JAVA IO

java.io is package that provides system input and output through data streams. Read write file, socket.

Important interface/abstract

Closable: to indicate that this class hold resource that needed to be closed. For example, opens file

• void close()

AutoCloseable : same as Closable but method close will be invoked if object is declared in try-with-resources header block

• void close()

graph LR;
    Closable-->|extends|AutoCloseable;

InputStream : abstract class that representing an input stream of byte

graph RL;
    ByteArrayInputStream-->|extends|InputStream;
    BufferedInputStream-->|extends|InputStream;
    etc[... and much more]-->|extends|InputStream;

OutputStream : abstract class that representing an output stream of byte

graph RL;
    ByteArrayOutputStream-->|extends|OutputStream;
    BufferedOutputStream-->|extends|OutputStream;
    etc[... and much more]-->|extends|OutputStream;

Example implementation

• BufferedInputStream : buffered bytes in to array that support make and reset so that user can repeatly read some bytes
• BufferedOutputStream : buffered bytes in to array before flush bytes to underling OutputStream, to achive a better performace by avoid costly action

Reader: abstract class for reading character streams Writer: abstract class for writing character streams

graph RL;
    BufferedReader-->|extends|Reader;
    FileReader-->|extends|Reader;
    etc[... and much more]-->|extends|Reader;

graph RL;
    ByteArrayOutputStream-->|extends|OutputStream;
    BufferedOutputStream-->|extends|OutputStream;
    etc[... and much more]-->|extends|OutputStream;

Example usage of Java IO with simple echo server

Server side code

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@Slf4j
class EchoServer {

    @SneakyThrows
    public static void main(String[] args) {
        final int port = 8080;
        //Create a Server socket and bound to specify port for TCP connection.
        final var ss = new ServerSocket(port);
        log.info("Start server at port : " + port);

        while (true) {
            //Put socket.accept and in/out stream in try() to execute autoClosable after all communication
            try (
                //ss.accept is block until connection is created.
                var socket = ss.accept();
                //Create BufferedReader so that we can use readLine(), It need 'Reader' in constructure which can created from 'InputStream' of socket
                var inReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                // PrintWriter so that we can print String into it.
                var outWriter = new PrintWriter(socket.getOutputStream(), true)
            ) {
                log.info("Get connection from : " + socket.getLocalAddress() + ':' + socket.getPort());

                String inputLine;
                //Get next line until it's null
                while ((inputLine = inReader.readLine()) != null ) {
                    log.info("Server received : " + inputLine);
                    outWriter.println(inputLine + " is echo");
                }

            }
        }
    }

}

Socket provide InputStream for incoming data and OutputStearm for outgoing data. With BufferedReader, we can read as String line by line. Similar to PrintWriter to warp so that we can print line into underlining stream.

Client side command

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# this will open TCP socket in interactive mode. we can send message with enter.
nc localhost 8080
...