httpcomponents-client/src/docbkx/connmgmt.xml

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<chapter id="connmgmt">
    <title>Connection management</title>
    <para>HttpClient has a complete control over the process of connection initialization and
        termination as well as I/O operations on active connections. However various aspects of
        connection operations can be controlled using a number of parameters.</para>
    <section>
        <title>Connection parameters</title>
        <para>These are parameters that can influence connection operations:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>'http.socket.timeout':</title>
                    <para>defines the socket timeout (<literal>SO_TIMEOUT</literal>) in
                        milliseconds, which is the timeout for waiting for data or, put differently,
                        a maximum period inactivity between two consecutive data packets). A timeout
                        value of zero is interpreted as an infinite timeout. This parameter expects
                        a value of type <classname>java.lang.Integer</classname>. If this parameter
                        is not set read operations will not time out (infinite timeout).</para>
                </formalpara>
                <formalpara>
                    <title>'http.tcp.nodelay':</title>
                    <para>determines whether Nagle's algorithm is to be used. The Nagle's algorithm
                        tries to conserve bandwidth by minimizing the number of segments that are
                        sent. When applications wish to decrease network latency and increase
                        performance, they can disable Nagle's algorithm (that is enable
                            <literal>TCP_NODELAY</literal>. Data will be sent earlier, at the cost
                        of an increase in bandwidth consumption. This parameter expects a value of
                        type <classname>java.lang.Boolean</classname>. If this parameter is not,
                            <literal>TCP_NODELAY</literal> will be enabled (no delay).</para>
                </formalpara>
                <formalpara>
                    <title>'http.socket.buffer-size':</title>
                    <para>determines the size of the internal socket buffer used to buffer data
                        while receiving / transmitting HTTP messages. This parameter expects a value
                        of type <classname>java.lang.Integer</classname>. If this parameter is not
                        set HttpClient will allocate 8192 byte socket buffers.</para>
                </formalpara>
                <formalpara>
                    <title>'http.socket.linger':</title>
                    <para>sets <literal>SO_LINGER</literal> with the specified linger time in
                        seconds. The maximum timeout value is platform specific. Value 0 implies
                        that the option is disabled. Value -1 implies that the JRE default is used.
                        The setting only affects the socket close operation. If this parameter is
                        not set value -1 (JRE default) will be assumed.</para>
                </formalpara>
                <formalpara>
                    <title>'http.connection.timeout':</title>
                    <para>determines the timeout in milliseconds until a connection is established.
                        A timeout value of zero is interpreted as an infinite timeout. This
                        parameter expects a value of type <classname>java.lang.Integer</classname>.
                        If this parameter is not set connect operations will not time out (infinite
                        timeout).</para>
                </formalpara>
                <formalpara>
                    <title>'http.connection.stalecheck':</title>
                    <para>determines whether stale connection check is to be used. Disabling stale
                        connection check may result in a noticeable performance improvement (the
                        check can cause up to 30 millisecond overhead per request) at the risk of
                        getting an I/O error when executing a request over a connection that has
                        been closed at the server side. This parameter expects a value of type
                            <classname>java.lang.Boolean</classname>. For performance critical
                        operations the check should be disabled. If this parameter is not set the
                        stale connection will be performed before each request execution.</para>
                </formalpara>
                <formalpara>
                    <title>'http.connection.max-line-length':</title>
                    <para>determines the maximum line length limit. If set to a positive value, any
                        HTTP line exceeding this limit will cause an
                            <exceptionname>java.io.IOException</exceptionname>. A negative or zero
                        value will effectively disable the check. This parameter expects a value of
                        type <classname>java.lang.Integer</classname>. If this parameter is not set,
                        no limit will be enforced.</para>
                </formalpara>
                <formalpara>
                    <title>'http.connection.max-header-count':</title>
                    <para>determines the maximum HTTP header count allowed. If set to a positive
                        value, the number of HTTP headers received from the data stream exceeding
                        this limit will cause an <exceptionname>java.io.IOException</exceptionname>.
                        A negative or zero value will effectively disable the check. This parameter
                        expects a value of type <classname>java.lang.Integer</classname>. If this
                        parameter is not set, no limit will be enforced.</para>
                </formalpara>
                <formalpara>
                    <title>'http.connection.max-status-line-garbage':</title>
                    <para>defines the maximum number of ignorable lines before we expect a HTTP
                        response's status line. With HTTP/1.1 persistent connections, the problem
                        arises that broken scripts could return a wrong
                            <literal>Content-Length</literal> (there are more bytes sent than
                        specified). Unfortunately, in some cases, this cannot be detected after the
                        bad response, but only before the next one. So HttpClient must be able to
                        skip those surplus lines this way. This parameter expects a value of type
                        java.lang.Integer. 0 disallows all garbage/empty lines before the status
                        line. Use <constant>java.lang.Integer#MAX_VALUE</constant> for unlimited
                        number. If this parameter is not set unlimited number will be
                        assumed.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
    </section>
    <section>
        <title>Connection persistence</title>
