diff --git a/src/java.base/share/classes/java/lang/Module.java b/src/java.base/share/classes/java/lang/Module.java
index 9ca560cfcba..6f4da2219c1 100644
--- a/src/java.base/share/classes/java/lang/Module.java
+++ b/src/java.base/share/classes/java/lang/Module.java
@@ -597,6 +597,14 @@ public final class Module implements AnnotatedElement {
      *
      * <p> This method does not check if the given module reads this module. </p>
      *
+     * @apiNote A package {@code p} opened to module {@code M} allows code in
+     * {@code M} do {@linkplain java.lang.reflect.AccessibleObject#setAccessible(boolean)
+     * deep reflection} on all types in the package.
+     * Further, if {@code M} reads this module, it can obtain a
+     * {@link java.lang.invoke.MethodHandles.Lookup Lookup} object that is allowed to
+     * {@link java.lang.invoke.MethodHandles.Lookup#defineClass(byte[]) define classes}
+     * in package {@code p}.
+     *
      * @param  pn
      *         The package name
      * @param  other
@@ -650,6 +658,14 @@ public final class Module implements AnnotatedElement {
      *
      * <p> This method does not check if the given module reads this module. </p>
      *
+     * @apiNote A package {@code p} opened to module {@code M} allows code in
+     * {@code M} do {@linkplain java.lang.reflect.AccessibleObject#setAccessible(boolean)
+     * deep reflection} on all types in the package.
+     * Further, if {@code M} reads this module, it can obtain a
+     * {@link java.lang.invoke.MethodHandles.Lookup Lookup} object that is allowed to
+     * {@link java.lang.invoke.MethodHandles.Lookup#defineClass(byte[]) define classes}
+     * in package {@code p}.
+     *
      * @param  pn
      *         The package name
      *
@@ -657,6 +673,8 @@ public final class Module implements AnnotatedElement {
      *         unconditionally
      *
      * @see ModuleDescriptor#opens()
+     * @see java.lang.reflect.AccessibleObject#setAccessible(boolean)
+     * @see java.lang.invoke.MethodHandles#privateLookupIn
      */
     public boolean isOpen(String pn) {
         Objects.requireNonNull(pn);
diff --git a/src/java.base/share/classes/java/lang/invoke/MethodHandles.java b/src/java.base/share/classes/java/lang/invoke/MethodHandles.java
index 7795ccadb6c..7199dd59476 100644
--- a/src/java.base/share/classes/java/lang/invoke/MethodHandles.java
+++ b/src/java.base/share/classes/java/lang/invoke/MethodHandles.java
@@ -233,6 +233,12 @@ public class MethodHandles {
      * <p>
      * The resulting {@code Lookup} object has no {@code ORIGINAL} access.
      *
+     * @apiNote The {@code Lookup} object returned by this method is allowed to
+     * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
+     * of {@code targetClass}. Extreme caution should be taken when opening a package
+     * to another module as such defined classes have the same full privilege
+     * access as other members in {@code targetClass}'s module.
+     *
      * @param targetClass the target class
      * @param caller the caller lookup object
      * @return a lookup object for the target class, with private access
@@ -851,7 +857,7 @@ public class MethodHandles {
      * <p>
      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
-     * and create a new {@code Lookup} with <a href="#privacc">private access</a>
+     * and produce a new {@code Lookup} with <a href="#privacc">private access</a>
      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
      * to call {@code privateLookupIn}.
@@ -869,6 +875,12 @@ public class MethodHandles {
      * it cannot be used to obtain another private {@code Lookup} by calling
      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
      * because it has no {@code MODULE} access.
+     * <p>
+     * The {@code Lookup} object returned by {@code privateLookupIn} is allowed to
+     * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
+     * of {@code T}. Extreme caution should be taken when opening a package
+     * to another module as such defined classes have the same full privilege
+     * access as other members in {@code M2}.
      *
      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
      *