Effective Foreign Function Interfaces: From JNI to JNR JavaOne 2013 CON4767 Ryan A. Sciampacone, Senior Software Developer, IBM JTC What do you do when your application needs access to platform features that aren’t available in the Java platform? You need a foreign function interface (FFI). The Java Native Interface (JNI) is the classic power tool for calling native code from your Java program. Using JNI means stepping out of the managed safety of the JVM into the wilds of native code. This session explains the most common JNI performance and correctness pitfalls and explains how to find and avoid them. As the JVM becomes the runtime of choice for more languages, the FFI landscape is also evolving. This session introduces alternative FFI approaches that minimizes effort (SWIG) and native code. It examines JNR in detail and shows how alternatives perform relative to handwritten JNI.

1. © 2013 IBM Corporation
Ryan
A.
Sciampacone
–
Managed
Run4me
Architect
25
September
2013
Effec4ve
Foreign
Func4on
Interfaces:
From
JNI
to
JNR

2. © 2013 IBM Corporation
Important
Disclaimers
THE
INFORMATION
CONTAINED
IN
THIS
PRESENTATION
IS
PROVIDED
FOR
INFORMATIONAL
PURPOSES
ONLY.
WHILST
EFFORTS
WERE
MADE
TO
VERIFY
THE
COMPLETENESS
AND
ACCURACY
OF
THE
INFORMATION
CONTAINED
IN
THIS
PRESENTATION,
IT
IS
PROVIDED
“AS
IS”,
WITHOUT
WARRANTY
OF
ANY
KIND,
EXPRESS
OR
IMPLIED.
ALL
PERFORMANCE
DATA
INCLUDED
IN
THIS
PRESENTATION
HAVE
BEEN
GATHERED
IN
A
CONTROLLED
ENVIRONMENT.
YOUR
OWN
TEST
RESULTS
MAY
VARY
BASED
ON
HARDWARE,
SOFTWARE
OR
INFRASTRUCTURE
DIFFERENCES.
ALL
DATA
INCLUDED
IN
THIS
PRESENTATION
ARE
MEANT
TO
BE
USED
ONLY
AS
A
GUIDE.
IN
ADDITION,
THE
INFORMATION
CONTAINED
IN
THIS
PRESENTATION
IS
BASED
ON
IBM’S
CURRENT
PRODUCT
PLANS
AND
STRATEGY,
WHICH
ARE
SUBJECT
TO
CHANGE
BY
IBM,
WITHOUT
NOTICE.
IBM
AND
ITS
AFFILIATED
COMPANIES
SHALL
NOT
BE
RESPONSIBLE
FOR
ANY
DAMAGES
ARISING
OUT
OF
THE
USE
OF,
OR
OTHERWISE
RELATED
TO,
THIS
PRESENTATION
OR
ANY
OTHER
DOCUMENTATION.
NOTHING
CONTAINED
IN
THIS
PRESENTATION
IS
INTENDED
TO,
OR
SHALL
HAVE
THE
EFFECT
OF:
-­‐
CREATING
ANY
WARRANT
OR
REPRESENTATION
FROM
IBM,
ITS
AFFILIATED
COMPANIES
OR
ITS
OR
THEIR
SUPPLIERS
AND/OR
LICENSORS
2

3. © 2013 IBM Corporation
Your
Speaker:
Ryan
A.
Sciampacone
• Run4me
Architect
@
IBM
(JTC)
• Interpreters
• Garbage
Collec4on
• Plaborm
interfacing
/
op4miza4on
• Hardware
exploita4on
(E)
ryan_sciampacone@ca.ibm.com
(T)
@rsciampacone
3

4. © 2013 IBM Corporation
What
should
you
get
from
this?
• Why
a
foreign
func4on
interface
(FFI)
is
important
• Performance
4ps
/
common
piballs
when
using
JNI
• Next
steps
in
the
evolu4on
of
the
FFI
for
Java
• At
the
very
least,
45
minutes
or
so
of
entertainment
4