        <para>The process of establishing a connection from one host to another is quite complex and
            involves multiple packet exchanges between two endpoints, which can be quite time
            consuming. The overhead of connection handshaking can be significant, especially for
            small HTTP messages. One can achieve a much higher data throughput if open connections
            can be re-used to execute multiple requests.</para>
        <para>HTTP/1.1 states that HTTP connections can be re-used for multiple requests per
            default. HTTP/1.0 compliant endpoints can also use similar mechanism to explicitly
            communicate their preference to keep connection alive and use it for multiple requests.
            HTTP agents can also keep idle connections alive for a certain period time in case a
            connection to the same target host may be needed for subsequent requests. The ability to
            keep connections alive is usually refered to as connection persistence. HttpClient fully
            supports connection persistence.</para>
    </section>
    <section>
        <title>HTTP connection routing</title>
        <para>HttpClient is capable of establishing connections to the target host either directly
            or via a route that may involve multiple intermediate connections also referred to as
            hops. HttpClient differentiates connections of a route into plain, tunneled and layered.
            The use of multiple intermediate proxies to tunnel connections to the target host is
            referred to as proxy chaining.</para>
        <para>Plain routes are established by connecting to the target or the first and only proxy.
            Tunnelled routes are established by connecting to the first and tunnelling through a
            chain of proxies to the target. Routes without a proxy cannot be tunnelled. Layered
            routes are established by layering a protocol over an existing connection. Protocols can
            only be layered over a tunnel to the target, or over a direct connection without
            proxies.</para>
        <section>
            <title>Route computation</title>
            <para><interfacename>RouteInfo</interfacename> interface represents information about a
                definitive route to a target host involving one or more intermediate steps or hops.
                    <classname>HttpRoute</classname> is a concrete implementation of
                    <interfacename>RouteInfo</interfacename>, which cannot be changed (is
                immutable). <classname>HttpTracker</classname> is a mutable
                    <interfacename>RouteInfo</interfacename> implementation used internally by
                HttpClient to track the remaining hops to the ultimate route target.
                    <classname>HttpTracker</classname> can be updated after a successful execution
                of the next hop towards the route target. <classname>HttpRouteDirector</classname>
                is a helper class that can be used to compute the next step in a route. This class
                is used internally by HttpClient.</para>
            <para><interfacename>HttpRoutePlanner</interfacename> is an interface representing a
                strategy to compute a complete route to a given target based on the execution
                context. HttpClient ships with two default
                    <interfacename>HttpRoutePlanner</interfacename> implementation.
                    <classname>ProxySelectorRoutePlanner</classname> is based on
                    <classname>java.net.ProxySelector</classname>. By default, it will pick up the
                proxy settings of the JVM, either from system properties or from the browser running
                the application. <classname>DefaultHttpRoutePlanner</classname> implementation does
                not make use of any Java system properties, nor of system or browser proxy settings.
                It computes routes based exclusively on HTTP parameters described below.</para>
        </section>
        <section>
            <title>Secure HTTP connections</title>
            <para>HTTP connections can be considered secure if information transmitted between two
                connection endpoints cannot be read or tampered with by an unauthorized third party.
                The SSL/TLS protocol is the most widely used technique to ensure HTTP transport
                security. However, other encryption techniques could be employed as well. Usually,
                HTTP transport is layered over the SSL/TLS encrypted connection.</para>
        </section>
    </section>
    <section>
        <title>HTTP route parameters</title>
        <para>These are parameters that can influence route computation:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>'http.route.default-proxy':</title>
                    <para>defines a proxy host to be used by default route planners that do not make
                        use of JRE settings. This parameter expects a value of type
                            <classname>HttpHost</classname>. If this parameter is not set direct
                        connections to the target will be attempted.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.route.local-address':</title>
                    <para>defines a local address to be used by all default route planner. On
                        machines with multiple network interfaces, this parameter can be used to
                        select the network interface from which the connection originates. This
                        parameter expects a value of type
                            <classname>java.net.InetAddress</classname>. If this parameter is not
                        set a default local address will be used automatically.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.route.forced-route':</title>
                    <para>defines an forced route to be used by all default route planner. Instead
                        of computing a route, the given forced route will be returned, even if it
                        points to a completely different target host. This parameter expects a value
                        of type <classname>HttpRoute</classname>.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
    </section>
    <section>
        <title>Socket factories</title>
        <para>HTTP connections make use of a <classname>java.net.Socket</classname> object
            internally to handle transmission of data across the wire. They, however, rely on
                <interfacename>SocketFactory</interfacename> interface to create, initialize and
            connect sockets. This enables the users of HttpClient to provide application specific
            socket initialization code at runtime. <classname>PlainSocketFactory</classname> is the
            default factory for creating and initializing plain (unencrypted) sockets.</para>
        <para>The process of creating a socket and that of connecting it to a host are decoupled, so
            that the socket could be closed while being blocked in the connect operation.</para>
        <programlisting><![CDATA[
PlainSocketFactory sf = PlainSocketFactory.getSocketFactory();
Socket socket = sf.createSocket();

