aaload (50,0x32)
从数组中装载引用类型
1. 操作数栈
pop:
arrayref,index
push:value
arrayref必须是引用类型R[]，并且必须指向元素为引用类型R的数组。索引必须是int类型。arrayref和index从操作数栈中弹出，数组中索引为index的引用值(value)被检索到并推入到操作数栈中。
-1. 字节码解读示例
在main函数中声明一个长度为1的数组，并给索引为0的项赋值，然后根据索引0调用该值赋给某个变量。代码如下：
package io.github.iamazy.asm.instructions.aaload;
import java.io.Serializable;
public class Aaload implements Serializable {
String name = "aaload";
public static void main(String[] args) {
Object[] objects = new Object[10];
objects[0] = "hello asm";
Object object = objects[0];
System.out.println(object);
}
}运行main函数或执行javac Aaload.java生成Aaload.class文件，并在Aaload.class文件同级目录执行javap -verbose Aaload，生成如下字节码信息。
Warning: File ./Aaload.class does not contain class Aaload
Classfile /Users/iamazy/Documents/GitHub/asm-tutorial/target/classes/io/github/iamazy/asm/instructions/aaload/Aaload.class
Last modified Mar 22, 2020; size 804 bytes
SHA-256 checksum 7ebc42d18bc4fc81d44ab57f7492aa00e500f1a0fb02c2f71f8be359ca78d102
Compiled from "Aaload.java"
public class io.github.iamazy.asm.instructions.aaload.Aaload implements java.io.Serializable
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #8 // io/github/iamazy/asm/instructions/aaload/Aaload
super_class: #4 // java/lang/Object
interfaces: 1, fields: 1, methods: 2, attributes: 1
Constant pool:
#1 = Methodref #4.#29 // java/lang/Object."<init>":()V
#2 = String #30 // aaload
#3 = Fieldref #8.#31 // io/github/iamazy/asm/instructions/aaload/Aaload.name:Ljava/lang/String;
#4 = Class #32 // java/lang/Object
#5 = String #33 // hello asm
#6 = Fieldref #34.#35 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #36.#37 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#8 = Class #38 // io/github/iamazy/asm/instructions/aaload/Aaload
#9 = Class #39 // java/io/Serializable
#10 = Utf8 name
#11 = Utf8 Ljava/lang/String;
#12 = Utf8 <init>
#13 = Utf8 ()V
#14 = Utf8 Code
#15 = Utf8 LineNumberTable
#16 = Utf8 LocalVariableTable
#17 = Utf8 this
#18 = Utf8 Lio/github/iamazy/asm/instructions/aaload/Aaload;
#19 = Utf8 main
#20 = Utf8 ([Ljava/lang/String;)V
#21 = Utf8 args
#22 = Utf8 [Ljava/lang/String;
#23 = Utf8 objects
#24 = Utf8 [Ljava/lang/Object;
#25 = Utf8 object
#26 = Utf8 Ljava/lang/Object;
#27 = Utf8 SourceFile
#28 = Utf8 Aaload.java
#29 = NameAndType #12:#13 // "<init>":()V
#30 = Utf8 aaload
#31 = NameAndType #10:#11 // name:Ljava/lang/String;
#32 = Utf8 java/lang/Object
#33 = Utf8 hello asm
#34 = Class #40 // java/lang/System
#35 = NameAndType #41:#42 // out:Ljava/io/PrintStream;
#36 = Class #43 // java/io/PrintStream
#37 = NameAndType #44:#45 // println:(Ljava/lang/Object;)V
#38 = Utf8 io/github/iamazy/asm/instructions/aaload/Aaload
#39 = Utf8 java/io/Serializable
#40 = Utf8 java/lang/System
#41 = Utf8 out
#42 = Utf8 Ljava/io/PrintStream;
#43 = Utf8 java/io/PrintStream
#44 = Utf8 println
#45 = Utf8 (Ljava/lang/Object;)V
{
java.lang.String name;
descriptor: Ljava/lang/String;
flags: (0x0000)
public io.github.iamazy.asm.instructions.aaload.Aaload();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String aaload
7: putfield #3 // Field name:Ljava/lang/String;
10: return
LineNumberTable:
line 5: 0
line 7: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lio/github/iamazy/asm/instructions/aaload/Aaload;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=3, args_size=1
0: iconst_1
1: anewarray #4 // class java/lang/Object
4: astore_1
5: aload_1
6: iconst_0
7: ldc #5 // String hello asm
9: aastore
10: aload_1
11: iconst_0
12: aaload
13: astore_2
14: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
17: aload_2
18: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
21: return
LineNumberTable:
line 10: 0
line 11: 5
line 12: 10
line 13: 14
line 14: 21
