Java Bytecode Crash Course [Code One 2019]

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.

Java Bytecode Crash Course [DEV4312]
Principal Member of Technical Staff
Java Platform Group
September 16, 2019
David Buck

JVM Sustaining Engineer
OpenJDK 8 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)

Agenda
Introduction
Foundational Concepts
Examples

Why?
lingua franca of Java platform

Why?
javap output

Why?
javap output
foundation for future study
bridge methods
type eraser
debugging

javap
-v option to “disassemble” bytecode
-p option to see private methods
use latest version

What is bytecode?
byte-sized opcodes
Only 256 possible opcodes
operands can be 8 or 16 bit

stack machine
Convenient
abstract away CPU register details
free liveness analysis

stack machine
Not that rare
RPN notation

stack machine
Not that rare
RPN notation
PostScript

stack machine
Not that rare
RPN notation
PostScript
Forth

local variables
store / load commands
used to access method arguments

Let’s go for a spin!
void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
?

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
?
0

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
0

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
0
0

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
0
0
100

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
1

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
2

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
3

void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
100

double spin
void dspin() {
double i;
for (i = 0.0; i < 100.0; i++) {
; //empty
}
}

double spin
void dspin() {
double i;
for (i = 0.0; i < 100.0; i++) {
; //empty
}
}
void dspin();
stack=4, locals=3,
args_size=1
0: dconst_0
1: dstore_1
2: dload_1
3: ldc2_w #2
// double 100.0d
6: dcmpg
7: ifge 17
10: dload_1
11: dconst_1
12: dadd
13: dstore_1
14: goto 2
17: return

data “categories”
Size 1
boolean
byte
char
short
int
float
reference
returnAddress
Size 2
long
double

int spin
void spin() {
int i;
for (i = 0; i < 100; i++) {
;// empty
}
}
void spin();
Code:
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 14
8: iinc 1, 1
11: goto 2
14: return
StackLocals
this
i
int 1
int 0

Double Spin
void dspin() {
double i;
for (i = 0.0; i < 100.0; i++) {
; //empty
}
}
void dspin();
stack=4, locals=3,
args_size=1
0: dconst_0
1: dstore_1
2: dload_1
3: ldc2_w #2
// double 100.0d
6: dcmpg
7: ifge 17
10: dload_1
11: dconst_1
12: dadd
13: dstore_1
14: goto 2
17: returnStackLocals
this
i (1 of 2)
d1 (2 of 2)
d1 (1 of 2)
i (2 of 2)
d0 (2 of 2)
d0 (1 of 2)

double spin
void dspin() {
double i;
for (i = 0.0; i < 100.0; i++) {
; //empty
}
}
void dspin();
stack=4, locals=3,
args_size=1
0: dconst_0
1: dstore_1
2: dload_1
3: ldc2_w #2
// double 100.0d
6: dcmpg
7: ifge 17
10: dload_1
11: dconst_1
12: dadd
13: dstore_1
14: goto 2
this
i (1 of 2)
d1 (2 of 2)
d1 (1 of 2)
i (2 of 2)
d0 (2 of 2)
d0 (1 of 2)

constant pool loads
Category 1
ldc
ldc_w
Category 2
ldc2_w

but some types are more equal than
others…opcode byte short int long float double char reference
Tipush bipush sipush
Tconst iconst lconst fconst dconst aconst
Tload iload lload fload dload aload
Tstore istore lstore fstore dstore astore
Tinc iinc
Taload baload saload iaload laload faload daload caload aaload
Tastore bastore sastore iastore lastore fastore dastore castore aastore
Tadd iadd ladd fadd dadd
Tsub isub lsub fsub dsub
Tmul imul lmul fmul dmul
Tdiv idiv ldiv fdiv ddiv
Trem irem lrem frem drem
Tneg ineg lneg fneg dneg
Tshl ishl lshl
Tshr ishr lshr
Tushr iushr lushr
Tand iand land
Tor ior lor
Txor ixor lxor
i2T i2b i2s i2l i2f i2d
l2T l2i l2f l2d
f2T f2i f2l f2d
d2T d2i d2l d2f
Tcmp lcmp
Tcmpl fcmpl dcmpl
Tcmpg fcmpg dcmpg
if_TcmpOP if_icmpOP if_acmpOP
Treturn ireturn lreturn freturn dreturn areturn

