Семантика final полей в java

Ñåìàíòèêà final ïîëåé â java
Âëàäèìèð Ñèòíèêîâ, Âàëåíòèí Êîâàëåíêî
sitnikov@netcracker.com, @VladimirSitnikv
NetCracker
Ñåíòÿáðü 2014

Ââåäåíèå
Ïðèìåðû
2 / 103 (c) Copyright 2014, NetCracker Technology Corp. All rights reserved

Çà÷åì final â JMM?

Áåçîïàñíîñòü String
String s = ...
if ( checkAccess (s)) {
return readFile (s);
}
Ïðàâèëüíî ëè ïðîâåðÿþòñÿ ïðàâà íà äîñòóï ê ôàéëó?

Áåç îïàñíîñòü String â java 1.4
String s = ...
}
Îòâåò çàâèñèò îò âåðñèè, è â java 1.4 âîçìîæíû ïðîáëåìû

String s = GLOBAL ;
}
HackThread
GLOBAL =
"/tmp/ etc / passwd "
. substring (4);
Íàïðèìåð: HackThread âûïîëíÿåò .substring(4) è êîâàðíî
ïåðåäà¼ò åãî â ïîòîê, ÷èòàþùèé ôàéëû

String s = GLOBAL ;
}
HackThread
GLOBAL =
. substring (4);
Â java 1.4 substring-ñòðîêà ññûëàåòñÿ íà òîò æå ìàññèâ
ñèìâîëîâ, è âñ¼ îïðåëåäÿåòñÿ ïîëÿìè String#offset è String#size

String s = GLOBAL ;
}
HackThread
GLOBAL =
. substring (4);
race
race
Ñèíõðîíèçàöèè ìåæäó ïîòîêàìè íåò, ïîýòîìó ÷èòàòåëü ìîæåò
óâèäåòü íåäîèíèöèàëèçèðîâàííûé îáúåêò-ñòðîêó

String s = GLOBAL ;
}
HackThread
GLOBAL =
. substring (4);
race
race
checkAccess ìîæåò óâèäåòü "/tmp/etc/passwd", à readFile óæå
"/etc/passwd"

String s = GLOBAL ;
}
HackThread
GLOBAL =
. substring (4);
race
race
checkAccess ìîæåò óâèäåòü "/tmp/etc/passwd", à readFile óæå
"/etc/passwd"
Äàæå ñèíõðîíèçàöèÿ íà s è volatile íå ñïàñóò!

Áåçîïàñíîñòü String â java 1.5+
String s = GLOBAL ;
}
HackThread
GLOBAL =
. substring (4);
hb
hb
Â java 1.5+ final çàùèùàåò îò íåäîñîçäàííûõ îáúåêòîâ è
HackTread

Çà÷åì íàì JMM?

Êîä-çàãàäêà
int x = 1;
public int neverTryThisAtHome () {
int i = this .x; // it is 1, isn ’t it?
this . setX (2); // just updates x to 2
return this .x - i; // 2 - 1 == ...?
}
Êàêèå çíà÷åíèÿ ìîãóò âåðíóòüñÿ? 1? 0? -1?

int x = 1;
return this .x - i; // 2 - 1 == ...?
}
Äà, òóò âåðí¼òñÿ 1

final int x = 1;
return this .x - i; // 2 - 1 == ...?
}
À òåïåðü äîáàâèì íîæåê final

final int x = 1;
return this .x - i; // 2 - 1 == ...?
}
Ñïåöèôèêàöèÿ ðàçðåøàåò âñå âàðèàíòû: 1, 0, è äàæå -1! (ñì.
ïðèìåð 17.5.3-1)

Íåìíîãî òåîðèè

Ïðîãðàììíûé ïîðÿäîê (program order)
I Îòðàæàåò òî, â êàêîì ïîðÿäêå íàïèñàí èñõîäíûé êîä

