摘要：調用計算的時間，這個方法會清理移除并過期的連接除了清理過期的連接外，還通過間接觸發，去清理關閉或異常的連接

本文主要研究一下jdk httpclient的ConnectionPool

java.net.http/jdk/internal/net/http/HttpConnection.java

    /**
     * Factory for retrieving HttpConnections. A connection can be retrieved
     * from the connection pool, or a new one created if none available.
     *
     * The given {@code addr} is the ultimate destination. Any proxies,
     * etc, are determined from the request. Returns a concrete instance which
     * is one of the following:
     *      {@link PlainHttpConnection}
     *      {@link PlainTunnelingConnection}
     *
     * The returned connection, if not from the connection pool, must have its,
     * connect() or connectAsync() method invoked, which ( when it completes
     * successfully ) renders the connection usable for requests.
     */
    public static HttpConnection getConnection(InetSocketAddress addr,
                                               HttpClientImpl client,
                                               HttpRequestImpl request,
                                               Version version) {
        // The default proxy selector may select a proxy whose  address is
        // unresolved. We must resolve the address before connecting to it.
        InetSocketAddress proxy = Utils.resolveAddress(request.proxy());
        HttpConnection c = null;
        boolean secure = request.secure();
        ConnectionPool pool = client.connectionPool();

        if (!secure) {
            c = pool.getConnection(false, addr, proxy);
            if (c != null && c.isOpen() /* may have been eof/closed when in the pool */) {
                final HttpConnection conn = c;
                if (DEBUG_LOGGER.on())
                    DEBUG_LOGGER.log(conn.getConnectionFlow()
                                     + ": plain connection retrieved from HTTP/1.1 pool");
                return c;
            } else {
                return getPlainConnection(addr, proxy, request, client);
            }
        } else {  // secure
            if (version != HTTP_2) { // only HTTP/1.1 connections are in the pool
                c = pool.getConnection(true, addr, proxy);
            }
            if (c != null && c.isOpen()) {
                final HttpConnection conn = c;
                if (DEBUG_LOGGER.on())
                    DEBUG_LOGGER.log(conn.getConnectionFlow()
                                     + ": SSL connection retrieved from HTTP/1.1 pool");
                return c;
            } else {
                String[] alpn = null;
                if (version == HTTP_2 && hasRequiredHTTP2TLSVersion(client)) {
                    alpn = new String[] { "h2", "http/1.1" };
                }
                return getSSLConnection(addr, proxy, alpn, request, client);
            }
        }
    }

這里非https、https1.1的，走pool.getConnection(true, addr, proxy)

java.net.http/jdk/internal/net/http/HttpConnection.java

    void closeOrReturnToCache(HttpHeaders hdrs) {
        if (hdrs == null) {
            // the connection was closed by server, eof
            close();
            return;
        }
        if (!isOpen()) {
            return;
        }
        HttpClientImpl client = client();
        if (client == null) {
            close();
            return;
        }
        ConnectionPool pool = client.connectionPool();
        boolean keepAlive = hdrs.firstValue("Connection")
                .map((s) -> !s.equalsIgnoreCase("close"))
                .orElse(true);

        if (keepAlive) {
            Log.logTrace("Returning connection to the pool: {0}", this);
            pool.returnToPool(this);
        } else {
            close();
        }
    }

調用pool.returnToPool(this)歸還連接

java.net.http/jdk/internal/net/http/ConnectionPool.java

/**
 * Http 1.1 connection pool.
 */
final class ConnectionPool {

    static final long KEEP_ALIVE = Utils.getIntegerNetProperty(
            "jdk.httpclient.keepalive.timeout", 1200); // seconds
    static final long MAX_POOL_SIZE = Utils.getIntegerNetProperty(
            "jdk.httpclient.connectionPoolSize", 0); // unbounded
    final Logger debug = Utils.getDebugLogger(this::dbgString, Utils.DEBUG);