HttpParams params = new BasicHttpParams();
params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000L);
sf.connectSocket(socket, "locahost", 8080, null, -1, params);
]]></programlisting>
        <section>
            <title>Secure socket layering</title>
            <para><interfacename>LayeredSocketFactory</interfacename> is an extension of
                    <interfacename>SocketFactory</interfacename> interface. Layered socket factories
                are capable of creating sockets that are layered over an existing plain socket.
                Socket layering is used primarily for creating secure sockets through proxies.
                HttpClient ships with SSLSocketFactory that implements SSL/TLS layering. Please note
                HttpClient does not use any custom encryption functionality. It is fully reliant on
                standard Java Cryptography (JCE) and Secure Sockets (JSEE) extensions.</para>
        </section>
        <section>
            <title>SSL/TLS customization</title>
            <para>HttpClient makes use of SSLSocketFactory to create SSL connections.
                    <classname>SSLSocketFactory</classname> allows for a high degree of
                customization. It can take an instance of
                    <interfacename>javax.net.ssl.SSLContext</interfacename> as a parameter and use
                it to create custom configured SSL connections.</para>
            <programlisting><![CDATA[
TrustManager easyTrustManager = new X509TrustManager() {

    @Override
    public void checkClientTrusted(
            X509Certificate[] chain,
            String authType) throws CertificateException {
        // Oh, I am easy!
    }

    @Override
    public void checkServerTrusted(
            X509Certificate[] chain,
            String authType) throws CertificateException {
        // Oh, I am easy!
    }

    @Override
    public X509Certificate[] getAcceptedIssuers() {
        return null;
    }
    
};

SSLContext sslcontext = SSLContext.getInstance("TLS");
sslcontext.init(null, new TrustManager[] { easyTrustManager }, null);

SSLSocketFactory sf = new SSLSocketFactory(sslcontext); 
SSLSocket socket = (SSLSocket) sf.createSocket();
socket.setEnabledCipherSuites(new String[] { "SSL_RSA_WITH_RC4_128_MD5" });

