Error sending file through java socket

Question

I have a problem sending files through socket java. Sometimes the code works, sometimes not. I test the while block in both, it seems the code is sending all the bytes, but the server is not receiving (but even in this test, the file is sent correctly). In this case, the server stopped receive data. All files are about 150Kb. I'm using the port 9191.

Server:

while (true) {
        try {
            Socket socket = ss.accept();
            ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
            String fileName = in.readUTF();

            FileOutputStream fos = new FileOutputStream(destinationPath + fileName);
            byte[] buf = new byte[1024];

            int len;
            while ((len = in.read(buf)) >= 0) {
                fos.write(buf, 0, len);
            }
            fos.flush();

        } catch (Exception ex) {
            ex.printStackTrace();
        }
}

Client:

    try {
        Socket socket = new Socket(host, port);
        ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());

        out.writeUTF(file.getName());
        out.writeLong(file.length());
        FileInputStream in = new FileInputStream(file);
        byte[] buf = new byte[1024];

        int len;
        while ((len = in.read(buf)) >= 0) {
            out.write(buf, 0, len);
        }

        out.close();
        socket.close();

    } catch (Exception e) {
        throw e;
    }

Smith_61 · Accepted Answer · 2015-05-13 02:11:59Z

1

First you should replace ObjectInputStream and ObjectOutputStream with DataInputStream and DataOutputStream respectively. You are not serializing java objects and it makes no sense to use stream classes meant for that purpose.

Second you are sending the file length in the client but not specifically reading it in the server. It is instead getting added to the beginning of the file.

answered May 13, 2015 at 2:11

Smith_61

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7 Comments

Marcelo Over a year ago

I made the changes and realize that it send 1 file, and the second (the same or different) file the code on server goes to a infinite loop in "while ((len = in.read(buf)) >= 0) {", because I put a println after the block and it never shows in second time.

Smith_61 Over a year ago

@Marcelo if you want to send multiple files from client to server, I would suggest first sending an int that tells the server how many files to expect. Then for each file start with the file name, followed by it's size, then by size bytes. On the server read in the file name, it's size then copy size bytes from the stream to the file and repeat until out of files.

Marcelo Over a year ago

In fact, in localhost works, but in a ubuntu server remotely doesn't. I clear iptables rules and nothing. In this case, I send one file, and after some seconds, another file.

Smith_61 Over a year ago

What is the error you get when attempting it remotely?

Marcelo Over a year ago

Nothing. It stop internally in the while loop and does nothing. I put messages internally and suddenly it stop showing message

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Elliott Frisch · Accepted Answer · 2015-05-13 02:12:40Z

1

On the client you say,

out.writeUTF(file.getName());
out.writeLong(file.length());

But on the server you say,

String fileName = in.readUTF();
FileOutputStream fos = new FileOutputStream(destinationPath + fileName);
byte[] buf = new byte[1024];
int len;
while ((len = in.read(buf)) >= 0) {
    fos.write(buf, 0, len);
}
fos.flush();

You didn't read the file length. And you need to make sure you read all of the sent bytes (or your file will be corrupt). Also, don't forget to close() the FileOutputStream (or use a try-with-resources to close it for you). Something like,

String fileName = in.readUTF();
try (FileOutputStream fos = new FileOutputStream(destinationPath
        + fileName)) {
    long size = in.readLong();
    byte[] buf = new byte[1024];
    long total = 0;
    int len;
    while ((len = in.read(buf)) >= 0) {
        fos.write(buf, 0, len);
        total += len;
    }
    if (total != size) {
        System.err.printf("Expected %d bytes, but received %d bytes%n",
                size, total);
    }
}

answered May 13, 2015 at 2:12

Elliott Frisch

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6 Comments

Marcelo Over a year ago

I made the changes and realize that it send 1 file, and the second (the same or different) file the code on server goes to a infinite loop in "while ((len = in.read(buf)) >= 0) {", because I put a println after the block and it never shows in second time.

Elliott Frisch Over a year ago

Make sure you're sending another file... the code above sends 1 file (not multiple files - and the client posted calls socket.close();)

Marcelo Over a year ago

In fact, in localhost works, but in a ubuntu server remotely doesn't. I clear iptables rules and nothing. In this case, I send one file, and after some seconds, another file, calling the SocketClient again

Elliott Frisch Over a year ago

What do you mean by calling the SocketClient again? Once you call socket.close() you can't use the socket without reconnecting. Similarly, once you call out.close() you can't use the OutputStream from socket.getOutputStream() without reconnecting (for the same reason).

Marcelo Over a year ago

So, I instantiate SocketClient each time I want to send a file. In this case I call SocketClient N times in a loop. I think it is not a problem call socket.close() in the SocketClient in this case.

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