    // Pools of idle connections

    private final HashMap> plainPool;
    private final HashMap> sslPool;
    private final ExpiryList expiryList;
    private final String dbgTag; // used for debug
    boolean stopped;

    //......
    /**
     * Entries in connection pool are keyed by destination address and/or
     * proxy address:
     * case 1: plain TCP not via proxy (destination only)
     * case 2: plain TCP via proxy (proxy only)
     * case 3: SSL not via proxy (destination only)
     * case 4: SSL over tunnel (destination and proxy)
     */
    static class CacheKey {
        final InetSocketAddress proxy;
        final InetSocketAddress destination;

        CacheKey(InetSocketAddress destination, InetSocketAddress proxy) {
            this.proxy = proxy;
            this.destination = destination;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final CacheKey other = (CacheKey) obj;
            if (!Objects.equals(this.proxy, other.proxy)) {
                return false;
            }
            if (!Objects.equals(this.destination, other.destination)) {
                return false;
            }
            return true;
        }

        @Override
        public int hashCode() {
            return Objects.hash(proxy, destination);
        }
    }

    synchronized HttpConnection getConnection(boolean secure,
                                              InetSocketAddress addr,
                                              InetSocketAddress proxy) {
        if (stopped) return null;
        CacheKey key = new CacheKey(addr, proxy);
        HttpConnection c = secure ? findConnection(key, sslPool)
                                  : findConnection(key, plainPool);
        //System.out.println ("getConnection returning: " + c);
        return c;
    }

    private HttpConnection
    findConnection(CacheKey key,
                   HashMap> pool) {
        LinkedList l = pool.get(key);
        if (l == null || l.isEmpty()) {
            return null;
        } else {
            HttpConnection c = l.removeFirst();
            expiryList.remove(c);
            return c;
        }
    }
    /**
     * Returns the connection to the pool.
     */
    void returnToPool(HttpConnection conn) {
        returnToPool(conn, Instant.now(), KEEP_ALIVE);
    }

    // Called also by whitebox tests
    void returnToPool(HttpConnection conn, Instant now, long keepAlive) {

        // Don"t call registerCleanupTrigger while holding a lock,
        // but register it before the connection is added to the pool,
        // since we don"t want to trigger the cleanup if the connection
        // is not in the pool.
        CleanupTrigger cleanup = registerCleanupTrigger(conn);

        // it"s possible that cleanup may have been called.
        HttpConnection toClose = null;
        synchronized(this) {
            if (cleanup.isDone()) {
                return;
            } else if (stopped) {
                conn.close();
                return;
            }
            if (MAX_POOL_SIZE > 0 && expiryList.size() >= MAX_POOL_SIZE) {
                toClose = expiryList.removeOldest();
                if (toClose != null) removeFromPool(toClose);
            }
            if (conn instanceof PlainHttpConnection) {
                putConnection(conn, plainPool);
            } else {
                assert conn.isSecure();
                putConnection(conn, sslPool);
            }
            expiryList.add(conn, now, keepAlive);
        }
        if (toClose != null) {
            if (debug.on()) {
                debug.log("Maximum pool size reached: removing oldest connection %s",
                          toClose.dbgString());
            }
            close(toClose);
        }
        //System.out.println("Return to pool: " + conn);
    }

    private void removeFromPool(HttpConnection c) {
        assert Thread.holdsLock(this);
        if (c instanceof PlainHttpConnection) {
            removeFromPool(c, plainPool);
        } else {
            assert c.isSecure();
            removeFromPool(c, sslPool);
        }
    }

    private boolean
    removeFromPool(HttpConnection c,
                   HashMap> pool) {
        //System.out.println("cacheCleaner removing: " + c);
        assert Thread.holdsLock(this);
        CacheKey k = c.cacheKey();
        List l = pool.get(k);
        if (l == null || l.isEmpty()) {
            pool.remove(k);
            return false;
        }
        return l.remove(c);
    }

    private void
    putConnection(HttpConnection c,
                  HashMap> pool) {
        CacheKey key = c.cacheKey();
        LinkedList l = pool.get(key);
        if (l == null) {
            l = new LinkedList<>();
            pool.put(key, l);
        }
        l.add(c);
    }

    void stop() {
        List closelist = Collections.emptyList();
        try {
            synchronized (this) {
                stopped = true;
                closelist = expiryList.stream()
                    .map(e -> e.connection)
                    .collect(Collectors.toList());
                expiryList.clear();
                plainPool.clear();
                sslPool.clear();
            }
        } finally {
            closelist.forEach(this::close);
        }
    }
}