HttpParams params = new BasicHttpParams();
params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000L);
sf.connectSocket(socket, "locahost", 443, null, -1, params);
]]></programlisting>
            <para>Customization of SSLSocketFactory implies a certain degree of familiarity with the
                concepts of the SSL/TLS protocol, a detailed explanation of which is out of scope
                for this document. Please refer to the <ulink
                    url="http://java.sun.com/j2se/1.5.0/docs/guide/security/jsse/JSSERefGuide.html"
                    >Java Secure Socket Extension</ulink> for a detailed description of
                    <interfacename>javax.net.ssl.SSLContext</interfacename> and related
                tools.</para>
        </section>
        <section>
            <title>Hostname verification</title>
            <para>In addition to the trust verification and the client authentication performed on
                the SSL/TLS protocol level, HttpClient can optionally verify whether the target
                hostname matches the names stored inside the server's X.509 certificate, once the
                connection has been established. This verification can provide additional guarantees
                of authenticity of the server trust material. X509HostnameVerifier interface
                represents a strategy for hostname verification. HttpClient ships with three
                X509HostnameVerifier. Important: hostname verification should not be confused with
                SSL trust verification.</para>
            <itemizedlist>
                <listitem>
                    <formalpara>
                        <title><classname>StrictHostnameVerifier</classname>:</title>
                        <para>The strict hostname verifier works the same way as Sun Java 1.4, Sun
                            Java 5, Sun Java 6. It's also pretty close to IE6. This implementation
                            appears to be compliant with RFC 2818 for dealing with wildcards. The
                            hostname must match either the first CN, or any of the subject-alts. A
                            wildcard can occur in the CN, and in any of the subject-alts.</para>
                    </formalpara>
                </listitem>
                <listitem>
                    <formalpara>
                        <title><classname>BrowserCompatHostnameVerifier</classname>:</title>
                        <para>The hostname verifier that works the same way as Curl and Firefox. The
                            hostname must match either the first CN, or any of the subject-alts. A
                            wildcard can occur in the CN, and in any of the subject-alts. The only
                            difference between <classname>BrowserCompatHostnameVerifier</classname>
                            and <classname>StrictHostnameVerifier</classname> is that a wildcard
                            (such as "*.foo.com") with
                                <classname>BrowserCompatHostnameVerifier</classname> matches all
                            subdomains, including "a.b.foo.com".</para>
                    </formalpara>
                </listitem>
                <listitem>
                    <formalpara>
                        <title><classname>AllowAllHostnameVerifier</classname>:</title>
                        <para>This hostname verifier essentially turns hostname verification off.
                            This implementation is a no-op, and never throws the
                                <exceptionname>javax.net.ssl.SSLException</exceptionname>.</para>
                    </formalpara>
                </listitem>
            </itemizedlist>
            <para>Per default HttpClient uses <classname>BrowserCompatHostnameVerifier</classname>
                implementation. One can specify a different hostname verifier implementation if
                desired</para>
            <programlisting><![CDATA[
SSLSocketFactory sf = new SSLSocketFactory(SSLContext.getInstance("TLS"));
sf.setHostnameVerifier(SSLSocketFactory.STRICT_HOSTNAME_VERIFIER);
]]></programlisting>
        </section>
    </section>
    <section>
        <title>Protocol schemes</title>
        <para><classname>Scheme</classname> class represents a protocol scheme such as "http" or
            "https" and contains a number of protocol properties such as the default port and the
            socket factory to be used to creating <classname>java.net.Socket</classname> instances
            for the given protocol. <classname>SchemeRegistry</classname> class is used to maintain
            a set of <classname>Scheme</classname>s HttpClient can choose from when trying to
            establish a connection by a request URI:</para>
        <programlisting><![CDATA[
Scheme http = new Scheme("http", PlainSocketFactory.getSocketFactory(), 80);

SSLSocketFactory sf = new SSLSocketFactory(SSLContext.getInstance("TLS"));
sf.setHostnameVerifier(SSLSocketFactory.STRICT_HOSTNAME_VERIFIER);
Scheme https = new Scheme("https", sf, 443);

SchemeRegistry sr = new SchemeRegistry();
sr.register(http);
sr.register(https);
]]></programlisting>
    </section>
    <section>
        <title>HttpClient proxy configuration</title>
        <para>Even though HttpClient is aware of complex routing scemes and proxy chaining, it
            supports only simple direct or one hop proxy connections out of the box.</para>
        <para>The simplest way to tell HttpClient to connect to the target host via a proxy is by
            setting the default proxy parameter:</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();

HttpHost proxy = new HttpHost("someproxy", 8080);
httpclient.getParams().setParameter(ConnRoutePNames.DEFAULT_PROXY, proxy);
]]></programlisting>
        <para>One can also instruct HttpClient to use standard JRE proxy selector to obtain proxy
            information:</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();

