The document outlines a product direction intended for information purposes only and is not binding. Any features or functionality described may change or be removed at Oracle's sole discretion. Statements relating to Oracle's plans are forward-looking and subject to risks. All information is current as of September 2019. Oracle undertakes no duty to update any statement in light of new information or future events. The document is intended to provide a non-binding overview of Oracle's general product direction and is not a commitment to deliver any functionality.

1. The following is intended to outline our general product direction. It is intended for information purposes
only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code,
or functionality, and should not be relied upon in making purchasing decisions. The development,
release, timing, and pricing of any features or functionality described for Oracle’s products may change
and remains at the sole discretion of Oracle Corporation.
Statements in this presentation relating to Oracle’s future plans, expectations, beliefs, intentions and
prospects are “forward-looking statements” and are subject to material risks and uncertainties. A detailed
discussion of these factors and other risks that affect our business is contained in Oracle’s Securities and
Exchange Commission (SEC) filings, including our most recent reports on Form 10-K and Form 10-Q
under the heading “Risk Factors.” These filings are available on the SEC’s website or on Oracle’s website
at http://www.oracle.com/investor. All information in this presentation is current as of September 2019
and Oracle undertakes no duty to update any statement in light of new information or future events.
Safe Harbor
Copyright © 2019 Oracle and/or its affiliates.

2. Java Concurrency,
A(nother) Peek Under the Hood
[DEV4211]
Principal Member of Technical Staff
Java Platform Group
September 19, 2019
David Buck
Copyright © 2019 Oracle and/or its affiliates.

3. IoT Microservices DevOps in the Cloud
Copyright © 2019 Oracle and/or its affiliates.

4. IoT Microservices DevOps in the Cloud
Copyright © 2019 Oracle and/or its affiliates.

5. IoT Microservices DevOps in the Cloud
Copyright © 2019 Oracle and/or its affiliates.

6. JVM Sustaining Engineer
OpenJDK Update Project
Maintainer
JavaOne Rock Star
Co-author of Oracle WebLogic
Server 11g 構築・運用ガイド
@DavidBuckJP
https://blogs.oracle.com/buck/
Who am I? David Buck (left)

7. Program Agenda
Introduction
Background
Tangent 1: Special Relativity
History
Implementation
Tangent 2: HSDIS
Future
Conclusions
1
2
3
4
5
7
6

8. Introduction

9. Motivation
“But I don’t write multithreaded code…”

10. Web Server
picture: Coolcaesar at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

11. GUI

12. Libraries
picture: David Vignoni / ICON KING (http://icon-king.com) [LGPL (http://www.gnu.org/licenses/lgpl.html) or LGPL (http://www.gnu.org/licenses/lgpl.html)]

13. Batch Processing
picture: US Social Security Administration (http://www.ssa.gov/history/acalcs.html) [Public domain]

14. “But I don’t write multithreaded code…”

15. “But I don’t write multithreaded code…”

16. Race Conditions
picture: Sakurambo at English Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

17. Heisenbugs
Bug Heisenberg
Heisenbug
picture: Bundesarchiv, Bild 183-R57262 / Unknown / CC-BY-SA 3.0 [CC BY-SA 3.0 de (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)]

18. Observer Effect
picture: Christian Schirm (Own work) [CC0]

19. Avoiding Heisenbugs
Java Memory Model
synchronized keyword
java.util.concurrent

20. Avoiding Heisenbugs
Java Memory Model
synchronized keyword
java.util.concurrent
• Don’t do ＸＸＸ
• You must do ＹＹＹ

21. a² + b² == c²

22. picture: WTF Public License, Version 2

23. Background

24. Memory Model

25. Memory Model
Definition 1
Specification that explains when we can guarantee that a written
value may be read correctly

26. void hogeMethod1() {
int localA = 42;
assert localA == 42;
}

27. static int staticA;
void hogeMethod2() {
staticA = 42;
assert staticA == 42;
}