I Êîìïèëÿòîðó çàïðåùåíî ïåðåóïîðÿäî÷èâàòü, èãíîðèðîâàòü è
ìåíÿòü îïåðàöèè, åñëè ýòî íàðóøèò program order

I Íî ýòî íå çíà÷èò, ÷òî âñ¼ âûïîëíÿåòñÿ èìåííî òàê, êàê â
èñõîäíîì êîäå

I Íî ýòî íå çíà÷èò, ÷òî âñ¼ âûïîëíÿåòñÿ èìåííî òàê, êàê â
èñõîäíîì êîäå
I Íàïðèìåð: äëÿ îïåðàöèé íàä ëîêàëüíûìè ïåðåìåííûìè
program order âîîáùå íå îïðåäåë¼í

×àñòè÷íûé ïîðÿäîê (partial order)
I Â ãëàâå 17 JLS 8 ðàç óïîìèíàåòñÿ ñëîâî "partial order"

I Â ãëàâå 17 JLS 8 ðàç óïîìèíàåòñÿ ñëîâî "partial order"
I ×àñòè÷íûì ïîðÿäêîì hb
! ÿâëÿåòñÿ áèíàðíîå îòíîøåíèå,
êîòîðîå:

I Â ãëàâå 17 JLS 8 ðàç óïîìèíàåòñÿ ñëîâî partial order
êîòîðîå:
I Ðåôëåêñèâíî: äëÿ ëþáîãî ýëåìåíòà x âûïîëíÿåòñÿ x
hb
! x

êîòîðîå:
hb
! x
I Àíòèñèììåòðè÷íî: åñëè x
hb
! y è y
hb
! x, òî x è y ýòî îäíî è
òî æå

êîòîðîå:
hb
! x
I Àíòèñèììåòðè÷íî: åñëè x
hb
! y è y
hb
! x, òî x è y ýòî îäíî è
òî æå
I Òðàíçèòèâíî: åñëè x
hb
! y è y
hb
! z, òî x
hb
! z

Happens-before
I Îñíîâíîé ïðèíöèï JMM: ðàññìîòðèì âñå âîçìîæíûå
âûïîëíåíèÿ è îòêèíåì íåïðàâèëüíûå

Happens-before
I Happens-before ýòî ÷àñòè÷íûé ïîðÿäîê, êîòîðûé äîëæåí
ñîáëþäàòüñÿ

Happens-before
I Çàïðåùåíî âèäåòü áóäóùèå çàïèñè (r hb
! w)

Happens-before
! w)
I È òå, êîòîðûå áûëè ïåðåò¼ðëèñü â hb ïîðÿäêå:
w1 hb
! w2 hb
! r

Happens-before
! w)
I È òå, êîòîðûå áûëè ïåðåò¼ðëèñü â hb ïîðÿäêå:
w1 hb
! w2 hb
! r
I È òå, êîòîðûå çàïðåùåíû ïðîñòî òàê: 17.4.8 executions and
causality requirements

Ñåìàíòèêà final ïîëåé îäíèì ñëàéäîì
w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2
I w è r2 – èíòåðåñóþùèå íàñ íàñ çàïèñü è ÷òåíèå

w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2
I w è r2 – èíòåðåñóþùèå íàñ íàñ çàïèñü è ÷òåíèå
I f – çàìîðîçêà final ïîëÿ, êîòîðîå ÷èòàåòñÿ â r1

w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2
I Åñëè åäèíñòâåííûé ïóòü îò çàïèñè ê ÷òåíèþ èä¼ò ÷åðåç âñå
ýòè ñòðåëî÷êè, òî ìû íå ìîæåì óâèäåòü áîëåå ðàííèå çàïèñè

w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2
I Åñëè åäèíñòâåííûé ïóòü îò çàïèñè ê ÷òåíèþ èä¼ò ÷åðåç âñå
ýòè ñòðåëî÷êè, òî ìû íå ìîæåì óâèäåòü áîëåå ðàííèå çàïèñè
I Åñëè ïóòåé áîëüøå îäíîãî – êàê ïîâåç¼ò