ProxySelectorRoutePlanner routePlanner = new ProxySelectorRoutePlanner(
        httpclient.getConnectionManager().getSchemeRegistry(),
        ProxySelector.getDefault());  
httpclient.setRoutePlanner(routePlanner);
]]></programlisting>
        <para>Alternatively, one can provide a custom <interfacename>RoutePlanner</interfacename>
            implementation in order to have a complete control over the process of HTTP route
            computation:</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();
httpclient.setRoutePlanner(new HttpRoutePlanner() {

    public HttpRoute determineRoute(
            HttpHost target, 
            HttpRequest request, 
            HttpContext context) throws HttpException {
        return new HttpRoute(target, null,  new HttpHost("someproxy", 8080), 
                "https".equalsIgnoreCase(target.getSchemeName()));
    }
    
});
]]></programlisting>
    </section>
    <section>
        <title>HTTP connection managers</title>
        <section>
            <title>Connection operators</title>
            <para>Operated connections are client side connections whose underlying socket or its
                state can be manipulated by an external entity, usually referred to as a connection
                operator. <interfacename>OperatedClientConnection</interfacename> interface extends
                    <interfacename>HttpClientConnection</interfacename> interface and define
                additional methods to manage connection socket. The
                    <interfacename>ClientConnectionOperator</interfacename> interface represents a
                strategy for creating <interfacename>OperatedClientConnection</interfacename>
                instances and updating the underlying socket of those objects. Implementations will
                most likely make use <interfacename>SocketFactory</interfacename>s to create
                    <classname>java.net.Socket</classname> instances. The
                    <interfacename>ClientConnectionOperator</interfacename> interface enables the
                users of HttpClient to provide a custom strategy for connection operators as well as
                an ability to provide alternative implementation of the
                    <interfacename>OperatedClientConnection</interfacename> interface.</para>
        </section>
        <section>
            <title>Managed connections and connection managers</title>
            <para>HTTP connections are complex, stateful, thread-unsafe objects which need to be
                properly managed to function correctly. HTTP connections can only be used by one
                execution thread at a time. HttpClient employs a special entity to manage access to
                HTTP connections called HTTP connection manager and represented by the
                    <interfacename>ClientConnectionManager</interfacename> interface. The purpose of
                an HTTP connection manager is to serve as a factory for new HTTP connections, manage
                persistent connections and synchronize access to persistent connections making sure
                that only one thread can have access to a connection at a time.</para>
            <para>Internally HTTP connection managers work with instances of
                    <interfacename>OperatedClientConnection</interfacename>, but they hands out
                instances of <interfacename>ManagedClientConnection</interfacename> to the service
                consumers. <interfacename>ManagedClientConnection</interfacename> acts as a wrapper
                for a <interfacename>OperatedClientConnection</interfacename> instance that manages
                its state and controls all I/O operations on that connection. It also abstracts away
                socket operations and provides convenience methods for opening and updating sockets
                in order to establish a route.
                    <interfacename>ManagedClientConnection</interfacename> instances are aware of
                their link to the connection manager that spawned them and of the fact that they
                must be returned back to the manager when no longer in use.
                    <interfacename>ManagedClientConnection</interfacename> classes also implement
                    <interfacename>ConnectionReleaseTrigger</interfacename> interface that can be
                used to trigger the release of the connection back to the manager. Once the
                connection release has been triggered the wrapped connection gets detached from the
                    <interfacename>ManagedClientConnection</interfacename> wrapper and the
                    <interfacename>OperatedClientConnection</interfacename> instance is returned
                back to the manager. Even though the service consumer still holds a reference to the
                    <interfacename>ManagedClientConnection</interfacename> instance, it is no longer
                able to execute any I/O operation or change the state of the
                    <interfacename>OperatedClientConnection</interfacename> either intentionally or
                unintentionally.</para>
            <para>This is an example of acquiring a connection from a connection manager:</para>
            <programlisting><![CDATA[
HttpParams params = new BasicHttpParams();
Scheme http = new Scheme("http", PlainSocketFactory.getSocketFactory(), 80);
SchemeRegistry sr = new SchemeRegistry();
sr.register(http);