28. int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

29. Culprit #1

30. Culprit #2

31. Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}

32. Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}
void hoge5() {
a = 2;
b = 2;
c = 3;
d = 4;
e = 5;
}

33. Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}
void hoge5() {
b = 2;
c = 3;
d = 4;
e = 5;
a = 2;
}

34. Rule
Single threaded behavior must never change

35. Out of Order Execution
picture: Amit6, original version (File:Superscalarpipeline.png) by User:Poil (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)]

36. CPU Differences
Type Alpha ARMv7
PA-
RISC
POWER
SPARC
RMO
SPARC
PSO
SPARC
TSO
x86
x86
oostore
AMD64 IA-64 zSeries
load-
load
Y Y Y Y Y Y Y
load-
store
Y Y Y Y Y Y Y
store-
store
Y Y Y Y Y Y Y Y
store-
load
Y Y Y Y Y Y Y Y Y Y Y Y
Atomic
(loads)
Y Y Y Y Y
Atomic
(stores)
Y Y Y Y Y Y
Depend
ent
loads
Y
instruct
ion
cache
Y Y Y Y Y Y Y Y Y Y
chart source: https://en.wikipedia.org/wiki/Memory_ordering

37. CPU Differences
Loose Strict
Alpha
X86

38. CPU Differences
Loose Strict

39. Memory Barriers
int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

40. Memory Barriers
int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

41. Memory Barriers
int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

42. Memory Barriers Types
store-store
store-load
load-store
load-load

43. Compiler Barriers
GCC
__asm__ __volatile__("":::"memory");
VC++
_ReadBarrier
_WriteBarrier
_ReadWriteBarrier

45. Tangent 1: Special Relativity
picture: Sakurambo (Own work) [Public domain]

46. Tangent 1: Special Relativity
A
B
C

47. Tangent 1: Special Relativity
A
B
C
A, B, C

48. Tangent 1: Special Relativity
A
B
C
A, B, C C, B, A

49. Tangent 1: Special Relativity
Observed order depends on viewer's frame of reference
A
B
C
A, B, C C, B, A

50. No single “correct” timeline
Special Relativity
Observed order of events depends on frame of reference

51. No single “correct” timeline
Special Relativity
Observed order of events depends on frame of reference
Multithreaded Code
Observed order of events depends on thread

52. Memory Model
Definition 2
Clarify what multithreaded behavior developers may depend on

53. Memory Model
Definition 2
Clarify what multithreaded behavior developers may depend on
Limits what types of optimizations the runtime may do

54. History

55. Java Memory Model (JMM)
Write once, run anywhere
1995
Part of the formal language
specification
Happened-before
picture: Peter Campbell [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/4.0-3.0-2.5-2.0-1.0)]

56. happened-before

57. Leslie Lamport
LaTex
happened-before
picture: Leslie Lamport [GFDL (http://en.wikipedia.org/wiki/GFDL]

58. happened-before
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}

59. happened-before
void hoge4() {
data = 42;
ready = true;
}
void hoge3() {
while (!ready) {};
assert data == 42;
}

60. happened-before
void hoge4() {
data = 42;
ready = true;
}
void hoge3() {
while (!ready) {};
assert data == 42;
}

61. happened-before
void synchronized hoge4() {
data = 42;
ready = true;
}
void synchronized hoge3() {
while (!ready) {};
assert data == 42;
}
Warning: if hoge3 is executed first, the above code will deadlock

62. happened-before
void synchronized hoge4()
{
data = 42;
ready = true;
}
void synchronized hoge3()
{
while (!ready) {};
assert data == 42;
}
Warning: if hoge3 is executed first, the above code will deadlock

63. JMM Specification

64. Original JMM Keywords
synchronized
mutual exclusion
happened-before

65. Original JMM Keywords
synchronized
mutual exclusion
happened-before
volatile
Visibility

66. Problems with the Original JMM
volatile does not establish a happened-before relationship
final values can change
Many important runtime optimization were not allowed

