Make Java agent work with JDK 9+

[dmitri-gb]

Hi!

I would like to propose an alternative strategy for getting the lambda class bytes. Starting with JDK 9, class file transformers are not invoked for generated lambda classes anymore [1], which is the reason Retrolambda currently fails when run with Java 11 with the -javaagent option. This PR makes Retrolambda patch JDK's InnerClassLambdaMetafactory instead, getting the bytes directly from its spinInnerClass method. This way, one can run Retrolambda with both Java 8 as well as Java 11 (input classes must still be compiled to Java 8 of course).

The primary motivation for this change was to be able to run a Maven build with Java 11 (with --release 8 or -source 8 -target 8) and have the Retrolambda Maven plugin transform the classes to Java 6 bytecode, without needing to also have JDK 8 available and passing its path in the <java8home> plugin configuration option.

[1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2016-January/038199.html

[luontola]

Thanks for the PR.

Instead of the <java8home> option, you can also use the maven-toolchains-plugin. Though you'll still need JDK 8 installed on the machine.

        <plugin>
            <artifactId>maven-toolchains-plugin</artifactId>
            <executions>
                <execution>
                    <goals>
                        <goal>toolchain</goal>
                    </goals>
                </execution>
            </executions>
            <configuration>
                <toolchains>
                    <jdk>
                        <version>1.8</version>
                    </jdk>
                </toolchains>
            </configuration>
        </plugin>

[luontola]

This has been included in Retrolambda 2.5.6