ClientConnectionManager connMrg = new SingleClientConnManager(params, sr);

// Request new connection. This can be a long process
ClientConnectionRequest connRequest = connMrg.requestConnection(
        new HttpRoute(new HttpHost("localhost", 80)), null);

// Wait for connection up to 10 sec
ManagedClientConnection conn = connRequest.getConnection(10, TimeUnit.SECONDS);
try {
    // Do useful things with the connection.
    // Release it when done.
    conn.releaseConnection();
} catch (IOException ex) {
    // Abort connection upon an I/O error.
    conn.abortConnection();
    throw ex;
}
]]></programlisting>
            <para>The connection request can be terminated prematurely by calling
                    <methodname>ClientConnectionRequest#abortRequest()</methodname> if necessary.
                This will unblock the thread blocked in the
                    <methodname>ClientConnectionRequest#getConnection()</methodname> method.</para>
            <para><classname>BasicManagedEntity</classname> wrapper class can be used to ensure
                automatic release of the underlying connection once the response content has been
                fully consumed. HttpClient uses this mechanism internally to achieve transparent
                connection release for all responses obtained from
                    <methodname>HttpClient#execute()</methodname> methods:</para>
            <programlisting><![CDATA[
ClientConnectionRequest connRequest = connMrg.requestConnection(
        new HttpRoute(new HttpHost("localhost", 80)), null);
ManagedClientConnection conn = connRequest.getConnection(10, TimeUnit.SECONDS);
try {
    BasicHttpRequest request = new BasicHttpRequest("GET", "/");
    conn.sendRequestHeader(request);
    HttpResponse response = conn.receiveResponseHeader();
    conn.receiveResponseEntity(response);
    HttpEntity entity = response.getEntity();
    if (entity != null) {
        BasicManagedEntity managedEntity = new BasicManagedEntity(entity, conn, true);
        // Replace entity
        response.setEntity(managedEntity);
    }
    // Do something useful with the response
    // The connection will be released automatically 
    // as soon as the response content has been consumed
} catch (IOException ex) {
    // Abort connection upon an I/O error.
    conn.abortConnection();
    throw ex;
}
]]></programlisting>
        </section>
        <section>
            <title>Simple connection manager</title>
            <para><classname>SingleClientConnManager</classname> is a simple connection manager that
                maintains only one connection at a time. Even though this class is thread-safe it
                ought to be used by one execution thread only.
                    <classname>SingleClientConnManager</classname> will make an effort to reuse the
                connection for subsequent requests with the same route. It will, however, close the
                existing connection and open it for the given route, if the route of the persistent
                connection does not match that of the connection request. If the connection has been
                already been allocated
                    <exceptionname>java.lang.IllegalStateException</exceptionname> is thrown.</para>
            <para><classname>SingleClientConnManager</classname> is used by HttpClient per
                default.</para>
        </section>
        <section>
            <title>Pooling connection manager</title>
            <para><classname>ThreadSafeClientConnManager</classname> is a more complex
                implementation that manages a pool of client connections and is able to service
                connection requests from multiple execution threads. Connections are pooled on a per
                route basis. A request for a route which already the manager has persistent
                connections for available in the pool will be services by leasing a connection from
                the pool rather than creating a brand new connection.</para>
            <para><classname>ThreadSafeClientConnManager</classname> maintains a maximum limit of
                connection on a per route basis and in total. Per default this implementation will
                create no more than than 2 concurrent connections per given route and no more 20
                connections in total. For many real-world applications these limits may prove too
                constraining, especially if they use HTTP as a transport protocol for their
                services. Connection limits, however, can be adjusted using HTTP parameters.</para>
            <para>This example shows how the connection pool parameters can be adjusted:</para>
            <programlisting><![CDATA[
HttpParams params = new BasicHttpParams();
// Increase max total connection to 200
ConnManagerParams.setMaxTotalConnections(params, 200);
// Increase default max connection per route to 20
ConnPerRouteBean connPerRoute = new ConnPerRouteBean(20);
// Increase max connections for localhost:80 to 50
HttpHost localhost = new HttpHost("locahost", 80);
connPerRoute.setMaxForRoute(new HttpRoute(localhost), 50);
ConnManagerParams.setMaxConnectionsPerRoute(params, connPerRoute);

