How will Java lambda functions be compiled?
Loop.times(5, () -> {
System.out.println(\"looping\");
});
Which of these would it effectively compile to?
for(int i = 0; i &
-
The VM decides how to implement lambda, not a compiler.
See
Translation strategysection in Translation of Lambda Expressions.Instead of generating bytecode to create the object that implements the lambda expression (such as calling a constructor for an inner class), we describe a recipe for constructing the lambda, and delegate the actual construction to the language runtime. That recipe is encoded in the static and dynamic argument lists of an invokedynamic instruction.
forconstruction from your example is most effective way in terms of simple compiling or perfomance (but the performance differences are very small, by the tests).Addon:
I created and disassemble two examples:
for (String string: Arrays.asList("hello")) { System.out.println(string); }Disassembled bytecode, constants and other information:
Classfile LambdaCode.class Last modified 30.05.2013; size 771 bytes MD5 checksum 79bf2821b5a14485934e5cebb60c99d6 Compiled from "LambdaCode.java" public class test.lambda.LambdaCode SourceFile: "LambdaCode.java" minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #11.#22 // java/lang/Object."<init>":()V #2 = Class #23 // java/lang/String #3 = String #24 // hello #4 = Methodref #25.#26 // java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; #5 = InterfaceMethodref #27.#28 // java/util/List.iterator:()Ljava/util/Iterator; #6 = InterfaceMethodref #29.#30 // java/util/Iterator.hasNext:()Z #7 = InterfaceMethodref #29.#31 // java/util/Iterator.next:()Ljava/lang/Object; #8 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream; #9 = Methodref #34.#35 // java/io/PrintStream.println:(Ljava/lang/String;)V #10 = Class #36 // test/lambda/LambdaCode #11 = Class #37 // java/lang/Object #12 = Utf8 <init> #13 = Utf8 ()V #14 = Utf8 Code #15 = Utf8 LineNumberTable #16 = Utf8 main #17 = Utf8 ([Ljava/lang/String;)V #18 = Utf8 StackMapTable #19 = Class #38 // java/util/Iterator #20 = Utf8 SourceFile #21 = Utf8 LambdaCode.java #22 = NameAndType #12:#13 // "<init>":()V #23 = Utf8 java/lang/String #24 = Utf8 hello #25 = Class #39 // java/util/Arrays #26 = NameAndType #40:#41 // asList:([Ljava/lang/Object;)Ljava/util/List; #27 = Class #42 // java/util/List #28 = NameAndType #43:#44 // iterator:()Ljava/util/Iterator; #29 = Class #38 // java/util/Iterator #30 = NameAndType #45:#46 // hasNext:()Z #31 = NameAndType #47:#48 // next:()Ljava/lang/Object; #32 = Class #49 // java/lang/System #33 = NameAndType #50:#51 // out:Ljava/io/PrintStream; #34 = Class #52 // java/io/PrintStream #35 = NameAndType #53:#54 // println:(Ljava/lang/String;)V #36 = Utf8 test/lambda/LambdaCode #37 = Utf8 java/lang/Object #38 = Utf8 java/util/Iterator #39 = Utf8 java/util/Arrays #40 = Utf8 asList #41 = Utf8 ([Ljava/lang/Object;)Ljava/util/List; #42 = Utf8 java/util/List #43 = Utf8 iterator #44 = Utf8 ()Ljava/util/Iterator; #45 = Utf8 hasNext #46 = Utf8 ()Z #47 = Utf8 next #48 = Utf8 ()Ljava/lang/Object; #49 = Utf8 java/lang/System #50 = Utf8 out #51 = Utf8 Ljava/io/PrintStream; #52 = Utf8 java/io/PrintStream #53 = Utf8 println #54 = Utf8 (Ljava/lang/String;)V { public test.lambda.LambdaCode(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return LineNumberTable: line 15: 0 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=3, args_size=1 0: iconst_1 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String hello 8: aastore 9: invokestatic #4 // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; 12: invokeinterface #5, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 17: astore_1 18: aload_1 19: invokeinterface #6, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 24: ifeq 47 27: aload_1 28: invokeinterface #7, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 33: checkcast #2 // class java/lang/String 36: astore_2 37: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream; 40: aload_2 41: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 44: goto 18 47: return LineNumberTable: line 35: 0 line 36: 37 line 37: 44 line 38: 47 StackMapTable: number_of_entries = 2 frame_type = 252 /* append */ offset_delta = 18 locals = [ class java/util/Iterator ] frame_type = 250 /* chop */ offset_delta = 28 }and
