字节码
enum Code {
_illegal = -1,
// Java bytecodes
_nop = 0, // 0x00
_aconst_null = 1, // 0x01
_iconst_m1 = 2, // 0x02
_iconst_0 = 3, // 0x03
_iconst_1 = 4, // 0x04
_iconst_2 = 5, // 0x05
_iconst_3 = 6, // 0x06
_iconst_4 = 7, // 0x07
_iconst_5 = 8, // 0x08
_lconst_0 = 9, // 0x09
_lconst_1 = 10, // 0x0a
_fconst_0 = 11, // 0x0b
_fconst_1 = 12, // 0x0c
_fconst_2 = 13, // 0x0d
_dconst_0 = 14, // 0x0e
_dconst_1 = 15, // 0x0f
_bipush = 16, // 0x10
_sipush = 17, // 0x11
_ldc = 18, // 0x12
_ldc_w = 19, // 0x13
_ldc2_w = 20, // 0x14
_iload = 21, // 0x15
_lload = 22, // 0x16
_fload = 23, // 0x17
_dload = 24, // 0x18
_aload = 25, // 0x19
_iload_0 = 26, // 0x1a
_iload_1 = 27, // 0x1b
_iload_2 = 28, // 0x1c
_iload_3 = 29, // 0x1d
_lload_0 = 30, // 0x1e
_lload_1 = 31, // 0x1f
_lload_2 = 32, // 0x20
_lload_3 = 33, // 0x21
_fload_0 = 34, // 0x22
_fload_1 = 35, // 0x23
_fload_2 = 36, // 0x24
_fload_3 = 37, // 0x25
_dload_0 = 38, // 0x26
_dload_1 = 39, // 0x27
_dload_2 = 40, // 0x28
_dload_3 = 41, // 0x29
_aload_0 = 42, // 0x2a
_aload_1 = 43, // 0x2b
_aload_2 = 44, // 0x2c
_aload_3 = 45, // 0x2d
_iaload = 46, // 0x2e
_laload = 47, // 0x2f
_faload = 48, // 0x30
_daload = 49, // 0x31
_aaload = 50, // 0x32
_baload = 51, // 0x33
_caload = 52, // 0x34
_saload = 53, // 0x35
_istore = 54, // 0x36
_lstore = 55, // 0x37
_fstore = 56, // 0x38
_dstore = 57, // 0x39
_astore = 58, // 0x3a
_istore_0 = 59, // 0x3b
_istore_1 = 60, // 0x3c
_istore_2 = 61, // 0x3d
_istore_3 = 62, // 0x3e
_lstore_0 = 63, // 0x3f
_lstore_1 = 64, // 0x40
_lstore_2 = 65, // 0x41
_lstore_3 = 66, // 0x42
_fstore_0 = 67, // 0x43
_fstore_1 = 68, // 0x44
_fstore_2 = 69, // 0x45
_fstore_3 = 70, // 0x46
_dstore_0 = 71, // 0x47
_dstore_1 = 72, // 0x48
_dstore_2 = 73, // 0x49
_dstore_3 = 74, // 0x4a
_astore_0 = 75, // 0x4b
_astore_1 = 76, // 0x4c
_astore_2 = 77, // 0x4d
_astore_3 = 78, // 0x4e
_iastore = 79, // 0x4f
_lastore = 80, // 0x50
_fastore = 81, // 0x51
_dastore = 82, // 0x52
_aastore = 83, // 0x53
_bastore = 84, // 0x54
_castore = 85, // 0x55
_sastore = 86, // 0x56
_pop = 87, // 0x57
_pop2 = 88, // 0x58
_dup = 89, // 0x59
_dup_x1 = 90, // 0x5a
_dup_x2 = 91, // 0x5b
_dup2 = 92, // 0x5c
_dup2_x1 = 93, // 0x5d
_dup2_x2 = 94, // 0x5e
_swap = 95, // 0x5f
_iadd = 96, // 0x60
_ladd = 97, // 0x61
_fadd = 98, // 0x62
_dadd = 99, // 0x63
_isub = 100, // 0x64
_lsub = 101, // 0x65
_fsub = 102, // 0x66
_dsub = 103, // 0x67
_imul = 104, // 0x68
_lmul = 105, // 0x69
_fmul = 106, // 0x6a
_dmul = 107, // 0x6b
_idiv = 108, // 0x6c
_ldiv = 109, // 0x6d
_fdiv = 110, // 0x6e
_ddiv = 111, // 0x6f
_irem = 112, // 0x70
_lrem = 113, // 0x71
_frem = 114, // 0x72
_drem = 115, // 0x73
_ineg = 116, // 0x74
_lneg = 117, // 0x75
_fneg = 118, // 0x76
_dneg = 119, // 0x77
_ishl = 120, // 0x78
_lshl = 121, // 0x79
_ishr = 122, // 0x7a
_lshr = 123, // 0x7b
_iushr = 124, // 0x7c
_lushr = 125, // 0x7d
_iand = 126, // 0x7e
_land = 127, // 0x7f
_ior = 128, // 0x80
_lor = 129, // 0x81
_ixor = 130, // 0x82
_lxor = 131, // 0x83
_iinc = 132, // 0x84
_i2l = 133, // 0x85
