public interface ConnectionKeepAliveStrategy {
    /**
     * Returns the duration of time which this connection can be safely kept
     * idle. If the connection is left idle for longer than this period of time,
     * it MUST not reused. A value of 0 or less may be returned to indicate that
     * there is no suitable suggestion.
     *
     * When coupled with a {@link org.apache.http.ConnectionReuseStrategy}, if
     * {@link org.apache.http.ConnectionReuseStrategy#keepAlive(
     *   HttpResponse, HttpContext)} returns true, this allows you to control
     * how long the reuse will last. If keepAlive returns false, this should
     * have no meaningful impact
     *
     * @param response
     *            The last response received over the connection.
     * @param context
     *            the context in which the connection is being used.
     *
     * @return the duration in ms for which it is safe to keep the connection
     *         idle, or <=0 if no suggested duration.
     */
    long getKeepAliveDuration(HttpResponse response, HttpContext context);
}

@Contract(threading = ThreadingBehavior.IMMUTABLE)
public class DefaultConnectionKeepAliveStrategy implements ConnectionKeepAliveStrategy {
    public static final DefaultConnectionKeepAliveStrategy INSTANCE = new DefaultConnectionKeepAliveStrategy();
    @Override
    public long getKeepAliveDuration(final HttpResponse response, final HttpContext context) {
        Args.notNull(response, "HTTP response");
        final HeaderElementIterator it = new BasicHeaderElementIterator(
                response.headerIterator(HTTP.CONN_KEEP_ALIVE));
        while (it.hasNext()) {
            final HeaderElement he = it.nextElement();
            final String param = he.getName();
            final String value = he.getValue();
            if (value != null && param.equalsIgnoreCase("timeout")) {
                try {
                    return Long.parseLong(value) * 1000;
                } catch(final NumberFormatException ignore) {
                }
            }
        }
        return -1;
    }
}

public interface ConnectionReuseStrategy {
    /**
     * Decides whether a connection can be kept open after a request.
     * If this method returns {@code false}, the caller MUST
     * close the connection to correctly comply with the HTTP protocol.
     * If it returns {@code true}, the caller SHOULD attempt to
     * keep the connection open for reuse with another request.
     * <p>
     * One can use the HTTP context to retrieve additional objects that
     * may be relevant for the keep-alive strategy: the actual HTTP
     * connection, the original HTTP request, target host if known,
     * number of times the connection has been reused already and so on.
     * </p>
     * <p>
     * If the connection is already closed, {@code false} is returned.
     * The stale connection check MUST NOT be triggered by a
     * connection reuse strategy.
     * </p>
     *
     * @param response
     *          The last response received over that connection.
     * @param context   the context in which the connection is being
     *          used.
     *
     * @return {@code true} if the connection is allowed to be reused, or
     *         {@code false} if it MUST NOT be reused
     */
    boolean keepAlive(HttpResponse response, HttpContext context);
}

public class DefaultClientConnectionReuseStrategy extends DefaultConnectionReuseStrategy {
    public static final DefaultClientConnectionReuseStrategy INSTANCE = new DefaultClientConnectionReuseStrategy();
    @Override
    public boolean keepAlive(final HttpResponse response, final HttpContext context) {
        final HttpRequest request = (HttpRequest) context.getAttribute(HttpCoreContext.HTTP_REQUEST);
        if (request != null) {
            final Header[] connHeaders = request.getHeaders(HttpHeaders.CONNECTION);
            if (connHeaders.length != 0) {
                final TokenIterator ti = new BasicTokenIterator(new BasicHeaderIterator(connHeaders, null));
                while (ti.hasNext()) {
                    final String token = ti.nextToken();
                    if (HTTP.CONN_CLOSE.equalsIgnoreCase(token)) {
                        return false;
                    }
                }
            }
        }
        return super.keepAlive(response, context);
    }
}

@Override
    public CloseableHttpResponse execute(
            final HttpRoute route,
            final HttpRequestWrapper request,
            final HttpClientContext context,
            final HttpExecutionAware execAware) throws IOException, HttpException {
            //......
               response = requestExecutor.execute(request, managedConn, context);
                // The connection is in or can be brought to a re-usable state.
                if (reuseStrategy.keepAlive(response, context)) {
                    // Set the idle duration of this connection
                    final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
                    if (this.log.isDebugEnabled()) {
                        final String s;
                        if (duration > 0) {
                            s = "for " + duration + " " + TimeUnit.MILLISECONDS;
                        } else {
                            s = "indefinitely";
                        }
                        this.log.debug("Connection can be kept alive " + s);
                    }
                    connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
                    connHolder.markReusable();
                } else {
                    connHolder.markNonReusable();
                }
            //......
    }

@Override
    public void releaseConnection(
            final HttpClientConnection managedConn,
            final Object state,
            final long keepalive, final TimeUnit timeUnit) {
        Args.notNull(managedConn, "Managed connection");
        synchronized (managedConn) {
            final CPoolEntry entry = CPoolProxy.detach(managedConn);
            if (entry == null) {
                return;
            }
            final ManagedHttpClientConnection conn = entry.getConnection();
            try {
                if (conn.isOpen()) {
                    final TimeUnit effectiveUnit = timeUnit != null ? timeUnit : TimeUnit.MILLISECONDS;
                    entry.setState(state);
                    entry.updateExpiry(keepalive, effectiveUnit);
                    if (this.log.isDebugEnabled()) {
                        final String s;
                        if (keepalive > 0) {
                            s = "for " + (double) effectiveUnit.toMillis(keepalive) / 1000 + " seconds";
                        } else {
                            s = "indefinitely";
                        }
                        this.log.debug("Connection " + format(entry) + " can be kept alive " + s);
                    }
                    conn.setSocketTimeout(0);
                }
            } finally {
                this.pool.release(entry, conn.isOpen() && entry.isRouteComplete());
                if (this.log.isDebugEnabled()) {
                    this.log.debug("Connection released: " + format(entry) + formatStats(entry.getRoute()));
                }
            }
        }
    }

public synchronized void updateExpiry(final long time, final TimeUnit timeUnit) {
        Args.notNull(timeUnit, "Time unit");
        this.updated = System.currentTimeMillis();
        final long newExpiry;
        if (time > 0) {
            newExpiry = this.updated + timeUnit.toMillis(time);
        } else {
            newExpiry = Long.MAX_VALUE;
        }
        this.expiry = Math.min(newExpiry, this.validityDeadline);
    }
    public synchronized boolean isExpired(final long now) {
        return now >= this.expiry;
    }

/**
     * Closes expired connections and evicts them from the pool.
     */
    public void closeExpired() {
        final long now = System.currentTimeMillis();
        enumAvailable(new PoolEntryCallback<T, C>() {
            @Override
            public void process(final PoolEntry<T, C> entry) {
                if (entry.isExpired(now)) {
                    entry.close();
                }
            }
        });
    }