Freeze action w hb
! f hb
! a mc
! r1 dr
! r2

Freeze action w hb
! f hb
! a mc
! r1 dr
! r2
Freeze action

Freeze action w hb
! f hb
! a mc
! r1 dr
! r2
Freeze action
Ïîëå
Freeze
ñëó÷àåòñÿ â êîíöå êîíñòðóêòîðà è ìîðîçèò final ïîëÿ

Dereference chain: ÷òî ýòî? w hb
! f hb
! a mc
! r1 dr
! r2
T local = GLOBAL ; r1
int localX = local .x; r2
I r2 ÷èòàåò ïîëå îáúåêòà

! f hb
! a mc
! r1 dr
! r2
I Ïîòîê íå ñîçäàâàë îáúåêò

! f hb
! a mc
! r1 dr
! r2
I Çíà÷èò, ãäå-òî ìû äîëæíû áûëè ÷èòàòü àäðåñ ýòîãî îáúåêòà

! f hb
! a mc
! r1 dr
! r2
dr
I Çíà÷èò, ãäå-òî ìû äîëæíû áûëè ÷èòàòü àäðåñ ýòîãî îáúåêòà
I Ýòî íàçûâàåòñÿ r1 dr
! r2

Dereference chain äâà ïîòîêà w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
T localA = new T();
GLOBAL = localA ;
Thread 2
T localB = GLOBAL ; r1
if ( localB != null ) {
int localX = localB .x; r2
}
dr
r1 dr
! r2 (÷èòàåì ïîëå íåñîçäàííîãî íàìè îáúåêòà)

! f hb
! a mc
! r1 dr
! r2
Thread 1
T localA = new T(); a
GLOBAL = localA ;
Thread 2
}
dr
dr?
Åñòü ëè a dr
! r2?

! f hb
! a mc
! r1 dr
! r2
Thread 1
T localA = new T(); a
GLOBAL = localA ;
Thread 2
}
dr
dr?
Ìåæäó ïîòîêàìè dr íå âîçíèêàåò!

Dereference chain: êîíòðîëüíûé âwûñòðåë hb
! f hb
! a mc
! r1 dr
! r2
T local = GLOBAL ; ra
local = GLOBAL ; rb
Åñòü ëè çäåñü dr ?

Dereference chain: êîíòðîëüíûé âwûñòðåë hb
! f hb
! a mc
! r1 dr
! r2
T local = GLOBAL ; ra
local = GLOBAL ; rb
dr?
dr?
Îäèí èç ra dr
! r2 èëè rb dr
! r2 òî÷íî äîëæåí áûòü, íî òî÷íåå
ñêàçàòü íåâîçìîæíî

Memory chain: ÷òî ýòî? w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o;
Thread 3
T o = GL2;
int r = o.x;
mc
Åñëè ÷òåíèå âèäèò çàïèñü, òî w1 mc
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o; w2
Thread 3
T o = GL2;
int r = o.x;
mc
mc
Åñëè ïèøåì àäðåñ ñîçäàííîãî âðàãîì îáúåêòà, òî r1 mc
! w2

! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o; w2
Thread 3
T o = GL2; r3
int r = o.x;
mc
mc
mc
r3 âèäèò w2 ) w2 mc
! r3

! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o; w2
Thread 3
T o = GL2; r3
int r = o.x; r4
mc
mc
mc dr
r3 dr
! r4 (÷èòàåì ïîëå îáúåêòà)

! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o; w2
Thread 3
T o = GL2; r3
int r = o.x; r4
mc
mc
mc dr
mc
r3 dr
! r4 (÷èòàåì ïîëå îáúåêòà) ) r3 mc
! r4

