httpcomponents-client/src/docbkx/fundamentals.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE preface PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
                 "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<!-- 
   ====================================================================
   Licensed to the Apache Software Foundation (ASF) under one
   or more contributor license agreements.  See the NOTICE file
   distributed with this work for additional information
   regarding copyright ownership.  The ASF licenses this file
   to you under the Apache License, Version 2.0 (the
   "License"); you may not use this file except in compliance
   with the License.  You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing,
   software distributed under the License is distributed on an
   "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   KIND, either express or implied.  See the License for the
   specific language governing permissions and limitations
   under the License.
   ====================================================================
-->
<chapter id="fundamentals">
    <title>Fundamentals</title>
    <section>
        <title>Request execution</title>
        <para> The most essential function of HttpClient is to execute HTTP methods. Execution of an
            HTTP method involves one or several HTTP request / HTTP response exchanges, usually
            handled internally by HttpClient. The user is expected to provide a request object to
            execute and HttpClient is expected to transmit the request to the target server return a
            corresponding response object, or throw an exception if execution was unsuccessful. </para>
        <para> Quite naturally, the main entry point of the HttpClient API is the HttpClient
            interface that defines the contract described above. </para>
        <para>Here is an example of request execution process in its simplest form:</para>
        <programlisting><![CDATA[
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet("http://localhost/");
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
if (entity != null) {
    InputStream instream = entity.getContent();
    int l;
    byte[] tmp = new byte[2048];
    while ((l = instream.read(tmp)) != -1) {
    }
}
]]></programlisting>
        <section>
            <title>HTTP request</title>
            <para>All HTTP requests have a request line consisting a method name, a request URI and
                a HTTP protocol version.</para>
            <para>HttpClient supports out of the box all HTTP methods defined in the HTTP/1.1
                specification: <literal>GET</literal>, <literal>HEAD</literal>,
                    <literal>POST</literal>, <literal>PUT</literal>, <literal>DELETE</literal>,
                    <literal>TRACE</literal> and <literal>OPTIONS</literal>. There is a special
                class for each method type.: <classname>HttpGet</classname>,
                    <classname>HttpHead</classname>, <classname>HttpPost</classname>,
                    <classname>HttpPut</classname>, <classname>HttpDelete</classname>,
                    <classname>HttpTrace</classname>, and <classname>HttpOptions</classname>.</para>
            <para>The Request-URI is a Uniform Resource Identifier that identifies the resource upon
                which to apply the request. HTTP request URIs consist of a protocol scheme, host
                name, optional port, resource path, optional query, and optional fragment.</para>
            <programlisting><![CDATA[
HttpGet httpget = new HttpGet(
     "http://www.google.com/search?hl=en&q=httpclient&btnG=Google+Search&aq=f&oq=");
]]></programlisting>
            <para>HttpClient provides a number of utility methods to simplify creation and
                modification of request URIs.</para>
            <para>URI can be assembled programmatically:</para>
            <programlisting><![CDATA[
URI uri = URIUtils.createURI("http", "www.google.com", -1, "/search", 
    "q=httpclient&btnG=Google+Search&aq=f&oq=", null);
HttpGet httpget = new HttpGet(uri);
System.out.println(httpget.getURI());
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
http://www.google.com/search?q=httpclient&btnG=Google+Search&aq=f&oq=
]]></programlisting>
            <para>Query string can also be generated from individual parameters:</para>
            <programlisting><![CDATA[
List<NameValuePair> qparams = new ArrayList<NameValuePair>();
qparams.add(new BasicNameValuePair("q", "httpclient"));
qparams.add(new BasicNameValuePair("btnG", "Google Search"));
qparams.add(new BasicNameValuePair("aq", "f"));
qparams.add(new BasicNameValuePair("oq", null));
URI uri = URIUtils.createURI("http", "www.google.com", -1, "/search", 
    URLEncodedUtils.format(qparams, "UTF-8"), null);
HttpGet httpget = new HttpGet(uri);
System.out.println(httpget.getURI());
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
http://www.google.com/search?q=httpclient&btnG=Google+Search&aq=f&oq=
]]></programlisting>
        </section>
        <section>
            <title>HTTP response</title>
            <para>HTTP response is a message sent by the server back to the client after having
                received and interpreted a request message. The first line of that message consists
                of the protocol version followed by a numeric status code and its associated textual
                phrase.</para>
            <programlisting><![CDATA[
HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
HttpStatus.SC_OK, "OK");

