JavaZone 2014 - goto java;

Martin SkarsauneJava Developer and Co-Owner

A peek into the OpenJDK compiler : goto java;

高馬丁

GOTO statements in

Java!

GOTO Statement –Objective• Syntax:

goto identifier;• Runtime

– Program control moves to target statement

• Other Semantics – Target (statement label) must be defined in

same scope, compilation error if not– Potential circular gotos should give compilation

warning.

GOTO Statement – Means

1. OpenJDK– Open source Java implementation – Compiler implemented in Java

2. Modify compiler source to support goto statements

Success case

public class GotoSuccess {

public static void main(String[] args) {one: System.out.print("goto ");two: System.out.print("Java");goto four;three: System.out.print("2014");goto five;four: System.out.print("Zone ");goto three;five: System.out.print("!");}

}

GOTOAHEAD

Dijkstra 1968:

What is a Compiler?

Compiler

What is a Compiler?

front back

Parse Enter Process Attribute Flow Desugar Generat

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processAnnotations(enterTrees(… parseFiles(…))),…)…generate(desugar(flow(attribute(…))))


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• Syntax: goto identifier;• First convert character stream to token

stream (Scanner.java)

GOTO IDENTIFIER SEMI

[g][o][t][o][ ][f][o][u][r][;]


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• goto is already a reserved word in Java!• Lucky for us, goto is therefore defined

as a TokenKind. • Tokens.java:141: GOTO("goto")• The scanner therefore works out of the

box!


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final Scanner scanner=factory.newScanner(”goto identifier;”, false);

...import com.sun.tools.javac.parser.Scanner;...


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scanner.nextToken();


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scanner.nextToken();assertThat(”First token is GOTO”, scanner.token().kind, is(GOTO));


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scanner.nextToken();assertThat("Second token is IDENTIFIER", scanner.token().kind,

is(IDENTIFIER));


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is(IDENTIFIER));



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is(IDENTIFIER));

scanner.nextToken(); assertThat("Third token is SEMI", scanner.token().kind, is(SEMI));

CompilationUnit

ClassDefinition

FieldDefintion

MethodDefinition

Signature

Body

IfStatement

Goto

Wikipedia: “the visitor design pattern is a way of separating an algorithm from an object structure on which it operates”

visitClassDef(..)

visitMethodDef(..)

visitIf(..)


eAbstract Syntax Tree

[g][o][t][o][ ][f][o][u][r][;]

GOTO IDENTIFIER SEMI


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Class Interface


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• Interface based visitors


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• Class based visitors

public void visitGoto(JCGoto tree) { try { print("goto " + tree.label + ";"); } catch (IOException e) { throw new UncheckedIOException(e); } }

public void visitGoto(JCGoto tree) { //TODO implement}


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public static class JCLabeledStatement extends JCStatement implements LabeledStatementTree {…public GotoResolver handler;…}

public class GotoResolver { Map<GotoTree, Name> gotos; Map<Name, LabeledStatementTree> targets; List<StatementTree> statementsInSequence; ...}


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• JavacParser.parseStatement()

case GOTO: { nextToken(); Name label = ident(); JCGoto t = to(F.at(pos).Goto(label, getGotoResolver())); accept(SEMI); return t; }

TreeMaker.java:

public JCGoto Goto(Name label, GotoResolver resolver) { JCGoto tree = new JCGoto(label, resolver); tree.pos = pos; return tree;}


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• Basic sanity testing of compilation unit– File name and folder location– Duplicate class names

• Corrections– Add default constructor if no constructors

are declared


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public class SimpleClass {

}

• Default constructor


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public class SimpleClass { public SimpleClass() { super(); }}

• Default constructor


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• Annotation processing API• Part of ordinary javac process since

Java 1.6• Plugin API (see javac documentation)


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@

- proc:only – only process annotations, do not compile

- proc:none – do not process annotations

• Output controlled by command line switches


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• Semantic checks– Types– References

