Java: note that classes with entirely private constructors can't be subclassed

java/ql/src/Likely Bugs/Concurrency/StartInConstructor.ql

@@ -15,6 +15,10 @@
 
 import java
 
+private predicate hasASubclass(RefType t) {
+  exists(RefType sub | sub != t | sub.getAnAncestor() = t)
+}
+
 /**
  * Holds if this type is either `final` or
  * `private` and without subtypes.
@@ -24,7 +28,11 @@ private predicate cannotBeExtended(RefType t) {
   or
   // If the class is private, all possible subclasses are known.
   t.isPrivate() and
-  not exists(RefType sub | sub != t | sub.getAnAncestor() = t)
+  not hasASubclass(t)
+  or
+  // If the class only has private constructors, all possible subclasses are known.
+  forex(Constructor c | c.getDeclaringType() = t | c.isPrivate()) and
+  not hasASubclass(t)
 }
 
 from MethodCall m, Constructor c, Class clazz

java/ql/test/query-tests/StartInConstructor/Test.java

@@ -30,4 +30,18 @@ public Private() {
 		}
 
 	}
-}
+
+	public static class AllPrivateConstructors {
+		Thread myThread;
+
+		private AllPrivateConstructors() {
+			myThread = new Thread("myThread");
+			// OK - class cannot be extended outside this file, and is not in fact extended
+			myThread.start();
+		}
+
+		public static AllPrivateConstructors create() {
+			return new AllPrivateConstructors();
+		}
+	}
+}