public class MethodAreaDemo {
public static void main(String[] args) {
int x = 500;
int y = 100;
int a = x / y;
int b = 50;
System.out.println(a + b);
}
}
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字节码
public class com.atguigu.java1.MethodAreaDemo
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #5.#24
#2 = Fieldref #25.#26
#3 = Methodref #27.#28
#4 = Class #29
#5 = Class #30
#6 = Utf8 <init>
#7 = Utf8 ()V
#8 = Utf8 Code
#9 = Utf8 LineNumberTable
#10 = Utf8 LocalVariableTable
#11 = Utf8 this
#12 = Utf8 Lcom/atguigu/java1/MethodAreaDemo;
#13 = Utf8 main
#14 = Utf8 ([Ljava/lang/String;)V
#15 = Utf8 args
#16 = Utf8 [Ljava/lang/String;
#17 = Utf8 x
#18 = Utf8 I
#19 = Utf8 y
#20 = Utf8 a
#21 = Utf8 b
#22 = Utf8 SourceFile
#23 = Utf8 MethodAreaDemo.java
#24 = NameAndType #6:#7
#25 = Class #31
#26 = NameAndType #32:#33
#27 = Class #34
#28 = NameAndType #35:#36
#29 = Utf8 com/atguigu/java1/MethodAreaDemo
#30 = Utf8 java/lang/Object
#31 = Utf8 java/lang/System
#32 = Utf8 out
#33 = Utf8 Ljava/io/PrintStream;
#34 = Utf8 java/io/PrintStream
#35 = Utf8 println
#36 = Utf8 (I)V
{
public com.atguigu.java1.MethodAreaDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/atguigu/java1/MethodAreaDemo;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=5, args_size=1
0: sipush 500
3: istore_1
4: bipush 100
6: istore_2
7: iload_1
8: iload_2
9: idiv
10: istore_3
11: bipush 50
13: istore 4
15: getstatic #2
18: iload_3
19: iload 4
21: iadd
22: invokevirtual #3
25: return
LineNumberTable:
line 9: 0
line 10: 4
line 11: 7
line 12: 11
line 13: 15
line 14: 25
LocalVariableTable:
Start Length Slot Name Signature
0 26 0 args [Ljava/lang/String;
4 22 1 x I
7 19 2 y I
11 15 3 a I
15 11 4 b I
}
SourceFile: "MethodAreaDemo.java"
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图解字节码指令执行流程
1、初始状态
2、首先将操作数500压入操作数栈中
3、然后操作数 500 从操作数栈中取出,存储到局部变量表中索引为 1 的位置
4、将操作数100压入操作数栈中
5、然后操作数 100 从操作数栈中取出,存储到局部变量表中索引为 2 的位置
6、读取本地变量 1 ,压入操作数栈
7、读取本地变量 2 ,压入操作数栈
8、两数相除,计算结果放在操作数栈顶,之后执行 istore_3 指令,将计算结果从操作数栈中弹出,存入本地变量表 3 中
9、将操作数 50 压入操作数栈
10、将操作数 50 从栈顶弹出,保存在局部变量表 4 中
11、获取 System.out 输出流的引用
12、将本地变量表 3 的值取出,压入操作数栈中,准备进行加法运算
13、将本地变量表 4 的值取出,压入操作数栈中,准备进行加法运算
15、执行加法运算后,将计算结果放在操作数栈顶
调用静态方法 println( ) ,输出加法结果
16、main( ) 方法执行结束
关于【符号引用 --> 直接引用】的理解
上面代码调用 System.out.println() 方法时,首先需要看看 System 类有没有加载,再看看 PrintStream 类有没有加载
如果没有加载,则执行加载,执行时,将常量池中的符号引用(字面量)转换为运行时常量池的直接引用(真正的地址值)
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