others…
opcode byte short int long float double char reference
Tipush bipush sipush
Tconst iconst lconst fconst dconst aconst
Tload iload lload fload dload aload
Tstore istore lstore fstore dstore astore
Tinc iinc
Taload baload saload iaload laload faload daload caload aaload
Tastore bastore sastore iastore lastore fastore dastore castore aastore
Tadd iadd ladd fadd dadd
Tsub isub lsub fsub dsub
Tmul imul lmul fmul dmul
Tdiv idiv ldiv fdiv ddiv
Trem irem lrem frem drem
Tneg ineg lneg fneg dneg
Tshl ishl lshl
Tshr ishr lshr
Tushr iushr lushr
Tand iand land
Tor ior lor
Txor ixor lxor
i2T i2b i2s i2l i2f i2d
l2T l2i l2f l2d
f2T f2i f2l f2d
d2T d2i d2l d2f
Tcmp lcmp
Tcmpl fcmpl dcmpl
Tcmpg fcmpg dcmpg
if_TcmpOP if_icmpOP if_acmpOP
Treturn ireturn lreturn freturn dreturn areturn

others…
Actual type
Computational
type
Category
Boolean int 1
byte int 1
char int 1
short int 1
int int 1
float float 1
reference reference 1
returnAddress returnAddress 1
long long 2
double double 2

short spin
void sspin() {
short i;
for (i = 0; i < 100; i++) {
; //empty
}
}

short spin
void sspin() {
short i;
for (i = 0; i < 100; i++) {
; //empty
}
}
void sspin();
stack=2, locals=2,
args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 16
8: iload_1
9: iconst_1
10: iadd
11: i2s
12: istore_1
13: goto 2
this
i
int 1
int 0

constant pool fun
void useManyNumeric() {
int i = 100;
int j = 1000000;
long l1 = 1;
long l2 = 0xffffffff;
double d = 2.2;
// ...do some calculations...
}
void useManyNumeric();
0: bipush 100
2: istore_1
3: ldc #2
// int 1000000
5: istore_2
6: lconst_1
7: lstore_3
8: ldc2_w #3
// long -1l
11: lstore 5
13: ldc2_w #5
// double 2.2d
16: dstore 7
18: return

Constant Pool Fun
void useManyNumeric() {
int i = 100;
int j = 1000000;
long l1 = 1;
long l2 = 0xffffffff;
double d = 2.2;
// ...do some calculations...
}
void useManyNumeric();
0: bipush 100
2: istore_1
3: ldc #2
// int 1000000
5: istore_2
6: lconst_1
7: lstore_3
8: ldc2_w #3
// long -1l
11: lstore 5
13: ldc2_w #5
// double 2.2d
16: dstore 7
18: return

argument passing (virtual)
int addTwo(int i, int j) {
return i + j;
}

return i + j;
}
int addTwo(int, int);
stack=2, locals=3,
args_size=3
0: iload_1
1: iload_2
2: iadd
3: ireturn

return i + j;
}
stack=2, locals=3,
args_size=3
0: iload_1
1: iload_2
2: iadd
3: ireturn
StackLocals
this
i
j

return i + j;
}
stack=2, locals=3,
args_size=3
0: iload_1
1: iload_2
2: iadd
3: ireturn
StackLocals
this
i
i
j

return i + j;
}
stack=2, locals=3,
args_size=3
0: iload_1
1: iload_2
2: iadd
3: ireturn
StackLocals
this
i
i
j j

return i + j;
}
stack=2, locals=3,
args_size=3
0: iload_1
1: iload_2
2: iadd
3: ireturn
StackLocals
this
i
i+j
j

argument passing (static)
static int addTwoStatic(int i, int j)
{
return i + j;
}