借用連接調用getConnection方法，最后是調用findConnection方法，從LinkedList>移除掉第一個，再從expiryList移除掉該連接

歸還連接調用returnToPool方法，如果當前expiryList超出MAX_POOL_SIZE，則移除掉最老的一個，再將其從ExpiryList、HashMap>移除并且close掉；之后調用putConnection往HashMap>添加該連接，最后再往expiryList添加該連接

可以看見ConnectionPool維護了HashMap>以及ExpiryList兩個重要的屬性，借用時從這兩個地方移除，歸還時往這兩個地方添加；不一樣的是歸還時如果MAX_POOL_SIZE大于0，則會對expiryList大小進行判斷，超過最大值則移除最老的連接，并將其從這兩個地方移除掉

MAX_POOL_SIZE讀取的是jdk.httpclient.connectionPoolSize，讀取不到默認為0，表示無限

ConnectionPool有個stop方法，在HttpClient的stop時候調用(SelectorManager線程退出時觸發)，stop方法會清除連接池并關閉連接

java.net.http/jdk/internal/net/http/ConnectionPool.java

    /**
     * Manages a LinkedList of sorted ExpiryEntry. The entry with the closer
     * deadline is at the tail of the list, and the entry with the farther
     * deadline is at the head. In the most common situation, new elements
     * will need to be added at the head (or close to it), and expired elements
     * will need to be purged from the tail.
     */
    private static final class ExpiryList {
        private final LinkedList list = new LinkedList<>();
        private volatile boolean mayContainEntries;

        int size() { return list.size(); }

        // A loosely accurate boolean whose value is computed
        // at the end of each operation performed on ExpiryList;
        // Does not require synchronizing on the ConnectionPool.
        boolean purgeMaybeRequired() {
            return mayContainEntries;
        }

        // Returns the next expiry deadline
        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        Optional nextExpiryDeadline() {
            if (list.isEmpty()) return Optional.empty();
            else return Optional.of(list.getLast().expiry);
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        HttpConnection removeOldest() {
            ExpiryEntry entry = list.pollLast();
            return entry == null ? null : entry.connection;
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        void add(HttpConnection conn) {
            add(conn, Instant.now(), KEEP_ALIVE);
        }

        // Used by whitebox test.
        void add(HttpConnection conn, Instant now, long keepAlive) {
            Instant then = now.truncatedTo(ChronoUnit.SECONDS)
                    .plus(keepAlive, ChronoUnit.SECONDS);

            // Elements with the farther deadline are at the head of
            // the list. It"s more likely that the new element will
            // have the farthest deadline, and will need to be inserted
            // at the head of the list, so we"re using an ascending
            // list iterator to find the right insertion point.
            ListIterator li = list.listIterator();
            while (li.hasNext()) {
                ExpiryEntry entry = li.next();

                if (then.isAfter(entry.expiry)) {
                    li.previous();
                    // insert here
                    li.add(new ExpiryEntry(conn, then));
                    mayContainEntries = true;
                    return;
                }
            }
            // last (or first) element of list (the last element is
            // the first when the list is empty)
            list.add(new ExpiryEntry(conn, then));
            mayContainEntries = true;
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        void remove(HttpConnection c) {
            if (c == null || list.isEmpty()) return;
            ListIterator li = list.listIterator();
            while (li.hasNext()) {
                ExpiryEntry e = li.next();
                if (e.connection.equals(c)) {
                    li.remove();
                    mayContainEntries = !list.isEmpty();
                    return;
                }
            }
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool.
        // Purge all elements whose deadline is before now (now included).
        List purgeUntil(Instant now) {
            if (list.isEmpty()) return Collections.emptyList();

