src/site/apt/httpcomponents-client-4.5.x/primer.apt

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    ----------
    Client HTTP Programming Primer
    ----------
    ----------
    ----------

Client HTTP Programming Primer

* {About}

    This document is intended for people who suddenly have to or want to implement
    an application that automates something usually done with a browser,
    but are missing the background to understand what they actually need to do.
    It provides guidance on the steps required to implement a program that
    interacts with a web site which is designed to be used with a browser.
    It does not save you from eventually learning the background of what
    you are doing, but it should help you to get started quickly and learn
    the details later.

    This document has evolved from discussions on the HttpClient mailing lists.
    Although it refers to HttpClient, the concepts described here apply equally
    to HttpComponents or Java's {{{http://docs.oracle.com/javase/7/docs/api/java/net/HttpURLConnection.html}HttpURLConnection}} 
    or any other HTTP communication library for any programming language. So you 
    might find it useful even if you're not using Java and HttpClient.

    The existence of this document does not imply that the HttpClient community
    feels responsible for teaching you how to program a client HTTP application.
    It is merely a way for us to reduce the noise on the mailing list without
    just leaving the newbies out in the cold.

* {Scenario}

    Let's assume that you have some kind of repetitive, web-based task that
    you want to automate. Something like:

    * goto page http://xxx.yyy.zzz/login.html

    * enter username and password in a web form and hit the "login" button

    * navigate to a specific page

    * check the number/headline/whatever shown on that page

    []

    At this time, we don't have a specific example which could be developed
    into a sample application. So this document is all bla-bla, and you will
    have to work out the details - all the details - yourself. Such is life.

* {Caveat}

    This scenario describes a hobbyist usage of HTTP, in other words:
    <<a bad practice>>. Web sites are designed for user interaction, not
    as an application programming interface (API). The interface of a
    web site is the user interface displayed by a browser. The HTTP
    communication between the browser and the server is an internal API,
    subject to change without notice.

    A web site can be redesigned at any point in time. The server then
    sends different documents and a browser will display the new content.
    The user easily adjusts to click the appropriate links, and the browser
    communicates via HTTP as specified by the new documents from the server.
    Your application that only mimicks a browser will simply break.

    Nevertheless, implementing this scenario will help you to get
    familiar with HTTP communication. It is also "good enough" for
    hobbyists applications, for example if you want to download the
    latest installment of your favorite daily webcomic to install
    it as the screen background. There is no big damage if such an
    application breaks.

    If you want to implement a solid application, you should use only
    published APIs. For example, to check for new mail on your webmail
    account, you should ask the webmail provider for POP or IMAP access.
    These are standardized protocols supported my most EMail client applications.
    If you want to have a newsticker, look for RSS feeds from the provider and
    applications that display them.

    As another example, if you want to perform a web search, there are
    search companies that provide an API for using their search engines.
    Unlike the examples before, such APIs are proprietary. You will still
    have to implement an application, but then you are using a published API
    that the provider will not change without notice.


* {Not a Browser}

    HttpClient is not a browser. It is an HTTP communication library 
    and as such it provides only a subset of functions expected from 
    a common browser application. The most fundamental difference is 
    absence of user interface in HttpClient. The browser needs a rendering
    engine to display pages, and to interpret user input such as mouse clicks
    somewhere on the displayed page. There is a layout engine which computes
    how an HTML page should be displayed, including cascading style sheets
    and images. A JavaScript interpreter runs JavaScript code embedded in
    or referenced from HTML pages. Events from the user interface are passed
    to the JavaScript interpreter for processing. On top of that, there are 
    interfaces for plugins that can handle Applets, embedded media objects 
    like PDF files, Quicktime movies and Flash animations, or ActiveX 
    controls that can do anything. HttpClient can only be used 
    programmatically through its APIs to transmit and receive HTTP messages. 
    HttpClient is also completely content agnostic. It can transfer message 
    content but it is unable to render or process it in any fashion.

    Another major difference is tolerance for bad input or HTTP standard
    violations. There needs to be tolerance for invalid user input to make
    the browser user friendly. There also needs to be tolerance for malformed
    documents retrieved from servers, and for flaws in server behavior when
    executing protocols, to make as many websites as possible accessible to
    the user. HttpClient is however strives to adhere to the HTTP standard
    specification and related standards as close and as possible by default.
    It also provides means to relaxing some of the restrictions imposed
    by the specification where permissible or required for compatibility
    with non-compliant HTTP origin or proxy servers.  