System.out.println(response.getProtocolVersion());
System.out.println(response.getStatusLine().getStatusCode());
System.out.println(response.getStatusLine().getReasonPhrase());
System.out.println(response.getStatusLine().toString());
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
HTTP/1.1
200
OK
HTTP/1.1 200 OK
]]></programlisting>
        </section>
        <section>
            <title>Working with message headers</title>
            <para>An HTTP message can contain a number of headers describing properties of the
                message such as the content length, content type and so on. HttpClient provides
                methods to retrieve, add, remove and enumerate headers.</para>
            <programlisting><![CDATA[
HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
    HttpStatus.SC_OK, "OK");
response.addHeader("Set-Cookie", 
    "c1=a; path=/; domain=localhost");
response.addHeader("Set-Cookie", 
    "c2=b; path=\"/\", c3=c; domain=\"localhost\"");
Header h1 = response.getFirstHeader("Set-Cookie");
System.out.println(h1);
Header h2 = response.getLastHeader("Set-Cookie");
System.out.println(h2);
Header[] hs = response.getHeaders("Set-Cookie");
System.out.println(hs.length);
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
Set-Cookie: c1=a; path=/; domain=localhost
Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
2
]]></programlisting>
            <para>The most efficient way to obtain all headers of a given type is by using the
                    <interfacename>HeaderIterator</interfacename> interface.</para>
            <programlisting><![CDATA[
HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
    HttpStatus.SC_OK, "OK");
response.addHeader("Set-Cookie", 
    "c1=a; path=/; domain=localhost");
response.addHeader("Set-Cookie", 
    "c2=b; path=\"/\", c3=c; domain=\"localhost\"");

HeaderIterator it = response.headerIterator("Set-Cookie");

while (it.hasNext()) {
    System.out.println(it.next());
}
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
Set-Cookie: c1=a; path=/; domain=localhost
Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
]]></programlisting>
            <para>It also provides convenience methods to parse HTTP messages into individual header
                elements.</para>
            <programlisting><![CDATA[
HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
    HttpStatus.SC_OK, "OK");
response.addHeader("Set-Cookie", 
    "c1=a; path=/; domain=localhost");
response.addHeader("Set-Cookie", 
    "c2=b; path=\"/\", c3=c; domain=\"localhost\"");

HeaderElementIterator it = new BasicHeaderElementIterator(
    response.headerIterator("Set-Cookie"));

while (it.hasNext()) {
    HeaderElement elem = it.nextElement(); 
    System.out.println(elem.getName() + " = " + elem.getValue());
    NameValuePair[] params = elem.getParameters();
    for (int i = 0; i < params.length; i++) {
        System.out.println(" " + params[i]);
    }
}
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
c1 = a
path=/
domain=localhost
c2 = b
path=/
c3 = c
domain=localhost
]]></programlisting>
        </section>
        <section>
            <title>HTTP entity</title>
            <para>HTTP messages can carry a content entity associated with the request or response.
                Entities can be found in some requests and in some responses, as they are optional.
                Requests that use entities are referred to as entity enclosing requests. The HTTP
                specification defines two entity enclosing methods: <literal>POST</literal> and
                    <literal>PUT</literal>. Responses are usually expected to enclose a content
                entity. There are exceptions to this rule such as responses to
                    <literal>HEAD</literal> method and <literal>204 No Content</literal>,
                    <literal>304 Not Modified</literal>, <literal>205 Reset Content</literal>
                responses.</para>
            <para>HttpClient distinguishes three kinds of entities, depending on where their content
                originates:</para>
            <itemizedlist>
                <listitem>
                    <formalpara>
                        <title>streamed:</title>
                        <para>The content is received from a stream, or generated on the fly. In
                            particular, this category includes entities being received from HTTP
                            responses. Streamed entities are generally not repeatable.</para>
                    </formalpara>
                </listitem>
                <listitem>
                    <formalpara>
                        <title>self-contained:</title>
                        <para>The content is in memory or obtained by means that are independent
                            from a connection or other entity. Self-contained entities are generally
                            repeatable. This type of entities will be mostly used for entity
                            enclosing HTTP requests.</para>
                    </formalpara>
                </listitem>
                <listitem>
                    <formalpara>
                        <title>wrapping:</title>
                        <para>The content is obtained from another entity.</para>
                    </formalpara>
                </listitem>
            </itemizedlist>
            <para>This distinction is important for connection management when streaming out content
                from an HTTP response. For request entities that are created by an application and
                only sent using HttpClient, the difference between streamed and self-contained is of
                little importance. In that case, it is suggested to consider non-repeatable entities
                as streamed, and those that are repeatable as self-contained.</para>
            <section>
                <title>Repeatable entities</title>
                <para>An entity can be repeatable, meaning its content can be read more than once.
                    This is only possible with self contained entities (like
                        <classname>ByteArrayEntity</classname> or
                        <classname>StringEntity</classname>)</para>
            </section>
            <section>
                <title>Using HTTP entities</title>
                <para>Since an entity can represent both binary and character content, it has
                    support for character encodings (to support the latter, ie. character
                    content).</para>
                <para>The entity is created when executing a request with enclosed content or when
                    the request was successful and the response body is used to send the result back
                    to the client.</para>
                <para>To read the content from the entity, one can either retrieve the input stream
                    via the <methodname>HttpEntity#getContent()</methodname> method, which returns
                    an <classname>java.io.InputStream</classname>, or one can supply an output
                    stream to the <methodname>HttpEntity#writeTo(OutputStream)</methodname> method,
                    which will return once all content has been written to the given stream.</para>
                <para>When the entity has been received with an incoming message, the methods
                        <methodname>HttpEntity#getContentType()</methodname> and
                        <methodname>HttpEntity#getContentLength()</methodname> methods can be used
                    for reading the common metadata such as <literal>Content-Type</literal> and
                        <literal>Content-Length</literal> headers (if they are available). Since the
                        <literal>Content-Type</literal> header can contain a character encoding for
                    text mime-types like text/plain or text/html, the
                        <methodname>HttpEntity#getContentEncoding()</methodname> method is used to
                    read this information. If the headers aren't available, a length of -1 will be
                    returned, and NULL for the content type. If the <literal>Content-Type</literal>
                    header is available, a <interfacename>Header</interfacename> object will be
                    returned.</para>
                <para>When creating an entity for a outgoing message, this meta data has to be
                    supplied by the creator of the entity.</para>
                <programlisting><![CDATA[
StringEntity myEntity = new StringEntity("important message", 
    "UTF-8");