67. Doug Lea
Author of the Java Multithreaded
bible
Introduced his own OSS high-
level multithreaded library

68. Java SE 5.0
Modern JMM introduced
Lea’s library added to official JDK
picture: Zvi Roger [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)]

69. Modern JMM
JSR-133
Fixed shortcomings in original JMM
volatile does establish happens-before
final values will never change

70. Modern JMM Keywords
synchronized
mutual exclusion
happened-before
volatile
visibility
happened-before
final
immutability

71. JSR-133 の volatile
void hoge4() {
data = 42;
ready = true;
}
void hoge3() {
while (!ready) {};
assert data == 42;
}
Warning: if hoge3 is executed first, the above code will deadlock
volatile boolean ready = false;

72. volatile != atomic
volatile int id = 0;
int incID() {
return id++;
}

73. volatile != atomic
volatile int id = 0;
int incID() {
return id++;
}
reg = [id]
reg++
[id] = reg

74. volatile != atomic
volatile int id = 0;
synchronized int incID() {
return id++;
}

75. 64-bit updates are not guaranteed atomic
• Reads and Writes to long and double values may not happen
atomically
• Possible to read a mix of two different writes
Copyright © 2019 Oracle and/or its affiliates.
Old value MSB New value LSB

76. happened-before
Monitor (acquisition / release)
Volatile (read / write)
final “freeze” (constructor)
JNI
System.out.println（）
Input / Output
thread start()/join()
operations on j.u.concurrent collections

77. java.util.concurent
JSR-166
Doug Lea’s OSS multithreading library

78. Multithreaded Control
java.util.concurent
synchronized
wait()/notify()
volatile / final
Abstract
Low-level

79. Fork/Join (JDK 7)

80. Functional Programming (JDK 8)
Lambda Forms
Stream API
λ

81. Var Handles
• Lower-level than volatile / final
• Allows a value to be volatile only when we want it to be
• Explicate fence and acquire / release semantics
• Added in JDK 9 to replace several use cases previously only
supported with sun.misc.Unsafe
Copyright © 2019 Oracle and/or its affiliates.

82. Implementation

83. Tangent 2: HSDIS (HotSpot
Disassembler)
Lets us see code generated by JIT
Requires a disassembler plugin
GNU binutils-based plugin
base-hsdis
Command line options
+PrintAssembly
+CompileCommand=print,*MyClass.myMethod

84. Demos

85. ARM Assembly Crash Course

86. ARM Assembly Crash Course
Op Target Source

87. ARM Assembly Crash Course
MOV r02 #42

88. ARM Assembly Crash Course
MOV r02 #42
r02 = 42

89. ARM Assembly Crash Course
Op Target Source

90. ARM Assembly Crash Course
Op Target Source
Exception
STR Source Target

91. ARM Assembly Crash Course
MOV
Copies registers
MOV r02 r03
Writes Literals
MOV r02 #42

92. ARM Assembly Crash Course
LDx
Loads data from memory to a register
ldr r03, [r06]
r03 = *r06

93. ARM Assembly Crash Course
CMP / BEQ
Compare and branch if equal
cmp r0, #0
beq 0x74178d08
if (r0 == 0) goto 0x74178d08

94. ARM Assembly Crash Course
DMB
Data Memory Barrier
dmb st
dmb sy

95. One more thing…

96. Safe Points
Methods proactively poll
JVM makes page unreadable to stop the world
movw ip, #45056 ; 0xb000
movt ip, #30461 ; 0x76fd
ldr ip, [ip] ; {poll_return}

97. Long Value
static long val = 0;
public static void main(String[] args) {
new Thread(() -> { while(true) val = -1L; } ).start();
new Thread(() -> { while(true) val = 0L; } ).start();
while (true) {
long temp = val;
if ( temp != -1 && temp != 0 ) {
System.out.println("temp (DEC): " + temp);
System.out.println("temp (BIN): " + Long.toBinaryString(temp));
System.exit(-1);
}
}
}