_i2f = 134, // 0x86
_i2d = 135, // 0x87
_l2i = 136, // 0x88
_l2f = 137, // 0x89
_l2d = 138, // 0x8a
_f2i = 139, // 0x8b
_f2l = 140, // 0x8c
_f2d = 141, // 0x8d
_d2i = 142, // 0x8e
_d2l = 143, // 0x8f
_d2f = 144, // 0x90
_i2b = 145, // 0x91
_i2c = 146, // 0x92
_i2s = 147, // 0x93
_lcmp = 148, // 0x94
_fcmpl = 149, // 0x95
_fcmpg = 150, // 0x96
_dcmpl = 151, // 0x97
_dcmpg = 152, // 0x98
_ifeq = 153, // 0x99
_ifne = 154, // 0x9a
_iflt = 155, // 0x9b
_ifge = 156, // 0x9c
_ifgt = 157, // 0x9d
_ifle = 158, // 0x9e
_if_icmpeq = 159, // 0x9f
_if_icmpne = 160, // 0xa0
_if_icmplt = 161, // 0xa1
_if_icmpge = 162, // 0xa2
_if_icmpgt = 163, // 0xa3
_if_icmple = 164, // 0xa4
_if_acmpeq = 165, // 0xa5
_if_acmpne = 166, // 0xa6
_goto = 167, // 0xa7
_jsr = 168, // 0xa8
_ret = 169, // 0xa9
_tableswitch = 170, // 0xaa
_lookupswitch = 171, // 0xab
_ireturn = 172, // 0xac
_lreturn = 173, // 0xad
_freturn = 174, // 0xae
_dreturn = 175, // 0xaf
_areturn = 176, // 0xb0
_return = 177, // 0xb1
_getstatic = 178, // 0xb2
_putstatic = 179, // 0xb3
_getfield = 180, // 0xb4
_putfield = 181, // 0xb5
_invokevirtual = 182, // 0xb6
_invokespecial = 183, // 0xb7
_invokestatic = 184, // 0xb8
_invokeinterface = 185, // 0xb9
_invokedynamic = 186, // 0xba // if EnableInvokeDynamic
_new = 187, // 0xbb
_newarray = 188, // 0xbc
_anewarray = 189, // 0xbd
_arraylength = 190, // 0xbe
_athrow = 191, // 0xbf
_checkcast = 192, // 0xc0
_instanceof = 193, // 0xc1
_monitorenter = 194, // 0xc2
_monitorexit = 195, // 0xc3
_wide = 196, // 0xc4
_multianewarray = 197, // 0xc5
_ifnull = 198, // 0xc6
_ifnonnull = 199, // 0xc7
_goto_w = 200, // 0xc8
_jsr_w = 201, // 0xc9
_breakpoint = 202, // 0xca
number_of_java_codes,
// JVM bytecodes
_fast_agetfield = number_of_java_codes,
_fast_bgetfield ,
_fast_cgetfield ,
_fast_dgetfield ,
_fast_fgetfield ,
_fast_igetfield ,
_fast_lgetfield ,
_fast_sgetfield ,
_fast_aputfield ,
_fast_bputfield ,
_fast_zputfield ,
_fast_cputfield ,
_fast_dputfield ,
_fast_fputfield ,
_fast_iputfield ,
_fast_lputfield ,
_fast_sputfield ,
_fast_aload_0 ,
_fast_iaccess_0 ,
_fast_aaccess_0 ,
_fast_faccess_0 ,
_fast_iload ,
_fast_iload2 ,
_fast_icaload ,
_fast_invokevfinal ,
_fast_linearswitch ,
_fast_binaryswitch ,
// special handling of oop constants:
_fast_aldc ,
_fast_aldc_w ,
_return_register_finalizer ,
_shouldnotreachhere, // For debugging
// Platform specific JVM bytecodes
#ifdef TARGET_ARCH_x86
# include "bytecodes_x86.hpp"
#endif
#ifdef TARGET_ARCH_sparc
# include "bytecodes_sparc.hpp"
#endif
#ifdef TARGET_ARCH_zero
# include "bytecodes_zero.hpp"
#endif
#ifdef TARGET_ARCH_arm
# include "bytecodes_arm.hpp"
#endif
#ifdef TARGET_ARCH_ppc
# include "bytecodes_ppc.hpp"
#endif
number_of_codes
};
将操作栈中的栈顶元素弹出并设值本地变量表对应的变量。
astore
String s = "greet";
0: ldc #2 // String greet
2: astore_1
astore_
指令_astore，_astore_0，_astore_1，_astore_2，_astore_3
dstore
dstore_
fstore
fstore_
istore
istore_
lstore
lstore_
int i = 10; long l = 100L; float f = 0.1F; double d = 0.1; String s = "abc"; char c = 'a';0: bipush 10
2: istore_1