System.out.println(myEntity.getContentType());
System.out.println(myEntity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(myEntity));
System.out.println(EntityUtils.toString(myEntity));
System.out.println(EntityUtils.toByteArray(myEntity).length);
]]></programlisting>
                <para>stdout &gt;</para>
                <programlisting><![CDATA[
Content-Type: text/plain; charset=UTF-8
17
UTF-8
important message
17
]]></programlisting>
            </section>
        </section>
        <section>
            <title>Ensuring release of low level resources</title>
            <para>When finished with a response entity, it's important to ensure that all entity
                content has been fully consumed, so that the connection could be safely returned to
                the connection pool and re-used by the connection manager for subsequent requests.
                The easiest way to do so is to call the
                    <methodname>HttpEntity#consumeContent(</methodname>) method to consume any
                available content on the stream. HttpClient will automatically release the
                underlying connection back to the connection manager as soon as it detects that the
                end of the content stream has been reached. The
                    <methodname>HttpEntity#consumeContent()</methodname> method is safe to call more
                than once.</para>
            <para>There can be situations, however, when only a small portion of the entire response
                content needs to be retrieved and the performance penalty for consuming the
                remaining content and making the connection reusable is too high, one can simply
                terminate the request by calling <methodname>HttpUriRequest#abort()</methodname>
                method.</para>
            <programlisting><![CDATA[
HttpGet httpget = new HttpGet("http://localhost/");
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
if (entity != null) {
    InputStream instream = entity.getContent();
    int byteOne = instream.read();
    int byteTwo = instream.read();
    // Do not need the rest
    httpget.abort();
}
]]></programlisting>
            <para>The connection will not be reused, but all level resources held by it will be
                correctly deallocated.</para>
        </section>
        <section>
            <title>Consuming entity content</title>
            <para>The recommended way to consume content of an entity is by using its
                    <methodname>HttpEntity#getContent()</methodname> or
                    <methodname>HttpEntity#writeTo(OutputStream)</methodname> methods. HttpClient
                also comes with the <classname>EntityUtils</classname> class, which exposes several
                static methods to more easily read the content or information from an entity.
                Instead of reading the <classname>java.io.InputStream</classname> directly, one can
                retrieve the whole content body in a string / byte array by using the methods from
                this class. However, the use of <classname>EntityUtils</classname> is
                strongly discouraged unless the response entities originate from a trusted HTTP
                server and are known to be of limited length.</para>
            <programlisting><![CDATA[
HttpGet httpget = new HttpGet("http://localhost/");
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
if (entity != null) {
    long len = entity.getContentLength();
    if (len != -1 && len < 2048) {
        System.out.println(EntityUtils.toString(entity));
    } else {
        // Stream content out
    }
}
]]></programlisting>
            <para>In some situations it may be necessary to be able to read entity content more than
                once. In this case entity content must be buffered in some way, either in memory or
                on disk. The simplest way to accomplish that is by wrapping the original entity with
                the <classname>BufferedHttpEntity</classname> class. This will cause the content of
                the original entity to be read into a in-memory buffer. In all other ways the entity
                wrapper will be have the original one.</para>
            <programlisting><![CDATA[
HttpGet httpget = new HttpGet("http://localhost/");
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
if (entity != null) {
    entity = new BufferedHttpEntity(entity);
}
]]></programlisting>
        </section>
        <section>
            <title>Producing entity content</title>
            <para>HttpClient provides several classes that can be used to efficiently stream out
                content though HTTP connections. Instances of those classes can be associated with
                entity enclosing requests such as <literal>POST</literal> and <literal>PUT</literal>
                in order to enclose entity content into outgoing HTTP requests. HttpClient provides
                several classes for most common data containers such as string, byte array, input
                stream, and file: <classname>StringEntity</classname>,
                    <classname>ByteArrayEntity</classname>,
                <classname>InputStreamEntity</classname>, and
                <classname>FileEntity</classname>.</para>
            <programlisting><![CDATA[
File file = new File("somefile.txt");
FileEntity entity = new FileEntity(file, "text/plain; charset=\"UTF-8\"");

