Are Java 8 lambdas compiled as inner classes, methods or something else? -

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• how java lambda functions compiled? 1 answer

i have read article today regarding lambdas:

http://www.infoq.com/articles/java-8-lambdas-a-peek-under-the-hood

the article suggests, lambdas are not implemented anon inner classes (due performance). gives example lambda expression can compiled (static) method of class.

i have tried simple snippet:

private void run() {     system.out.println(this);     givehello(system.out::println); }  private void givehello(consumer<string> consumer) {     system.out.println(consumer);     consumer.accept("hello"); } 

and output :

sample.main@14ae5a5 sample.main$$lambda$1/168423058@4a574795 hello 

so it's not same instance. it's not central "lambda factory" instance either..

how lambdas implemented then?

the expression itself, assuming pass actual lambda expression , not method reference, compiled separate, synthetic method. in addition formal arguments expected functional interface (e.g., single string in case of consumer<string>), include arguments captured values.

at code location lambda expression or method reference appears, invokedynamic instruction emitted. first time instruction hit, call made bootstrap method on lambdametafactory. bootstrap method fix actual implementation of target functional interface delegates target method, , gets returned. target method either synthetic method representing lambda body or whichever named method provided using :: operator. while class implements functional interface is being created, process deferred; not happen @ compile time.

finally, runtime patches invokedynamic site bootstrap result¹, constructor call generated delegate captured values passed in, including (possibly) invocation target². alleviates performance hit removing bootstrapping process subsequent calls.

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¹ see java.lang.invoke end of chapter "timing of linkage", courtesy of @holger.

² in case of lambda no captures, invokedynamic instruction resolve shared delegate instance can reused during subsequent calls, though implementation detail.