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<?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">
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<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();
try {
// do something useful
} finally {
instream.close();
}
}
]]></programlisting>
<section>
<title>HTTP request</title>
<para>All HTTP requests have a request line consisting a method name, a request URI and
an 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 specific
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 <classname>URIBuilder</classname> utility class to simplify
creation and modification of request URIs.</para>
<programlisting><![CDATA[
URIBuilder builder = new URIBuilder();
builder.setScheme("http").setHost("www.google.com").setPath("/search")
.setParameter("q", "httpclient")
.setParameter("btnG", "Google Search")
.setParameter("aq", "f")
.setParameter("oq", "");
URI uri = builder.build();
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 request 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",
ContentType.create("text/plain", "UTF-8"));
System.out.println(myEntity.getContentType());
System.out.println(myEntity.getContentLength());
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
important message
17
]]></programlisting>
</section>
</section>
<section>
<title>Ensuring release of low level resources</title>
<para> In order to ensure proper release of system resources one must close the content
stream associated with the entity.</para>
<programlisting><![CDATA[
HttpResponse response;
HttpEntity entity = response.getEntity();
if (entity != null) {
InputStream instream = entity.getContent();
try {
// do something useful
} finally {
instream.close();
}
}
]]></programlisting>
<para>Please note that the <methodname>HttpEntity#writeTo(OutputStream)</methodname>
method is also required to ensure proper release of system resources once the
entity has been fully written out. If this method obtains an instance of
<classname>java.io.InputStream</classname> by calling
<methodname>HttpEntity#getContent()</methodname>, it is also expected to close
the stream in a finally clause.</para>
<para>When working with streaming entities, one can use the
<methodname>EntityUtils#consume(HttpEntity)</methodname> method to ensure that
the entity content has been fully consumed and the underlying stream has been
closed.</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, in which case
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 the 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, ContentType.create("text/plain", "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 the generic <classname>InputStreamEntity</classname>.
<classname>FileEntity</classname> can be a good starting point.</para>
<section>
<title>HTML forms</title>
<para>Many applications 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 the 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>The <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 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 will 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
the <interfacename>ResponseHandler</interfacename> interface, which includes
the <methodname>handleResponse(HttpResponse response)</methodname> method.
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
a <interfacename>java.util.Map&lt;String, Object&gt;</interfacename>. It is
simply a collection of arbitrary named values. An application can populate context
attributes prior to request execution or examine the context after the execution has
been completed.</para>
<para><interfacename>HttpContext</interfacename> can contain arbitrary objects and
therefore may be unsafe to share between multiple threads. It is recommended that
each thread of execution maintains its own context.</para>
<para>In the course of HTTP request execution HttpClient adds the following attributes to
the execution context:</para>
<itemizedlist>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_CONNECTION</constant>='http.connection':</title>
<para><interfacename>HttpConnection</interfacename> instance representing the
actual connection to the target server.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_TARGET_HOST</constant>='http.target_host':</title>
<para><classname>HttpHost</classname> instance representing the connection
target.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_PROXY_HOST</constant>='http.proxy_host':</title>
<para><classname>HttpHost</classname> instance representing the connection
proxy, if used</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_REQUEST</constant>='http.request':</title>
<para><interfacename>HttpRequest</interfacename> instance representing the
actual HTTP request.
The final HttpRequest object in the execution context always represents
the state of the message _exactly_ as it was sent to the target server.
Per default HTTP/1.0 and HTTP/1.1 use relative request URIs.
However if the request is sent via a proxy in a non-tunneling mode then
the URI will be absolute.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_RESPONSE</constant>='http.response':</title>
<para><interfacename>HttpResponse</interfacename> instance representing the
actual HTTP response.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>ExecutionContext.HTTP_REQ_SENT</constant>='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();
EntityUtils.consume(entity);
}
]]></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 to 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 attempt 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 an 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>
</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 InterruptedIOException) {
// Timeout
return false;
}
if (exception instanceof UnknownHostException) {
// Unknown host
return false;
}
if (exception instanceof ConnectException) {
// Connection refused
return false;
}
if (exception instanceof SSLException) {
// 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 fails 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
- even if currently 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>Th 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();
EntityUtils.consume(entity);
}
]]></programlisting>
</section>
<section>
<title>HTTP parameters</title>
<para>The 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 to 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); // Default to 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); // Use HTTP 1.1 for this request only
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>The <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><constant>CoreProtocolPNames.PROTOCOL_VERSION</constant>='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><constant>CoreProtocolPNames.HTTP_ELEMENT_CHARSET</constant>='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><constant>CoreProtocolPNames.HTTP_CONTENT_CHARSET</constant>='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><constant>CoreProtocolPNames.USER_AGENT</constant>='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><constant>CoreProtocolPNames.STRICT_TRANSFER_ENCODING</constant>='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><constant>CoreProtocolPNames.USE_EXPECT_CONTINUE</constant>='http.protocol.expect-continue':</title>
<para>activates the <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.
The <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 not attempt to use the handshake.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title><constant>CoreProtocolPNames.WAIT_FOR_CONTINUE</constant>='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>