5. © 2013 IBM Corporation
WHY
IS
AN
FFI
IMPORTANT?
What
the
Java
Na4ve
Interface
(JNI)
does
for
you…
5

6. © 2013 IBM Corporation
Why
have
a
FFI?
• Plaborm
interfacing
• Legacy
System
Support
• Cross
Language
Interop
• Speed
6

7. © 2013 IBM Corporation
Mo4va4ng
Example:
Missed
some
API
• New
things
come
up
/
Some
things
missed
7

8. © 2013 IBM Corporation
JNI
Call
Example
Java:
8
C:

9. © 2013 IBM Corporation
Benefits
of
the
JNI
implementa4on
• API
based
• GC
Safety
• Java
/
JVM
based
ac4vity
not
blocked
• Plaborm
agnos4c
• Clear
lines
between
Java
/
plaborm
• Founda4on
for
JVM
Tooling
(JVMTI)
9

10. © 2013 IBM Corporation
JNI
API
Building
Blocks
10

11. © 2013 IBM Corporation
JNIEnv
is
your
lifeline
• JNIEnv
is
the
connec4on
back
to
the
“Java
world”
• Calls
through
the
JNIEnv
are
effec4vely
round
trip
• Performance
is
dependent
on
keeping
round
trips
down
11

12. © 2013 IBM Corporation
PERFORMANCE
PITFALLS
But
it’s
already
slow,
what’s
a
few
more
instruc4ons?
12

13. © 2013 IBM Corporation
Caching
Classes,
Method
IDs
and
Field
IDs
• Core
elements
when
interac4ng
with
objects
• Effec4vely
your
na4ve
“resolve”
point
• Not
as
cheap
to
acquire
• So
cache
them!
13

14. © 2013 IBM Corporation
Basic
ID
use
14

15. © 2013 IBM Corporation
Caching
Classes,
Method
IDs
and
Field
IDs
15

16. © 2013 IBM Corporation
Caching
Classes,
Method
IDs
and
Field
IDs
16

17. © 2013 IBM Corporation
Caching
Classes,
Method
IDs
and
Field
IDs
24x!
17
NoCache Cache
Field ID Caching

18. © 2013 IBM Corporation
Triggering
Array
Copies
• Remember:
No
direct
manipula4on
of
Java
Objects
• Basically
3
categories
of
API:
• Some4mes
copying
the
en4re
array
(Get*ArrayElements)
• Always
copy
a
por4on
of
the
array
(Get*ArrayRegion)
• (Usually)
Direct
on-­‐heap
access
(GetPrimi4veArrayCri4cal)
• Be
aware
of
the
cost
of
copying!
18

19. © 2013 IBM Corporation
Triggering
Array
Copies
19

20. © 2013 IBM Corporation
Triggering
Array
Copies
20

21. © 2013 IBM Corporation
Triggering
Array
Copies
21
Elements Region
GetElements vs. GetRegion
8.5x!

22. © 2013 IBM Corporation
Reaching
Back
for
Parameters
• We
all
hate
huge
func4on
prototypes
• Basic
teaching
is
pass
the
object
go
from
there
• Think
–
what
is
the
cost
of
accessing
fields
in
JNI?
• Pass
what
you
need
as
arguments
22

23. © 2013 IBM Corporation
Reaching
Back
for
Parameters
23

24. © 2013 IBM Corporation
Reaching
Back
for
Parameters
24

25. © 2013 IBM Corporation
Reaching
Back
for
Parameters
25
Fetch Parameter
Fetching vs. Parameters
6x!

