Avoid synchronizing on this and use an internal monitor instead

Where possible, code that previously synchronized on this (or on the
class in the case of static methods) has been updated to use an
internal monitor object instead. This allows the locking model that's
employed to be an implementation detail rather than part of the
class's API.

Classes that override a synchronized method continue to declare
the overriding method as synchronized. This ensures that locking
is consistent across the superclass and its subclass.

Closes gh-6262

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/audit/InMemoryAuditEventRepository.java

@@ -33,6 +33,8 @@ public class InMemoryAuditEventRepository implements AuditEventRepository {
 
 	private static final int DEFAULT_CAPACITY = 4000;
 
+	private final Object monitor = new Object();
+
 	/**
 	 * Circular buffer of the event with tail pointing to the last element.
 	 */
@@ -52,34 +54,40 @@ public class InMemoryAuditEventRepository implements AuditEventRepository {
 	 * Set the capacity of this event repository.
 	 * @param capacity the capacity
 	 */
-	public synchronized void setCapacity(int capacity) {
-		this.events = new AuditEvent[capacity];
+	public void setCapacity(int capacity) {
+		synchronized (this.monitor) {
+			this.events = new AuditEvent[capacity];
+		}
 	}
 
 	@Override
-	public synchronized void add(AuditEvent event) {
+	public void add(AuditEvent event) {
 		Assert.notNull(event, "AuditEvent must not be null");
-		this.tail = (this.tail + 1) % this.events.length;
-		this.events[this.tail] = event;
+		synchronized (this.monitor) {
+			this.tail = (this.tail + 1) % this.events.length;
+			this.events[this.tail] = event;
+		}
 	}
 
 	@Override
-	public synchronized List<AuditEvent> find(Date after) {
+	public List<AuditEvent> find(Date after) {
 		return find(null, after, null);
 	}
 
 	@Override
-	public synchronized List<AuditEvent> find(String principal, Date after) {
+	public List<AuditEvent> find(String principal, Date after) {
 		return find(principal, after, null);
 	}
 
 	@Override
-	public synchronized List<AuditEvent> find(String principal, Date after, String type) {
+	public List<AuditEvent> find(String principal, Date after, String type) {
 		LinkedList<AuditEvent> events = new LinkedList<AuditEvent>();
-		for (int i = 0; i < this.events.length; i++) {
-			AuditEvent event = resolveTailEvent(i);
-			if (event != null && isMatch(principal, after, type, event)) {
-				events.addFirst(event);
+		synchronized (this.events) {
+			for (int i = 0; i < this.events.length; i++) {
+				AuditEvent event = resolveTailEvent(i);
+				if (event != null && isMatch(principal, after, type, event)) {
+					events.addFirst(event);
+				}
 			}
 		}
 		return events;

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/health/MailHealthIndicatorTests.java

@@ -88,8 +88,8 @@ public class MailHealthIndicatorTests {
 		}
 
 		@Override
-		public synchronized void connect(String host, int port, String user,
-				String password) throws MessagingException {
+		public void connect(String host, int port, String user, String password)
+				throws MessagingException {
 		}
 
 		@Override