HttpPost httppost = new HttpPost("http://localhost/action.do");
httppost.setEntity(entity);
]]></programlisting>
            <para>Please note <classname>InputStreamEntity</classname> is not repeatable, because it
                can only read from the underlying data stream once. Generally it is recommended to
                implement a custom <interfacename>HttpEntity</interfacename> class which is
                self-contained instead of using generic <classname>InputStreamEntity</classname>.
                    <classname>FileEntity</classname> can be a good starting point.</para>
            <section>
                <title>Dynamic content entities</title>
                <para>Often HTTP entities need to be generated dynamically based a particular
                    execution context. HttpClient provides support for dynamic entities by using
                        <classname>EntityTemplate</classname> entity class and
                        <interfacename>ContentProducer</interfacename> interface. Content producers
                    are objects which produce their content on demand, by writing it out to an
                    output stream. They are expected to be able produce their content every time
                    they are requested to do so. So entities created with
                        <classname>EntityTemplate</classname> are generally self-contained and
                    repeatable.</para>
                <programlisting><![CDATA[
ContentProducer cp = new ContentProducer() {
    public void writeTo(OutputStream outstream) throws IOException {
        Writer writer = new OutputStreamWriter(outstream, "UTF-8");
        writer.write("<response>");
        writer.write("  <content>");
        writer.write("    important stuff");
        writer.write("  </content>");
        writer.write("</response>");
        writer.flush();
    }
};
HttpEntity entity = new EntityTemplate(cp);
HttpPost httppost = new HttpPost("http://localhost/handler.do");
httppost.setEntity(entity);
]]></programlisting>
            </section>
            <section>
                <title>HTML forms</title>
                <para>Many applications frequently need to simulate the process of submitting an
                    HTML form, for instance, in order to log in to a web application or submit input
                    data. HttpClient provides special entity class
                        <classname>UrlEncodedFormEntity</classname> to facilitate the
                    process.</para>
                <programlisting><![CDATA[
List<NameValuePair> formparams = new ArrayList<NameValuePair>();
formparams.add(new BasicNameValuePair("param1", "value1"));
formparams.add(new BasicNameValuePair("param2", "value2"));
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formparams, "UTF-8");
HttpPost httppost = new HttpPost("http://localhost/handler.do");
httppost.setEntity(entity);
]]></programlisting>
                <para>This <classname>UrlEncodedFormEntity</classname> instance will use the so
                    called URL encoding to encode parameters and produce the following
                    content:</para>
                <programlisting><![CDATA[
param1=value1&param2=value2
]]></programlisting>
            </section>
            <section>
                <title>Content chunking</title>
                <para>Generally it is recommended to let HttpClient choose the most appropriate
                    transfer encoding based on the properties of the HTTP message being transferred.
                    It is possible, however, to inform HttpClient that the chunk coding is preferred
                    by setting <methodname>HttpEntity#setChunked()</methodname> to true. Please note
                    that HttpClient will use this flag as a hint only. This value well be ignored
                    when using HTTP protocol versions that do not support chunk coding, such as
                    HTTP/1.0.</para>
                <programlisting><![CDATA[
StringEntity entity = new StringEntity("important message", 
    "text/plain; charset=\"UTF-8\"");
entity.setChunked(true);
HttpPost httppost = new HttpPost("http://localhost/acrtion.do");
httppost.setEntity(entity);
]]></programlisting>
            </section>
        </section>
        <section>
            <title>Response handlers</title>
            <para>The simplest and the most convenient way to handle responses is by using
                    <interfacename>ResponseHandler</interfacename> interface. This method completely
                relieves the user from having to worry about connection management. When using a
                    <interfacename>ResponseHandler</interfacename> HttpClient will automatically
                take care of ensuring release of the connection back to the connection manager
                regardless whether the request execution succeeds or causes an exception.</para>
            <programlisting><![CDATA[
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet("http://localhost/");

