diff --git a/core/src/main/java/hudson/ExtensionList.java b/core/src/main/java/hudson/ExtensionList.java
index 262327f05b..2420f05057 100644
--- a/core/src/main/java/hudson/ExtensionList.java
+++ b/core/src/main/java/hudson/ExtensionList.java
@@ -419,6 +419,25 @@ public class ExtensionList<T> extends AbstractList<T> implements OnMaster {
         return j == null ? create((Jenkins) null, type) : j.getExtensionList(type);
     }
 
+    /**
+     * Convenience method allowing lookup of the only instance of a given type.
+     * Equivalent to {@code ExtensionList.lookup(Class).get(Class)} if there is one instance,
+     * and throws an {@code IllegalStateException} otherwise.
+     *
+     * @param type The type to look up.
+     * @return the singleton instance of the given type in its list.
+     * @throws IllegalStateException if there are no instances, or more than one
+     *
+     * @since TODO
+     */
+    public static @Nonnull <U> U lookupSingleton(Class<U> type) {
+        ExtensionList<U> all = lookup(type);
+        if (all.size() != 1) {
+            throw new IllegalStateException("Expected 1 instance of " + type.getName() + " but got " + all.size());
+        }
+        return all.get(0);
+    }
+
     /**
      * Places to store static-scope legacy instances.
      */
diff --git a/core/src/main/java/hudson/PluginManager.java b/core/src/main/java/hudson/PluginManager.java
index 933b747457..c816805ba6 100644
--- a/core/src/main/java/hudson/PluginManager.java
+++ b/core/src/main/java/hudson/PluginManager.java
@@ -1995,8 +1995,8 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
          * Convenience method to ease access to this monitor, this allows other plugins to register required updates.
          * @return this monitor.
          */
-        public static final PluginUpdateMonitor getInstance() {
-            return ExtensionList.lookup(PluginUpdateMonitor.class).get(0);
+        public static PluginUpdateMonitor getInstance() {
+            return ExtensionList.lookupSingleton(PluginUpdateMonitor.class);
         }
 
         /**
diff --git a/core/src/main/java/hudson/model/UpdateSite.java b/core/src/main/java/hudson/model/UpdateSite.java
index 3aded0c135..5771bf1e54 100644
--- a/core/src/main/java/hudson/model/UpdateSite.java
+++ b/core/src/main/java/hudson/model/UpdateSite.java
@@ -1122,15 +1122,9 @@ public class UpdateSite {
         @CheckForNull
         @Restricted(NoExternalUse.class)
         public Set<Warning> getWarnings() {
-            ExtensionList<UpdateSiteWarningsConfiguration> list = ExtensionList.lookup(UpdateSiteWarningsConfiguration.class);
-            if (list.size() == 0) {
-                return Collections.emptySet();
-            }
-
+            UpdateSiteWarningsConfiguration configuration = ExtensionList.lookupSingleton(UpdateSiteWarningsConfiguration.class);
             Set<Warning> warnings = new HashSet<>();
 
-            UpdateSiteWarningsConfiguration configuration = list.get(0);
-
             for (Warning warning: configuration.getAllWarnings()) {
                 if (configuration.isIgnored(warning)) {
                     // warning is currently being ignored
diff --git a/core/src/main/java/hudson/model/User.java b/core/src/main/java/hudson/model/User.java
index 10094bcb02..86ba7b9d17 100644
--- a/core/src/main/java/hudson/model/User.java
+++ b/core/src/main/java/hudson/model/User.java
@@ -1026,11 +1026,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
          */
         @GuardedBy("User.byNameLock")
         static ConcurrentMap<String,User> byName() {
-            ExtensionList<AllUsers> instances = ExtensionList.lookup(AllUsers.class);
-            if (instances.size() != 1) {
-                throw new IllegalStateException();
-            }
-            return instances.get(0).byName;
+            return ExtensionList.lookupSingleton(AllUsers.class).byName;
         }
 
     }
diff --git a/core/src/main/java/jenkins/model/Jenkins.java b/core/src/main/java/jenkins/model/Jenkins.java
index a9a5b1af2d..e3e890151e 100644
--- a/core/src/main/java/jenkins/model/Jenkins.java
+++ b/core/src/main/java/jenkins/model/Jenkins.java
@@ -3717,9 +3717,7 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
         try {
             JSONObject json = req.getSubmittedForm();
 