            List closelist = new ArrayList<>();

            // elements with the closest deadlines are at the tail
            // of the queue, so we"re going to use a descending iterator
            // to remove them, and stop when we find the first element
            // that has not expired yet.
            Iterator li = list.descendingIterator();
            while (li.hasNext()) {
                ExpiryEntry entry = li.next();
                // use !isAfter instead of isBefore in order to
                // remove the entry if its expiry == now
                if (!entry.expiry.isAfter(now)) {
                    li.remove();
                    HttpConnection c = entry.connection;
                    closelist.add(c);
                } else break; // the list is sorted
            }
            mayContainEntries = !list.isEmpty();
            return closelist;
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        java.util.stream.Stream stream() {
            return list.stream();
        }

        // should only be called while holding a synchronization
        // lock on the ConnectionPool
        void clear() {
            list.clear();
            mayContainEntries = false;
        }
    }

    static final class ExpiryEntry {
        final HttpConnection connection;
        final Instant expiry; // absolute time in seconds of expiry time
        ExpiryEntry(HttpConnection connection, Instant expiry) {
            this.connection = connection;
            this.expiry = expiry;
        }
    }

ExpiryList內部使用了LinkedList，而且使用ExpiryEntry對connection進行包裝

ExpiryEntry里頭除了HttpConnection，還維護了expiry時間，表示該連接的失效時間

對ExpiryList的添加操作是根據當前時間的秒數+KEEP_ALIVE參數計算出expiry時間，KEEP_ALIVE讀取的是jdk.httpclient.keepalive.timeout，讀取不到默認是1200秒；之后根據失效時間插入到LinkedList，失效時間長的在list頭部，快失效的在list尾部

對ExpiryList的移除操作有兩類，一類是移除最老的，通過pollLast操作完成，一類是移除指定連接，即使用ListIterator遍歷LinkedList進行匹配再移除

這里維護了mayContainEntries變量，在LinkedList進行操作時更新，用于返回ExpiryList是否有連接，避免需要時同步調用ConnectionPool來計算

java.net.http/jdk/internal/net/http/ConnectionPool.java

   /**
     * Purge expired connection and return the number of milliseconds
     * in which the next connection is scheduled to expire.
     * If no connections are scheduled to be purged return 0.
     * @return the delay in milliseconds in which the next connection will
     *         expire.
     */
    long purgeExpiredConnectionsAndReturnNextDeadline() {
        if (!expiryList.purgeMaybeRequired()) return 0;
        return purgeExpiredConnectionsAndReturnNextDeadline(Instant.now());
    }

    // Used for whitebox testing
    long purgeExpiredConnectionsAndReturnNextDeadline(Instant now) {
        long nextPurge = 0;

        // We may be in the process of adding new elements
        // to the expiry list - but those elements will not
        // have outlast their keep alive timer yet since we"re
        // just adding them.
        if (!expiryList.purgeMaybeRequired()) return nextPurge;

        List closelist;
        synchronized (this) {
            closelist = expiryList.purgeUntil(now);
            for (HttpConnection c : closelist) {
                if (c instanceof PlainHttpConnection) {
                    boolean wasPresent = removeFromPool(c, plainPool);
                    assert wasPresent;
                } else {
                    boolean wasPresent = removeFromPool(c, sslPool);
                    assert wasPresent;
                }
            }
            nextPurge = now.until(
                    expiryList.nextExpiryDeadline().orElse(now),
                    ChronoUnit.MILLIS);
        }
        closelist.forEach(this::close);
        return nextPurge;
    }

由于ExpiryList的connection具有失效時間，因而存在清理失效連接的步驟，這個步驟是通過purgeExpiredConnectionsAndReturnNextDeadline來完成

purgeExpiredConnectionsAndReturnNextDeadline方法被SelectorManager調用，用于計算selector.select的timeout時間