LocalVariableTable:
Start Length Slot Name Signature
0 22 0 args [Ljava/lang/String;
5 17 1 objects [Ljava/lang/Object;
14 8 2 object Ljava/lang/Object;
}
SourceFile: "Aaload.java"有上述信息可知：
1. 指定class文件格式版本号
Aaload.class文件的class文件格式版本号是52.0
2. 显示类访问标记符的值
类的access_flags为ACC_PUBLIC+ACC_SUPER=0x0001+0x0020=0x0021 (ACC_XXX对应的值见access_flags)
3. 显示类的基本信息
interfaces: 1 (impl Serializable), fields: 1 (name), methods: 2 (<init>,main), attributes: 1 ()
4. 显示了常量池中具体的信息
如常量池第一行
#1 = Methodref #4.#29 // java/lang/Object."<init>":()V
含义：常量池中第一项的类型为CONSTANT_Methodref_info，它的值是#4.#29(往下找第4项和第29项)，即java/lang/Object."<init>":()V
5. 类函数的执行过程
在生成的Aaload.class文件中，有两个函数，一个是类Aaload的无参构造函数，一个是程序入口main函数。
Aaload的无参构造函数对应的字节码信息public io.github.iamazy.asm.instructions.aaload.Aaload(); descriptor: ()V flags: (0x0001) ACC_PUBLIC Code: stack=2, locals=1, args_size=1 //可知操作数栈长度为2，本地变量表长度为1，参数列表长度为1 0: aload_0 //从本地变量表中加载索引为0变量的值，也即this的引用，压入栈 1: invokespecial #1 //出栈，使用invokespecial指令调用构造函数(java/lang/Object."<init>":()V),并初始化对象 4: aload_0 //同上，调用this.name="aaload",并将this的引用压入栈 5: ldc #2 //将字符串"aaload"常量压入栈 7: putfield #3 //出栈前面压入的两个值(this引用，字符串"aaload")，并将字符串"aaload"赋值给this.name 10: return LineNumberTable: line 5: 0 line 7: 4 LocalVariableTable: Start Length Slot Name Signature 0 11 0 this Lio/github/iamazy/asm/instructions/aaload/Aaload;其中：
LineNumberTable表示代码行号与指令的对应关系，前面一个数字表示代码行号，后面一个数字表示前面code对应的指令代号。LocalVariableTable表示本地变量表，这里只存了一个this的引用，类型描述为Lio/github/iamazy/asm/instructions/aaload/Aaload，start+length表示这个变量在字节码中的生命周期起始和结束的偏移量(this声明周期为开始0到结束11)，slot表示这个变量在局部变量表中的槽位(槽位可以复用)。main函数对应的字节码信息public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: (0x0009) ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=3, args_size=1 0: iconst_1 //将int类型的常量1压入操作数栈 1: anewarray #4 //从栈顶弹出数组长度(指定数组长度为1)，并实例化数组，并将其压栈 4: astore_1 //将栈顶的引用型数值弹出栈，并保存到局部变量表中的1的位置 5: aload_1 //从局部变量表中装载索引值为1(slot=1)的引用型数值，并压栈 6: iconst_0 //将int类型的常量0压栈 7: ldc #5 //将字符串"hello asm"常量压栈 9: aastore //将栈顶的数值依次弹出，并引用型数值存入指定数组的指定索引位置 10: aload_1 //同上，从局部变量表中装载索引值为1(slot=1)的引用型数值，并压栈 11: iconst_0 //将int类型的常量0压栈 12: aaload //将引用型数组指定索引的值(0)推送至栈顶 13: astore_2 //将栈顶的引用型数值(object[0])弹出栈，并保存到局部变量表中slot=2的位置 14: getstatic #6 //访问类的静态变量，类型为(java/lang/System.out:Ljava/io/PrintStream;) 17: aload_2 //从局部变量表中装载slot=2的引用型数值，并压栈 18: invokevirtual #7 //将栈顶的引用型数值弹出栈，并调用类成员方法，方法描述符为java/io/PrintStream.println:(Ljava/lang/Object;)V 21: return LineNumberTable: line 10: 0 line 11: 5 line 12: 10 line 13: 14 line 14: 21 LocalVariableTable: Start Length Slot Name Signature 0 22 0 args [Ljava/lang/String; 5 17 1 objects [Ljava/lang/Object; 14 8 2 object Ljava/lang/Object;
aaload
aastore
aconst_null
aload
aload_<n>
anewarray
areturn
arraylength
astore
astore_<n>
athrow
baload
bastore
bipush
caload
castore
checkcast
d2f
d2i
d2l
dadd
daload
dastore
dcmp<op>
dconst_<d>
ddiv
dload
dload_<n>
dmul
dneg
drem
dreturn
dstore
dstore_<n>
dsub
dup
dup_x1
dup_x2
dup2
dup2_x1
dup2_x2
f2d
f2i
f2l
fadd
faload
fastore
fcmp<op>
fconst_<f>
fdiv
fload
fload_<n>
fmul
fneg
frem
freturn
fstore
fstore_<n>
fsub
getfield
getstatic
goto
goto_w
i2b
i2c
i2d
i2f
i2l
i2s
iadd
iaload
iand
iastore
iconst_<i>
idiv
if_acmp<cond>
if_icmp<cond>
if<cond>
ifnonnull
ifnull
iinc
iload
iload_<n>
imul
ineg
instanceof
invokedynamic
invokeinterface
invokespecial
invokestatic
invokevirtual
ior
irem
ireturn
ishl
ishr
istore
istore_<n>
isub
iushr
ixor
jsr
jsr_w
l2d
l2f
l2i
ladd
laload
land
lastore
lcmp
lconst_<l>
ldc
ldc_w
ldc2_w
ldiv
lload
lload_<n>
lmul
lneg
lookupswitch
lor
lrem
lreturn
lshl
lshr
lstore
lstore_<n>
lsub
lushr
lxor
monitorenter
monitorexit
multianewarray
new
newarray
nop
pop
pop2
putfield
putstatic
ret
return
saload
sastore
sipush
swap
tableswitch
wide
