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Lambda&
Mechanics&
Marc&R.&Hoffmann&
mtrail&GmbH&
@marcandsweep&
&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
1&
(Source)&
Source&
compile&
Class&Files&
execute&
Run>me&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
2&
Lambda&Types&
!
List<String>!greetings!=!
! !Arrays.asList("Hello",!"Lambda");!
! !!
greetings.forEach((item)!A>!System.out.println(item));!
! !!
greetings.forEach(System.out::println);!
! !!
Consumer<String>!printer!=!System.out::println;!
greetings.forEach(printer);!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
3&
Func>onal&Interfaces&
•  SAM&=&Single&Abstract&Method&
•  Java&uses&strict&types&for&lambdas&
•  No&generic&method&type&
(like&in&other&languages)&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
4&
Lambda&vs.&Inner&Classes&
!
List<String>!greetings!=!
! !Arrays.asList("Hello",!"Lambda");!
! !!
greetings.forEach(new!Consumer<String>()!{!
!@Override!
!public!void!accept(String!item)!{!
! !System.out.println(item);!
!}!
});!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
5&
Desugaring&
!
greetings.forEach((item)!A>!System.out.println(item));!
!
$>#javap#LambdaExample#
#
...#
#
private#static#synthetic#void#lambda$0(java.lang.String#item);#
####getstatic#java/lang/System.out:Ljava/io/PrintStream;#
####aload_0########
####invokevirtual#java/io/PrintStream.println(Ljava/lang/String;)V#
####return#########
##
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
6&
Lambda&Instance&Crea>on&
!
greetings.forEach((item)!A>!System.out.println(item));!
!
$>#javap#LambdaExample#
#
...#
#
aload_1########
invokedynamic#InvokeDynamic##0:accept()Ljava/util/function/Consumer;#
invokeinterface#java/util/List.forEach(Ljava/util/function/Consumer;)V#
#
...#
##
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
7&
Method&Invoca>ons&in&Bytecode&
invokevirtual!java/io/PrintStream.println(C)V!
invokeinterface!java/lang/Runnable.run()V!
invokestatic!java/lang/System.exit(I)V!
invokespecial!java/lang/Object.<init>()V!
!
•  All&staOcally&typed&
•  invokedynamic&added&with&Java&7&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
8&
CallSite&
Target&Method&
invokedynamic&
Bootstrap&Method&
Bootstrap&Method&Parameters&
Dynamic&
Call&Site&
Bootstrap&
Method&
Bootstrap&Method&
Handle&
Caller&
Method&Name&
Method&Name&
Method&Descriptor&
Method&Descriptor&
n&Sta>c&Arguments&
n&Sta>c&Arguments&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
10&
java.lang.invoke.LambdaMetafactory&
•  Part&of&the&JDK&API&
•  Can&also&be&used&by&other&languages&
•  Why&„Meta“?&
Factory&for&funcOons&which&create&the&lambda&
instances&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
11&
java.lang.invoke.LambdaMetafactory&
public!static!CallSite!metafactory(!
!!!!MethodHandles.Lookup!caller,!
!!!!String!invokedName,!
!!!!MethodType!invokedType,!
!!!!MethodType!samMethodType,!
!!!!MethodHandle!implMethod,!
!!!!MethodType!instantiatedMethodType)!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
12&
java.lang.invoke.LambdaMetafactory&
public!static!CallSite!altMetafactory(!
!!!!MethodHandles.Lookup!caller,!
!!!!String!invokedName,!
!!!!MethodType!invokedType,!
!!!!Object...!args)!
•  Serializable&Lambdas&
•  AddiOonal&Marker&Interfaces&
•  Bridge&Methods&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
13&
Don‘t&Ever&Do&This&in&Your&Produc>on&Code!&
(At&least&don‘t&blame&me)&
//!Parameters!to!the!lambda!factory:!
Lookup!caller!=!MethodHandles.lookup();!
String!invokedName!=!"accept";!
MethodType!invokedType!=!MethodType.methodType(Consumer.class);!
MethodType!samMethodType!=!MethodType.methodType(Void.TYPE,!
!
!Object.class);!
MethodHandle!implMethod!=!caller.findStatic(!
!