! f hb
! a mc
! r1 dr
! r2
Thread 1
T o = new T ();
GL = o; w1
Thread 2
T o = GL; r1
GL2 = o; w2
Thread 3
T o = GL2; r3
int r = o.x; r4
mc
mc
mc dr
mc
mc
mc òðàíçèòèâíî (ò.ê. ÷àñòè÷íûé ïîðÿäîê) ) w1 mc
! r4

Ââåäåíèå
Ïðèìåðû

Disclaimer
I Âñå ïðèìåðû îïàñíî ñèíõðîíèçèðîâàíû (èíà÷å çà÷åì ìû
ñîáðàëèñü?)

Disclaimer
I Ñëó÷àè ÷òåíèÿ null íå ðàññìàòðèâàåì (äàæå åñëè îíè
âîçìîæíû)

Disclaimer
I Ñëó÷àè ÷òåíèÿ null íå ðàññìàòðèâàåì (äàæå åñëè îíè
âîçìîæíû)
I Ïðè ñîñòàâëåíèè ñëàéäîâ ïîñòðàäàë íå îäèí ìîçã

Ýòàëîííûé final (1) w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}};
GLOBAL = l;
Thread 2
T o = GLOBAL ;
if (o != null ) {
int result = o.fx; r2
}
Âîçìîæíî ëè â result ïîëó÷èòü 0?

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ;
if (o != null ) {
int result = o.fx;
}
hb
hb
Äåéñòâèÿ â îäíîì ïîòîêå îáðàçóþò happens-before:
w hb
! f , f hb
! a

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ; r0
if (o != null ) {
int result = o.fx;
}
hb
hb
mc
r0 âèäèò çàïèñü a :
a mc
! r0

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ; r0
if (o != null ) {
}
hb
hb
mc
dr
Ïîòîê 2 íå ñîçäàâàë îáúåêò, r1 ÷èòàåò åãî ïîëå, à r ýòî
åäèíñòâåííîå ÷òåíèå àäðåñà îáúåêòà, ïîýòîìó dereference chain:
r0 dr
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ; r0
if (o != null ) {
}
hb
hb
mc
dr
mc
r0 dr
! r1 ) r0 mc
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ; r0
if (o != null ) {
}
hb
hb
mc
mc
mc
a mc
! r1 ( mc òðàíçèòèâíî)

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ;
if (o != null ) {
int result = o.fx; r1 r2
}
hb
hb
mc
dr
Âîçüì¼ì r2 = r 1, òîãäà r1 dr
! r2 ( dr ðåôëåêñèâíî)

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb*
Íàøëè âñ¼ íåîáõîäèìîå äëÿ HB:
w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
}}; f
GLOBAL = l; a
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb*
(w hb
! r2) ) result 2 f42g

Ìàññèâ âíóòðè final (2) w hb
! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
int [] u = new int [1];
u [0] = 42; w
fx = u;
}};
GLOBAL = l;
T o = GLOBAL ;
if (o != null ) {
int [] lfx = o.fx;
int result = lfx [0]; r2
}

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
u [0] = 42; w
fx = u;
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
int [] lfx = o.fx;
}
hb
hb
w hb
! f , f hb
! a

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
u [0] = 42; w
fx = u;
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
int [] lfx = o.fx; r1
}
hb
hb
mc
r1 âèäèò çàïèñü a (ò.ê. ïðîñòîå final ïîëå):
a mc
! r1

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
u [0] = 42; w
fx = u;
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
Ïîòîê 2 íå ñîçäàâàë ìàññèâ, r2 ÷èòàåò åãî ýëåìåíò, r1 ýòî
åäèíñòâåííîå ÷òåíèå àäðåñà ìàññèâà, ïîýòîìó dereference chain:
r1 dr
! r2

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
u [0] = 42; w
fx = u;
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb*
w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
u [0] = 42; w
fx = u;
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb*
(w hb