* {Terminology}

    This section introduces some important terms you have to know to
    understand the rest of this document.

    <<<{HTTP Message}>>>
    
    consists of a header section and an optional entity. There are two kinds 
    of messages, requests and responses. They differ in the format of the 
    first line, but both can have header fields and an optional entity.

    <<<{HTTP Request}>>> 
    
    is sent from a client to a server. The first line includes the URI for 
    which the request is sent, and a method that the server should execute 
    for the client.

    <<<{HTTP Response}>>>
    
    is sent from a server to a client in response to a request. The first
    line includes a status code that tells about success or failure of
    the request. HTTP defines a set of status codes, like 200 for success
    and 404 for not found. Other protocols based on HTTP can define
    additional status codes.

    <<<{Method}>>>
    
    is an operation requested from the server. HTTP defines a set of
    operations, the most frequent being GET and POST. Other protocols
    based on HTTP can define additional methods.

    <<<{Header Fields}>>>
    
    are name-value pairs, where both name and value are text. The name of
    a header field is not case sensitive. Multiple values can be assigned
    to the same name. RFC 2616 defines a wide range
    of header fields for handling various aspects of the HTTP protocol.
    Other specifications, like RFC 2617 and RFC 2965, define additional
    headers. Some of the defined headers are for general use, others are
    meant for exclusive use with either requests or responses, still others
    are meant for use only with an entity.

    <<<{Entity}>>>
    
    is data sent with an HTTP message. For example, a response can contain
    the page or image you are downloading as an entity, or a request can
    include the parameters that you entered into a web form.
    The entity of an HTTP message can have an arbitrary data format, which
    is usually specified as a MIME type in a header field.

    <<<{Session}>>>
    
    is a series of requests from a single source to a server. The server
    can keep session data, and needs to recognize the session to which
    each incoming request belongs. For example, if you execute a web search,
    the server will only return one page of search results. But it keeps
    track of the other results and makes them available when you click on
    the link to the "next" page. The server needs to know from the request
    that it is you and your session for which more results are requested,
    and not me and my session. That's because I searched for something else.

    <<<{Cookies}>>>
    
    are the preferred way for servers to track sessions. The server supplies
    a piece of data, called a cookie, in response to a request. The server
    expects the client to send that piece of data in a header field with each
    following request of the same session.
    The cookie is different for each session, so the server can identify to
    which session a request belongs by looking at the cookie. If the cookie
    is missing from a request, the server will not respond as expected.

* {Step by Step}

** {GET the Login Page}

    Create and execute a GET request for the login page.
    Just use the link you would type into the browser as the URL.
    This is what a browser does when you enter a URL in the address bar
    or when you click on a link that points to another web page.

    Inspect the response from the server:

    * do you get the page you expected?

    []

    It should be sent as the entity of the response to your request.
    The entity is also referred to as the response body.

    * do you get a session cookie?

    []

    Cookies are sent in a header field named Set-Cookie or Set-Cookie2.
    It is possible that you don't get a session cookie until you log in.
    If there is no session cookie in the response, you'll have to do perform
    step 2 later, after you reach the point where the cookie is set.

    If you do not get the page you expect, check the URL you are requesting.
    If it is correct, the server may use a browser detection. You will have
    to set the header field User-Agent to a value used by a popular browser
    to pretend that the request is coming from that browser.

    If you can't get the login page, get the home page instead now.
    Get the login page in the next step, when you establish the session.

** {Establish the Session}

    Create and execute another GET request for a page.
    You can simply request the login page again, or some other page
    of which you know the URL. Do NOT try to get a page which would
    be returned in response to submitting a web form. Use something
    you can reach simply by clicking on a link in the browser. Something
    where you can see the URL in the browser status line while the
    mouse pointer is hovering over the link.

    This step is important when developing the application. Once you know
    that your application does establish the session correctly, you may
    be able to remove it. Only if you couldn't get the login page directly
    and had to get the home page first, you know you have to leave it in.

    Inspect the request being sent to the server.

    * is the session cookie sent with the request?

    []

    You can see what is sent to the server by enabling the wire log
    for HttpClient. You only need to see the request headers, not the body.
    The session cookie should be sent in a header field called Cookie.
    There may be several of those, and other cookies might be sent as well.

    Inspect the response from the server:

    * do you get another session cookie?