!LambdaMetaFactoryTest.class,!"myLambda",!samMethodType);!
!
//!Create!the!factory:!
CallSite!factory!=!LambdaMetafactory.metafactory(caller,!
!
!invokedName,!invokedType,!samMethodType,!implMethod,!
!
!samMethodType);!
!
//!Create!the!actual!lambda!instance:!
Consumer<String>!printer!=!(Consumer<String>)!
!
!factory.dynamicInvoker().invoke();!
!
//!Use!the!lambda!instance:!
List<String>!greetings!=!Arrays.asList("Hello",!"Lambda");!
greetings.forEach(printer);!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
14&
Lambda&Crea>on&Process&
Call&Bootstrap&Method&
Create&Lambda&Impl.&
Create&Factory&Method&
Link&Factory&Method&
First&Call&Only&
invokedynamic&
Call&Factory&Method&
Create&Lambda&Instance*&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
15&
Lambda&Implementa>on?&
!
greetings.forEach((item)!A>!{!
!throw!new!RuntimeException("Where!are!we?");!
});!
$>#java#LambdaExample#
#
Exception#in#thread#"main"#java.lang.RuntimeException:#Where#are#we?#
#at#LambdaWithException.lambda$0(LambdaWithException.java:14)#
#at#LambdaWithException$$Lambda$1/149928006.accept(Unknown#Source)#
#at#java.lang.Iterable.forEach(Iterable.java:75)#
#at#LambdaWithException.main(LambdaWithException.java:13)#
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
16&
Lambda&Implementa>on&Classes&I&
•  Byte&Code&Created&on&the&Fly&
•  sun.misc.Unsafe.defineAnonymousClass()&
•  Can&be&dumped&with&
ADjdk.internal.lambda.dumpProxyClasses=<path>!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
17&
Lambda&Implementa>on&Classes&II&
•  Members&
–  SAM&ImplementaOon&
–  Bridge&Methods&
–  Constructor&to&Retrieve&Capture&
–  Members&to&Store&Capture&&
–  StaOc&Factory&Method&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
18&
Inspec>ng&Lambda&
Implementa>on&Classes&
Class<?>!c!=!instance.getClass();!
show("toString()",!!!instance);!
show("Class!Name",!!!c.getName());!
show("Class!Modifier“,!
! ! !Modifier.toString(c.getModifiers()));!
show("Super!Class",!!c.getSuperclass().getName());!
show("Interfaces",!!!c.getInterfaces());!
show("Class!Loader",!c.getClassLoader());!
show(“Field",!!!!!!!!c.getDeclaredFields());!
show(“Constructor",!!c.getDeclaredConstructors());!
show(“Method",!!!!!!!c.getDeclaredMethods());!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
19&
Inspec>ng:&Simple&Lambda&
Runnable!lambda1!=!()!A>!System.out.println("Lambda");3
!
toString()#####:#LambdaInstanceInspection$$Lambda$1/821270929@2f2c9b19#
Class#Name#####:#LambdaInstanceInspection$$Lambda$1/821270929#
Class#Modifier#:#final#
Super#Class####:#java.lang.Object#
Interface######:#interface#java.lang.Runnable#
Class#Loader###:#sun.misc.Launcher$AppClassLoader@1540e19d#
Constructor####:#private#.../821270929()#
Method#########:#public#void#.../821270929.run()#
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
20&
Inspec>ng:&Lambda&with&Capture&
String!capturedValue!=!"Lambda";!
Runnable!lambda2!=!
! !()!A>!System.out.println(capturedValue);!
Interface######:#interface#java.lang.Runnable#
Field##########:#private#final#java.lang.String#.../303563356.arg$1#
Constructor####:#private#.../303563356(java.lang.String)#
Method#########:#public#void#.../303563356.run()#
Method#########:#private#static#java.lang.Runnable#
#
#
#
#
#
#.../303563356.get$Lambda(java.lang.String)#
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
21&
Inspec>ng:&Serializable&Lambda&
Runnable!lambda3!=!(Runnable!&!Serializable)!
! !()!A>!System.out.println();!
