Java socket does not always receive all bytes

Question

Java socket does not always receive all bytes

I have a problem with my Java application using sockets. After connecting to the socket on the server, we can download the file.

The procedure is as follows:

the client sends a message to the server with the file name to download
server responds with file size
then the server sends all bytes of the file to the socket

Here is my Java code:

//init keyManagerFactory and trustManager
SSLContext sslContext = SSLContext.getInstance("TLS");
sslContext.init(keyManagerFactory.getKeyManagers(), new TrustManager[] { trustManager }, new SecureRandom());
SSLSocketFactory socketFactory = sslContext.getSocketFactory();
SSLSocket socket = (SSLSocket) socketFactory.createSocket(new Socket(ipAddress, port), ipAddress, port, false);
socket.startHandshake();

DataInputStream in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
... 

int toRead = xxxx; //received from server response
List<Byte> bytes = new ArrayList<>();
while(toRead > 0) {
    byte b = in.readByte();
    bytes.add(b);
    toRead--;
}

The problem is that sometimes I get all bytes from the server, but in other cases I get only part of it, and there is no more data, for example. the server sends the file size = 1500, my application tries to get it from the socket, but there are only 1100 bytes, at another time there are 1200 and at another time 1500.

It can be said - this is a problem with the server. But I wrote a Python program with the same functionality, and it always works fine!

Here's the python code:

def recv(sock, toread):
    buf = bytearray(toread)
    view = memoryview(buf)
    while toread:
        nbytes = sock.recv_into(view, toread)
        if nbytes == 0:
            sock.close()
            raise Disconnected
        view = view[nbytes:]
        toread -= nbytes
    return buf

plainsock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock = ssl.wrap_socket(plainsock, cert_reqs=ssl.CERT_NONE, ssl_version=ssl.PROTOCOL_TLSv1)
sock.connect(someIP, somePort)

...

toRead = xxxx //received from server response

data = recv(sock, toRead)