diff --git a/src/main/java/org/jenkins/plugins/lockableresources/LockableResourcesManager.java b/src/main/java/org/jenkins/plugins/lockableresources/LockableResourcesManager.java
index 29ee75fac..9d16e36af 100644
--- a/src/main/java/org/jenkins/plugins/lockableresources/LockableResourcesManager.java
+++ b/src/main/java/org/jenkins/plugins/lockableresources/LockableResourcesManager.java
@@ -161,13 +161,15 @@ public List<LockableResource> getReadOnlyResources() {
     /** Get declared resources, means only defined in config file (xml or JCaC yaml). */
     @Restricted(NoExternalUse.class)
     public List<LockableResource> getDeclaredResources() {
-        ArrayList<LockableResource> declaredResources = new ArrayList<>();
-        for (LockableResource r : this.getResources()) {
-            if (!r.isEphemeral() && !r.isNodeResource()) {
-                declaredResources.add(r);
+        synchronized (syncResources) {
+            ArrayList<LockableResource> declaredResources = new ArrayList<>();
+            for (LockableResource r : this.resources) {
+                if (!r.isEphemeral() && !r.isNodeResource()) {
+                    declaredResources.add(r);
+                }
             }
+            return declaredResources;
         }
-        return declaredResources;
     }
 
     // ---------------------------------------------------------------------------
@@ -231,14 +233,16 @@ public void setDeclaredResources(List<LockableResource> declaredResources) {
     /** Get all resources used by project. */
     @Restricted(NoExternalUse.class)
     public List<LockableResource> getResourcesFromProject(String fullName) {
-        List<LockableResource> matching = new ArrayList<>();
-        for (LockableResource r : this.getResources()) {
-            String rName = r.getQueueItemProject();
-            if (rName != null && rName.equals(fullName)) {
-                matching.add(r);
+        synchronized (syncResources) {
+            List<LockableResource> matching = new ArrayList<>();
+            for (LockableResource r : this.resources) {
+                String rName = r.getQueueItemProject();
+                if (rName != null && rName.equals(fullName)) {
+                    matching.add(r);
+                }
             }
+            return matching;
         }
-        return matching;
     }
 
     // ---------------------------------------------------------------------------