該方法首先調用expiryList.purgeMaybeRequired()訪問mayContainEntries，看expiryList有無連接，沒有連接直接返回0；之后調用expiryList.purgeUntil(now)移除并獲取目前過期的連接，然后挨個從HashMap>移除并計算nextPurge，最后挨個close掉移除的連接

java.net.http/jdk/internal/net/http/ConnectionPool.java

    private CleanupTrigger registerCleanupTrigger(HttpConnection conn) {
        // Connect the connection flow to a pub/sub pair that will take the
        // connection out of the pool and close it if anything happens
        // while the connection is sitting in the pool.
        CleanupTrigger cleanup = new CleanupTrigger(conn);
        FlowTube flow = conn.getConnectionFlow();
        if (debug.on()) debug.log("registering %s", cleanup);
        flow.connectFlows(cleanup, cleanup);
        return cleanup;
    }

    void cleanup(HttpConnection c, Throwable error) {
        if (debug.on())
            debug.log("%s : ConnectionPool.cleanup(%s)",
                    String.valueOf(c.getConnectionFlow()), error);
        synchronized(this) {
            removeFromPool(c);
            expiryList.remove(c);
        }
        c.close();
    }

    /**
     * An object that subscribes to the flow while the connection is in
     * the pool. Anything that comes in will cause the connection to be closed
     * and removed from the pool.
     */
    private final class CleanupTrigger implements
            FlowTube.TubeSubscriber, FlowTube.TubePublisher,
            Flow.Subscription {

        private final HttpConnection connection;
        private volatile boolean done;

        public CleanupTrigger(HttpConnection connection) {
            this.connection = connection;
        }

        public boolean isDone() { return done;}

        private void triggerCleanup(Throwable error) {
            done = true;
            cleanup(connection, error);
        }

        @Override public void request(long n) {}
        @Override public void cancel() {}

        @Override
        public void onSubscribe(Flow.Subscription subscription) {
            subscription.request(1);
        }
        @Override
        public void onError(Throwable error) { triggerCleanup(error); }
        @Override
        public void onComplete() { triggerCleanup(null); }
        @Override
        public void onNext(List item) {
            triggerCleanup(new IOException("Data received while in pool"));
        }

        @Override
        public void subscribe(Flow.Subscriber> subscriber) {
            subscriber.onSubscribe(this);
        }

        @Override
        public String toString() {
            return "CleanupTrigger(" + connection.getConnectionFlow() + ")";
        }
    }

在調用returnToPool的時候，會調用registerCleanupTrigger，創建一個CleanupTrigger，然后調用conn.getConnectionFlow()獲取flow，再調用flow.connectFlows(cleanup, cleanup)

CleanupTrigger既是FlowTube.TubeSubscriber也是FlowTube.TubePublisher，在onComplete及onError方法里頭調用了cleanup方法，將連接從HashMap>及expiryList移除

這個CleanupTrigger的功能可能類似于主動式的連接健康檢查，在底層連接發生異常關閉的時候，通知到連接池這邊，觸發清理這些臟的連接

jdk httpclient的ConnectionPool相對于apache common pools而言比較簡單，有幾個參數(實際作用于ExpiryList)：

MAX_POOL_SIZE(jdk.httpclient.connectionPoolSize)，默認為0，表示無限
KEEP_ALIVE(jdk.httpclient.keepalive.timeout)，默認是1200秒

ConnectionPool同時維護了兩個屬性：HashMap>及expiryList，前者使用目標ip地址及代理地址作為CacheKey，每個地址維護一個連接池；后者不分cacheKey，對每個在連接池中的connection進行包裝，根據KEEP_ALIVE記錄了失效時間。

SelectorManager調用purgeExpiredConnectionsAndReturnNextDeadline計算select的timeout時間，這個方法會清理(移除并close)過期的連接

除了SelectorManager清理過期的連接外，connection還通過FlowTube間接觸發CleanupTrigger，去清理關閉或異常的連接

java.net.http javadoc