Summary

Update MethodHandle to be a sealed class.

Problem

The MethodHandle class is not intended to have user-defined subclasses and sealing is an explicit way to indicate this.

Solution

Update the class to be declared 'sealed`. Other needed changes are implementation-only and not part of the user-visible API change listed in this CSR.

Specification

--- a/src/java.base/share/classes/java/lang/invoke/MethodHandle.java
+++ b/src/java.base/share/classes/java/lang/invoke/MethodHandle.java
@@ -440,7 +440,9 @@ mh.invokeExact(System.out, "Hello, world.");
  * @author John Rose, JSR 292 EG
  * @since 1.7
  */
-public abstract class MethodHandle implements Constable {
+public abstract sealed class MethodHandle implements Constable
+    permits NativeMethodHandle, DirectMethodHandle,
+            DelegatingMethodHandle, BoundMethodHandle {

     /**
      * Internal marker interface which distinguishes (to the Java compiler)