-            MasterBuildConfiguration mbc = MasterBuildConfiguration.all().get(MasterBuildConfiguration.class);
-            if (mbc!=null)
-                mbc.configure(req,json);
+            ExtensionList.lookupSingleton(MasterBuildConfiguration.class).configure(req,json);
 
             getNodeProperties().rebuild(req, json.optJSONObject("nodeProperties"), NodeProperty.all());
         } finally {
@@ -4528,7 +4526,7 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
     @RestrictedSince("2.37")
     @Deprecated
     public FormValidation doCheckURIEncoding(StaplerRequest request) throws IOException {
-        return ExtensionList.lookup(URICheckEncodingMonitor.class).get(0).doCheckURIEncoding(request);
+        return ExtensionList.lookupSingleton(URICheckEncodingMonitor.class).doCheckURIEncoding(request);
     }
 
     /**
@@ -4538,7 +4536,7 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
     @RestrictedSince("2.37")
     @Deprecated
     public static boolean isCheckURIEncodingEnabled() {
-        return ExtensionList.lookup(URICheckEncodingMonitor.class).get(0).isCheckEnabled();
+        return ExtensionList.lookupSingleton(URICheckEncodingMonitor.class).isCheckEnabled();
     }
 
     /**
diff --git a/core/src/main/java/jenkins/security/UpdateSiteWarningsMonitor.java b/core/src/main/java/jenkins/security/UpdateSiteWarningsMonitor.java
index 83c2828f1b..ef376b0e5b 100644
--- a/core/src/main/java/jenkins/security/UpdateSiteWarningsMonitor.java
+++ b/core/src/main/java/jenkins/security/UpdateSiteWarningsMonitor.java
@@ -121,12 +121,7 @@ public class UpdateSiteWarningsMonitor extends AdministrativeMonitor {
     }
 
     private Set<UpdateSite.Warning> getActiveWarnings() {
-        ExtensionList<UpdateSiteWarningsConfiguration> configurations = ExtensionList.lookup(UpdateSiteWarningsConfiguration.class);
-        if (configurations.isEmpty()) {
-            return Collections.emptySet();
-        }
-        UpdateSiteWarningsConfiguration configuration = configurations.get(0);
-
+        UpdateSiteWarningsConfiguration configuration = ExtensionList.lookupSingleton(UpdateSiteWarningsConfiguration.class);
         HashSet<UpdateSite.Warning> activeWarnings = new HashSet<>();
 
         for (UpdateSite.Warning warning : configuration.getApplicableWarnings()) {
@@ -160,13 +155,7 @@ public class UpdateSiteWarningsMonitor extends AdministrativeMonitor {
      * @return true iff there are applicable but ignored (i.e. hidden) warnings.
      */
     public boolean hasApplicableHiddenWarnings() {
-        ExtensionList<UpdateSiteWarningsConfiguration> configurations = ExtensionList.lookup(UpdateSiteWarningsConfiguration.class);
-        if (configurations.isEmpty()) {
-            return false;
-        }
-
-        UpdateSiteWarningsConfiguration configuration = configurations.get(0);
-
+        UpdateSiteWarningsConfiguration configuration = ExtensionList.lookupSingleton(UpdateSiteWarningsConfiguration.class);
         return getActiveWarnings().size() < configuration.getApplicableWarnings().size();
     }
 
diff --git a/core/src/main/java/jenkins/security/UserDetailsCache.java b/core/src/main/java/jenkins/security/UserDetailsCache.java
index 1d5f221e43..3e5394c205 100644
--- a/core/src/main/java/jenkins/security/UserDetailsCache.java
+++ b/core/src/main/java/jenkins/security/UserDetailsCache.java
@@ -83,7 +83,7 @@ public final class UserDetailsCache {
      * @return the cache
      */
     public static UserDetailsCache get() {
-        return ExtensionList.lookup(UserDetailsCache.class).get(UserDetailsCache.class);
+        return ExtensionList.lookupSingleton(UserDetailsCache.class);
     }
 
     /**