3: ldc2_w #2 // long 100l
6: lstore_2
7: ldc #4 // float 0.1f
9: fstore 4
11: ldc2_w #5 // double 0.1d
14: dstore 5
16: ldc #7 // String abc
18: astore 7
20: bipush 97
22: istore 8
aastore
bastore
castore
dastore
fastore
iastore
lastore
sastore
将本地变量表中对应的变量压入到操作栈中。
aload
String s = "greet";
0: ldc #2 // String greet
2: astore_1
String s2 = s;
3: aload_1
4: astore_2
aload_
指令_aload_0，_aload_1，_aload_2，_aload_3，_fast_aload_0
dload
dload_
fload
fload_
iload
iload_
lload
lload_
aaload
baload
caload
daload
faload
iaload
laload
saload
int i2 = i; long l2 = l; float f2 = f; double d2 = d; String s2 = s; char c2 = c;
24: iload_1
25: istore 9
27: lload_2
28: lstore 10
30: fload 4
32: fstore 12
34: dload 5
36: dstore 13
38: aload 7
40: astore 15
42: iload 8
44: istore 16
putfield
假设对象实例i有个字段value，对value赋值如下：
private int value;
value = 1;
0: aload_0
1: iconst_1
2: putfield #2 // Field value:I
getfield
假设对象实例i有个字段value，对value赋值如下：
private int value;
value++;
0: aload_0
1: dup
2: getfield #2 // Field value:I
5: iconst_1
6: iadd
7: putfield #2 // Field value:I
int value2 = value;
0: aload_0
1: getfield #2 // Field value:I
4: istore_1
putstatic
getstatic
new
格式：new
indexbyte1
indexbyte2
在Heap上为指定的类对象先分配一块内存，并初始化其中的变量为默认的初始值，最后将该对象的一个引用objectref压入操作栈的栈顶。
这里的类由
new字节码随后的
indexbyte1，indexbyte2两个参数决：(indexbyte1 << 8) | indexbyte2，其值表示要实例化的类在常量池中的索引
创建对象new例子：
public class JavaNew {
public static void main(String[] args) {
Object obj = new Object();
}
} 反汇编结果：
>javap -c JavaNew
Compiled from "JavaNew.java"
public class JavaNew {
public JavaNew();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":
()V
4: return
public static void main(java.lang.String[]);
Code:
0: new #2 // class java/lang/Object
3: dup
4: invokespecial #1 // Method java/lang/Object."":
()V
7: astore_1
8: return
}
从反编译的结果看，生成了一个构造方法，源代码中我没有为JavaNew定义任何构造方法，如果没有显式定义构造方法，Java会隐式定义一个默认的无参构造方法。在public JavaNew();下面是这个构造方法的字节码实现指令，其中一条重要的指令：invokespecial #1，在JVM解释执行遇到这个指令的时候，JVM会调用一个特殊方法：，这从后面的注释也可以看出来：// Method java/lang/Object."":()V，它对应Java层面的构造方法：public JavaNew() {}，在这里我们可以认为构造方法也是一个特殊的构造方法，特殊在于在Java层面上它的方法名在Java层面看和类型一样，但在JVM层面却对应的是：，另外没有显式的返回类型，只有参数类型是一样的。最后要注意的事后面的()V，它是构造方法的签名，其形式如下：
(参数类型签名列表)返回类型签名
在另一篇文章： https://lobin.iteye.com/blog/2437928，讲GetMethodID函数的时候也会传入对应方法的签名。
创建数组newarray例子：
newarray
public class JavaNewArray {
public static void main(String[] args) {
int test[] = new int[123];
}
} 反汇编结果：
>javap -c JavaNewArray
Compiled from "JavaNewArray.java"
public class JavaNewArray {
public JavaNewArray();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":
()V
4: return
public static void main(java.lang.String[]);
Code:
0: bipush 123
2: newarray int
4: astore_1
5: return
}
dup
拷贝操作栈顶元素（值），并将拷贝的值压入到操作栈成为新的栈顶。
调用指令
invokedynamic
Java7开始支持
invokedynamic指令。
public class MainKlass5 {
public static void main(String[] args) {
Runnable x = () -> {
System.out.println("greet!");
};
x.run();
new Thread(x).start();
}
}
public static void main(java.lang.String[]);
Code:
0: invokedynamic #2, 0 // InvokeDynamic #0:run:()Ljava/lang/Runnable;