SchemeRegistry schemeRegistry = new SchemeRegistry();
schemeRegistry.register(
        new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
schemeRegistry.register(
        new Scheme("https", SSLSocketFactory.getSocketFactory(), 443));

ClientConnectionManager cm = new ThreadSafeClientConnManager(params, schemeRegistry);
HttpClient httpClient = new DefaultHttpClient(cm, params);
]]></programlisting>
        </section>
        <section>
            <title>Connection manager shutdown</title>
            <para>When an HttpClient instance is no longer needed and is about to go out of scope it
                is important to shut down its connection manager to ensure that all connections kept
                alive by the manager get closed and system resources allocated by those connections
                are released.</para>
            <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet("http://www.google.com/");
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
System.out.println(response.getStatusLine());
if (entity != null) {
    entity.consumeContent();
}
httpclient.getConnectionManager().shutdown();
]]></programlisting>
        </section>
    </section>
    <section>
        <title>Connection management parameters</title>
        <para>These are parameters that be used to customize standard HTTP connection manager
            implementations:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>'http.conn-manager.timeout':</title>
                    <para>defines the timeout in milliseconds used when retrieving an instance of
                            <interfacename>ManagedClientConnection</interfacename> from the
                            <interfacename>ClientConnectionManager</interfacename> This parameter
                        expects a value of type <classname>java.lang.Long</classname>. If this
                        parameter is not set connection requests will not time out (infinite
                        timeout).</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.conn-manager.max-per-route':</title>
                    <para>defines the maximum number of connections per route. This limit is
                        interpreted by client connection managers and applies to individual manager
                        instances. This parameter expects a value of type
                            <interfacename>ConnPerRoute</interfacename>.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>'http.conn-manager.max-total':</title>
                    <para>defines the maximum number of connections in total. This limit is
                        interpreted by client connection managers and applies to individual manager
                        instances. This parameter expects a value of type
                            <classname>java.lang.Integer</classname>.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
    </section>
    <section>
        <title>Multithreaded request execution</title>
        <para>When equipped with a pooling connection manager such as ThreadSafeClientConnManager
            HttpClient can be used to execute multiple requests simultaneously using multiple
            threads of execution.</para>
        <para><classname>ThreadSafeClientConnManager</classname> will allocate connections based on
            its configuration. If all connections for a given route have already been leased, a
            request for connection will block until a connection is released back to the pool. One
            can ensure the connection manager does not block indefinitely in the connection request
            operation by setting <literal>'http.conn-manager.timeout'</literal> to a positive value.
            If the connection request cannot be serviced within the given time period
                <exceptionname>ConnectionPoolTimeoutException</exceptionname> will be thrown.</para>
        <programlisting><![CDATA[
HttpParams params = new BasicHttpParams();
SchemeRegistry schemeRegistry = new SchemeRegistry();
schemeRegistry.register(
        new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));

ClientConnectionManager cm = new ThreadSafeClientConnManager(params, schemeRegistry);
HttpClient httpClient = new DefaultHttpClient(cm, params);

// URIs to perform GETs on
String[] urisToGet = {
    "http://www.domain1.com/",
    "http://www.domain2.com/",
    "http://www.domain3.com/",
    "http://www.domain4.com/"
};

// create a thread for each URI
GetThread[] threads = new GetThread[urisToGet.length];
for (int i = 0; i < threads.length; i++) {
    HttpGet httpget = new HttpGet(urisToGet[i]);
    threads[i] = new GetThread(httpClient, httpget);
}

// start the threads
for (int j = 0; j < threads.length; j++) {
    threads[j].start();
}

// join the threads
for (int j = 0; j < threads.length; j++) {
    threads[j].join();
}

]]></programlisting>
        <programlisting><![CDATA[
static class GetThread extends Thread {
    
    private final HttpClient httpClient;
    private final HttpContext context;
    private final HttpGet httpget;
    
    public GetThread(HttpClient httpClient, HttpGet httpget) {
        this.httpClient = httpClient;
        this.context = new BasicHttpContext();
        this.httpget = httpget;
    }
    
