Add maxSessions, getSessions, removeExpiredSessions

This commit removes the session threshold check added recently which
is not effective since maxIdleTime is usually much longer than the
frequency of checks. The lazy triggering of expiration checks during
create or retreive are simple and the most effective

This commit also adds a maxSessions limit on the total number of
sessions that can be created at any one time, a getSessions method
for management purposes, and a removeExpiredSessions public API
for manual triggering of expiration checks.

Issue: SPR-17020, SPR-16713

spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java

@@ -21,10 +21,10 @@ import java.time.Duration;
 import java.time.Instant;
 import java.time.ZoneId;
 import java.time.temporal.ChronoUnit;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.concurrent.locks.ReentrantLock;
 
@@ -47,13 +47,35 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 	private static final IdGenerator idGenerator = new JdkIdGenerator();
 
 
+	private int maxSessions = 10000;
+
 	private Clock clock = Clock.system(ZoneId.of("GMT"));
 
-	private final ConcurrentMap<String, InMemoryWebSession> sessions = new ConcurrentHashMap<>();
+	private final Map<String, InMemoryWebSession> sessions = new ConcurrentHashMap<>();
 
 	private final ExpiredSessionChecker expiredSessionChecker = new ExpiredSessionChecker();
 
 
+	/**
+	 * Set the maximum number of sessions that can be stored. Once the limit is
+	 * reached, any attempt to store an additional session will result in an
+	 * {@link IllegalStateException}.
+	 * <p>By default set to 10000.
+	 * @param maxSessions the maximum number of sessions
+	 * @since 5.1
+	 */
+	public void setMaxSessions(int maxSessions) {
+		this.maxSessions = maxSessions;
+	}
+
+	/**
+	 * Return the maximum number of sessions that can be stored.
+	 * @since 5.1
+	 */
+	public int getMaxSessions() {
+		return this.maxSessions;
+	}
+
 	/**
 	 * Configure the {@link Clock} to use to set lastAccessTime on every created
 	 * session and to calculate if it is expired.
@@ -66,7 +88,7 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 	public void setClock(Clock clock) {
 		Assert.notNull(clock, "Clock is required");
 		this.clock = clock;
-		this.expiredSessionChecker.removeExpiredSessions(clock.instant());
+		removeExpiredSessions();
 	}
 
 	/**
@@ -76,6 +98,16 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 		return this.clock;
 	}
 
+	/**
+	 * Return the map of sessions with an {@link Collections#unmodifiableMap
+	 * unmodifiable} wrapper. This could be used for management purposes, to
+	 * list active sessions, invalidate expired ones, etc.
+	 * @since 5.1
+	 */
+	public Map<String, InMemoryWebSession> getSessions() {
+		return Collections.unmodifiableMap(this.sessions);
+	}
+
 
 	@Override
 	public Mono<WebSession> createWebSession() {
@@ -108,15 +140,25 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 		return Mono.empty();
 	}
 
-	public Mono<WebSession> updateLastAccessTime(WebSession webSession) {
+	public Mono<WebSession> updateLastAccessTime(WebSession session) {
 		return Mono.fromSupplier(() -> {
-			Assert.isInstanceOf(InMemoryWebSession.class, webSession);
-			InMemoryWebSession session = (InMemoryWebSession) webSession;
-			session.updateLastAccessTime(getClock().instant());
+			Assert.isInstanceOf(InMemoryWebSession.class, session);
+			((InMemoryWebSession) session).updateLastAccessTime(this.clock.instant());
 			return session;
 		});
 	}
 
+	/**
+	 * Check for expired sessions and remove them. Typically such checks are
+	 * kicked off lazily during calls to {@link #createWebSession() create} or
+	 * {@link #retrieveSession retrieve}, no less than 60 seconds apart.
+	 * This method can be called to force a check at a specific time.
+	 * @since 5.1
+	 */
+	public void removeExpiredSessions() {
+		this.expiredSessionChecker.removeExpiredSessions(this.clock.instant());
+	}
+
 
 	private class InMemoryWebSession implements WebSession {
 
@@ -198,6 +240,12 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 		@Override
 		public Mono<Void> save() {
+			if (sessions.size() >= maxSessions) {
+				expiredSessionChecker.removeExpiredSessions(clock.instant());
+				if (sessions.size() >= maxSessions) {
+					return Mono.error(new IllegalStateException("Max sessions limit reached: " + sessions.size()));
+				}
+			}
 			if (!getAttributes().isEmpty()) {
 				this.state.compareAndSet(State.NEW, State.STARTED);
 			}
@@ -207,14 +255,14 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 		@Override
 		public boolean isExpired() {
-			return isExpired(Instant.now(getClock()));
+			return isExpired(clock.instant());
 		}
 
-		private boolean isExpired(Instant currentTime) {
+		private boolean isExpired(Instant now) {
 			if (this.state.get().equals(State.EXPIRED)) {
 				return true;
 			}
-			if (checkExpired(currentTime)) {
+			if (checkExpired(now)) {
 				this.state.set(State.EXPIRED);
 				return true;
 			}
@@ -234,30 +282,21 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 	private class ExpiredSessionChecker {
 
-		/** Max time before next expiration checks. */
-		private static final int CHECK_PERIOD = 60;
-
-		/** Max sessions that can be created before next expiration checks. */
-		private static final int SESSION_COUNT_THRESHOLD = 500;
+		/** Max time between expiration checks. */
+		private static final int CHECK_PERIOD = 60 * 1000;
 
 
 		private final ReentrantLock lock = new ReentrantLock();
 
-		private Instant nextCheckTime = Instant.now(clock).plus(CHECK_PERIOD, ChronoUnit.SECONDS);
-
-		private long lastSessionCount;
+		private Instant checkTime = clock.instant().plus(CHECK_PERIOD, ChronoUnit.MILLIS);
 
 
 		public void checkIfNecessary(Instant now) {
-			if (howManyCreated() > SESSION_COUNT_THRESHOLD || this.nextCheckTime.isBefore(now)) {
-				removeExpiredSessions(Instant.now(clock));
+			if (this.checkTime.isBefore(now)) {
+				removeExpiredSessions(now);
 			}
 		}
 
-		private long howManyCreated() {
-			return sessions.size() - this.lastSessionCount;
-		}
-
 		public void removeExpiredSessions(Instant now) {
 			if (sessions.isEmpty()) {
 				return;
@@ -274,8 +313,7 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 					}
 				}
 				finally {
-					this.nextCheckTime = clock.instant().plus(CHECK_PERIOD, ChronoUnit.SECONDS);
-					this.lastSessionCount = sessions.size();
+					this.checkTime = now.plus(CHECK_PERIOD, ChronoUnit.MILLIS);
 					this.lock.unlock();
 				}
 			}