5: astore_1
6: aload_1
7: invokeinterface #3, 1 // InterfaceMethod java/lang/Runnable.run:()V
12: new #4 // class java/lang/Thread
15: dup
16: aload_1
17: invokespecial #5 // Method java/lang/Thread."":(Ljava/lang/Runnable;)V
20: invokevirtual #6 // Method java/lang/Thread.start:()V
23: return
invokeinterface
public class MainKlass5 {
public static void main(String[] args) {
Runnable x = () -> {
System.out.println("greet!");
};
x.run();
new Thread(x).start();
}
}
重点是
7: invokeinterface #3, 1这一行。
public static void main(java.lang.String[]);
Code:
0: invokedynamic #2, 0 // InvokeDynamic #0:run:()Ljava/lang/Runnable;
5: astore_1
6: aload_1
7: invokeinterface #3, 1 // InterfaceMethod java/lang/Runnable.run:()V
12: new #4 // class java/lang/Thread
15: dup
16: aload_1
17: invokespecial #5 // Method java/lang/Thread."":(Ljava/lang/Runnable;)V
20: invokevirtual #6 // Method java/lang/Thread.start:()V
23: return
invokespecial
特殊调用。
special理解为“特殊”其实并不准确，应该理解为“特定”，表示特定方法的调用，它其实是相对于“多态”的。
多态是运行时根据引用的对象实例，动态调用特定对象实例的对应方法。这种情况是invokevirtual。
而这里的
“特定”并不需要运行时动态决定调用特定对象
实例的对应方法，它是在编译时就已经决定了调用了哪一个方法。
隐式构造：
public class Klass0 {
}
public asm.invokespecial.Klass0();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
显式构造
public class Klass1 {
public Klass1() {
}
public Klass1(int i) {
int i2 = i;
}
}
public asm.invokespecial.Klass1();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public asm.invokespecial.Klass1(int);
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: iload_1
5: istore_2
6: return
显式调用父类构造
public class SuperKlass1 extends Klass1 {
public SuperKlass1(int i) {
super(i);
}
}
public asm.invokespecial.SuperKlass1(int);
Code:
0: aload_0
1: iload_1
2: invokespecial #2 // Method asm/invokespecial/Klass1."":(I)V
5: return
调用父类方法：
public class SuperKlass2 extends Klass2 {
public void test0() {
super.test0();
}
}
public void test0();
Code:
0: aload_0
1: invokespecial #2 // Method asm/invokespecial/Klass2.test0:()V
4: return
构造（创建对象实例）
public class MainKlass1 {
public static void main(String[] args) {
SuperKlass1 klass0 = new Klass1(1);
Klass1 klass1 = new Klass1(1);
}
}
public static void main(java.lang.String[]);
Code:
0: new #2 // class asm/invokespecial/Klass1
3: dup
4: iconst_1
5: invokespecial #3 // Method asm/invokespecial/Klass1."":(I)V
8: astore_1
9: new #2 // class asm/invokespecial/Klass1
12: dup
13: iconst_1
14: invokespecial #3 // Method asm/invokespecial/Klass1."":(I)V
17: astore_2
18: return
私有方法调用：
public class Klass3 {
private void test0() {
}
public void test1() {
test0();
}
}
public void test1();
Code:
0: aload_0
1: invokespecial #2 // Method test0:()V
4: return
invokestatic
invokevirtual
public class MainKlass4 {
public static void main(String[] args) {
SuperKlass4 klass4 = new Klass4_1();
klass4.test();
klass4 = new Klass4_2();
klass4.test();
}
}
public static void main(java.lang.String[]);
Code:
0: new #2 // class asm/invokespecial/Klass4_1
3: dup
4: invokespecial #3 // Method asm/invokespecial/Klass4_1."":()V
7: astore_1
8: aload_1