98. Object Initialization
public class ObjInit {
static ObjInit globalRef = new ObjInit();
long number;
public ObjInit() {
number = 42;
}

99. Object Initialization
public static void main(String[] args) {
new Thread(()
-> { while(true) {
long temp = globalRef.number;
if (temp != 42) {
System.out.println(temp);
System.exit(-1);
}
}
} ).start();
new Thread(()
-> { while(true) globalRef = new ObjInit(); } ).start(); }}

100. public class Loop extends Thread {
static boolean done = false;
public static void main(String[] args) {
Thread childThread = new Loop();
childThread.start();
System.out.println("Starting loop");
spinWait();
System.out.println("Exited loop");
}
private static void spinWait() {
while (!done) ;
}
public void run() {
try {
Thread.sleep(5 * 1000);
} catch (Exception e) {}
done = true;
}
} // class Loop
Loop

101. Future

102. JEP-188/JMM9
Not delivered with JDK 9 (JMM9 != JDK 9)
JVM-level specification
Compatibility with C11/C++11
Cover new features added since JDK 5
(e.g. AtomicX.weakCompareAndSet())

103. Conclusions

104. Avoid low-level API
java.util.concurent
synchronized
wait()/notify()
volatile / final
Var handles
Abstract
Low-level

105. Code against the specification
Intermittent bugs are common
Behavior changes with JRE / platform
Stuff to avoid
synchronized (new Object()) { }
Randomly adding volatile declarations until it magically starts working

106. Testing
As close to production environment as possible
Same hardware
Same JRE (including version）
Same settings

107. Testing
As much variety as possible
Various hardware
Various JREs (including version）
Various settings

108. Java Concurrency in Practice
Java Concurrency Bible
Covers JSR-133 / JSR-166

109. Summary
Avoid low-level APIs
Code against the standard, not the implementation
Testing is always indispensible
Must read Java Concurrency in Practice (JCiP)

110. Thank You!!!

111. [ JSR 133 (Java Memory Model) FAQ ]
https://www.cs.umd.edu/~pugh/java/
memoryModel/jsr-133-faq.html
[ Java Concurrency in Practice ]
http://jcip.net/
[ Concurrent Programming in Java ]
http://gee.cs.oswego.edu/dl/cpj/
[ The JSR-133 Cookbook for Compiler Writers ]
http://gee.cs.oswego.edu/dl/jmm/cook
book.html
[ PrintAssembly ]
https://wiki.openjdk.java.net/display/H
otSpot/PrintAssembly
[ BASIC DISASSEMBLER PLUGIN FOR
HOTSPOT DOWNLOADS ]
https://kenai.com/projects/base-
hsdis/downloads
References

112. Session Survey
Help us make the content
even better. Please complete
the session survey in the
Mobile App.
Copyright © 2019 Oracle and/or its affiliates.

113. The preceding is intended to outline our general product direction. It is intended for information purposes
only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code,
or functionality, and should not be relied upon in making purchasing decisions. The development,
release, timing, and pricing of any features or functionality described for Oracle’s products may change
and remains at the sole discretion of Oracle Corporation.
Statements in this presentation relating to Oracle’s future plans, expectations, beliefs, intentions and
prospects are “forward-looking statements” and are subject to material risks and uncertainties. A detailed
discussion of these factors and other risks that affect our business is contained in Oracle’s Securities and
Exchange Commission (SEC) filings, including our most recent reports on Form 10-K and Form 10-Q
under the heading “Risk Factors.” These filings are available on the SEC’s website or on Oracle’s website
at http://www.oracle.com/investor. All information in this presentation is current as of September 2019
and Oracle undertakes no duty to update any statement in light of new information or future events.
Safe Harbor
Copyright © 2019 Oracle and/or its affiliates.