ResponseHandler<byte[]> handler = new ResponseHandler<byte[]>() {
    public byte[] handleResponse(
            HttpResponse response) throws ClientProtocolException, IOException {
        HttpEntity entity = response.getEntity();
        if (entity != null) {
            return EntityUtils.toByteArray(entity);
        } else {
            return null;
        }
    }
};

byte[] response = httpclient.execute(httpget, handler);
]]></programlisting>
        </section>
    </section>
    <section>
        <title>HTTP execution context</title>
        <para>Originally HTTP has been designed as a stateless, response-request oriented protocol.
            However, real world applications often need to be able to persist state information
            through several logically related request-response exchanges. In order to enable
            applications to maintain a processing state HttpClient allows HTTP requests to be
            executed within a particular execution context, referred to as HTTP context. Multiple
            logically related requests can participate in a logical session if the same context is
            reused between consecutive requests. HTTP context functions similarly to
                <interfacename>java.util.Map&lt;String, Object&gt;</interfacename>. It is
            simply a collection of arbitrary named values. Application can populate context
            attributes prior to a request execution or examine the context after the execution has
            been completed.</para>
        <para>In the course of HTTP request execution HttpClient adds the following attributes to
            the execution context:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>'http.connection':</title>
                    <para><interfacename>HttpConnection</interfacename> instance representing the
                        actual connection to the target server.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.target_host':</title>
                    <para><classname>HttpHost</classname> instance representing the connection
                        target.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.proxy_host':</title>
                    <para><classname>HttpHost</classname> instance representing the connection
                        proxy, if used</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.request':</title>
                    <para><interfacename>HttpRequest</interfacename> instance representing the
                        actual HTTP request.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.response':</title>
                    <para><interfacename>HttpResponse</interfacename> instance representing the
                        actual HTTP response.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.request_sent':</title>
                    <para><classname>java.lang.Boolean</classname> object representing the flag
                        indicating whether the actual request has been fully transmitted to the
                        connection target.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
        <para>For instance, in order to determine the final redirect target, one can examine the
            value of the <literal>http.target_host</literal> attribute after the request
            execution:</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();

HttpContext localContext = new BasicHttpContext();
HttpGet httpget = new HttpGet("http://www.google.com/");

HttpResponse response = httpclient.execute(httpget, localContext);

HttpHost target = (HttpHost) localContext.getAttribute(
    ExecutionContext.HTTP_TARGET_HOST);

System.out.println("Final target: " + target);