9: invokevirtual #4 // Method asm/invokespecial/SuperKlass4.test:()V
12: new #5 // class asm/invokespecial/Klass4_2
15: dup
16: invokespecial #6 // Method asm/invokespecial/Klass4_2."":()V
19: astore_1
20: aload_1
21: invokevirtual #4 // Method asm/invokespecial/SuperKlass4.test:()V
24: return
ldc指令
将运行时常量池中的item压入到操作栈。
String str = "string abc";0: ldc #2 // String string abc
2: astore_1
String str = new String("string abc");
0: new #2 // class java/lang/String
3: dup
4: ldc #3 // String string abc
6: invokespecial #4 // Method java/lang/String."":(Ljava/lang/String;)V
9: astore_1
在这个例子中，将字符串"string abc"压入到操作栈。
iconst_指令
将int常量i压入操作栈。如iconst_m1 = 2 (0x2), iconst_0 = 3 (0x3), iconst_1 = 4 (0x4), iconst_2 = 5 (0x5), iconst_3 = 6 (0x6), iconst_4 = 7 (0x7), iconst_5 = 8 (0x8)分别表示将-1, 0, 1, 2, 3, 4, 5压入操作栈。
5: iconst_1
类加载时，在验证阶段针对指令的类型检查，参考 https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.10.1.9,其中列举了所有在验证阶段会进行针对指令的类型检查。aaload、aastore、aconst_null、aload, aload_、anewarray、areturn、arraylength、astore, astore_、athrow等.
其他指令码
下面的这些指令并不在java中定义：
LoadLoad
StoreStore
LoadStore
StoreLoad
EnterLoad
EnterStore
ExitLoad
ExitStore
LoadEnter
LoadExit
StoreEnter
StoreExit
EnterEnter
ExitExit
EnterExit
ExitEnter
synchronized
public class SynchronizedTest {
private int value;
public synchronized void set(int value) {
this.value = value;
}
public void set0(int value) {
synchronized (this) {
this.value = value;
}
}
}
public class instruction.SynchronizedTest {
public instruction.SynchronizedTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public synchronized void set(int);
Code:
0: aload_0
1: iload_1
2: putfield #2 // Field value:I
5: return
public void set0(int);
Code:
0: aload_0
1: dup
2: astore_2
3: monitorenter
4: aload_0
5: iload_1
6: putfield #2 // Field value:I
9: aload_2
10: monitorexit
11: goto 19
14: astore_3
15: aload_2
16: monitorexit
17: aload_3
18: athrow
19: return
Exception table:
from to target type
4 11 14 any
14 17 14 any
}
从反编译的字节码结果看，
synchronized以修饰符作用在方法上，在字节码上是不体现的。但在方法内部作为同步原语使用的话，在字节码上体现在
monitorenter/
monitorexit上。
赋值
private int value;
public void set() {
value = 100;
}
0: aload_0
1: bipush 100
3: putfield #2 // Field value:I
6: return
volatile变量的赋值：
private volatile int j;
public void test_assign() {
j = 11;
}
0: aload_0
1: bipush 11
3: putfield #2 // Field j:I
6: return
volatile的作用并没有体现在反编译的字节码上。这个需要在反编译到汇编才能看得到效果：
0x00c69fbd: mov $0xb,%esi
0x00c69fc2: mov %esi,0x8(%eax)
0x00c69fc5: lock addl $0x0,(%esp) ;*putfield j
; - ins.VolatileTest::test_assign@3 (line 11)
; - ins.VolatileTest::main@9 (line 16)
lock addl $0x0,(%esp)实际上就是个内存屏障，它能保证变量的可见性。至于volatile的禁止重排序作用，这里还看不到效果。
关于
lock指令，x86下是这么描述的：
来自https://www.felixcloutier.com/x86/lock的资料：
LOCK — Assert LOCK# Signal Prefix 写道
Description
Causes the processor’s LOCK# signal to be asserted during execution of the accompanying instruction (turns the instruction into an atomic instruction). In a multiprocessor environment, the LOCK# signal ensures that the processor has exclusive use of any shared memory while the signal is asserted.
