Lambda Expressions Backported to Java 7, 6 and 5

Do you want to use lambda expressions already today, but you are forced to use Java and a stable JRE in production? Now that's possible with Retrolambda, which will take bytecode compiled with Java 8 and convert it to run on Java 7, 6 and 5 runtimes, letting you use lambda expressions and method references on those platforms. It won't give you the improved Java 8 Collections API, but fortunately there are multiple alternative libraries which will benefit from lambda expressions.

Behind the Scenes

A couple of days ago in a café it popped into my head to find out whether somebody had made this already, but after speaking into the air, I did it myself over a weekend.

The original plan of copying the classes from OpenJDK didn't work (LambdaMetafactory depends on some package-private classes and would have required modifications), but I figured out a better way to do it without additional runtime dependencies.

Retrolambda uses a Java agent to find out what bytecode LambdaMetafactory generates dynamically, and saves it as class files, after which it replaces the invokedynamic instructions to instantiate those classes directly. It also changes some private synthetic methods to be package-private, so that normal bytecode can access them without method handles.

After the conversion you'll have just a bunch of normal .class files - but with less typing.

P.S. If you hear about experiences of using Retrolambda for Android development, please leave a comment.


  1. Congrats! That's a pretty awesome idea for early adopters...

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  3. Hi, i'm Sergey, maintainer of HoloEverywhere library for Android.
    Dalvik VM works nicely with retrolambda :)
    Many thanks for library and expirence.

    I use next code for testing: http://pastebin.com/MrXUPn5B
    Screenshot from Android Debug Bridge: http://i.imgur.com/ram0ecB.png

  4. This comment has been removed by the author.

  5. So it basically let you use lambda expressiona in development for JRE5+ project using JDK8 if I understand ?

    So how is the bytecode looking ? Is it the same as if you were using a a single method interface and an anonymous class instance to simulate lambdas ?

  6. Yes, it's pretty much the same as what an anonymous class would look like. There are a couple of wrapper methods causing indirection (so that future JVMs can in the improve the lambda implementation), but the JIT will easily inline them.

    One difference over anonymous classes is that there is an optimization (as is also in JDK 8) that if the lambda doesn't capture values from its enclosing scope, only one instance of the class is ever instantiated (it's stored in a static final field). This optimization will be included in Retrolambda 1.1.0 which I'll release later today or tomorrow.

  7. Thanks for the explanations, I will definitively give it a try !

    1. I've updated my comment in response to your Stack Overflow question about a backport to Java 7. I really didn't believe it could be done so efficiently!

    2. Cool. Im downloading JDK8 now :)

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  9. (This is basically a repeat of a reddit comment in case this is a better place for you to catch it.)

    I'm impressed across the board -- concept, approach, code, explanation, and dealing calmly with redditors that missed the point. :)

    I note that you've spent some time with Scala and are now in to Clojure. Have you heard of Perl 6, which is, very loosely speaking, in the same sort of space (as compared to the older generation of dynamic langs such as python, ruby, and the original perl)?

    All of this leads me to:

    Perhaps you would be willing to visit the IRC channel #perl6 on freenode [1] and chat for a while? A guy called jnthn is finishing up an initial port of Perl 6 to the JVM [2] and is digging in to java interop, multicores, optimizations, dalvik, and so on. I know they could do with pointers from someone with the level of JVM expertise you must have and I bet he'd enjoy a chat with you even if only for 15 minutes. Here's hoping...

    [1] https://kiwiirc.com/client/irc.freenode.net/perl6?nick=orfjackal
    [2] http://6guts.wordpress.com/2013/07/15/rakudo-jvm-news-more-tests-plus-thread-and-promise-prototypes/

  10. I'm not using Perl, so I haven't been following the news surrounding it. (I mainly use Ruby and a little Bash for scripting.)

    Sure, I can chat for a while and try to help what I can. But regarding JVM language development there should be much more knowledgeable people at https://groups.google.com/forum/#!forum/jvm-languages

  11. Esko, I just saw the exchange in the public log [1]. Thank you. :)

    Fyi, two leading P6ers live in Sweden and work at http://edument.se/english/ including jnthn. I've already seen enough of your work and personality to feel confident they'd be interested in you, as an employee or in their network (aiui their network is not limited to folk living in Sweden). If you're interested, I suggest the same approach, i.e. point jnthn (or masak) at this blog post, mention that I've left two comments, and go from there.

    jnthn, masak, check out http://lets-code.orfjackal.net/

    [1] http://irclog.perlgeek.de/perl6/2013-07-25#i_7370179

  12. Thanks Esko, my concern is with performance. Have you benchmarked it between a Java 6 and 8 VMs?

  13. Nope. You're welcome to benchmark it yourselves (*insert here the usual warnings about microbenchmarks* [1]). Please write an article and post a link here.

    [1] http://stackoverflow.com/questions/504103/how-do-i-write-a-correct-micro-benchmark-in-java https://code.google.com/p/caliper/wiki/JavaMicrobenchmarks

    1. I'm guessing that the performance will be the same as with the original lambda expressions (or slightly better because of removing the indirection of method handles). The most probable cause of any performance difference is that the older JVMs don't do as many optimizations as the newer JVMs. So make sure to run all benchmarks using the same JVM version.