Interface######:#interface#java.lang.Runnable#
Interface######:#interface#java.io.Serializable#
Class#Loader###:#sun.misc.Launcher$AppClassLoader@1540e19d#
Constructor####:#private#.../250421012()#
Method#########:#public#void#.../250421012.run()#
Method#########:#private#final#java.lang.Object#
###############################.../250421012.writeReplace()#
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
22&
java.lang.invoke.SerializedLambda&
public!final!class!SerializedLambda!implements!Serializable!{!
!!!!!
!!!!private!final!Class<?>!capturingClass;!
!!!!private!final!String!functionalInterfaceClass;!
!!!!private!final!String!functionalInterfaceMethodName;!
!!!!private!final!String!functionalInterfaceMethodSignature;!
!!!!private!final!String!implClass;!
!!!!private!final!String!implMethodName;!
!!!!private!final!String!implMethodSignature;!
!!!!private!final!int!implMethodKind;!
!!!!private!final!String!instantiatedMethodType;!
!!!!private!final!Object[]!capturedArgs;!
!
!!!!...!
}!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
23&
Inspec>ng:&Bridge&Methods&
static!interface!StringSupplier!extends!Supplier<String>!
{!
!@Override!
!String!get();!
}!
!
StringSupplier!lambda4!=!()!A>!"Lambda";!
!
Constructor####:#private#.../1096979270()#
Method#########:#public#java.lang.String#.../1096979270.get()#
Method#########:#public#java.lang.Object#.../1096979270.get()#
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
24&
Lambda&Instance&ReWUse&
Runnable!createLambda()!{!
!return!LambdaInstancesTest::staticMethod;!
}!
!
@Test!
public!void!same_call_site()!{!
!Runnable!r1!=!createLambda();!
!Runnable!r2!=!createLambda();!
!assertSame(r1,!r2);!
!assertSame(r1.getClass(),!r2.getClass());!
}!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
25&
One&Implementa>on&per&Call&Site&
@Test!
public!void!different_call_site()!{!
!Runnable!r1!=!LambdaInstancesTest::staticMethod;!
!Runnable!r2!=!LambdaInstancesTest::staticMethod;!
!assertNotSame(r1,!r2);!
!assertNotSame(r1.getClass(),!r2.getClass());!
}!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
26&
Capturing&Lambdas&
Runnable!createLambdaWithCapture()!{!
!return!System.out::println;!
}!
!
@Test!
public!void!same_call_site_with_capture()!{!
!Runnable!r1!=!createLambdaWithCapture();!
!Runnable!r2!=!createLambdaWithCapture();!
!assertNotSame(r1,!r2);!
!assertSame(r1.getClass(),!r2.getClass());!
}!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
27&
Bonus&Ques>on:&
To&Capture&or&Not&to&Capture?&
Runnable!createLambdaWithCaptureOrNot()!{!
!return!()!A>!System.out.println();!
}!
!
@Test!
public!void!same_call_site_with_capture_or_not()!{!
!Runnable!r1!=!createLambdaWithCaptureOrNot();!
!Runnable!r2!=!createLambdaWithCaptureOrNot();!
!assertSame(r1,!r2);!
!assertSame(r1.getClass(),!r2.getClass());!
}!
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
28&
References&
hZps://www.eclipsecon.org/na2015/session/lambdaWmechanics&
&
&
•  Code&Examples:&hbps://github.com/marchof/lambdadmechanics&
•  JLS8:&FuncOonal&Interfaces,&Chapter&9.8&
•  JLS8:&CompiledTime&Step&3:&Is&the&Chosen&Method&Appropriate?&
Chapter&15.12.3&
•  JLS8:&RunOmedEvaluaOon&of&Lambda&Expressions,&Chapter&15.27.4&
•  The&Java&Virtual&Machine&SpecificaOon,&Java&SE&8&EdiOon&
•  Java&8&API:&LambdaMetafactory&
•  Brian&Götz:&Lambda&d&A&Peek&Under&The&Hood&
•  Anton&Arihpov:&Java&8&Revealed&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
29&
Ques>ons?&
Marc&R.&Hoffmann&
mtrail&GmbH&
@marcandsweep&
&
Lambda&Mechanics&&&&&Marc&R.&Hoffmann&&&&&EclipseCon&NA&2015&&&&&San&Francisco&
30&
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