Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

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 <div align="right"><small>&copy; Copyright 2023 Contributors. All rights reserved.</small></div>

 <h1>AspectJ 1.9.21</h1>

 <p>
  Please note that Bugzilla for issue management is deprecated and new issues should be filed as
  <a href="https://github.com/eclipse/org.aspectj/issues/new">GitHub issues</a>.
  The list of issues addressed for 1.9.21 can be found here:
  <ul>
    <li><a href="https://github.com/eclipse/org.aspectj/issues?q=is%3Aissue+is%3Aclosed++milestone%3A1.9.21">GitHub 1.9.21</a></li>
    <li><a href="https://bugs.eclipse.org/bugs/buglist.cgi?bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&f0=OP&f1=OP&f3=CP&f4=CP&j1=OR&list_id=16866879&product=AspectJ&query_format=advanced&target_milestone=1.9.21">Bugzilla 1.9.21</a></li>
  </ul>
 </p>

 <h2>New features</h2>

 <p>
  AspectJ 1.9.21 supports <a href="https://openjdk.java.net/projects/jdk/21/">Java 21</a>, its final features and a
  subset of preview features, such as:
 </p>
 <ul>
  <li>Record patterns</li>
  <li>Pattern matching for <tt>switch</tt></li>
  <li>Virtual threads</li>
  <li>Sequenced collections</li>
  <li>Structured concurrency (preview)</li>
  <li>String templates (preview)</li>
  <li>Instance main methods (preview)</li>
 </ul>

 <p>
  Unfortunately, even after holding back the AspectJ release for 3 months after JDK 21 general availability, waiting for
  Eclipse JDT Core and the Eclipse Java Compiler (ECJ) to catch up with Java 21 language features, even with the Java 21
  officially supported in Eclipse 2023-12, some preview features are still unimplemented in ECJ:
 </p>
 <ul>
  <li>Unnamed classes (preview)</li>
  <li>Unnamed patterns and variables (preview)</li>
 </ul>

 <p>
  As soon as these preview features are part of the upstream ECJ we depend on, we hope to publish another AspectJ
  release to support them in the AspectJ Compiler (AJC), too.
 </p>

 <h2>Improvements</h2>

 <ul>
  <li>
    In <a href="https://github.com/eclipse-aspectj/aspectj/issues/266">GitHub issue 266</a>, exception cause reporting
    has been improved in <tt>ExtensibleURLClassLoader</tt>. Thanks to Andy Russell (@euclio) for his contribution.
  </li>
 </ul>

 <h2>Other changes and bug fixes</h2>

 <ul>
  <li>
    No major bug fixes
  </li>
 </ul>

 <h2>AspectJ usage hints</h2>

 <h3>AspectJ compiler build system requirements</h3>

 <p>
  Since 1.9.21, the AspectJ compiler <tt>ajc</tt> (contained in the <tt>aspectjtools</tt> library) no longer works on
  JDKs 11 to 16. The minimum compile-time requirement is now JDK 17 due to upstream changes in the Eclipse Java Compiler
  (subset of JDT Core), which AspectJ is a fork of. You can still compile to legacy target versions as low as Java 1.3
  when compiling plain Java code or using plain Java ITD constructs which do not require the AspectJ runtime
  <tt>aspectjrt</tt>, but the compiler itself needs JDK 17+. Just like in previous AspectJ versions, both the runtime
  <tt>aspectjrt</tt> and the load-time weaver <tt>aspectjweaver</tt> still only require JRE 8+.
 </p>
 <p>
  History: Since 1.9.7, the AspectJ compiler <tt>ajc</tt> needed JDK 11+, before then JDK 8+.
 </p>

 <h3>Use LTW on Java 16+</h3>

 <p>
  Please note that if you want to use load-time weaving on Java 16+, the weaving agent collides with
  <a href="https://openjdk.java.net/jeps/396">JEP 396 (Strongly Encapsulate JDK Internals by Default)</a> and related
  subsequent JEPs. Therefore, you need to set the JVM parameter <tt>--add-opens java.base/java.lang=ALL-UNNAMED</tt> in
  order to enable aspect weaving. This is due to the fact that the weaver uses internal APIs for which we have not found
  an adequate replacement yet when defining classes in different classloaders.
 </p>

 <h3>Compile with Java preview features</h3>

 <p>
  For features marked as preview on a given JDK, you need to compile with <tt>ajc --enable-preview</tt> and run with
  <tt>java --enable-preview</tt> on that JDK.
 </p>
 <p>
  Please note that you cannot run code compiled with preview features on any other JDK than the one used for
  compilation. For example, records compiled with preview on JDK 15 cannot be used on JDK 16 without recompilation. This
  is a JVM limitation unrelated to AspectJ. Also, e.g. sealed classes are preview-1 on JDK 15 and preview-2 on JDK 16.
  You still need to recompile, no matter what.
 </p>

 <hr>

 <p>
  <b>Available:</b> 1.9.21 on 11-Dec-2023
 </p>

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 <tr> <td>README's
     </td>
     <td>Changes and porting guide for AspectJ
        <a href="README-1921.html">1.9.21</a>,
        <a href="README-1920.html">1.9.20 and 1.9.20.1</a>,
        <a href="README-1919.html">1.9.19</a>,
        <a href="README-199.html">1.9.9 and 1.9.9.1</a>,