Arrays.asList("hello").forEach(p -> {System.out.println(p);});Disassembled bytecode, constants and other information:
Classfile LambdaCode.class Last modified 30.05.2013; size 1262 bytes MD5 checksum 4804e0a37b73141d5791cc39d51d649c Compiled from "LambdaCode.java" public class test.lambda.LambdaCode SourceFile: "LambdaCode.java" InnerClasses: public static final #64= #63 of #70; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles BootstrapMethods: 0: #27 invokestatic java/lang/invoke/LambdaMetafactory.metaFactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; Method arguments: #28 invokeinterface java/util/function/Consumer.accept:(Ljava/lang/Object;)V #29 invokestatic test/lambda/LambdaCode.lambda$0:(Ljava/lang/String;)V #30 (Ljava/lang/String;)V minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #10.#21 // java/lang/Object."<init>":()V #2 = Class #22 // java/lang/String #3 = String #23 // hello #4 = Methodref #24.#25 // java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; #5 = InvokeDynamic #0:#31 // #0:lambda$:()Ljava/util/function/Consumer; #6 = InterfaceMethodref #32.#33 // java/util/List.forEach:(Ljava/util/function/Consumer;)V #7 = Fieldref #34.#35 // java/lang/System.out:Ljava/io/PrintStream; #8 = Methodref #36.#37 // java/io/PrintStream.println:(Ljava/lang/String;)V #9 = Class #38 // test/lambda/LambdaCode #10 = Class #39 // java/lang/Object #11 = Utf8 <init> #12 = Utf8 ()V #13 = Utf8 Code #14 = Utf8 LineNumberTable #15 = Utf8 main #16 = Utf8 ([Ljava/lang/String;)V #17 = Utf8 lambda$0 #18 = Utf8 (Ljava/lang/String;)V #19 = Utf8 SourceFile #20 = Utf8 LambdaCode.java #21 = NameAndType #11:#12 // "<init>":()V #22 = Utf8 java/lang/String #23 = Utf8 hello #24 = Class #40 // java/util/Arrays #25 = NameAndType #41:#42 // asList:([Ljava/lang/Object;)Ljava/util/List; #26 = Utf8 BootstrapMethods #27 = MethodHandle #6:#43 // invokestatic java/lang/invoke/LambdaMetafactory.metaFactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #28 = MethodHandle #9:#44 // invokeinterface java/util/function/Consumer.accept:(Ljava/lang/Object;)V #29 = MethodHandle #6:#45 // invokestatic test/lambda/LambdaCode.lambda$0:(Ljava/lang/String;)V #30 = MethodType #18 // (Ljava/lang/String;)V #31 = NameAndType #46:#47 // lambda$:()Ljava/util/function/Consumer; #32 = Class #48 // java/util/List #33 = NameAndType #49:#50 // forEach:(Ljava/util/function/Consumer;)V #34 = Class #51 // java/lang/System #35 = NameAndType #52:#53 // out:Ljava/io/PrintStream; #36 = Class #54 // java/io/PrintStream #37 = NameAndType #55:#18 // println:(Ljava/lang/String;)V #38 = Utf8 test/lambda/LambdaCode #39 = Utf8 java/lang/Object #40 = Utf8 java/util/Arrays #41 = Utf8 asList #42 = Utf8 ([Ljava/lang/Object;)Ljava/util/List; #43 = Methodref #56.#57 // java/lang/invoke/LambdaMetafactory.metaFactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #44 = InterfaceMethodref #58.#59 // java/util/function/Consumer.accept:(Ljava/lang/Object;)V #45 = Methodref #9.