    []

    You should not get another session cookie. If you get the same session
    cookie as before, the server behaves a little strange but that should
    not be a problem. If you get a new session cookie, then the server did
    not recognize the session for the request. Usually, this happens if the
    request did not contain the session cookie. But servers might use other
    means to track sessions, or to detect session hijacking.

    If the session cookie is not sent in the request, one of two things
    has gone wrong. Either the cookie was not detected in the previous
    response, or the cookie was not selected for being sent with the new
    request.

    HttpClient automatically parses cookies sent in responses and puts them
    into a cookie store. HttpClient uses a configurable cookie policy
    to decide whether a cookie being sent from a server is correct.
    The default policy complies strictly with RFC 2109, but many servers
    do not. Play around with the cookie policies until the cookie is
    accepted and put into the cookie store.

    If the cookie is accepted from the previous response but still not
    sent with the new request, make sure that HttpClient uses the same
    cookie store object. Unless you explicitly manage cookie store 
    objects (not recommended for newbies!), this will be the case if you 
    use the same HttpClient object to execute both requests.

    If the cookie is still not sent with the request, make sure that the
    URL you are requesting is in the scope for the cookie. Cookies are
    only sent to the domain and path specified in the cookie scope.
    A cookie for host "jakarta.apache.org" will not be sent to host
    "tomcat.apache.org". A cookie for domain ".apache.org" will be sent
    to both. A cookie for host "apache.org", without the leading dot,
    will not be sent to "jakarta.apache.org". The latter case can be
    resolved by using a different cookie spec that adds the leading dot.
    In the other cases, use a URL that in the cookie scope to establish
    the session.

    If the session cookie is sent with the request, but a new session cookie
    is set in the response anyway, check whether there are cookies other
    than the session cookie in the request. Some servers are incapable of
    detecting multiple cookies sent in individual header fields. HttpClient
    can be advised to put all cookies into a single header field.

    If that doesn't help, you are in trouble. The server may use additional
    means to track the session, for example the header field named Referer.
    Set that field to the URL of the previous request.
    ({{{http://mail-archives.apache.org/mod_mbox/jakarta-httpclient-user/200602.mbox/%3c19b.44e04b45.31166eaa@aol.com%3e}see this mail}})

    If that doesn't help either, you will have to compare the request from
    your application to a corresponding one generated by a browser. The
    instructions in step 5 for POST requests apply for GET requests as well.
    It's even simpler with GET, since you don't have an entity.

** {Analyze the Form}

    Now it is time to analyze the form defined in the HTML markup of the page.
    A form in HTML is a set of name-value-pairs called parameters, where some
    of the values can be entered in the browser. By analyzing the HTML markup,
    you can learn which parameters you have to define and how to send them
    to the server.

    Look for the <form> tag in the page source. There may be several forms in
    the page, but they can not be nested. Locate the form you want to submit.
    Locate the matching </form> tag. Everything in between the two may be
    relevant. Let's start with the {attributes of the <form> tag}:

    <<<{method}=>>>
 
    specifies the method used for submitting the form. If it is GET or
    not specified at all, then you need to create a GET request. The parameters
    will be added as a query string to the URL. If the method is POST, you
    need to create a POST request. The parameters will be put in the entity
    of the request, also referred to as the request body.
    How to do that is discussed in step 5.

    <<<{action}=>>>
 
    specifies the URL to which the request has to be sent. Do not try to
    get this URL from the address bar of your browser! A browser will
    automatically follow redirects and only displays the final URL, which
    can be different from the URL in this attribute.
    It is possible that the URL includes a query string that specifies
    some parameters. If so, keep that in mind.

    <<<{enctype}=>>>
 
    specifies the MIME type for the entity of the request generated by the
    form. The two common cases are url-encoded (default) and multipart-mime.
    Note that these terms are just informally used here, the exact values
    that need to be written in an HTML document are specified elsewhere.
    This attribute is only used for the POST method. If the method is GET,
    the parameters will always be url-encoded, but not in an entity.

    <<<{accept-charset}=>>>
    
    specifies the character set that the browser should allow for user input.
    It will not be discussed here, but you will have to consider this value
    if you experience charset related problems.

    Except for optional query parameters in the action attribute, the parameters
    of a form are specified by HTML tags between <form> and </form>.
    The following is a list of tags that can be used to define parameters.
    Except where stated otherwise, they have a name attribute which specifies
    the name of the parameter. The value of the parameter usually depends on
    user input.