{
return i + j;
}
static int
addTwoStatic(int, int);
flags: ACC_STATIC
Code:
stack=2, locals=2,
args_size=2
0: iload_0
1: iload_1
2: iadd
3: ireturn

{
return i + j;
}
static int
addTwoStatic(int, int);
flags: ACC_STATIC
Code:
stack=2, locals=2,
args_size=2
0: iload_0
1: iload_1
2: iadd
3: ireturn
StackLocals
i
j
int 1
int 0

method call (virtual)
int add12and13() {
return addTwo(12, 13);
}
int add12and13();
Code:
stack=3, locals=1,
args_size=1
0: aload_0
1: bipush 12
3: bipush 13
5: invokevirtual #2
// Method addTwo:(II)I
8: ireturn
StackLocals
this

int add12and13() {
}
int add12and13();
Code:
stack=3, locals=1,
args_size=1
0: aload_0
1: bipush 12
3: bipush 13
5: invokevirtual #2
8: ireturn
StackLocals
this this

int add12and13() {
}
int add12and13();
Code:
stack=3, locals=1,
args_size=1
0: aload_0
1: bipush 12
3: bipush 13
5: invokevirtual #2
8: ireturn
StackLocals
this this
12

int add12and13() {
}
int add12and13();
Code:
stack=3, locals=1,
args_size=1
0: aload_0
1: bipush 12
3: bipush 13
5: invokevirtual #2
8: ireturn
StackLocals
this this
12
13

int add12and13() {
}
int add12and13();
Code:
stack=3, locals=1,
args_size=1
0: aload_0
1: bipush 12
3: bipush 13
5: invokevirtual #2
8: ireturn
StackLocals
this 25

method call (static)
int add12and13_static() {
return addTwoStatic(12, 13);
}
int add12and13_static();
Code:
stack=2, locals=1,
args_size=1
0: bipush 12
2: bipush 13
4: invokestatic #3
// Method
addTwoStatic:(II)I
7: ireturn
StackLocals
25

}
Code:
stack=2, locals=1,
args_size=1
0: bipush 12
2: bipush 13
4: invokestatic #3
// Method
addTwoStatic:(II)I
7: ireturn
StackLocals
12

}
Code:
stack=2, locals=1,
args_size=1
0: bipush 12
2: bipush 13
4: invokestatic #3
// Method
addTwoStatic:(II)I
7: ireturn
StackLocals
12
13

method call (private)
class Near {
int it;
public int getItNear() {
return getIt();
}
private int getIt() {
return it;
}
}

class Near {
int it;
public int getItNear() {
return getIt();
}
private int getIt() {
return it;
}
}
public int getItNear();
stack=1, locals=1,
args_size=1
0: aload_0
1: invokespecial #3
// Method getIt:()I
4: ireturn

method call (super class)
class Far extends Near {
int getItFar() {
return super.getItNear();
}
}

class Far extends Near {
int getItFar() {
return super.getItNear();
}
}
int getItFar();
Code:
stack=1, locals=1,
args_size=1
0: aload_0
1: invokespecial #3
// Method
Examples3_7$Near.getItNaer:
()I
4: ireturn

types of calls
invokevirtual
invokestatic
invokeinterface
invokespecial
invokedynamic

types of calls
invokevirtual
instance method
this pointer
virtual lookup

types of calls
invokevirtual
instance method
this pointer
virtual lookup
invokestatic
class method
no this pointer
static linking

types of calls
invokevirtual
instance method
this pointer
virtual lookup
invokestatic
class method
no this pointer
static linking
invokeinterface
interface method
this pointer
virtual lookup

types of calls
invokevirtual
instance method
this pointer
virtual lookup
invokestatic
class method
no this pointer
static linking
invokeinterface
interface method
this pointer
virtual lookup
invokespecial
Everything else
constructors
super class
private
this pointer
static linking