26. © 2013 IBM Corporation
Local
Reference
Abuse
• Local
references
are
beau4ful
for
hiding
JVM-­‐isms
• And
they
get
blissfully
ignored
by
a
lot
of
na4ve
code
26

27. © 2013 IBM Corporation
Local
Reference
Abuse
27

28. © 2013 IBM Corporation
Local
Reference
Abuse
28

29. © 2013 IBM Corporation
CORRECTNESS
PITFALLS
Hey
it
ran
didn’t
it?
29

30. © 2013 IBM Corporation
Using
Array
API
Correctly
• Recap:
Basically
3
categories
of
API
• Some4mes
copying
the
en4re
array
(Get*ArrayElements)
• Always
copy
a
por4on
of
the
array
(Get*ArrayRegion)
• (Usually)
Direct
on-­‐heap
access
(GetPrimi4veArrayCri4cal)
• You
need
to
be
certain
that
you
are
geqng
a
copy!
• Affected
by
VM
vendor,
GC
technology,
and
version
of
JDK
30

31. © 2013 IBM Corporation
Using
Array
API
Correctly
31

32. © 2013 IBM Corporation
Using
Array
API
Correctly
32

33. © 2013 IBM Corporation
Respec4ng
“Cri4cal”
Sec4ons
• Cri4cal
API:
Get/Release
for
(almost)
assured
direct
access
• Idea:
Direct
on-­‐heap
access
which
locks
out
certain
JVM
events
• General
advice:
Do
not
use
JNI
API
between
Get
and
Release
33

34. © 2013 IBM Corporation
Respec4ng
“Cri4cal”
Sec4ons
34

35. © 2013 IBM Corporation
Check
your
excep4ons!
• Java
code
and
IDEs
really
help
you
with
excep4ons
• Na4ve
code
doesn’t
give
you
this
feedback
• JNI
API
can
raise
excep4ons
-­‐
Check
and
act
accordingly
• You
are
the
try/catch
block
now!
35

36. © 2013 IBM Corporation
Check
your
excep4ons!
36

37. © 2013 IBM Corporation
Check
your
excep4ons!
37

38. © 2013 IBM Corporation
Check
your
return
values!
38

39. © 2013 IBM Corporation
Maintain
your
Global
References
• Global
references
aren’t
cleaned
up
like
Local
references
• Great
for
caching,
but
watch
out
for
leaks
39

40. © 2013 IBM Corporation
Use
the
correct
JNIEnv
• JNIEnv
are
unique
to
your
thread
• Using
the
wrong
one
can
cause
subtle
problems
• GetEnv
/
GetJavaVM
exist
from
the
invoca4on
API
• For
callbacks,
make
sure
you
have
the
correct
one
available
40

41. © 2013 IBM Corporation
EVOLUTION
If
it
is
broken
you
should
probably
fix
it
41

42. © 2013 IBM Corporation
JNI
is
good,
but
some4mes
its
in
the
way
• Direct
func4on
calls
• Speed
• Marshalling
data
is
expensive
• Unsigned
types?
• Op4mizing
the
JNI
interface
can
be
tricky
42

43. © 2013 IBM Corporation
Packed
Objects
• A
solu4on
to
accessing
na4ve
data
• Session:
An
Introduc4on
to
Packed
Objects
(CON5758)
43
int
int
Java
Na4ve
…
int[]
d
anObject
int
int
int
…
Object
header
Object
field
/
data

44. © 2013 IBM Corporation
References
• Introduc4on
to
Packed
Objects
(CON5758)
• JVMLS
2013:
Packed
Objects
(Sciampacone)
hup://goo.gl/jWv2Jx
• John
Duïmovich
(IBM
Java
CTO)
hup://goo.gl/OGeDix
• JNI
Best
Prac4ces
(Dawson,Johnson,Low)
hup://goo.gl/hvyGDY
44

45. © 2013 IBM Corporation
?
45

46. © 2013 IBM Corporation
Copyright
and
Trademarks
©
IBM
Corpora4on
2013.
All
Rights
Reserved.
IBM,
the
IBM
logo,
and
ibm.com
are
trademarks
or
registered
trademarks
of
Interna4onal
Business
Machines
Corp.,
and
registered
in
many
jurisdic4ons
worldwide.
Other
product
and
service
names
might
be
trademarks
of
IBM
or
other
companies.
A
current
list
of
IBM
trademarks
is
available
on
the
Web
–
see
the
IBM
“Copyright
and
trademark
informa4on”
page
at
URL:
www.ibm.com/legal/copytrade.shtml
46