Thursday, October 21, 2010

Extracting HTTP headers, handling partial headers correctly

Here is the correct code for the previous post:

    //extract all HTTP request response headers locally and return the size of the headers
    private int extractHttpHeaders(byte[] httpHeader) {
        final String delimiter = ": ";
        List<StringBuilder> headers = new ArrayList<StringBuilder>();
        for (int i=0; i<httpHeader.length; i++) {
            StringBuilder line = new StringBuilder();
            for (; i<httpHeader.length && (char)httpHeader[i]!='\n'; i++) {
            if (i==httpHeader.length) {
                //partial header, full headers have not been received yet
                //this will break out of the loop
            else if (line.length()==0 || (line.length()==1 && (char)httpHeader[i-1]=='\r')) {
                //all headers received
                httpHeaders.length = i+1;
                return i+1;
            } else {
                //line has a header, add it
                int colonAt = line.indexOf(delimiter);
                if (colonAt != -1) {
                    String value = line.substring(colonAt+delimiter.length(), line.charAt(line.length()-1)=='\r' ? line.length()-1 : line.length()).trim();
                    if (value.length() > 0)
                        httpHeaders.put(line.substring(0,colonAt), value);
        //partial header, full headers have not been received yet
        return -1; //full headers have not been received yet

The only difference is unconditionally clearing the headers map if we don't see a blank line in the headers. The correct termination of the headers is a single CRLF pair (blank line). If that has not been seen, then it is a partial header, even though each line in the partial header may have been fully received.

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