• Corrections– Add required calls to super constructor


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• We want to verify that goto target exists in current scope

public class GotoMissingLabel {

public static void main(String[] args) {one: System.out.print("goto ");two: System.out.print("Java");goto six;three: System.out.print("2014");goto five;four: System.out.print("Zone ");goto three;five: System.out.print("!");}

}


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• Attr.java:

@Override public void visitGoto(JCGoto that) {

that.findTarget();if(that.target==null)

log.error(that.pos(), "undef.label", that.label);result = null;

}

class JCGoto: …public void findTarget() { this.target = (JCLabeledStatement)this.handler.findTarget(this);}


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Flow analysis• Detect unreachable code• Verify assignments• Ensure proper method return• Verify (effectively) final• Check exception flow


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We want to detect circular gotos:

public class GotoCircularWarning {public static void main(String[] args) {

one: System.out.print("goto "); two: System.out.print("Java"); goto four; three: System.out.print("2014"); goto five; four: System.out.print("Zone "); goto three; five: System.out.println("!"); goto one;//forms infinite loop }}


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Flow.FlowAnalyzer class:

@Override public void visitGoto(JCGoto tree) { if (tree.handler.detectCircularGotoPosition(tree))

log.warning(tree.pos, "circular.goto"); }

compiler.properties:

...compiler.warn.circular.goto=circular goto...


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TransTypes

Unlambda Lower

• Erase generic types

public class Bridge implements Comparator {

}


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}

public interface Comparator<T> {tint compare(T o1, T o2);

}or<T> {


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TransTypes

Unlambda Lower


public class Bridge implements Comparator<Integer> {

}


}or<T> {


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TransTypes

Unlambda Lower


public class Bridge implements Comparator<Integer> {

public int compare(Integer first, Integer second) {return first - second;

}}


}or<T> {


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}}


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} /*synthetic*/ public int compare(Object first, Object second) { return this.compare((Integer)first, (Integer)second); }}


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TransTypes

Unlambda Lower

• Extract Inner Class

public class Outer {private void foo() { }

public Runnable fooRunner() {

return new Runnable() {

public void run() {

foo();}

};}

}


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TransTypes

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public Runnable fooRunner() {

return new Runnable() {

public void run() {

foo();}

};}

}


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TransTypes

Unlambda Lower



public Runnable fooRunner() {return new Outer$1(this);

}}

class Outer$1 implements Runnable {final Outer this$0;

public void run() {

this$0.foo(); }}


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TransTypes

Unlambda Lower




}}


Outer$1(final Outer this$0) { this.this$0 = this$0; super(); } public void run() {

this$0.foo(); }}


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TransTypes

Unlambda Lower




}}


Outer$1(final Outer this$0) { this.this$0 = this$0; super(); } public void run() {

this$0.foo(); }}


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TransTypes

Unlambda Lower




}

static void access$000( Outer x0) { x0.foo(); }}


Outer$1(final Outer this$0) { this.this$0 = this$0; super(); } public void run() { Outer.access$000(this$0); }}


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TransTypes

Unlambda Lower

• Boxing

List<Integer> list = Arrays.asList(1, 2);

for (Integer i : list) {System.out.println("Double: " + i * 2);

}


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TransTypes

Unlambda Lower

• Boxing

List<Integer> list = Arrays.asList(1, 2);


}

public static <T> List<T> asList(T... a) { return new ArrayList<>(a); }



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TransTypes

Unlambda Lower

• Boxing

List<Integer> list = Arrays.asList(Integer.valueOf(1), Integer.valueOf(2));

for (Integer i : list) { System.out.println("Double: " + i * 2);

}


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TransTypes

Unlambda Lower

• Unboxing



}


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TransTypes

Unlambda Lower

• Unboxing


for (Integer i : list) {System.out.println("Double: " + i.intValue() * 2);

