The hack for modifying a final field doesn't work on Java 12 and newer

org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal net.orfjackal.retrolambda:retrolambda-maven-plugin:2.5.7-SNAPSHOT:process-main (default) on project end-to-end-tests: Failed to run Retrolambda
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:216)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
	at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:120)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:355)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:155)
	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:584)
	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:216)
	at org.apache.maven.cli.MavenCli.main(MavenCli.java:160)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
Caused by: org.apache.maven.plugin.MojoExecutionException: Failed to run Retrolambda
	at net.orfjackal.retrolambda.maven.ProcessClassesMojo.processClassesInCurrentProcess(ProcessClassesMojo.java:161)
	at net.orfjackal.retrolambda.maven.ProcessClassesMojo.execute(ProcessClassesMojo.java:133)
	at net.orfjackal.retrolambda.maven.ProcessMainClassesMojo.execute(ProcessMainClassesMojo.java:17)
	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:132)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
	... 19 more
Caused by: java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	at net.orfjackal.retrolambda.maven.ProcessClassesMojo.processClassesInCurrentProcess(ProcessClassesMojo.java:157)
	... 23 more
Caused by: java.lang.IllegalStateException: Cannot initialize dumper; unexpected JDK implementation. Please run Retrolambda using the Java agent (enable forking in the Maven plugin).
	at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.install(LambdaClassDumper.java:38)
	at net.orfjackal.retrolambda.Retrolambda.run(Retrolambda.java:67)
	at net.orfjackal.retrolambda.Retrolambda.run(Retrolambda.java:25)
	... 28 more
	Suppressed: java.lang.RuntimeException: java.lang.IllegalAccessException: class net.orfjackal.retrolambda.lambdas.LambdaClassDumper cannot access a member of class java.lang.invoke.InnerClassLambdaMetafactory (in module java.base) with modifiers "private static final"
		at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.uninstall(LambdaClassDumper.java:48)
		at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.close(LambdaClassDumper.java:55)
		at net.orfjackal.retrolambda.Retrolambda.run(Retrolambda.java:102)
		... 29 more
	Caused by: java.lang.IllegalAccessException: class net.orfjackal.retrolambda.lambdas.LambdaClassDumper cannot access a member of class java.lang.invoke.InnerClassLambdaMetafactory (in module java.base) with modifiers "private static final"
		at java.base/jdk.internal.reflect.Reflection.newIllegalAccessException(Reflection.java:376)
		at java.base/java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:639)
		at java.base/java.lang.reflect.Field.checkAccess(Field.java:1075)
		at java.base/java.lang.reflect.Field.set(Field.java:778)
		at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.uninstall(LambdaClassDumper.java:46)
		... 31 more
Caused by: java.lang.NoSuchFieldException: modifiers
	at java.base/java.lang.Class.getDeclaredField(Class.java:2416)
	at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.makeNonFinal(LambdaClassDumper.java:59)
	at net.orfjackal.retrolambda.lambdas.LambdaClassDumper.install(LambdaClassDumper.java:32)
	... 30 more

README.md

@@ -241,6 +241,11 @@ yourself or pay someone to do it, if you think it's worth the effort. ;)
 Version History
 ---------------
 
+### Upcoming
+
+- Improved error messages for Java 12 and newer, which have been confirmed
+  to not work without the Java agent
+
 ### Retrolambda 2.5.6 (2018-11-30)
 
 - Fix a `NullPointerException` crash in the Maven plugin on Java 10 & 11

retrolambda/src/main/java/net/orfjackal/retrolambda/lambdas/LambdaClassDumper.java

@@ -1,4 +1,4 @@
-// Copyright © 2013-2014 Esko Luontola <www.orfjackal.net>
+// Copyright © 2013-2020 Esko Luontola and other Retrolambda contributors
 // This software is released under the Apache License 2.0.
 // The license text is at http://www.apache.org/licenses/LICENSE-2.0
 
@@ -56,10 +56,16 @@ public void close() {
     }
 
     private static void makeNonFinal(Field field) throws Exception {
-        Field modifiers = field.getClass().getDeclaredField("modifiers");
-        modifiers.setAccessible(true);
-        int mod = modifiers.getInt(field);
-        modifiers.setInt(field, mod & ~Modifier.FINAL);
+        try {
+            Field modifiers = field.getClass().getDeclaredField("modifiers");
+            modifiers.setAccessible(true);
+            int mod = modifiers.getInt(field);
+            modifiers.setInt(field, mod & ~Modifier.FINAL);
+        } catch (NoSuchFieldException e) {
+            throw new RuntimeException("Failed to make a field non-final (" + field + "). " +
+                    "This known to fail on Java 12 and newer. Prefer using Java 8 or try using the Java agent " +
+                    "(fork=true in the Maven plugin).", e);
+        }
     }
 
     private static Object newProxyClassesDumper(Path dumpDir) throws Exception {

scripts/build.sh

@@ -83,13 +83,7 @@ JAVA_HOME="$JAVA11_HOME" mvn clean verify \
     --errors \
     | tee build.log && check_build_log
 
-JAVA_HOME="$JAVA12_HOME" mvn clean verify \
-    --errors \
-    | tee build.log && check_build_log
-
-JAVA_HOME="$JAVA13_HOME" mvn clean verify \
-    --errors \
-    | tee build.log && check_build_log
+# (Java 12+ fails without forking because java.lang.invoke.InnerClassLambdaMetafactory#dumper cannot be made non-final)
 
 # Make sure that the Java agent works on all new Java versions