1. java程序源代码与字节码
源代码
public class StackHeapAnalysis {
// java 运行堆栈分析
public static void main(String[] args) {
//define my wallet totel balance
int balance = 500;
// birthday cake
int cakeVal = 99;
int cakeNum = 1;
// birthday flower
int flower = 2;
int flowerNum = 99;
int cakeSpent = cakeNum * cakeVal;
int flowerSpent = flower * flowerNum;
int totalSpent = flowerSpent + cakeSpent;
int currentBalance = balance - totalSpent;
System.out.printf("in order to celebrate birthday for my gift friends, i have spent %d\n", totalSpent);
System.out.printf("now my wallet balance is %d \n", currentBalance);
}
字节码文件
public class com.xiaokunliu.homework.thread.base.StackHeapAnalysis
minor version: 0 // 最低版本号
major version: 52 // 主版本号 52 = 0x34 意味着是jdk8版本
flags: ACC_PUBLIC, ACC_SUPER // 访问标识符
Constant pool: // 常量池
#1 = Methodref #4.#32 // java/lang/Object."<init>":()V
#2 = Fieldref #33.#34 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #35 // in order to celebrate birthday for my gift friends, i have spent %d\n
#4 = Class #36 // java/lang/Object
#5 = Methodref #37.#38 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#6 = Methodref #39.#40 // java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
#7 = String #41 // now my wallet balance is %d \n
#8 = Class #42 // com/xiaokunliu/homework/thread/base/StackHeapAnalysis
#9 = Utf8 <init>
#10 = Utf8 ()V
#11 = Utf8 Code
#12 = Utf8 LineNumberTable
#13 = Utf8 LocalVariableTable
#14 = Utf8 this
#15 = Utf8 Lcom/xiaokunliu/homework/thread/base/StackHeapAnalysis;
#16 = Utf8 main
#17 = Utf8 ([Ljava/lang/String;)V
#18 = Utf8 args
// 常量池省略....
{
public com.xiaokunliu.homework.thread.base.StackHeapAnalysis(); // 类的构造器
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 8: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/xiaokunliu/homework/thread/base/StackHeapAnalysis;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=6, locals=10, args_size=1 // 以下是main线程的字节码操作指令
0: sipush 500
3: istore_1
4: bipush 99
6: istore_2
7: iconst_1
8: istore_3
9: iconst_2
10: istore 4
12: bipush 99
14: istore 5
16: iload_3
17: iload_2
18: imul
19: istore 6
21: iload 4
23: iload 5
25: imul
26: istore 7
28: iload 7
30: iload 6
32: iadd
33: istore 8
35: iload_1
36: iload 8
38: isub
39: istore 9
41: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
44: ldc #3 // String in order to celebrate birthday for my gift friends, i have spent %d\n
46: iconst_1
47: anewarray #4 // class java/lang/Object
50: dup
51: iconst_0
52: iload 8
54: invokestatic #5 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
57: aastore
58: invokevirtual #6 // Method java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
61: pop
62: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
65: ldc #7 // String now my wallet balance is %d \n
67: iconst_1
68: anewarray #4 // class java/lang/Object
71: dup
72: iconst_0
73: iload 9
75: invokestatic #5 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
78: aastore
79: invokevirtual #6 // Method java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
82: pop
83: return
LineNumberTable: // 字节码存储代码在代码表的数组中,LineNumberTable通过定位源文件的代码行数与代码表数据长度对应起来
line 15: 0
// ......
LocalVariableTable: // 线程本地局部变量表,存储main线程中的局部变量信息
Start Length Slot Name Signature
0 84 0 args [Ljava/lang/String;
// ....
}
2. java运行堆栈分析
类执行分析准备
- 定义的class文件编译为二进制.class的时候,产生幻数标识,访问标识
- 执行main的时候先执行当前类的init构造器完成初始化
- 线程执行包含程序计数器以及虚拟机栈(多个栈帧[操作数栈 + 局部变量表 + 动态链接 + 方法返回])
JVM的int入栈操作指令参考
## 参考链接
https://en.wikipedia.org/wiki/Java_bytecode_instruction_listings
## int入栈指令
iconst_<i> 操作常量 -1 - 5
ipush 操作一个字节的int数据
sipush 操作两个字节的int数据
ldc 超过两个字节的int数据
main方法执行分析
int balance = 500分析
// source.java
int balance = 500;
// 字节码文件
0: sipush 500
3: istore_1
// 其他定义变量的操作与上述一致
将500压入操作数栈中并存储在本地变量表,然后弹出操作数栈,同时程序计数器记录当前代码执行的位置
int cakeNum = 1;与int flower = 2;分析
// source.java
int cakeNum = 1;
int flower = 2;
// 字节码文件
7: iconst_1
8: istore_3
9: iconst_2
10: istore 4
JVM将常量1和2压入操作数栈并存储在本地变量表中,然后弹出操作数栈,更新程序计数器当前执行的代码的位置
- 进行乘法运算
// source.java
int cakeSpent = cakeNum * cakeVal;
// 字节码
16: iload_3 // 1
17: iload_2 // 99
18: imul
19: istore 6
将本地变量表中的数据1 和 数据99分别压入操作数栈中然后进行乘法运算最后将运算结果存储在本地变量表中
其他的数学基本运算也是依次类推
- 打印输出
//source.java
System.out.printf("in order to celebrate birthday for my gift friends, i have spent %d\n", totalSpent)
// 字节码文件
41: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
44: ldc #3 // String in order to celebrate birthday for my gift friends, i have spent %d\n
46: iconst_1
47: anewarray #4 // class java/lang/Object
50: dup
51: iconst_0
52: iload 8
54: invokestatic #5 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
57: aastore
58: invokevirtual #6 // Method java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
61: pop
- 分析:
- getstatic #2 表示从常量池获取静态资源#2并将其加载到操作数栈中,同时创建新的栈帧,因为属于一个新的方法调用
- ldc 将常量池中将字符串数据压入操作数栈中
- anewarray #4 从常量池中获取静态资源#4并将其创建一个新的对象数组引用
- dup 上述的对象引用复制到操作数栈的顶部
- iload 8 从本地变量表中加载totalSpent的数据
- invokestatic 执行整数转换为字符串的方法,将totalSpent转换为字符串
- aastore 将上述计算得到的结果存储到本地变量表中
- invokevirtual 调用方法输出字符串到控制台中
- main的线程中弹出getstatic的栈帧
3. 小结
- java程序代码转换为.class字节码的时候是按照指定的字节码指令进行操作
- 线程栈中定义的局部变量数据将会压入操作栈中进行计算然后存储到本地变量表中,并且会将执行代码的行数记录到程序计数器中
- 线程调用方法的时候会新创建一个新的栈帧执行对应的方法中的代码,当方法执行完成之后栈帧将会从当前的线程中弹出