HttpEntity entity = response.getEntity();
if (entity != null) {
entity.consumeContent();
}
]]></programlisting>
        <para>stdout &gt;</para>
        <programlisting><![CDATA[
Final target: http://www.google.ch
]]></programlisting>
    </section>
    <section>
        <title>Exception handling</title>
        <para>HttpClient can throw two types of exceptions:
                <exceptionname>java.io.IOException</exceptionname> in case of an I/O failure such as
            socket timeout or an socket reset and <exceptionname>HttpException</exceptionname> that
            signals an HTTP failure such as a violation of the HTTP protocol. Usually I/O errors are
            considered non-fatal and recoverable, whereas HTTP protocol errors are considered fatal
            and cannot be automatically recovered from.</para>
        <section>
            <title>HTTP transport safety</title>
            <para>It is important to understand that the HTTP protocol is not well suited for all
                types of applications. HTTP is a simple request/response oriented protocol which was
                initially designed to support static or dynamically generated content retrieval. It
                has never been intended to support transactional operations. For instance, the HTTP
                server will consider its part of the contract fulfilled if it succeeds in receiving
                and processing the request, generating a response and sending a status code back to
                the client. The server will make no attempts to roll back the transaction if the
                client fails to receive the response in its entirety due to a read timeout, a
                request cancellation or a system crash. If the client decides to retry the same
                request, the server will inevitably end up executing the same transaction more than
                once. In some cases this may lead to application data corruption or inconsistent
                application state.</para>
            <para>Even though HTTP has never been designed to support transactional processing, it
                can still be used as a transport protocol for mission critical applications provided
                certain conditions are met. To ensure HTTP transport layer safety the system must
                ensure the idempotency of HTTP methods on the application layer.</para>
        </section>
        <section>
            <title>Idempotent methods</title>
            <para>HTTP/1.1 specification defines idempotent method as</para>
            <para>
                <citation>Methods can also have the property of &quot;idempotence&quot; in
                    that (aside from error or expiration issues) the side-effects of N &gt; 0
                    identical requests is the same as for a single request</citation>
            </para>
            <para>In other words the application ought to ensure that it is prepared to deal with
                the implications of multiple execution of the same method. This can be achieved, for
                instance, by providing a unique transaction id and by other means of avoiding
                execution of the same logical operation.</para>
            <para>Please note that this problem is not specific to HttpClient. Browser based
                applications are subject to exactly the same issues related to HTTP methods
                non-idempotency.</para>
            <para>HttpClient assumes non-entity enclosing methods such as <literal>GET</literal> and
                    <literal>HEAD</literal> to be idempotent and entity enclosing methods such as
                    <literal>POST</literal> and <literal>PUT</literal> to be not.</para>
        </section>
        <section>
            <title>Automatic exception recovery</title>
            <para>By default HttpClient attempts to automatically recover from I/O exceptions. The
                default auto-recovery mechanism is limited to just a few exceptions that are known
                to be safe.</para>
            <itemizedlist>
                <listitem>
                    <para>HttpClient will make no attempt to recover from any logical or HTTP
                        protocol errors (those derived from
                            <exceptionname>HttpException</exceptionname> class).</para>
                </listitem>
                <listitem>
                    <para>HttpClient will automatically retry those methods that are assumed to be
                        idempotent.</para>
                </listitem>
                <listitem>
                    <para>HttpClient will automatically retry those methods that fail with a
                        transport exception while the HTTP request is still being transmitted to the
                        target server (i.e. the request has not been fully transmitted to the
                        server).</para>
                </listitem>
                <listitem>
                    <para>HttpClient will automatically retry those methods that have been fully
                        transmitted to the server, but the server failed to respond with an HTTP
                        status code (the server simply drops the connection without sending anything
                        back). In this case it is assumed that the request has not been processed by
                        the server and the application state has not changed. If this assumption may
                        not hold true for the web server your application is targeting it is highly
                        recommended to provide a custom exception handler.</para>
                </listitem>
            </itemizedlist>
        </section>
        <section>
            <title>Request retry handler</title>
            <para>In order to enable a custom exception recovery mechanism one should provide an
                implementation of the <interfacename>HttpRequestRetryHandler</interfacename>
                interface.</para>
            <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();

HttpRequestRetryHandler myRetryHandler = new HttpRequestRetryHandler() {

    public boolean retryRequest(
            IOException exception, 
            int executionCount,
            HttpContext context) {
        if (executionCount >= 5) {
            // Do not retry if over max retry count
            return false;
        }
        if (exception instanceof NoHttpResponseException) {
            // Retry if the server dropped connection on us
            return true;
        }
        if (exception instanceof SSLHandshakeException) {
            // Do not retry on SSL handshake exception
            return false;
        }
        HttpRequest request = (HttpRequest) context.getAttribute(
                ExecutionContext.HTTP_REQUEST);
        boolean idempotent = !(request instanceof HttpEntityEnclosingRequest); 
        if (idempotent) {
            // Retry if the request is considered idempotent 
            return true;
        }
        return false;
    }

};

httpclient.setHttpRequestRetryHandler(myRetryHandler);
]]></programlisting>
        </section>
    </section>
    <section>
        <title>Aborting requests</title>
        <para>In some situations HTTP request execution fail to complete within the expected time
            frame due to high load on the target server or too many concurrent requests issued on
            the client side. In such cases it may be necessary to terminate the request prematurely
            and unblock the execution thread blocked in a I/O operation. HTTP requests being
            executed by HttpClient can be aborted at any stage of execution by invoking
                <methodname>HttpUriRequest#abort()</methodname> method. This method is thread-safe
            and can be called from any thread. When an HTTP request is aborted its execution thread
            blocked in an I/O operation is guaranteed to unblock by throwing a
                <exceptionname>InterruptedIOException</exceptionname></para>
    </section>
    <section id="protocol_interceptors">
        <title>HTTP protocol interceptors</title>
        <para>HTTP protocol interceptor is a routine that implements a specific aspect of the HTTP
            protocol. Usually protocol interceptors are expected to act upon one specific header or
            a group of related headers of the incoming message or populate the outgoing message with
            one specific header or a group of related headers. Protocol interceptors can also
            manipulate content entities enclosed with messages, transparent content compression /
            decompression being a good example. Usually this is accomplished by using the
            'Decorator' pattern where a wrapper entity class is used to decorate the original
            entity. Several protocol interceptors can be combined to form one logical unit.</para>
        <para>Protocol interceptors can collaborate by sharing information - such as a processing
            state - through the HTTP execution context. Protocol interceptors can use HTTP context
            to store a processing state for one request or several consecutive requests.</para>
        <para>Usually the order in which interceptors are executed should not matter as long as they
            do not depend on a particular state of the execution context. If protocol interceptors
            have interdependencies and therefore must be executed in a particular order, they should
            be added to the protocol processor in the same sequence as their expected execution
            order.</para>
        <para>Protocol interceptors must be implemented as thread-safe. Similarly to servlets,
            protocol interceptors should not use instance variables unless access to those variables
            is synchronized.</para>
        <para>This is an example of how local context can be used to persist a processing state
            between consecutive requests:</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();