----------------------------------------
<input type="text" name="...">
<input type="password" name="...">
----------------------------------------

    specify single-line input fields. Using the return key in one of these
    fields will submit the form, so the value really is a single line of
    input from the user.

----------------------------------------
<input type="text" readonly name="..." value="...">
<input type="hidden" name="..." value="...">
----------------------------------------

    specify a parameter that can not be changed by the user.
    The value of the parameter is given by the value attribute.

----------------------------------------
<input type="radio" name="..." value="...">
<input type="checkbox" name="..." value="...">
----------------------------------------

    specify a parameter that can be included or omitted. There usually is
    more than one tag with the same name. For radio buttons, only one can
    be selected and the value of the parameter is the value of the selected
    radio button. For checkboxes, more than one can be selected. There will
    be one name-value-pair for each selected checkbox, with the same name
    for all of them.

----------------------------------------
<input type="submit" name="..." value="...">
<button type="submit" name="..." value="...">
----------------------------------------

    specify a button to submit the form. The parameter will only be added
    to the form if that button is used to submit. If another button is used,
    or the form is submitted by pressing the return key in a text input field,
    the parameter is not part of the submitted form data. If the name attribute
    is missing, no parameter is added to the form data for that button.

----------------------------------------
<textarea name="...">
<textarea value="..." readonly>
----------------------------------------

    specify a multi-line input field. In the readonly case, the value of
    the parameter is the text between the <textarea> and </textarea> tags.

----------------------------------------
<select name="..." multiple>}}}
  <option value="...">...</option>}}}
  <option value="...">...</option>}}}
  ...
</select>
----------------------------------------

    specify a selection list or drop-down menu. If the multiple attribute is
    not present, only one option can be selected. There will be one
    name-value-pair for each selected option, with the same name for all of them.
    If there is no value attribute, the value for that option is
    the text between <option> and </option>.

----------------------------------------
<input type="image" name="...">
----------------------------------------

    specifies an image that can be clicked to submit the form. If that image
    is clicked to submit the form, two parameters are added to the form data.
    The name attribute is suffixed with ".x" and ".y", the values for the
    parameters are the relative coordinates of the mouse pointer within the
    image at the time of the click, in pixel. If the name attribute is missing,
    no parameters will be added to the form data.

----------------------------------------
<input type="file" name="...">
----------------------------------------
    
    specifies a file selection box. The user can select a file that should
    be sent as part of the form data. This is only possible if the encoding
    is multipart-mime. Unlike other parameters, the file is not mapped to a
    simple name-value-pair. File upload is not a topic for beginners.

    These tags are used to define parameters in static HTML. With dynamic HTML,
    in particular JavaScript, the parameter values can be changed before the
    form is submitted. If that is the case, you are in trouble. Learn JavaScript,
    analyze the code that is executed, and modify your application to match
    that behavior.


** {Analyze the Form, Again}

    After you have determined the action URL and name-value-pairs of
    a form, you should exit the program you used to get the HTML source,
    start it again and repeat the analysis with the new page.

    Most parameters will be the same for both pages. But some parameters,
    in particular those from hidden input fields, may change from session
    to session, or even with every request. The same can be the case with
    the action URL.

    Parameters that remain the same can be hard-coded in your program.
    If parameters change (except for user input), then your application
    has to request the page with the form and extract the dynamic parameters
    at runtime. If you're lucky you can locate them by simple string searches.
    If you're unlucky, you need an HTML parser to make sense of the page.
    HTML parsing is out of scope for HttpClient, but you'll find some
    HTML parsers mentioned in the mailing list archives.

    Note that a redesign of the form on the server can break your application
    at any time. Whenever that happens, you have to repeat the analysis with
    the new form returned by the server after the redesign, and adjust your
    application accordingly.


** {POST the Form}

    After analyzing the form, it is time to create a request that matches
    what a browser would generate. If the method is GET, just add the
    name-value-pairs for all parameters to the query string. If the method
    is POST, things are a little more complicated.

    It depends on the server how closely you have to match browser behavior.
    For example, a servlet will not distinguish between parameters in the
    query string and url-encoded parameters of the entity. But other server
    side code might make that distinction. The safe way is always to match
    browser behavior exactly.

