CODEWITHSUNDEEP
javasocketsio
user499054
user499054

Reputation:

InputStream's available() tends to halt

This is a bit of an obscure problem that only seems to happen when I'm on certain computers. I was having this issue today on our school's XP computers and I can't seem to replicate this on my home computer (W7).

Anyway, reading/writing to sockets in Java tends to be problematic whenever I use this code (where: int avail, InputStream socket, byte[] buffer, String output):

                   while( (avail = input.available()) > 0 )
                    {
                        read = input.read( buffer );
                        output += new String( buffer, 0, read );
                    }

It seems to make sense (reading all the data until no data is available to a temporary buffer, then to a string), but on our school computers (testing it using IE7), the whole thing somehow pauses. I'm thinking input.available() is causing it to somehow block because the thread just keeps running without ever reaching an endpoint... effectively just pausing somewhere.

OH, I forgot to mention: whenever I run this in debug mode and perform each line step-by-step, it works completely like it should... which just confuses me even more.

When I got home to replicate this issue, it works just fine (just using Firefox and IE8). I have no idea what would be a better alternative to this.

PS: If the buffer is large enough and I just use:

                    read = input.read( buffer );
                    output += new String( buffer, 0, read );

It works just fine, but there's always a worry that the data sent will exceed the buffer size.

Upvotes: 3

Views: 1413

Answers (3)

user207421
user207421

Reputation: 310916

Your code is invalid. This is a misuse of available(). All it does is tell you how many bytes may be available for reading without blocking. It cannot be used to indicate how many bytes will ever be sent by the peer, and it has no necessary relationship with peer messages. There are no messages in TCP, only a byte stream. If you want to read to EOS, just remove the available() test and read until it returns -1. If you want to read a message, the peer will have to delimit it for you somehow, e.g. by an out-of-band terminator, a length word prefix, or a self-describing protocol such as Object Serialization or XML.

It 'works' in debug mode because you are radically changing the timing with breakpoints. This is further proof that what you are doing is incorrect.

Upvotes: 1

Ryan Stewart
Ryan Stewart

Reputation: 128829

You're thinking about available() the wrong way. That method tells you approximately how many bytes can be read right now, without blocking. The commonly accepted idiom for what you're trying to do is

int length;
while ((length = in.read(buffer)) != -1) {
    output += new String(buffer, 0, length);
}

or something along those lines (not compiled/tested).

Update: I think you misunderstand the concept of "end of the stream". "End of the stream" doesn't mean that all the data you want to read has been read. It means that there isn't, and won't ever be, anything else to read. For instance, it might mean that you were reading a file and have come to the end of it, or it might mean you were reading from an in-memory byte array and came to the end of that. Those are "end of streams".

In your question, you indicated, or at least implied, that you're reading from a Socket. Are you aware that you'll never get to the end of that stream until the associated Socket or the remote end of the connection is closed? Just because you received a bit of data from it doesn't make it the end of the stream.

Upvotes: 4

Jeshurun
Jeshurun

Reputation: 23186

Why not use a buffered reader? Something like:

    BufferedReader reader = new BufferedReader(new InputStreamReader(input));
    String output = "";
    try {
        String readLine = null;
        while ((readLine = reader.readLine()) != null) {
            output += readLine + "\n";
        }
    } catch (IOException e) {
        System.err.println("Error: " + e);
    }
    System.out.println("Read from Socket:" + output);

Upvotes: 1

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