HttpContext localContext = new BasicHttpContext();

AtomicInteger count = new AtomicInteger(1);

localContext.setAttribute("count", count);

httpclient.addRequestInterceptor(new HttpRequestInterceptor() {

    public void process(
            final HttpRequest request, 
            final HttpContext context) throws HttpException, IOException {
        AtomicInteger count = (AtomicInteger) context.getAttribute("count");
        request.addHeader("Count", Integer.toString(count.getAndIncrement()));
    }
    
});

HttpGet httpget = new HttpGet("http://localhost/");
for (int i = 0; i < 10; i++) {
    HttpResponse response = httpclient.execute(httpget, localContext);
    
    HttpEntity entity = response.getEntity();
    if (entity != null) {
        entity.consumeContent();
    }
}
]]></programlisting>
    </section>
    <section>
        <title>HTTP parameters</title>
        <para>HttpParams interface represents a collection of immutable values that define a runtime
            behavior of a component. In many ways <interfacename>HttpParams</interfacename> is
            similar to <interfacename>HttpContext</interfacename>. The main distinction between the
            two lies in their use at runtime. Both interfaces represent a collection of objects that
            are organized as a map of keys to object values, but serve distinct purposes:</para>
        <itemizedlist>
            <listitem>
                <para><interfacename>HttpParams</interfacename> is intended to contain simple
                    objects: integers, doubles, strings, collections and objects that remain
                    immutable at runtime.</para>
            </listitem>
            <listitem>
                <para>
                    <interfacename>HttpParams</interfacename> is expected to be used in the 'write
                    once - ready many' mode. <interfacename>HttpContext</interfacename> is intended
                    to contain complex objects that are very likely to mutate in the course of HTTP
                    message processing. </para>
            </listitem>
            <listitem>
                <para>The purpose of <interfacename>HttpParams</interfacename> is to define a
                    behavior of other components. Usually each complex component has its own
                        <interfacename>HttpParams</interfacename> object. The purpose of
                        <interfacename>HttpContext</interfacename> is to represent an execution
                    state of an HTTP process. Usually the same execution context is shared among
                    many collaborating objects.</para>
            </listitem>
        </itemizedlist>
        <section>
            <title>Parameter hierarchies</title>
            <para>In the course of HTTP request execution <interfacename>HttpParams</interfacename>
                of the <interfacename>HttpRequest</interfacename> object are linked together with
                    <interfacename>HttpParams</interfacename> of the
                    <interfacename>HttpClient</interfacename> instance used to execute the request.
                This enables parameters set at the HTTP request level take precedence over
                    <interfacename>HttpParams</interfacename> set at the HTTP client level. The
                recommended practice is to set common parameters shared by all HTTP requests at the
                HTTP client level and selectively override specific parameters at the HTTP request
                level.</para>
            <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();
httpclient.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, 
    HttpVersion.HTTP_1_0);
httpclient.getParams().setParameter(CoreProtocolPNames.HTTP_CONTENT_CHARSET, 
    "UTF-8");

HttpGet httpget = new HttpGet("http://www.google.com/");
httpget.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, 
    HttpVersion.HTTP_1_1);
httpget.getParams().setParameter(CoreProtocolPNames.USE_EXPECT_CONTINUE, 
    Boolean.FALSE);

httpclient.addRequestInterceptor(new HttpRequestInterceptor() {

    public void process(
            final HttpRequest request, 
            final HttpContext context) throws HttpException, IOException {
        System.out.println(request.getParams().getParameter(
                CoreProtocolPNames.PROTOCOL_VERSION));
        System.out.println(request.getParams().getParameter(
                CoreProtocolPNames.HTTP_CONTENT_CHARSET));
        System.out.println(request.getParams().getParameter(
                CoreProtocolPNames.USE_EXPECT_CONTINUE));
        System.out.println(request.getParams().getParameter(
                CoreProtocolPNames.STRICT_TRANSFER_ENCODING));
    }
    
