Since Groovy 2.0, support was added for the JVMinvokedynamic instruction. This instruction is supported since Java 7 and is a new bytecode instruction in the JVM that allows easier implementation of dynamic languages. This instruction will also be used internally, by the JVM, for the lambda support in Java 8.
This means that unlike APIs, AST transformations or syntactic sugar, this feature isnot visible to the developer or the end user. It is a compilation and runtime feature only. This means that given two programs written in Groovy, you have the choice to compile it with or without invokedynamic support. Whatever you choose, it comes with pros and cons:
it is possible to mix classes compiled with and without invokedynamic in the same project, as long as you run JDK 1.7+
depending on the JVM (even different minor versions of the JVM), you can target close to Java performance for dynamic Groovy with invokedynamic support activated
The Groovy distribution comes withtwo jars:
groovy-x.y.z.jar : contains Groovy sources compiled with call site caching
groovy-x-y-z-indy.jar : contains Groovy sources compiled with invokedynamic instructions
As Groovy core and the Groovy modules are sometimes written in Groovy, we currently have no choice but to distribute twodistinct versions of Groovy. This means that if you pick the "normal" jar, the Groovy classes of Groovy itself arecompiled with call site caching (1.6+), while if you use the "indy" jar, the Groovy classes of Groovy itself arecompiled using invokedynamic.
Both jars contain a fully working Groovy implementation that is capable of compiling user supplied Groovy sources using eitherinvokedynamic or call site caching. The sets of jars are mutually exclusive (don’t put both on classpath) and the key difference betweenthem has to do with how the Groovy source files that make up Groovy itself are compiled.
If you download the distribution and use the command line, it’s always the "normal" version of Groovy which is picked up in classpath. This means that whatever command you use (groovy,groovyc,groovysh orgroovyConsole), invokedynamic support is not available out of the box. To use a Groovy distribution that was compiled with invokedynamic for its Groovy sources you have to switch the jars manually. The distribution makes use of the jars in the lib directory, while the indy jars are available in the indy directory. You have three things to do:
remove or rename the groovy-*.jar files in the lib directory
replace them with the indy version from the indy directory
remove the -indy classifier from jar names
Here’s a bash script that would do it all at once:
$ for f in `ls lib/groovy*.jar | cut -d/ -f2`;do k=`basename $f .jar`; mv lib/$k.jar lib/$k.jar.old; cp indy/$k-indy.jar lib/$k.jar ; doneThe usual way to run a script from the command line is bygroovy foo.groovy, wherefoo.groovy is the Groovy programin source form. To use indy for this you have to use the indy compilation flag,groovy --indy foo.groovy.
Independently of the jar version that you use (and after having exchanged the jars as described), invokedynamic support requires a specific compilation flag (indy). If you want to compile your classes with invokedynamic support, this flag must be set at compile time. The following tables show you what happens with user compiled classes and Groovy core classes depending on the jar you use and the compilation flag:
| indy flag | off | on |
|---|---|---|
normal jar | call site caching | invokedynamic |
indy jar | call site caching | invokedynamic |
| indy flag | off | on |
|---|---|---|
normal jar | call site caching | call site caching |
indy jar | invokedynamic | invokedynamic |
So even if you use the indy jar, if you don’t use the invokedynamic flag at compile time, then the compiled classes will use the "old" format.