime(2039): java.lang.NullPointerException
11-13 17:03:56.789: E/AndroidRuntime(2039):     at com.AUIEE.client_test.Client_TestActivity$1.onClick(Client_TestActivity.java:103)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.view.View.performClick(View.java:2485)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.view.View$PerformClick.run(View.java:9089)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.os.Handler.handleCallback(Handler.java:587)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.os.Handler.dispatchMessage(Handler.java:92)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.os.Looper.loop(Looper.java:130)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at android.app.ActivityThread.main(ActivityThread.java:3859)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at java.lang.reflect.Method.invokeNative(Native Method)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at java.lang.reflect.Method.invoke(Method.java:507)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:840)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:598)
11-13 17:03:56.789: E/AndroidRuntime(2039):     at dalvik.system.NativeStart.main(Native Method)

回答1:

Firstly, your

if(s.isBound()){
    connected = true;
    cThread = new Thread(new ClientThread());
    cThread.setName("Client Connection Thread");
    cThread.start();
}

block is at a wrong place. If any catch block triggered, then it will execute the above block. But if an exception is raised that means that s will probably be null, so you have a certain NullPointerException. (crash of the application) The appropriate place is inside the try block.

Secondly, as you can see from the logcat log, the connection that you are trying to establish refuses. Perhaps there is something wrong with the ip/port or a firewall.

Also logcat informs you that you probably have an uncaught exception.

1-13 17:03:56.781: W/dalvikvm(2039): threadid=1: thread exiting with uncaught exception (group=0x4001e560)

Fix that problems and if the problem persist come again to discuss it.

回答2:

I agree that your code misplaced in the TRY/CATCH loop is probably the root of your problem.

In answer to your question about isBound and isConnected, here are their definitions.

public boolean isBound ()

Returns whether this socket is bound to a local address and port.
Returns true if the socket is bound to a local address, false otherwise.