});
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
HTTP/1.1
UTF-8
false
null
]]></programlisting>
        </section>
        <section>
            <title>HTTP parameters beans</title>
            <para><interfacename>HttpParams</interfacename> interface allows for a great deal of
                flexibility in handling configuration of components. Most importantly, new
                parameters can be introduced without affecting binary compatibility with older
                versions. However, <interfacename>HttpParams</interfacename> also has a certain
                disadvantage compared to regular Java beans:
                    <interfacename>HttpParams</interfacename> cannot be assembled using a DI
                framework. To mitigate the limitation, HttpClient includes a number of bean classes
                that can used in order to initialize <interfacename>HttpParams</interfacename>
                objects using standard Java bean conventions.</para>
            <programlisting><![CDATA[
HttpParams params = new BasicHttpParams();
HttpProtocolParamBean paramsBean = new HttpProtocolParamBean(params);
paramsBean.setVersion(HttpVersion.HTTP_1_1);
paramsBean.setContentCharset("UTF-8");
paramsBean.setUseExpectContinue(true);

System.out.println(params.getParameter(
    CoreProtocolPNames.PROTOCOL_VERSION));
System.out.println(params.getParameter(
    CoreProtocolPNames.HTTP_CONTENT_CHARSET));
System.out.println(params.getParameter(
    CoreProtocolPNames.USE_EXPECT_CONTINUE));
System.out.println(params.getParameter(
    CoreProtocolPNames.USER_AGENT));
]]></programlisting>
            <para>stdout &gt;</para>
            <programlisting><![CDATA[
HTTP/1.1
UTF-8
false
null
]]></programlisting>
        </section>
    </section>
    <section>
        <title>HTTP request execution parameters</title>
        <para>These are parameters that can impact the process of request execution:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>'http.protocol.version':</title>
                    <para>defines HTTP protocol version used if not set explicitly on the request
                        object. This parameter expects a value of type
                            <interfacename>ProtocolVersion</interfacename>. If this parameter is not
                        set HTTP/1.1 will be used.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.protocol.element-charset':</title>
                    <para>defines the charset to be used for encoding HTTP protocol elements. This
                        parameter expects a value of type <classname>java.lang.String</classname>.
                        If this parameter is not set <literal>US-ASCII</literal> will be
                        used.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.protocol.content-charset':</title>
                    <para>defines the charset to be used per default for content body coding. This
                        parameter expects a value of type <classname>java.lang.String</classname>.
                        If this parameter is not set <literal>ISO-8859-1</literal> will be
                        used.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.useragent':</title>
                    <para>defines the content of the <literal>User-Agent</literal> header. This
                        parameter expects a value of type <classname>java.lang.String</classname>.
                        If this parameter is not set, HttpClient will automatically generate a value
                        for it.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.protocol.strict-transfer-encoding':</title>
                    <para>defines whether responses with an invalid
                            <literal>Transfer-Encoding</literal> header should be rejected. This
                        parameter expects a value of type <classname>java.lang.Boolean</classname>.
                        If this parameter is not set invalid <literal>Transfer-Encoding</literal>
                        values will be ignored.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.protocol.expect-continue':</title>
                    <para>activates <literal>Expect: 100-Continue</literal> handshake for the entity
                        enclosing methods. The purpose of the <literal>Expect:
                            100-Continue</literal> handshake is to allow the client that is sending
                        a request message with a request body to determine if the origin server is
                        willing to accept the request (based on the request headers) before the
                        client sends the request body. The use of the <literal>Expect:
                            100-continue</literal> handshake can result in a noticeable performance
                        improvement for entity enclosing requests (such as <literal>POST</literal>
                        and <literal>PUT</literal>) that require the target server's authentication.
                            <literal>Expect: 100-continue</literal> handshake should be used with
                        caution, as it may cause problems with HTTP servers and proxies that do not
                        support HTTP/1.1 protocol. This parameter expects a value of type
                            <classname>java.lang.Boolean</classname>. If this parameter is not set
                        HttpClient will attempt to use the handshake.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.protocol.wait-for-continue':</title>
                    <para>defines the maximum period of time in milliseconds the client should spend
                        waiting for a <literal>100-continue</literal> response. This parameter
                        expects a value of type <classname>java.lang.Integer</classname>. If this
                        parameter is not set HttpClient will wait 3 seconds for a confirmation
                        before resuming the transmission of the request body.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
    </section>
</chapter>