    @Override
    public void run() {
        try {
            HttpResponse response = this.httpClient.execute(this.httpget, this.context);
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                // do something useful with the entity
                // ...
                // ensure the connection gets released to the manager
                entity.consumeContent();
            }
        } catch (Exception ex) {
            this.httpget.abort();
        }
    }
   
}
]]></programlisting>
    </section>
    <section>
        <title>Connection eviction policy</title>
        <para>One of the major shortcoming of the classic blocking I/O model is that the network
            socket can react to I/O events only when blocked in an I/O operation. When a connection
            is released back to the manager, it can be kept alive however it is unable to monitor
            the status of the socket and react to any I/O events. If the connection gets closed on
            the server side, the client side connection is unable to detect the change in the
            connection state and react appropriately by closing the socket on its end.</para>
        <para>HttpClient tries to mitigate the problem by testing whether the connection is 'stale',
            that is no longer valid because it was closed on the server side, prior to using the
            connection for executing an HTTP request. The stale connection check is not 100%
            reliable and adds 10 to 30 ms overhead to each request execution. The only feasible
            solution that does not involve a one thread per socket model for idle connections is a
            dedicated monitor thread used to evict connections that are considered expired due to a
            long period of inactivity. The monitor thread can periodically call
                <methodname>ClientConnectionManager#closeExpiredConnections()</methodname> method to
            close all expired connections and evict closed connections from the pool. It can also
            optionally call <methodname>ClientConnectionManager#closeIdleConnections()</methodname>
            method to close all connections that have been idle over a given period of time.</para>
        <programlisting><![CDATA[
public static class IdleConnectionMonitorThread extends Thread {
    
    private final ClientConnectionManager connMgr;
    private volatile boolean shutdown;
    
    public IdleConnectionMonitorThread(ClientConnectionManager connMgr) {
        super();
        this.connMgr = connMgr;
    }

    @Override
    public void run() {
        try {
            while (!shutdown) {
                synchronized (this) {
                    wait(5000);
                    // Close expired connections
                    connMgr.closeExpiredConnections();
                    // Optionally, close connections
                    // that have been idle longer than 30 sec
                    connMgr.closeIdleConnections(30, TimeUnit.SECONDS);
                }
            }
        } catch (InterruptedException ex) {
            // terminate
        }
    }
    
    public void shutdown() {
        shutdown = true;
        synchronized (this) {
            notifyAll();
        }
    }
    
}
]]></programlisting>
    </section>
    <section>
        <title>Connection keep alive strategy</title>
        <para>The HTTP specification does not specify how long a persistent connection may be and
            should be kept alive. Some HTTP servers use non-standard <literal>Keep-Alive</literal>
            header to communicate to the client the period of time in seconds they intend to keep
            the connection alive on the server side. HttpClient makes use of this information if
            available. If the <literal>Keep-Alive</literal> header is not present in the response,
            HttpClient assumes the connection can be kept alive indefinitely. However, many HTTP
            servers out there are configured to drop persistent connections after a certain period
            of inactivity in order to conserve system resources, quite often without informing the
            client. In case the default strategy turns out to be too optimistic, one may want to
            provide a custom keep-alive strategy.</para>
        <programlisting><![CDATA[
DefaultHttpClient httpclient = new DefaultHttpClient();
httpclient.setKeepAliveStrategy(new ConnectionKeepAliveStrategy() {

    public long getKeepAliveDuration(HttpResponse response, HttpContext context) {
        // Honor 'keep-alive' header
        HeaderElementIterator it = new BasicHeaderElementIterator(
                response.headerIterator(HTTP.CONN_KEEP_ALIVE));
        while (it.hasNext()) {
            HeaderElement he = it.nextElement();
            String param = he.getName(); 
            String value = he.getValue();
            if (value != null && param.equalsIgnoreCase("timeout")) {
                try {
                    return Long.parseLong(value) * 1000;
                } catch(NumberFormatException ignore) {
                }
            }
        }
        HttpHost target = (HttpHost) context.getAttribute(
                ExecutionContext.HTTP_TARGET_HOST);
        if ("www.naughty-server.com".equalsIgnoreCase(target.getHostName())) {
            // Keep alive for 5 seconds only
            return 5 * 1000;
        } else {
            // otherwise keep alive for 30 seconds
            return 30 * 1000;
        }
    }
    
});
]]></programlisting>
    </section>
</chapter>