Why does an SSL Server Socket connection block in Java whereas a non SSL Server Socket does not?

Consider the following code for a non-SSL Socket server and client all on the one thread:

import java.io.DataInputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerClient {
    public static void main(String[] args) throws IOException {
        ServerSocket ss = new ServerSocket(0); // open a random free port.

        Socket c = new Socket(ss.getInetAddress(), ss.getLocalPort());

        Socket s = ss.accept();

        final byte[] bytes = "Hello World!".getBytes();
        final OutputStream out = c.getOutputStream();
        System.out.println("writing to stream");
        out.write(bytes.length);
        out.write(bytes);

        System.out.println("reading from stream");

        final DataInputStream in = new DataInputStream(s.getInputStream());
        int len = in.read();
        final byte[] b = new byte[len];
        in.readFully(b);
        System.out.println(new String(b));

        c.close();
        ss.close();
    }
}

This produces the following output:

writing to stream
reading from stream
Hello World!

This process opened a server socket - connected with a client socket. Passed data down the socket and then closed down. There was no issue passing the data.

Consider a version to a prove a point with SSL Sockets:

import javax.net.ssl.SSLServerSocket;
import javax.net.ssl.SSLServerSocketFactory;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;
import java.io.DataInputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.Socket;
import java.util.logging.Logger;

public class SSLServerClient {

    private static Logger log = Logger.getLogger("InfoLogging");

    public static void main(String[] args) throws IOException {

        System.setProperty("javax.net.ssl.keyStore", "/path/KeyStore.jks");
        System.setProperty("javax.net.ssl.keyStorePassword", "password");

        SSLServerSocketFactory sslserversocketfactory = (SSLServerSocketFactory) SSLServerSocketFactory.getDefault();

        SSLServerSocket serverListeningSSLSocket = (SSLServerSocket) sslserversocketfactory.createServerSocket(4380);
        log.info("Server started");

        SSLSocketFactory sslSocketFactory=(SSLSocketFactory) SSLSocketFactory.getDefault();
        SSLSocket clientSocket =  (SSLSocket) sslSocketFactory.createSocket(serverListeningSSLSocket.getInetAddress(),
                serverListeningSSLSocket.getLocalPort());

        SSLSocket serverCommsSSLSocket = (SSLSocket) serverListeningSSLSocket.accept();
        log.info("new client");

        final byte[] bytes = "Hello World!".getBytes();
        final OutputStream out = clientSocket.getOutputStream();
        System.out.println("writing to stream");
        out.write(bytes.length);
        out.write(bytes);

        System.out.println("reading from stream");

        final DataInputStream in = new DataInputStream(serverCommsSSLSocket.getInputStream());
        int len = in.read();
        final byte[] b = new byte[len];
        in.readFully(b);
        System.out.println(new String(b));

        clientSocket.close();
        serverCommsSSLSocket.close();
        serverListeningSSLSocket.close();
    }
}

This gives the following output:

Nov 21, 2018 10:23:51 PM com.gamble.ssl.SSLServerClient main INFO: Server started
Nov 21, 2018 10:23:52 PM com.gamble.ssl.SSLServerClient main INFO: new client
writing to stream

ie it blocks on the server socket starting.

My question is: Why does an SSL Server Socket connection block in Java whereas a non SSL Server Socket does not?

The simple answer to the question is:

Because an SSL Handshake is asynchronous and the non SSL socket doesn't have to do that step.

Ok - there were two issues with the original SSL Code:

1. There was no TrustStore:

   System.setProperty( "javax.net.ssl.trustStore", "/path/KeyStore.jks");
   

2. Because the Handshake is synchronous - you have to start the client on a different thread. ie:

   (new Thread() {
     public void run() {
       // do stuff
     }
    }).start();
   

So the code looks like:

    import javax.net.ssl.SSLServerSocket;
    import javax.net.ssl.SSLServerSocketFactory;
    import javax.net.ssl.SSLSocket;
    import javax.net.ssl.SSLSocketFactory;
    import java.io.DataInputStream;
    import java.io.IOException;
    import java.io.OutputStream;
    import java.util.logging.Logger;

    public class SSLServerClient {

        public static void main(String[] args) throws IOException {

            System.setProperty("javax.net.ssl.keyStore", "/path/KeyStore.jks");
            System.setProperty( "javax.net.ssl.trustStore", "/path/KeyStore.jks");

            System.setProperty("javax.net.ssl.keyStorePassword", "password");

            SSLServerSocketFactory sslserversocketfactory = (SSLServerSocketFactory) SSLServerSocketFactory.getDefault();

            SSLServerSocket serverListeningSSLSocket = (SSLServerSocket) sslserversocketfactory.createServerSocket(4380);
            System.out.println("--server started");

            SSLSocketFactory sslSocketFactory=(SSLSocketFactory) SSLSocketFactory.getDefault();
            SSLSocket clientSocket =  (SSLSocket) sslSocketFactory.createSocket(serverListeningSSLSocket.getInetAddress(),
                    serverListeningSSLSocket.getLocalPort());

            SSLSocket serverCommsSSLSocket = (SSLSocket) serverListeningSSLSocket.accept();
            System.out.println("--new client");

            final byte[] bytes = "--Hello World!".getBytes();
            final OutputStream out = clientSocket.getOutputStream();
            System.out.println("--Gotten output stream");
            final DataInputStream in = new DataInputStream(serverCommsSSLSocket.getInputStream());

            (new Thread() {
                public void run() {
                    System.out.println("--reading from stream");

                    int len = 0;
                    try {
                        len = in.read();
                        final byte[] b = new byte[len];
                        in.readFully(b);
                        System.out.println(new String(b));

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


            System.out.println("--writing to stream");
            out.write(bytes.length);
            System.out.println("--writing to stream - length");
            out.write(bytes);

            clientSocket.close();
            serverCommsSSLSocket.close();
            serverListeningSSLSocket.close();
        }
    }

And the output looks like

    --server started
    --new client
    --Gotten output stream
    --writing to stream
    --reading from stream
    --writing to stream - length
    --Hello World!

    Process finished with exit code 0

Done

(Note I added the leading -- to the output to aid when reading SSL Debug output.