    HttpClient supports both encoding types, url-encoded and multipart-mime.
    To send parameters url-encoded, use the POST request and add the parameters
    directly there. To send parameters in multipart-mime, collect the parameters
    in a multipart-encoded request entity and add set the entity for the 
    POST request. You will also find support for file upload in the multipart 
    package. Note that these techniques are mutually exclusive, they can not be 
    combined. Parameters defined in the query string of the URL can remain there.

    Send the request. Inspect the response from the server:

    * do you get a status code 303 or 307?
    
    []

    That is called a redirect. Follow redirects to the ultimate page
    and inspect that response. See step 6 on following redirects.

    * do you get the page you expected?

    []

    If the server response to your POST request indicates a problem,
    try to enable or disable the expect-continue handshake, or switch
    the protocol version to HTTP/1.0. If that doesn't help...

    Inspect the request you are sending:

    * are there significant differences to the request of a browser?

    []

    There is a variety of sniffer programs you can use to grep the
    browser request. Some of them are mentioned in the responses
    to {{{http://mail-archives.apache.org/mod_mbox/jakarta-httpclient-user/200603.mbox/%3c981224FF5B88B349B7C1FED584D2620E02A2CBB2@CORPUSMX50B.corp.emc.com%3e this question}on the mailing list}}.

    Candidates for problems are missing or wrong parameters, and differences
    in the header fields. The parameters are all up to you. As a general rule
    for the header fields, you should send the same as the browser does. The
    order of the fields does not matter.

    But there's a caveat: some header fields are controlled by HttpClient and
    can not be set explicitly. Other header fields are used to indicate
    capabilities which a browser has, but your application probably has not.
    For these, the request from your application has to and should differ.
    Here is a possibly incomplete {list of headers that need special consideration}:

    <<<{Host}:>>>

    controlled by HttpClient. The value is usually obtained from the URL
    you are posting to. It is possible to set a different value, called
    a "virtual host".

    <<<{Content-Type}:>>>
    
    <<<{Content-Length}:>>>
    
    <<<{Transfer-Encoding}:>>>
    
    controlled by HttpClient. The values are obtained from the request entity.

    <<<{Connection}:>>>
    
    usually controlled by HttpClient to handle connection keep-alive.
    Leave it alone or set the value to "close".

    <<<{Content-Encoding}:>>>
    
    used to indicate the capability to process compressed responses.
    Do not set this, unless you are prepared to implement decompression.

** {Follow Redirects}

    It is quite common for servers to respond with a 303 or 307 status code
    to a POST request. These redirects indicate that your application has to
    send another request to retrieve the actual result of the operation you
    have triggered with the POST request.

    HttpClient can be configured to follow some redirects automatically.
    Others it is not allowed to follow automatically, since RFC 2616 specifies
    that a user interaction should take place. We will make sure that HttpClient
    is compliant with this requirement, but we can't stop you from implementing
    a different behavior in your application. The Location header field in the
    redirect response indicates the URL from which to fetch the actual page.
    It is common practice that servers return a relative URL as the location,
    although the specification requires an absolute URL.

    Note that there may be more than one redirect in succession. Your
    application then has to follow the redirect for a redirect, but make sure
    that you do not enter an infinite loop. If you find that there are more
    than two redirects in succession, something probably is fishy.


** {Logout}

    Your application can send as many GET and POST requests and follow as many
    redirects as is required. But you should remember that there is a session
    tracked by the server. Once your application is done, and if the web site
    does provide a logout link, you should send a final request to log out.
    This will tell the server that the session data can be discarded. If the
    server prevents multiple logins with the same user ID and your application
    has to run repeatedly, logout may even be required.

* {Further Reading}

    ReferenceMaterials: a list of technical specifications for HTTP and related 
    stuff.

    * {{{http://www.w3.org/TR/html4/interact/forms.html} HTML 4.01 Specification, 
    Section on Forms}}: Includes how browsers have to generate the data to submit 
    to the server.

    * {{{https://digital.com/tools/html-cheatsheet/} Giving Form to Forms}}:
    Explains how to define HTML forms and what is submitted to the server.
    Probably easier to digest than the HTML 4.01 Specification.

    * {{{http://jakarta.apache.org/commons/fileupload/} Commons File Upload}}:
    Server-side library for parsing multipart requests.

    * {{{http://www.cs.tut.fi/~jkorpela/forms/file.html} Tutorial on File Upload 
    in HTML}}
    
    []