In most IA-32 and all Intel 64 processors, locking may occur without the LOCK# signal being asserted. See the “IA-32 Architecture Compatibility” section below for more details.
The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B, CMPXCHG16B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. If the LOCK prefix is used with one of these instructions and the source operand is a memory operand, an undefined opcode exception (#UD) may be generated. An undefined opcode exception will also be generated if the LOCK prefix is used with any instruction not in the above list. The XCHG instruction always asserts the LOCK# signal regardless of the presence or absence of the LOCK prefix.
The LOCK prefix is typically used with the BTS instruction to perform a read-modify-write operation on a memory location in shared memory environment.
The integrity of the LOCK prefix is not affected by the alignment of the memory field. Memory locking is observed for arbitrarily misaligned fields.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Causes the processor’s LOCK# signal to be asserted during execution of the accompanying instruction (turns the instruction into an atomic instruction). In a multiprocessor environment, the LOCK# signal ensures that the processor has exclusive use of any shared memory while the signal is asserted.
In most IA-32 and all Intel 64 processors, locking may occur without the LOCK# signal being asserted. See the “IA-32 Architecture Compatibility” section below for more details.
The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B, CMPXCHG16B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. If the LOCK prefix is used with one of these instructions and the source operand is a memory operand, an undefined opcode exception (#UD) may be generated. An undefined opcode exception will also be generated if the LOCK prefix is used with any instruction not in the above list. The XCHG instruction always asserts the LOCK# signal regardless of the presence or absence of the LOCK prefix.
The LOCK prefix is typically used with the BTS instruction to perform a read-modify-write operation on a memory location in shared memory environment.
The integrity of the LOCK prefix is not affected by the alignment of the memory field. Memory locking is observed for arbitrarily misaligned fields.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
而如果没有volatile修饰，反编译的汇编效果类似：
0x00c69fbd: movl $0xa,0x8(%eax) ;*putfield i
; - ins.NonvolatileTest::test_assign@3 (line 15)
; - ins.NonvolatileTest::main@9 (line 22)
如果是本地变量的赋值：
public void assign() {
int i = 100;
}
0: bipush 100
2: istore_1
3: return
i++操作：
public class VariableSetTest {
private int value;
public static void main(String[] args) {
VariableSetTest test = new VariableSetTest();
test.value++;
System.out.println(test.value);
}
} public class VariableSetTest {
public VariableSetTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: new #2 // class VariableSetTest
3: dup
4: invokespecial #3 // Method "":()V
7: astore_1
8: aload_1
9: dup
10: getfield #4 // Field value:I
13: iconst_1
14: iadd
15: putfield #4 // Field value:I
18: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
21: aload_1
22: getfield #4 // Field value:I
25: invokevirtual #6 // Method java/io/PrintStream.println:(I)V
28: return
}
局部变量的i++操作：
public class VariableSetTest2 {
public static void main(String[] args) {
int value = 0;
value++;
System.out.println(value);
}
} public class VariableSetTest2 {
public VariableSetTest2();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iinc 1, 1
5: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
8: iload_1
9: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
12: return
}
1、VariableSetTest中的++操作对应的指令为14: iadd，但
VariableSetTest2中的++操作对应的指令为
2: iinc 1, 1。
2、
VariableSetTest，
VariableSetTest2中的++操作会更新value的值，但在反编译后的指令码中没有astore类似的指令更新本地变量表中对应变量value的值。