Replace synchronized usage with ReentrantLock

Closes gh-702

spring-vault-core/src/main/java/org/springframework/vault/authentication/LifecycleAwareSessionManager.java

@@ -18,6 +18,7 @@ package org.springframework.vault.authentication;
 import java.time.Duration;
 import java.util.Map;
 import java.util.Optional;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.http.HttpEntity;
@@ -81,7 +82,7 @@ public class LifecycleAwareSessionManager extends LifecycleAwareSessionManagerSu
 	 */
 	private final RestOperations restOperations;
 
-	private final Object lock = new Object();
+	private final ReentrantLock lock = new ReentrantLock();
 
 	/**
 	 * The token state: Contains the currently valid token that identifies the Vault
@@ -254,12 +255,15 @@ public class LifecycleAwareSessionManager extends LifecycleAwareSessionManagerSu
 
 		if (!getToken().isPresent()) {
 
-			synchronized (this.lock) {
-
+			this.lock.lock();
+			try {
 				if (!getToken().isPresent()) {
 					doGetSessionToken();
 				}
 			}
+			finally {
+				this.lock.unlock();
+			}
 		}
 
 		return getToken().map(TokenWrapper::getToken)

spring-vault-core/src/main/java/org/springframework/vault/authentication/SimpleSessionManager.java

@@ -16,6 +16,7 @@
 package org.springframework.vault.authentication;
 
 import java.util.Optional;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.util.Assert;
 import org.springframework.vault.support.VaultToken;
@@ -34,7 +35,7 @@ public class SimpleSessionManager implements SessionManager {
 
 	private final ClientAuthentication clientAuthentication;
 
-	private final Object lock = new Object();
+	private final ReentrantLock lock = new ReentrantLock();
 
 	private volatile Optional<VaultToken> token = Optional.empty();
 
@@ -53,11 +54,16 @@ public class SimpleSessionManager implements SessionManager {
 	public VaultToken getSessionToken() {
 
 		if (!this.token.isPresent()) {
-			synchronized (this.lock) {
+
+			this.lock.lock();
+			try {
 				if (!this.token.isPresent()) {
 					this.token = Optional.of(this.clientAuthentication.login());
 				}
 			}
+			finally {
+				this.lock.unlock();
+			}
 		}
 
 		return this.token.orElseThrow(() -> new IllegalStateException("Cannot obtain VaultToken"));

spring-vault-core/src/main/java/org/springframework/vault/core/env/VaultPropertySource.java

@@ -18,6 +18,7 @@ package org.springframework.vault.core.env;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -60,7 +61,7 @@ public class VaultPropertySource extends EnumerablePropertySource<VaultOperation
 
 	private final boolean ignoreSecretNotFound;
 
-	private final Object lock = new Object();
+	private final ReentrantLock lock = new ReentrantLock();
 
 	/**
 	 * Create a new {@link VaultPropertySource} given a {@link VaultTemplate} and
@@ -141,7 +142,9 @@ public class VaultPropertySource extends EnumerablePropertySource<VaultOperation
 	 */
 	protected void loadProperties() {
 
-		synchronized (this.lock) {
+		this.lock.lock();
+
+		try {
 			if (logger.isDebugEnabled()) {
 				logger.debug(String.format("Fetching properties from Vault at %s", this.path));
 			}
@@ -176,6 +179,9 @@ public class VaultPropertySource extends EnumerablePropertySource<VaultOperation
 				this.properties.putAll(doTransformProperties(properties));
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	@Override