#60 // test/lambda/LambdaCode.lambda$0:(Ljava/lang/String;)V #46 = Utf8 lambda$ #47 = Utf8 ()Ljava/util/function/Consumer; #48 = Utf8 java/util/List #49 = Utf8 forEach #50 = Utf8 (Ljava/util/function/Consumer;)V #51 = Utf8 java/lang/System #52 = Utf8 out #53 = Utf8 Ljava/io/PrintStream; #54 = Utf8 java/io/PrintStream #55 = Utf8 println #56 = Class #61 // java/lang/invoke/LambdaMetafactory #57 = NameAndType #62:#66 // metaFactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #58 = Class #67 // java/util/function/Consumer #59 = NameAndType #68:#69 // accept:(Ljava/lang/Object;)V #60 = NameAndType #17:#18 // lambda$0:(Ljava/lang/String;)V #61 = Utf8 java/lang/invoke/LambdaMetafactory #62 = Utf8 metaFactory #63 = Class #71 // java/lang/invoke/MethodHandles$Lookup #64 = Utf8 Lookup #65 = Utf8 InnerClasses #66 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #67 = Utf8 java/util/function/Consumer #68 = Utf8 accept #69 = Utf8 (Ljava/lang/Object;)V #70 = Class #72 // java/lang/invoke/MethodHandles #71 = Utf8 java/lang/invoke/MethodHandles$Lookup #72 = Utf8 java/lang/invoke/MethodHandles { public test.lambda.LambdaCode(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return LineNumberTable: line 15: 0 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=1, args_size=1 0: iconst_1 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String hello 8: aastore 9: invokestatic #4 // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; 12: invokedynamic #5, 0 // InvokeDynamic #0:lambda$:()Ljava/util/function/Consumer; 17: invokeinterface #6, 2 // InterfaceMethod java/util/List.forEach:(Ljava/util/function/Consumer;)V 22: return LineNumberTable: line 28: 0 line 38: 22 }Compiler generated class-file is more complicated and larger (771b vs 1262b) for Lambda example.
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Java compiler will generate synthetic methods for the code construct that is neither explicitly nor implicitly declared.
As we are aware, lambda expression/function is an anonymous class method implementation for abstract method in functional interface and if we see the byte code of a compiled class file with lambda expression, Instead of creating a new object that will wrap the Lambda function, it uses the new INVOKEDYNAMIC instruction to dynamically link this call site to the actual Lambda function which is converted to
private static synthetic lambda$0(Ljava/lang/String;)Vwhich will accept string as a parameter.private static synthetic lambda$0(Ljava/lang/String;)V GETSTAIC java/lang/System.out: Ljava/io/PrintStream; ALOAD 0 INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V RETURNExample:
list.forEach(x-> System.out.println(x));This lambda expression
x-> System.out.println(x)is converted to private static synthetic block as mentioned above. But how this will be invoked for each element in the list when we run java Class? Refer the below byte code of lambda expression linkage as forEach accepts Consumer functional interface object.INVOKEDYNAMIC accept()Ljava/util/function/Consumer; [ java/lang/invoke/LambdaMetaFactory.metafactory(Ljava/lang/invokeMethodHandler$Lookup.Ljava/lang/invoke/CallSite.. //arguments (Ljava/lang/Object;)V //INVOKESTATIC com/<Classname>.lambda$)(Ljava/lang/String;)V, (Ljava/lang/String;)V ]java.lang.invoke.LambdaMetaFactory: This class provides two forms of linkage methods:
- A standard version (metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)) using an optimized protocol,
- An alternate version altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)).
These linkage methods are designed to support the evaluation of lambda expressions and method references in the Java Language. For every lambda expressions or method reference in the source code, there is a target type which is a functional interface. Evaluating a lambda expression produces an object of its target type. The recommended mechanism for evaluating lambda expressions is to desugar the lambda body to a method, invoke an invokedynamic call site whose static argument list describes the sole method of the functional interface and the desugared implementation method, and returns an object (the lambda object) that implements the target type. Note(For method references, the implementation method is simply the referenced method; no desugaring is needed.)
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