}


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TransTypes

Unlambda Lower

• Varargs



}


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TransTypes

Unlambda Lower

• Varargs



}




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TransTypes

Unlambda Lower

• Varargs

List<Integer> list = Arrays.asList(new Integer[]{Integer.valueOf(1),

Integer.valueOf(2)});for (Integer i : list) {

System.out.println("Double: " + i.intValue() * 2);}


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TransTypes

Unlambda Lower

• For each loop





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TransTypes

Unlambda Lower

• For each loop





public interface Iterable<T> { Iterator<T> iterator();}


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TransTypes

Unlambda Lower

• For each loop


Integer.valueOf(2)});for (;;) {

Integer i System.out.println("Double: " + i.intValue() * 2);

}


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TransTypes

Unlambda Lower

• For each loop


Integer.valueOf(2)});for (Iterator i$ = list.iterator();;) {


}


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TransTypes

Unlambda Lower

• For each loop


Integer.valueOf(2)});for (Iterator i$ = list.iterator(); i$.hasNext();) {


}


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TransTypes

Unlambda Lower

• For each loop


Integer.valueOf(2)});for (Iterator i$ = list.iterator(); i$.hasNext();) {

Integer i = (Integer)i$.next();System.out.println("Double: " + i.intValue() * 2);

}


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TransTypes

Unlambda Lower

• Enumspublic enum Status {

YES, NO, MAYBE}


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TransTypes

Unlambda Lower


YES, NO, MAYBE private Status(String $enum$name, int $enum$ordinal) { super($enum$name, $enum$ordinal); }}


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TransTypes

Unlambda Lower


YES, NO, MAYBE private Status(String $enum$name, int $enum$ordinal) { super($enum$name, $enum$ordinal); } public static Status valueOf(String name) { return (Status)Enum.valueOf(Status.class, name); }}


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TransTypes

Unlambda Lower


YES, NO, MAYBE private Status(String $enum$name, int $enum$ordinal) { super($enum$name, $enum$ordinal); } public static Status valueOf(String name) { return (Status)Enum.valueOf(Status.class, name); } private static final Status[] $VALUES = new Status[]{ Status.YES, Status.NO, Status.MAYBE};

public static Status[] values() { return (Status[])$VALUES.clone(); }}


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TransTypes

Unlambda Lower

• Enum based switch statements

public class SwitchStatus {

void switchStatus(Status status) {

switch (status) { case MAYBE: return;

default: break; } }}


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TransTypes

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switch (status) { case MAYBE: return;



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TransTypes

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switch (SwitchStatus$1.$SwitchMap$Status[(status).ordinal()]) { case 1: return;



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TransTypes

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class SwitchStatus$1 {

}


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TransTypes

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class SwitchStatus$1 { static final int[] $SwitchMap$Status = new int[Status.values().length];

}


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TransTypes

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class SwitchStatus$1 { static final int[] $SwitchMap$Status = new int[Status.values().length]; [0][0][0]

}


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TransTypes

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class SwitchStatus$1 { static final int[] $SwitchMap$Status = new int[Status.values().length]; [0][0][1] static { try { SwitchStatus$1.$SwitchMap$Status[Status.MAYBE.ordinal()] = 1; } catch (NoSuchFieldError ex) { } }}


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TransTypes

Unlambda Lower






class SwitchStatus$1 { static final int[] $SwitchMap$Status = new int[Status.values().length]; [0][0][1] static { try { SwitchStatus$1.$SwitchMap$Status[Status.MAYBE.ordinal()] = 1; } catch (NoSuchFieldError ex) { } }}


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• Generate bytecodes• Organize initializers• String concatenation• Some optimizations


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• Organize <init> (Constructor)

public class InstanceInitialization { String key="key"; String value; public InstanceInitialization(String value) { this.value = value; }

}


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public class InstanceInitialization { String key="key"; String value; public InstanceInitialization(String value) { super(); this.value = value; }

}


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public class InstanceInitialization { String key; String value; public InstanceInitialization(String value) { super(); key = ”key”; this.value = value; }