When two Objects love each other very
much…
Object create() {
return new Object();
}
StackLocals

much…
Object create() {
}
java.lang.Object create();
stack=2, locals=1,
args_size=1
0: new #2
// class java/lang/Object
3: dup
4: invokespecial #1
// Method
java/lang/Object."<init>":(
)V
7: areturn
StackLocals

much…
Object create() {
}
stack=2, locals=1,
args_size=1
0: new #2
3: dup
4: invokespecial #1
// Method
)V
7: areturn
StackLocals
baby

much…
Object create() {
}
stack=2, locals=1,
args_size=1
0: new #2
3: dup
4: invokespecial #1
// Method
)V
7: areturn
StackLocals
baby
baby

field access
int i
void setIt(int value) {
i = value;
}
int getIt() {
return i;
}
StackLocals

field access (set)
int i
i = value;
}
int getIt() {
return i;
}
void setIt(int);
stack=2, locals=2,
args_size=2
0: aload_0
1: iload_1
2: putfield #6
// Field i:I
5: return
StackLocals

field access (set)
int i
i = value;
}
int getIt() {
return i;
}
void setIt(int);
stack=2, locals=2,
args_size=2
0: aload_0
1: iload_1
2: putfield #6
// Field i:I
5: return
StackLocals
this

field access (set)
int i
i = value;
}
int getIt() {
return i;
}
void setIt(int);
stack=2, locals=2,
args_size=2
0: aload_0
1: iload_1
2: putfield #6
// Field i:I
5: return
StackLocals
this
value

field access (get)
int i
i = value;
}
int getIt() {
return i;
}
int getIt();
stack=1, locals=1,
args_size=1
0: aload_0
1: getfield #6
// Field i:I
4: ireturn
StackLocals

array opcodes
newarray (primitive type)
anewarray
multianewarray
Tastore (e.g. iastore)
Taload (e.g. faload)

switch statement (table)
int chooseNear(int i) {
switch (i) {
case 0: return 0;
case 1: return 1;
case 2: return 2;
default: return -1;
}
}

switch statement (table)
int chooseNear(int i) {
switch (i) {
case 0: return 0;
case 1: return 1;
case 2: return 2;
default: return -1;
}
}
int chooseNear(int);
0: iload_1
1: tableswitch { // 0 to 2
0: 28
1: 30
2: 32
default: 34
}
28: iconst_0
29: ireturn
30: iconst_1
31: ireturn
32: iconst_2
33: ireturn
34: iconst_m1
35: ireturn

switch statement (sparse)
int chooseFar(int i) {
switch (i) {
case -100: return -1;
case 0: return 0;
case 100: return 1;
default: return -1;
}
}

switch statement (sparse)
int chooseFar(int i) {
switch (i) {
case -100: return -1;
case 0: return 0;
case 100: return 1;
default: return -1;
}
}
int chooseFar(int);
0: iload_1
1: lookupswitch { // 3
-100: 36
0: 38
100: 40
default: 42
}
36: iconst_m1
37: ireturn
38: iconst_0
39: ireturn
40: iconst_1
41: ireturn
42: iconst_m1
43: ireturn

untyped stack operations
pop
pop2
dup
dup2
dup_x1
dup2_x1
dup_x2
dup2_x2
swap

Exceptions
void catchOne();
stack=2, locals=2, args_size=1
0: aload_0
1: invokevirtual #4 // Method tryItOut:()V
4: goto 13
7: astore_1
8: aload_0
9: aload_1
10: invokevirtual #5
// Method handleExc:(Ljava/lang/Exception;)V
13: return
Exception table:
from to target type
0 4 7 Class TestExc

Exceptions
void catchOne() {
try {
tryItOut();
} catch (TestExc e) {
handleExc(e);
}
}

about finally
jsr / ret removed in jdk7 (cass file ver. 51)
now finally block code is just copied to each possible exit point

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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

Java Bytecode Crash Course [Code One 2019]

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