Chapter 3. Advanced topics
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## Chapter 3. Advanced topics

## 3.1. HTTP message parsing and formatting framework

HTTP message processing framework is designed to be expressive and flexible while remaining memory efficient and fast. HttpCore HTTP message processing code achieves near zero intermediate garbage and near zero-copy buffering for its parsing and formatting operations. The same HTTP message parsing and formatting API and implementations are used by both the blocking and non-blocking transport implementations, which helps ensure a consistent behavior of HTTP services regardless of the I/O model.

### 3.1.1. HTTP line parsing and formatting

HttpCore utilizes a number of low level components for all its line parsing and formatting methods.

`CharArrayBuffer` represents a sequence of characters, usually a single line in an HTTP message stream such as a request line, a status line or a header. Internally `CharArrayBuffer` is backed by an array of chars, which can be expanded to accommodate more input if needed. `CharArrayBuffer` also provides a number of utility methods for manipulating content of the buffer, storing more data and retrieving subsets of data.

    CharArrayBuffer buf = new CharArrayBuffer(64);
    buf.append("header:  data ");
    int i = buf.indexOf(':');
    String s = buf.substringTrimmed(i + 1, buf.length());
    System.out.println(s);
    System.out.println(s.length());

stdout >

    data
    4

`ParserCursor` represents a context of a parsing operation: the bounds limiting the scope of the parsing operation and the current position the parsing operation is expected to start at.

    CharArrayBuffer buf = new CharArrayBuffer(64);
    buf.append("header:  data ");
    int i = buf.indexOf(':');
    ParserCursor cursor = new ParserCursor(0, buf.length());
    cursor.updatePos(i + 1);
    System.out.println(cursor);

stdout >

    [0>7>14]

`LineParser` is the interface for parsing lines in the head section of an HTTP message. There are individual methods for parsing a request line, a status line, or a header line. The lines to parse are passed in-memory, the parser does not depend on any specific I/O mechanism.

    CharArrayBuffer buf = new CharArrayBuffer(64);
    buf.append("HTTP/1.1 200");
    ParserCursor cursor = new ParserCursor(0, buf.length());

    LineParser parser = new BasicLineParser();
    ProtocolVersion ver = parser.parseProtocolVersion(buf, cursor);
    System.out.println(ver);
    System.out.println(buf.substringTrimmed(
        cursor.getPos(),
        cursor.getUpperBound()));

stdout >

    HTTP/1.1
    200

    CharArrayBuffer buf = new CharArrayBuffer(64);
    buf.append("HTTP/1.1 200 OK");
    ParserCursor cursor = new ParserCursor(0, buf.length());
    LineParser parser = new BasicLineParser();
    StatusLine sl = parser.parseStatusLine(buf, cursor);
    System.out.println(sl.getReasonPhrase());

stdout >

    OK

`LineFormatter` for formatting elements of the head section of an HTTP message. This is the complement to `LineParser` . There are individual methods for formatting a request line, a status line, or a header line.

Please note the formatting does not include the trailing line break sequence `CR-LF`.

    CharArrayBuffer buf = new CharArrayBuffer(64);
    LineFormatter formatter = new BasicLineFormatter();
    formatter.formatRequestLine(buf,
        new BasicRequestLine("GET", "/", HttpVersion.HTTP_1_1));
    System.out.println(buf.toString());
    formatter.formatHeader(buf,
        new BasicHeader("Content-Type", "text/plain"));
    System.out.println(buf.toString());

stdout >

    GET / HTTP/1.1
    Content-Type: text/plain

`HeaderValueParser` is the interface for parsing header values into elements.

    CharArrayBuffer buf = new CharArrayBuffer(64);
    HeaderValueParser parser = new BasicHeaderValueParser();
    buf.append("name1=value1; param1=p1, " +
        "name2 = \"value2\", name3  = value3");
    ParserCursor cursor = new ParserCursor(0, buf.length());
    System.out.println(parser.parseHeaderElement(buf, cursor));
    System.out.println(parser.parseHeaderElement(buf, cursor));
    System.out.println(parser.parseHeaderElement(buf, cursor));

stdout >

    name1=value1; param1=p1
    name2=value2
    name3=value3

`HeaderValueFormatter` is the interface for formatting elements of a header value. This is the complement to `HeaderValueParser` .

