[GitHub] [lucene-solr] uschindler commented on a change in pull request #1837: LUCENE-7882: First idea of using Java 15 hidden anonymous classes for Lucene expressions

[GitBox <gi...@apache.org>]

uschindler commented on a change in pull request #1837:
URL: https://github.com/apache/lucene-solr/pull/1837#discussion_r484117905



##########
File path: lucene/expressions/src/java/org/apache/lucene/expressions/js/JavascriptCompiler.java
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@@ -85,7 +88,33 @@
     }
   }
   
-  private static final int CLASSFILE_VERSION = Opcodes.V1_8;
+  /** Method handle to invoke Java 15's way to define hidden classes.
+   * The method handle uses a private lookup of {@link JavascriptCompiler} to define
+   * classes (this ensures we can create classes in our package without extra permissions).
+   * The classes are initialized and have no strong relationship to our classloader.
+   * This ensures they can be unloaded.
+   * Signature of MH: {@code static Lookup defineHiddenClass(byte[] bc)}
+   * The MH is {@code null} if an earlier JDK is used. 
+   * @see "https://openjdk.java.net/jeps/371"
+   */
+  private static final MethodHandle MH_defineHiddenClass;
+  static {
+    final Lookup publicLookup = MethodHandles.publicLookup();
+    MethodHandle mh;
+    try {
+      final Object emptyOptions = Array.newInstance(
+          publicLookup.findClass(Lookup.class.getName().concat("$ClassOption")), 0);
+      mh = publicLookup.findVirtual(Lookup.class, "defineHiddenClass",
+          MethodType.methodType(Lookup.class, byte[].class, boolean.class, emptyOptions.getClass()));
+      mh = mh.bindTo(MethodHandles.lookup()); // private lookup of JavascriptCompiler!
+      mh = MethodHandles.insertArguments(mh.asFixedArity(), 1, true, emptyOptions);
+    } catch (ReflectiveOperationException e) {
+      mh = null;
+    }
+    MH_defineHiddenClass = mh;
+  }
+  
+  private static final int CLASSFILE_VERSION = Opcodes.V11;

Review comment:
       Actually this should have been done on master already! We are on Java 11, so classfile format should be Java 11, too.




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