How to Correctly Transfer Files Over Sockets in Java?

Java File Transfer over Sockets: Sending and Receiving Byte Arrays

In Java, transferring files over sockets involves converting the file into byte arrays, sending them via the socket, and then converting the bytes back into a file at the receiving end. This article addresses an issue encountered by a Java developer in implementing this file transfer functionality.

Server-Side Issue

The server code appears to create an empty file upon receiving data from the client. To resolve this, the server should use a loop to read the data sent by the client in chunks, using a buffer to temporarily store the data. Once all data has been received, the server can write the complete file. The corrected server code is as follows:

<code class="java">byte[] buffer = new byte[1024];
int bytesRead = 0;
while ((bytesRead = in.read(buffer)) != -1) {
    fos.write(buffer, 0, bytesRead);
}</code>

Client-Side Issue

The client code initially sends an empty byte array to the server. To send the actual file content, the following code should be used:

<code class="java">FileInputStream is = new FileInputStream(file);
byte[] bytes = new byte[(int) length];
is.read(bytes);
out.write(bytes);</code>

Improved Code

With the aforementioned corrections, the full code for server and client is as follows:

Server:

<code class="java">...
byte[] buffer = new byte[1024];
DataOutputStream out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));
DataInputStream in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));

FileOutputStream fos = new FileOutputStream("C:\test2.xml");
int bytesRead = 0;
while ((bytesRead = in.read(buffer)) != -1) {
    fos.write(buffer, 0, bytesRead);
}
fos.close();
...</code>

Client:

<code class="java">...
Socket socket = new Socket(host, 4444);
DataOutputStream out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));
DataInputStream in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));

File file = new File("C:\test.xml");
FileInputStream is = new FileInputStream(file);
long length = file.length();
if (length > Integer.MAX_VALUE) {
    System.out.println("File is too large.");
}
byte[] bytes = new byte[(int) length];
is.read(bytes);
out.write(bytes);
...</code>

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