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Java example source code file (connmgmt.xml)

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The connmgmt.xml example source code

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE preface PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at
    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.
<chapter id="connmgmt">
    <title>Connection management
    <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>
        <title>Connection parameters
        <para>These are parameters that can influence connection operations:
                    <para>defines the socket timeout (SO_TIMEOUT) 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. If this parameter
                        is not set read operations will not time out (infinite timeout).</para>
                    <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. 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. If this parameter is not,
                            <literal>TCP_NODELAY will be enabled (no delay).
                    <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. If this parameter is not
                        set HttpClient will allocate 8192 byte socket buffers.</para>
                    <para>sets SO_LINGER 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>
                    <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.
                        If this parameter is not set connect operations will not time out (infinite
                    <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. 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>
                    <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. A negative or zero
                        value will effectively disable the check. This parameter expects a value of
                        type <classname>java.lang.Integer. If this parameter is not set,
                        no limit will be enforced.</para>
                    <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.
                        A negative or zero value will effectively disable the check. This parameter
                        expects a value of type <classname>java.lang.Integer. If this
                        parameter is not set, no limit will be enforced.</para>
                    <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 (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 for unlimited
                        number. If this parameter is not set unlimited number will be
        <title>Connection persistence
        <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>
        <title>HTTP connection routing
        <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
            <title>Route computation
            <para>RouteInfo interface represents information about a
                definitive route to a target host involving one or more intermediate steps or hops.
                    <classname>HttpRoute is a concrete implementation of
                    <interfacename>RouteInfo, which cannot be changed (is
                immutable). <classname>HttpTracker is a mutable
                    <interfacename>RouteInfo implementation used internally by
                HttpClient to track the remaining hops to the ultimate route target.
                    <classname>HttpTracker can be updated after a successful execution
                of the next hop towards the route target. <classname>HttpRouteDirector
                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>HttpRoutePlanner 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 implementation.
                    <classname>ProxySelectorRoutePlanner is based on
                    <classname>java.net.ProxySelector. 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 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>
            <title>Secure HTTP connections
            <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>
        <title>HTTP route parameters
        <para>These are parameters that can influence route computation:
                    <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. If this parameter is not set direct
                        connections to the target will be attempted.</para>
                    <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. If this parameter is not
                        set a default local address will be used automatically.</para>
                    <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.
        <title>Socket factories
        <para>HTTP connections make use of a java.net.Socket object
            internally to handle transmission of data across the wire. They, however, rely on
                <interfacename>SocketFactory interface to create, initialize and
            connect sockets. This enables the users of HttpClient to provide application specific
            socket initialization code at runtime. <classname>PlainSocketFactory 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>
            <title>Secure socket layering
            <para>LayeredSocketFactory is an extension of
                    <interfacename>SocketFactory 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>
            <title>SSL/TLS customization
            <para>HttpClient makes use of SSLSocketFactory to create SSL connections.
                    <classname>SSLSocketFactory allows for a high degree of
                customization. It can take an instance of
                    <interfacename>javax.net.ssl.SSLContext as a parameter and use
                it to create custom configured SSL connections.</para>
            <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
                    >Java Secure Socket Extension</ulink> for a detailed description of
                    <interfacename>javax.net.ssl.SSLContext and related
            <title>Hostname verification
            <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>
                        <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>
                        <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
                            and <classname>StrictHostnameVerifier is that a wildcard
                            (such as "*.foo.com") with
                                <classname>BrowserCompatHostnameVerifier matches all
                            subdomains, including "a.b.foo.com".</para>
                        <para>This hostname verifier essentially turns hostname verification off.
                            This implementation is a no-op, and never throws the
            <para>Per default HttpClient uses BrowserCompatHostnameVerifier
                implementation. One can specify a different hostname verifier implementation if
        <title>Protocol schemes
        <para>Scheme 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 instances
            for the given protocol. <classname>SchemeRegistry class is used to maintain
            a set of <classname>Schemes HttpClient can choose from when trying to
            establish a connection by a request URI:</para>
        <title>HttpClient proxy configuration
        <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>
        <para>One can also instruct HttpClient to use standard JRE proxy selector to obtain proxy
        <para>Alternatively, one can provide a custom RoutePlanner
            implementation in order to have a complete control over the process of HTTP route
        <title>HTTP connection managers
            <title>Connection operators
            <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 interface extends
                    <interfacename>HttpClientConnection interface and define
                additional methods to manage connection socket. The
                    <interfacename>ClientConnectionOperator interface represents a
                strategy for creating <interfacename>OperatedClientConnection
                instances and updating the underlying socket of those objects. Implementations will
                most likely make use <interfacename>SocketFactorys to create
                    <classname>java.net.Socket instances. The
                    <interfacename>ClientConnectionOperator 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 interface.
            <title>Managed connections and connection managers
            <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 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, but they hands out
                instances of <interfacename>ManagedClientConnection to the service
                consumers. <interfacename>ManagedClientConnection acts as a wrapper
                for a <interfacename>OperatedClientConnection 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 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 classes also implement
                    <interfacename>ConnectionReleaseTrigger 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 wrapper and the
                    <interfacename>OperatedClientConnection instance is returned
                back to the manager. Even though the service consumer still holds a reference to the
                    <interfacename>ManagedClientConnection instance, it is no longer
                able to execute any I/O operation or change the state of the
                    <interfacename>OperatedClientConnection either intentionally or
            <para>This is an example of acquiring a connection from a connection manager:
            <para>The connection request can be terminated prematurely by calling
                    <methodname>ClientConnectionRequest#abortRequest() if necessary.
                This will unblock the thread blocked in the
                    <methodname>ClientConnectionRequest#getConnection() method.
            <para>BasicManagedEntity 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() methods:
            <title>Simple connection manager
            <para>SingleClientConnManager 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 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 is thrown.
            <para>SingleClientConnManager is used by HttpClient per
            <title>Pooling connection manager
            <para>ThreadSafeClientConnManager 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>ThreadSafeClientConnManager 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:
            <title>Connection manager shutdown
            <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>
        <title>Connection management parameters
        <para>These are parameters that be used to customize standard HTTP connection manager
                    <para>defines the timeout in milliseconds used when retrieving an instance of
                            <interfacename>ManagedClientConnection from the
                            <interfacename>ClientConnectionManager This parameter
                        expects a value of type <classname>java.lang.Long. If this
                        parameter is not set connection requests will not time out (infinite
                    <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
                    <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
        <title>Multithreaded request execution
        <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>ThreadSafeClientConnManager 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' to a positive value.
            If the connection request cannot be serviced within the given time period
                <exceptionname>ConnectionPoolTimeoutException will be thrown.
        <title>Connection eviction policy
        <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() method to
            close all expired connections and evict closed connections from the pool. It can also
            optionally call <methodname>ClientConnectionManager#closeIdleConnections()
            method to close all connections that have been idle over a given period of time.</para>
        <title>Connection keep alive strategy
        <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
            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 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>

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