    CharArrayBuffer buf = new CharArrayBuffer(64);
    HeaderValueFormatter formatter = new BasicHeaderValueFormatter();
    HeaderElement[] hes = new HeaderElement[] {
            new BasicHeaderElement("name1", "value1",
                    new NameValuePair[] {
                        new BasicNameValuePair("param1", "p1")} ),
            new BasicHeaderElement("name2", "value2"),
            new BasicHeaderElement("name3", "value3"),
    };
    formatter.formatElements(buf, hes, true);
    System.out.println(buf.toString());

stdout >

    name1="value1"; param1="p1", name2="value2", name3="value3"

### 3.1.2. HTTP message streams and session I/O buffers

HttpCore provides a number of utility classes for the blocking and non-blocking I/O models that facilitate the processing of HTTP message streams, simplify handling of `CR-LF` delimited lines in HTTP messages and manage intermediate data buffering.

HTTP connection implementations usually rely on session input/output buffers for reading and writing data from and to an HTTP message stream. Session input/output buffer implementations are I/O model specific and are optimized either for blocking or non-blocking operations.

Blocking HTTP connections use socket bound session buffers to transfer data. Session buffer interfaces are similar to `java.io.InputStream` / `java.io.OutputStream` classes, but they also provide methods for reading and writing `CR-LF` delimited lines.

    Socket socket1;
    Socket socket2;
    HttpParams params = new BasicHttpParams();
    SessionInputBuffer inbuffer = new SocketInputBuffer(
        socket1, 4096, params);
    SessionOutputBuffer outbuffer = new SocketOutputBuffer(
        socket2, 4096, params);

    CharArrayBuffer linebuf = new CharArrayBuffer(1024);
    inbuffer.readLine(linebuf);
    outbuffer.writeLine(linebuf);

Non-blocking HTTP connections use session buffers optimized for reading and writing data from and to non-blocking NIO channels. NIO session input/output sessions help deal with `CR-LF` delimited lines in a non-blocking I/O mode.

    ReadableByteChannel channel1;
    WritableByteChannel channel2;

    HttpParams params = new BasicHttpParams();
    SessionInputBuffer inbuffer = new SessionInputBufferImpl(
        4096, 1024, params);
    SessionOutputBuffer outbuffer = new SessionOutputBufferImpl(
        4096, 1024, params);

    CharArrayBuffer linebuf = new CharArrayBuffer(1024);
    boolean endOfStream = false;
    int bytesRead = inbuffer.fill(channel1);
    if (bytesRead == -1) {
        endOfStream = true;
    }
    if (inbuffer.readLine(linebuf, endOfStream)) {
        outbuffer.writeLine(linebuf);
    }
    if (outbuffer.hasData()) {
        outbuffer.flush(channel2);
    }

### 3.1.3. HTTP message parsers and formatters

HttpCore also provides coarse-grained facade type interfaces for parsing and formatting of HTTP messages. Default implementations of those interfaces build upon the functionality provided by `SessionInputBuffer` / `SessionOutputBuffer` and `HttpLineParser` / `HttpLineFormatter` implementations.

Example of HTTP request parsing / writing for blocking HTTP connections:

    SessionInputBuffer inbuffer;
    SessionOutputBuffer outbuffer;

    HttpParams params = new BasicHttpParams();

    HttpMessageParser requestParser = new HttpRequestParser(
            inbuffer,
            new BasicLineParser(),
            new DefaultHttpRequestFactory(),
            params);

    HttpRequest request = (HttpRequest) requestParser.parse();

    HttpMessageWriter requestWriter = new HttpRequestWriter(
            outbuffer,
            new BasicLineFormatter(),
            params);

    requestWriter.write(request);

Example of HTTP response parsing / writing for blocking HTTP connections:

    SessionInputBuffer inbuffer;
    SessionOutputBuffer outbuffer;

    HttpParams params = new BasicHttpParams();

    HttpMessageParser responseParser = new HttpResponseParser(
            inbuffer,
            new BasicLineParser(),
            new DefaultHttpResponseFactory(),
            params);

    HttpResponse response = (HttpResponse) responseParser.parse();