}


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public class InstanceInitialization { String key; String value; public <init>(String value) { super(); key = ”key”; this.value = value; }

}


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• Organize <clinit> (Static initialization)

public class StaticInitialization { static String key="key"; static String value; static { value = value; }}


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public class StaticInitialization { static String key; static String value; static { key="key”; value = value; }}


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public class StaticInitialization { static String key; static String value; static void <clinit>{ key="key”; value = value; }}


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• String concatenation

”string” + value

Source code “Generated code”


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”string” + value new StringBuilder()



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”string” + value new StringBuilder() .append(”string”)



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”string” + value new StringBuilder() .append(”string”) .append(value)



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”string” + value new StringBuilder() .append(”string”) .append(value) .toString()


Goto Generation

• Lucky for us there is a byte code for goto

• goto <addr>


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…if (<test>) { <ifblock>} else { <elseblock>}<codeafter>

Source code Byte codePC… …9 ifeq

… <elseblock>

… goto

<stackmap>

22 <ifblock>

22

<stackmap>

29 <codeafter>

29

Java >= 1.6:Stack map frames must

be embedded at target of jump instruction

Code generator (Code.java) marked as not

alive.Goto instruction added to

list of pending jumps (Chain.java)

Pending jumps processed


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…label: <somecode>…goto label;

Source code Byte codePC…20

… goto 20

<stackmap>

<somecode>

Used by goto?Must emit stackmap

• Scenario 1: go back

Emit goto to label and turn code generation on

again


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…goto label; …label: <somecode>

Source code Byte codePC……

29 <somecode>

goto

<stackmap>

Label position not yet known?• emit goto• add to list of pending gotos• turn generation on again

• Scenario 2: go forward

Label used? • emit stack frame • patch pending gotos

29


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Gen.java - Labelled

public void visitLabelled(JCLabeledStatement tree) { // if the label is used from gotos, have to emit stack map if (tree.handler.isUsed(tree)) code.emitStackMap(); … }


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Gen.java - Goto

public void visitGoto(JCGoto tree) { tree.handler.addBranch(new Chain(code.emitJump(goto_), null, code.state.dup()), tree.target); //normally goto marks code as not alive, turn generation on code.entryPoint();}

Target position known?• Yes – patch immediately• No – add to list of pending

gotos

Mission Accompli

shed!

λambd

a


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TransTypes

Unlambda Lower

• Lambda implementation in Java 8– Language change– Compilation– Runtime support

• Many interesting design considerations


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TransTypes

Unlambda Lower

• Simple example

public Comparator<String> lambdaExample() { return (String a, String b) -> a.compareTo(b);}


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TransTypes

Unlambda Lower

• LambdaToMethod.java

public Comparator<String> lambdaExample() { return (String a, String b) -> a.compareTo(b);}

/*synthetic*/ private static int lambda$lambdaExample$0( , ) { return ;}

final String afinal String b

a.compareTo(b)


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TransTypes

Unlambda Lower

• Runtime

public Comparator<String> lambdaExample() { return <invokedynamic>LambdaMetafactory.metafactory(}

/*synthetic*/ private static int lambda$lambdaExample$0( final String a, final String b) { return a.compareTo(b);}

public interface Comparator {/*erased*/ int compare(Object o1, Object o2);}

Lookup(LambdaExample), /*caller*/ "compare", ()Comparator, /*MethodType*/ (Object,Object)int, /*MethodType*/ lambda$lambdaExample$0, /*MethodHandle*/ (String,String)int); /*MethodType*/

final class LambdaExample$$Lambda$1/1834188994 implements Comparator { private LambdaExample$$Lambda$1/1834188994() public int compare(Object,Object)}


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TransTypes

Unlambda Lower

• Runtime class generation

LambdaMetaFactory• metaFactory(…)• altMetaFactory(…)

