Add check if sun.misc.Unsafe is present but throws on use, to use the preexisting no-Unsafe paths if it does.

java/core/src/main/java/com/google/protobuf/UnsafeUtil.java

@@ -284,7 +284,30 @@ public sun.misc.Unsafe run() throws Exception {
       // Catching Throwable here due to the fact that Google AppEngine raises NoClassDefFoundError
       // for Unsafe.
     }
-    return unsafe;
+
+    if (unsafe == null) {
+      return null;
+    }
+
+    // Unsafe is terminally deprecated and will be removed in a future version. Our runtime is
+    // anyway compatible with environments where Unsafe is not available, but as part of the removal
+    // some environments will have Unsafe available but throw on use, which would break us. To check
+    // if the JDK is in this state, simply try using a method and see if it does throw.
+    try {
+      int unused = unsafe.arrayBaseOffset(byte[].class);
+      return unsafe;
+    } catch (Exception unused) {
+      Logger.getLogger(UnsafeUtil.class.getName())
+          .log(
+              Level.WARNING,
+              "As part of the planned removal, sun.misc.Unsafe is available in the current"
+                  + " environment but configured to throw on use. Protobuf will continue without"
+                  + " using it, but with slightly reduced performance."
+                  + " --sun-misc-unsafe-memory-access=allow is likely available to opt back in if"
+                  + " desired. A later Protobuf version release will stop using sun.misc.Unsafe"
+                  + " entirely.");
+      return null;
+    }
   }
 
   /** Get a {@link MemoryAccessor} appropriate for the platform, or null if not supported. */