How do I switch between String I/O and Object I/O streams in Java?

Question

I'm creating a client-server application wherein the server or client sends strings using PrintStream and reads strings using BufferedReader/InputStreamReader. Eventually, I need to either send an object from the server to the client or vice versa using ObjectInputStream/ObjectOutputStream.

How do I switch from sending/receiving strings to sending/receiving objects? I am getting "invalid stream header: 7372000E".

Here are the stream portions of the client (I cut out all the exceptions for brevity):

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
BufferedReader fromServer;
PrintStream clientToServer;
try {
    fromServer = new BufferedReader(new InputStreamReader(sock.getInputStream()));
    clientToServer = new PrintStream(sock.getOutputStream());
} catch (IOException e) {
    System.out.println(e.getMessage());
    e.printStackTrace();
    return;
}

String username;
String opcode;
System.out.print(fromServer.readLine()); // MESSAGE 1
username = in.readLine();
clientToServer.println(username); // MESSAGE 2
System.out.println(fromServer.readLine()); // MESSAGE 3
if (!username.matches("[a-zA-Z]\\w+")) {
    return;
}
opcode = fromServer.readLine(); // MESSAGE 4

If statement and file stuff for opcode1, then:

ObjectInputStream ois;
ObjectOutputStream oos;
UUID u = null;
ois = new ObjectInputStream(new FileInputStream(f));
u = (UUID) ois.readObject();
oos = new ObjectOutputStream(sock.getOutputStream());
oos.writeObject(u); // MESSAGE 5

Else statement and more file stuff for opcode2, then:

ObjectOutputStream oos;
ObjectInputStream ois;
UUID u;
ois = new ObjectInputStream(sock.getInputStream());
u = (UUID) ois.readObject(); // MESSAGE 5
System.out.println("UUID " + u.toString() + " received.");
oos = new ObjectOutputStream(new FileOutputStream(f));
oos.writeObject(u);
System.out.println("UUID " + u.toString() + " written to file.");

The server does the following:

PrintStream output = new PrintStream(sock.getOutputStream()); 
BufferedReader input = new BufferedReader(new InputStreamReader(sock.getInputStream()));
output.println("Please enter your username: "); // MESSAGE 1
username = input.readLine(); // MESSAGE 2
output.println("Welcome back!"); // MESSAGE 3
output.println("opcode1") OR output.println("opcode2") // MESSAGE 4

opcode1 section:

ObjectInputStream ois = new ObjectInputStream(sock.getInputStream());
UUID local = (UUID) ois.readObject(); // MESSAGE 5
if (user.getUUID().equals(local))
output.println("Your UUID is valid."); // MESSAGE 6

opcode2 section:

ObjectOutputStream oos = new ObjectOutputStream(sock.getOutputStream());
oos.writeObject(u.getUUID()); // MESSAGE 5
output.println("You now have a UUID."); // MESSAGE 6

Strings are also objects. Can't you send objects all the time? — NilsH, Apr 21 '13 at 14:01
Just... don't do this. Streams can have a whole bunch of internal state, if you open two higher-level streams on a single lower-level one there's pretty much no way you can keep the internal state in sync. Is there any reason why you can't just send the string commands themselves through the OOS? — millimoose, Apr 21 '13 at 15:47
In your case, the culprit is probably the `BufferedReader`. Which does exactly as it says on the can, **buffers** data internally. I.e. it reads whole chunks of characters from the underlying reader and then splits the buffered data into say lines later. This means that at the point where you try to "switch" to the `ObjectInputStream`, the `BufferedReader` has already swallowed some of the serialised data. (Including the expected header the error is about.) — millimoose, Apr 21 '13 at 15:52

mason · Answer 1 · 2013-04-28T14:37:14.933

If you know your going to be sending different datatypes through the stream, you could make your own "YourMessage" class which would record what the datatype is stored.

private ObjectOutputStream sOutput;
private ObjectInputStream sInput;

class YourMessage {
    static final int STRINGTYPE = 0; OBJECTTYPE = 1; // etc
    int type;
    String str;
    // Any other message types

    // Constructor
    public YourMessage(int type, String str){
        this.type = type;
        this.str = str;
        // etc
    }

    // Create Getters here
    String getMessage(){
        return str;
    }

}

Then send messages through the objectoutput stream like this:

sOutput.writeObject(new YourMessage(YourMessage.STRINGTYPE, message));

Then on receiving this on the server,

ym = (YourMessage) sInput.readObject();

switch (ym.getType()) {
    case YourMessage.STRINGTYPE:
        // do something with -
        ym.getMessage();
        break;
    case // other types of messages

How do I switch between String I/O and Object I/O streams in Java?

1 Answers1