  14. That's a really Nice Development. Would like to try it out.

  15. Hi,
    this really looks helpful, but I can't figure out why it doesn't work for me. Am I doing some basic mistake?
    I'm using jdk8-b116 on linux ubuntu 12.04

    Thanks for help

    I have very simple HelloLambda class:

    package com.test.hello;

    public class HelloLambda {
    public static void main(String[] args) {
    process("This is Lambda", s -> System.out.println(s));

    private static void process(String msg, Printer p) {

    public interface Printer {
    void out(T t);

    java -jar -Dretrolambda.inputDir=build/classes/main -Dretrolambda.classpath=build/classes/main -Dretrolambda.bytecodeVersion=50 -Dretrolambda.outputDir=build/libs/output -javaagent:retrolambda-1.1.0.jar retrolambda-1.1.0.jar

    and it produces exception:

    Error! Failed to transform some classes
    java.lang.RuntimeException: java.util.NoSuchElementException
    at net.orfjackal.retrolambda.LambdaReifier.reifyLambdaClass(LambdaReifier.java:32)
    at net.orfjackal.retrolambda.LambdaUsageBackporter$InvokeDynamicInsnConvertingMethodVisitor.backportLambda(LambdaUsageBackporter.java:107)
    at net.orfjackal.retrolambda.LambdaUsageBackporter$InvokeDynamicInsnConvertingMethodVisitor.visitInvokeDynamicInsn(LambdaUsageBackporter.java:99)
    at org.objectweb.asm.ClassReader.readCode(ClassReader.java:1343)
    at org.objectweb.asm.ClassReader.readMethod(ClassReader.java:938)
    at org.objectweb.asm.ClassReader.accept(ClassReader.java:669)
    at org.objectweb.asm.ClassReader.accept(ClassReader.java:506)
    at net.orfjackal.retrolambda.LambdaUsageBackporter.transform(LambdaUsageBackporter.java:28)
    at net.orfjackal.retrolambda.Main$1.transform(Main.java:40)
    at net.orfjackal.retrolambda.BytecodeTransformingFileVisitor.visitFile(BytecodeTransformingFileVisitor.java:25)
    at net.orfjackal.retrolambda.BytecodeTransformingFileVisitor.visitFile(BytecodeTransformingFileVisitor.java:11)
    at java.nio.file.Files.walkFileTree(Files.java:2667)
    at java.nio.file.Files.walkFileTree(Files.java:2739)
    at net.orfjackal.retrolambda.Main.main(Main.java:38)
    Caused by: java.util.NoSuchElementException
    at java.util.concurrent.LinkedBlockingDeque.getFirst(LinkedBlockingDeque.java:553)
    at net.orfjackal.retrolambda.LambdaReifier.getLambdaFactoryMethod(LambdaReifier.java:47)
    at net.orfjackal.retrolambda.LambdaReifier.reifyLambdaClass(LambdaReifier.java:29)
    ... 13 more

    1. The "-jar" must be the last argument just before the jar file.

    2. I've just released version 1.1.1 which gives a proper error message in this situation (it shows the usage help if the Java agent is not loaded).

  16. Cool it works. I haven't noticed wrong -jar option. Thank you.

  17. Must be some stupid mistake, but I can't see it. I tried to reproduce the example for Dalvik mentioned above and caught a java.util.NoSuchElementException. I used the latest JDK8 from their site.

    Here's the code I used and commands I ran - http://pastebin.com/vVbSkBBh

    1. Thanks for reporting the problem. A recent JDK 8 had changes which made it incompatible with Retrolambda 1.1.1. I have now released Retrolambda 1.1.2 which fixes the problem.

  18. Hi Esko
    haven't you tried to resolve problem of default interfaces? This is another feature that breaks our backporting from jdk 8 to 6.


    1. No. I think that it would be hard to fully backport default interfaces, because they support polymorphism. One option might be to find all implementors of an interface that uses default methods, and copy the method implementation there, unless that class or its superclasses has an implementation. But I don't think that it would be worth the effort, because that alone would not give you the new JDK 8 APIs that were the reason for adding default methods in the first place.

    2. ... but still I think it's great idea :)

      And it will be useful for e.g. some API development on java 8 and re-distribute to older version. There is less code maintenance.

      I think its good approach to scan superclasses and interfaces for default method, and after collecting them all, import them in 'root' class. I'm not so much in bytecode but who knows with proper motivation and time...

      In short..you do pretty awesome yob!
      best regards

  19. Nice project, but unfortunately doesnt support "invokespecial" which is my main usecase...

    so i started a sourcecode patching project https://github.com/wolfposd/to7andbeyond

    currently its really rudimentary and only changes oneliners, but already suits my needs

    1. I don't quite follow you that what do you mean by it not supporting invokespecial. Please give an example of Java 8 source code that doesn't work with Retrolambda.

    2. myjButton.addActionlistener( e -> someMethod());

      tells me invokespecial is not supported

    3. Now that's funny, because I have absolutely no problems doing that with with Retrolambda: https://gist.github.com/orfjackal/9879520

    4. i could narrow it down to the ant task i was building, turns out the classpath-property wasn't set correctly