    HttpMessageWriter responseWriter = new HttpResponseWriter(
            outbuffer,
            new BasicLineFormatter(),
            params);

    responseWriter.write(response);

Example of HTTP request parsing / writing for non-blocking HTTP connections:

    SessionInputBuffer inbuffer;
    SessionOutputBuffer outbuffer;

    HttpParams params = new BasicHttpParams();

    NHttpMessageParser requestParser = new DefaultHttpRequestParser(
            inbuffer,
            new BasicLineParser(),
            new DefaultHttpRequestFactory(),
            params);

    HttpRequest request = (HttpRequest) requestParser.parse();

    NHttpMessageWriter requestWriter = new DefaultHttpRequestWriter(
            outbuffer,
            new BasicLineFormatter(),
            params);

    requestWriter.write(request);

Example of HTTP response parsing / writing for non-blocking HTTP connections:

    SessionInputBuffer inbuffer;
    SessionOutputBuffer outbuffer;

    HttpParams params = new BasicHttpParams();

    NHttpMessageParser responseParser = new DefaultHttpResponseParser(
            inbuffer,
            new BasicLineParser(),
            new DefaultHttpResponseFactory(),
            params);

    HttpResponse response = (HttpResponse) responseParser.parse();

    NHttpMessageWriter responseWriter = new DefaultHttpResponseWriter(
            outbuffer,
            new BasicLineFormatter(),
            params);

    responseWriter.write(response);

### 3.1.4. HTTP header parsing on demand

The default implementations of `HttpMessageParser` and `NHttpMessageParser` interfaces do not parse HTTP headers immediately. Parsing of header value is deferred until its properties are accessed. Those headers that are never used by the application will not be parsed at all. The `CharArrayBuffer` backing the header can be obtained through an optional `FormattedHeader` interface.

    Header h1 = response.getFirstHeader("Content-Type");
    if (h1 instanceof FormattedHeader) {
        CharArrayBuffer buf = ((FormattedHeader) h1).getBuffer();
        System.out.println(buf);
    }

## 3.2. Customizing HTTP connections

One can customize the way HTTP connections parse and format HTTP messages by extending the default implementations and overriding factory methods and replacing the default parser or formatter implementations with a custom one.

For blocking HTTP connections one also can provide custom implementation of session input/output buffers.

    class MyDefaultHttpClientConnection
                            extends DefaultHttpClientConnection {

        @Override
        protected SessionInputBuffer createSessionInputBuffer(
                Socket socket,
                int buffersize,
                HttpParams params) throws IOException {
            return new MySocketInputBuffer(socket, buffersize, params);
        }

        @Override
        protected SessionOutputBuffer createSessionOutputBuffer(
                Socket socket,
                int buffersize,
                HttpParams params) throws IOException {
            return new MySocketOutputBuffer(socket, buffersize, params);
        }

        @Override
        protected HttpMessageWriter createRequestWriter(
                SessionOutputBuffer buffer,
                HttpParams params) {
            return new MyHttpRequestWriter(
                buffer, new BasicLineFormatter(), params);
        }

        @Override
        protected HttpMessageParser createResponseParser(
                SessionInputBuffer buffer,
                HttpResponseFactory responseFactory,
                HttpParams params) {
            return new MyHttpResponseParser(
            buffer, new BasicLineParser(), responseFactory, params);
        }

    };

For non-blocking HTTP connection implementation one can replace the default HTTP message parser and formatter implementations. The session input/output buffer implementations can be overridden at the I/O reactor level.

    class MyDefaultNHttpClientConnection
                            extends DefaultNHttpClientConnection {

        public MyDefaultNHttpClientConnection(
                IOSession session,
                HttpResponseFactory responseFactory,
                ByteBufferAllocator allocator,
                HttpParams params) {
            super(session, responseFactory, allocator, params);
        }

        @Override
        protected NHttpMessageWriter createRequestWriter(
                SessionOutputBuffer buffer,
                HttpParams params) {
            return new HttpRequestWriter(
                buffer, new BasicLineFormatter(), params);
        }

        @Override
        protected NHttpMessageParser createResponseParser(
                SessionInputBuffer buffer,
                HttpResponseFactory responseFactory,
                HttpParams params) {
            return new HttpResponseParser(
                buffer, new BasicLineParser(), responseFactory, params);
        }

    };

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