InnerClassLambdaMetafactory

ASM

Lambda Class


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TransTypes

Unlambda Lower

• SerializationLambda Instance

Serialize

SerializedLambda

Deserialize

LambdaMeta

Factory


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TransTypes

Unlambda Lower

• Example with instance and local capture

private int c = 3; public Comparator<String> withInstanceAndLocalCapture() { int a = 1, b = 2; return (String d, String e) -> d.compareTo(e) + a + b - c;}


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TransTypes

Unlambda Lower


private int c = 3; public Comparator<String> withInstanceAndLocalCapture() { int a = 1, b = 2; return (String d, String e) -> d.compareTo(e) + a + b - c;}/*synthetic*/ private int lambda$withInstanceAndLocalCapture$1(final int cap$0, final int cap$1, final String d, final String e) { return d.compareTo(e) + cap$0 + cap$1 - c;}


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TransTypes

Unlambda Lower


private int c = 3; public Comparator<String> withInstanceAndLocalCapture() { int a = 1, b = 2; return <invokedynamic>LambdaMetafactory.metafactory(this,a,b); }/*synthetic*/ private int lambda$withInstanceAndLocalCapture$1(final int cap$0, final int cap$1, final String d, final String e) { return d.compareTo(e) + cap$0 + cap$1 - c;}


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TransTypes

Unlambda Lower


final class LambdaExample$$Lambda$2/2117255219 extends Object implements Comparator { public int compare(Object, Object) private final LambdaExample arg$1; private final int arg$2; private final int arg$3; private LambdaExample$$Lambda$2/2117255219(LambdaExample,int,int) private static Comparator get$Lambda(LambdaExample,int,int)}

.lambda$withInstanceAndLocalCapture$1(arg$2, arg$3,(String)d,(String)e) arg$1


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TransTypes

Unlambda Lower

• Possible to back port ?– Capture generated class ?– Compile time generation of inner class!


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TransTypes

Unlambda Lower

• Possible to back port ?– Capture generated class ?– Compile time generation of inner class!


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TransTypes

Unlambda Lower

• Step 1: Enable lambda : Source.java

public boolean allowLambda() { return compareTo( ) >= 0;}

JDK1_8JDK1_5

• Step 2: Enable special handling

boolean mustBackportLambda() { return this.target.compareTo(Target.JDK1_8) < 0;}


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TransTypes

Unlambda Lower


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TransTypes

Unlambda Lower

• Step 3: Implement and call lambda method

if(!this.attr.mustBackportLambda()) else{ result = new LambdaBackPorter(...).implementLambdaClass(...);}

result = makeMetafactoryIndyCall(...);


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TransTypes

Unlambda Lower

• Step 3: Example

private static final class Lambda$$2 implements Comparator<String> {

Lambda$$2 { super(); } public int compare(String arg0, String arg1) { return LambdaExample.lambda$lambdaExample$0(arg0, arg1); } public int compare(Object o1, Object o2) { return this.compare((String)o1, (String)o2); }}


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Unlambda Lower

• Step 3: Example

public Comparator<String> lambdaExample() { }

return LambdaMetafactory.metafactory();return new Lambda$$289();

λambd

a

Resources

• OpenJDK – Source: http://hg.openjdk.java.net/

• Compiler hacking tutorial– http://www.ahristov.com/tutorial/java-compiler.html

• Lambda design discussion– http://cr.openjdk.java.net/~briangoetz/lambda/lambda-

translation.html

• Sample code:– https://github.com/skarsaune/goto

• Slide pack:– http://www.slideshare.net/MartinSkarsaune

http://hg.openjdk.java.net/

http://hg.openjdk.java.net/

http://www.ahristov.com/tutorial/java-compiler.html

http://www.ahristov.com/tutorial/java-compiler.html

http://cr.openjdk.java.net/~briangoetz/lambda/lambda-translation.html



https://github.com/skarsaune/goto

https://github.com/skarsaune/goto

http://www.slideshare.net/MartinSkarsaune

http://www.slideshare.net/MartinSkarsaune

Questions or Comments?


Thank You for Your Time!


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JavaZone 2014 - goto java;