Ìàññèâ íàîáîðîò (2.1) w hb
! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
fx = u; // (!)
u [0] = 42; w
}};
GLOBAL = l;
T o = GLOBAL ;
if (o != null ) {
int [] lfx = o.fx;
int result = lfx [0];
}

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
fx = u; // (!)
u [0] = 42; w
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb
Ñòðîèì hb òàê æå êàê è â ïðåäûäóùåì ñëó÷àå

! f hb
! a mc
! r1 dr
! r2
T l = new T() {{
fx = u; // (!)
u [0] = 42; w
}}; f
GLOBAL = l; a
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
mc
dr
hb
(w hb
Ðåçóëüòàò íå çàâèñèò îò ïîðÿäêà çàïèñè final ïîëåé!

Óòåêàíèå this (3) w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLOBAL = this ;
}};
Thread 2
T o = GLOBAL ;
if (o != null ) {
}

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLOBAL = this ; a
}}; f
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb
w hb
! f , a hb
! f

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLOBAL = this ; a
}}; f
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb?
hb
Íî íàì-òî íóæíî f hb
! a!

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLOBAL = this ; a
}}; f
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb?
hb
Åñëè a hb
! f è f hb
! a, òî a = f (àíòèñèììåòðè÷íîñòü hb )
Íî ïóáëèêàöèÿ ññûëêè ýòî íèêàê íå freeze action!

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLOBAL = this ; a
}}; f
Thread 2
T o = GLOBAL ;
if (o != null ) {
}
hb
hb?
hb
Íåò hb , ïîýòîìó âîçìîæíû âñå âàðèàíòû: result 2 f0; 42g

Âðåäèòåëüñêîå óòåêàíèå this (4) w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ;
}};
GLa = l;
Thread 2
T u = GLb;
T o = GLa;
if (o != null ) {
}

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ; b
}}; f
GLa = l; a
Thread 2
T u = GLb;
T o = GLa;
if (o != null ) {
int result = o.fx;
}
hb
hb
hb?
w hb
! f , f hb
! a

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ; b
}}; f
GLa = l; a
Thread 2
T u = GLb;
T o = GLa; ra
if (o != null ) {
int result = o.fx;
}
hb
hb
hb?
mc
Ïóñòü âòîðîé ïîòîê óâèäåë GLa, òîãäà a mc
! ra!

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ; b
}}; f
GLa = l; a
Thread 2
T u = GLb; rb
T o = GLa; ra
if (o != null ) {
}
hb
hb
hb?
mc
dr?
dr?
Ãäå-òî äîëæíî áûòü dr : rb dr
! r1 èëè ra dr
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ; b
}}; f
GLa = l; a
Thread 2
T u = GLb; rb
T o = GLa; ra
if (o != null ) {
}
hb
hb
hb?
mc
dr?
dr
! r 1, òî ìû ïîïàëè, ò.ê. w hb
Åñëè rb dr
! r1 íå ñòðîèòñÿ

! f hb
! a mc
! r1 dr
! r2
Thread 1
T l = new T() {{
fx = 42; w
GLb = this ; b
}}; f
GLa = l; a
Thread 2
T u = GLb; rb
T o = GLa; ra
if (o != null ) {
}
hb
hb
hb?
mc
dr?
dr
Âûâîä: åñëè íàø ïîòîê óæå ÷èòàë îáúåêò ñ íåçàìîðîæåííûìè
ïîëÿìè, òî ãàðàíòèé final semantics íåò!

Reflection in action (5) w hb
! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1; w1
}};
GLOBAL = t;
U w = new U ();
w.x = 42; w2
reflectSet (t.fu , w);
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
int result = u.x; r2
}

! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}};
GLOBAL = t;
U w = new U ();
w.x = 42; w2
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
}
hb*?
Ïî-õîðîøåìó, íàì íóæíî w2 hb
! r2 (èíà÷å íå çàïðåùåíî âèäåòü
çàïèñü 0 â w.x)

! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}};
GLOBAL = t;
U w = new U ();
w.x = 42; w2
reflectSet (t.fu , w); f2
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
}
hb*?
f2 íå ïîäõîäèò äëÿ hb : íåò ïîäõîäÿùåãî f 2 hb
! a

! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}}; f1
GLOBAL = t;
U w = new U ();
w.x = 42; w2
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
}
hb*?
hb?
f1 òîæå íå ïîäõîäèò äëÿ hb : äîëæíî áûòü w2 hb
! f 1

! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}}; f1
GLOBAL = t;
U w = new U ();
w.x = 42; w2
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
}
hb*?
hb?
Ïîëó÷àåòñÿ, ÷òî ÷òåíèþ r2 íå çàïðåùåíî âèäåòü 0

! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}}; f1
GLOBAL = t; a
U w = new U();
w.x = 42; w2
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
}
hb*?
hb?
Èòîãî: ïîñëå ïóáëèêàöèè ìåíÿòü final ïîëÿ îïàñíî
result 2 f0; 1; 42g

×èíèì reflection (5.1) w hb
! f hb
! a mc
! r1 dr
! r2
T t = new T() {{
fu = new U ();
fu.x = 1;
}};
U w = new U ();
w.x = 42;
GLOBAL = t; a
Thread 2
T t = GLOBAL ;
if (t != null ) {
U u = t.fu;
int result = u.x;
}
Åñëè ïóáëèêîâàòü ññûëêó ïîñëå âñåõ èçìåíåíèé final ïîëåé, òî
âñ¼ ðàáîòàåò: result 2 f1; 42g

Final wrapper (6) w hb
! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a;
Thread 2
A a = GL;
B = new B() {{
fb = o;
}};
GL2 = b;
Thread 3
B B = GL2;
A a = b.fb;
int r = a.fx; r2

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
mc
A a = b.fb; rb
int r = a.fx; r1
mc
rb âèäèò wb ) wb mc
! rb
ra âèäèò çàïèñü wa : wa mc
! ra

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
mc
A a = b.fb; rb
int r = a.fx; r1
mc
mc
Ïîòîê 2 ïèøåò àäðåñ ñîçäàííîãî âðàãîì îáúåêòà, ïîýòîìó
ra mc
! wb

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
mc
A a = b.fb; rb
int r = a.fx; r1
mc
mc
dr
Ïîòîê 3 íå ñîçäàâàë îáúåêò A, r ÷èòàåò åãî ïîëå, à rb ýòî
åäèíñòâåííîå ÷òåíèå àäðåñà îáúåêòà, ïîýòîìó dereference chain:
rb dr
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
mc
A a = b.fb; rb
int r = a.fx; r1
mc
mc
dr
mc
rb dr
! r1 ) rb mc
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}};
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
mc
A a = b.fb; rb
int r = a.fx; r1
mc
mc
mc
mc
mc òðàíçèòèâíî (ò.ê. ÷àñòè÷íûé ïîðÿäîê) ) wa mc
! r1

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}}; f
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
A a = b.fb; rb
int r = a.fx; r1 r2
mc
dr
Âîçüì¼ì r2 = r 1, òîãäà r1 dr
! r2 ( dr ðåôëåêñèâíî)

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}}; f
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
A a = b.fb; rb
int r = a.fx; r1 r2
mc
dr
hb
hb
hb*
w hb
! f hb
! a mc
! r2 ) w hb
! r1 dr
! r2

! f hb
! a mc
! r1 dr
! r2
Thread 1
A a = new O() {{
fx = 42; w
}}; f
GL = a; wa
Thread 2
A a = GL; ra
B = new B() {{
fb = o; wb
}};
GL2 = b;
Thread 3
B B = GL2;
A a = b.fb; rb
int r = a.fx; r1 r2
mc
dr
hb
hb
hb*
(w hb

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Семантика final полей в java

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