Java字节码的一段旅行经历——提升硬实力1

字节码指令

1.1 什么是字节码

字节码（Byte-code）是一种包含执行程序，由一序列 op 代码/数据对组成的二进制文件，是一种中间码。字节是电脑里的数据量单位。

1.2 javap 工具

自己分析类文件（字节码）太麻烦了，Oracle提供了javap工具来反编译class文件

javap -v HelloWorld.class
  1. /Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/bin/javap -v com.jvm.t07_bytecode.T01_HelloWorld
  2. Classfile /Users/lei/IdeaProjects/JvmLearn/target/classes/com/jvm/t07_bytecode/T01_HelloWorld.class
  3.   Last modified 2020-8-10; size 586 bytes
  4.   MD5 checksum 916c3ebdbfef6b6fab63cac6a085aaa1
  5.   Compiled from "T01_HelloWorld.java"
  6. public class com.jvm.t07_bytecode.T01_HelloWorld
  7.   minor version: 0
  8.   major version: 52
  9.   flags: ACC_PUBLIC, ACC_SUPER
  10. Constant pool:
  11.    #1 = Methodref          #6.#20         // java/lang/Object."":()V
  12.    #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
  13.    #3 = String             #23            // HelloWorld
  14.    #4 = Methodref          #24.#25        // java/io/PrintStream.println:(Ljava/lang/String;)V
  15.    #5 = Class              #26            // com/jvm/t07_bytecode/T01_HelloWorld
  16.    #6 = Class              #27            // java/lang/Object
  17.    #7 = Utf8               
  18.    #8 = Utf8               ()V
  19.    #9 = Utf8               Code
  20.   #10 = Utf8               LineNumberTable
  21.   #11 = Utf8               LocalVariableTable
  22.   #12 = Utf8               this
  23.   #13 = Utf8               Lcom/jvm/t07_bytecode/T01_HelloWorld;
  24.   #14 = Utf8               main
  25.   #15 = Utf8               ([Ljava/lang/String;)V
  26.   #16 = Utf8               args
  27.   #17 = Utf8               [Ljava/lang/String;
  28.   #18 = Utf8               SourceFile
  29.   #19 = Utf8               T01_HelloWorld.java
  30.   #20 = NameAndType        #7:#8          // "":()V
  31.   #21 = Class              #28            // java/lang/System
  32.   #22 = NameAndType        #29:#30        // out:Ljava/io/PrintStream;
  33.   #23 = Utf8               HelloWorld
  34.   #24 = Class              #31            // java/io/PrintStream
  35.   #25 = NameAndType        #32:#33        // println:(Ljava/lang/String;)V
  36.   #26 = Utf8               com/jvm/t07_bytecode/T01_HelloWorld
  37.   #27 = Utf8               java/lang/Object
  38.   #28 = Utf8               java/lang/System
  39.   #29 = Utf8               out
  40.   #30 = Utf8               Ljava/io/PrintStream;
  41.   #31 = Utf8               java/io/PrintStream
  42.   #32 = Utf8               println
  43.   #33 = Utf8               (Ljava/lang/String;)V
  44. {
  45.   public com.jvm.t07_bytecode.T01_HelloWorld();
  46.     descriptor: ()V
  47.     flags: ACC_PUBLIC
  48.     Code:
  49.       stack=1, locals=1, args_size=1
  50.          0: aload_0
  51.          1: invokespecial #1                  // Method java/lang/Object."":()V
  52.          4: return
  53.       LineNumberTable:
  54.         line 11: 0
  55.       LocalVariableTable:
  56.         Start  Length  Slot  Name   Signature
  57.             0       5     0  this   Lcom/jvm/t07_bytecode/T01_HelloWorld;
  58.  
  59.   public static void main(java.lang.String[]);
  60.     descriptor: ([Ljava/lang/String;)V
  61.     flags: ACC_PUBLIC, ACC_STATIC
  62.     Code:
  63.       stack=2, locals=1, args_size=1
  64.          0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
  65.          3: ldc           #3                  // String HelloWorld
  66.          5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  67.          8: return
  68.       LineNumberTable:
  69.         line 13: 0
  70.         line 14: 8
  71.       LocalVariableTable:
  72.         Start  Length  Slot  Name   Signature
  73.             0       9     0  args   [Ljava/lang/String;
  74. }
  75. SourceFile: "T01_HelloWorld.java"
  76.  
  77. Process finished with exit code 0

2.3 图解方法执行流程

1）原始java 代码

  1. /*
  2.     演示 字节码指令 和 操作数栈、常量池的关系
  3.  */
  4. public class T02_ByteCommandStackConstantPoolRel {
  5.     public static void main(String[] args) {
  6.         int a = 10;
  7.         int b = Short.MAX_VALUE + 1;
  8.         int c = a + b;
  9.         System.out.println(c);
  10.     }
  11. }

2）编译后的字节码文件

  1. /Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/bin/javap -v com.jvm.t07_bytecode.T02_ByteCommandStackConstantPoolRel
  2. Classfile /Users/lei/IdeaProjects/JvmLearn/target/classes/com/jvm/t07_bytecode/T02_ByteCommandStackConstantPoolRel.class
  3.   Last modified 2020-8-10; size 709 bytes
  4.   MD5 checksum cc8fc12b6e178b8f28e787497e993363
  5.   Compiled from "T02_ByteCommandStackConstantPoolRel.java"
  6. public class com.jvm.t07_bytecode.T02_ByteCommandStackConstantPoolRel
  7.   minor version: 0
  8.   major version: 52
  9.   flags: ACC_PUBLIC, ACC_SUPER
  10. Constant pool:
  11.    #1 = Methodref          #7.#25         // java/lang/Object."":()V
  12.    #2 = Class              #26            // java/lang/Short
  13.    #3 = Integer            32768
  14.    #4 = Fieldref           #27.#28        // java/lang/System.out:Ljava/io/PrintStream;
  15.    #5 = Methodref          #29.#30        // java/io/PrintStream.println:(I)V
  16.    #6 = Class              #31            // com/jvm/t07_bytecode/T02_ByteCommandStackConstantPoolRel
  17.    #7 = Class              #32            // java/lang/Object
  18.    #8 = Utf8               
  19.    #9 = Utf8               ()V
  20.   #10 = Utf8               Code
  21.   #11 = Utf8               LineNumberTable
  22.   #12 = Utf8               LocalVariableTable
  23.   #13 = Utf8               this
  24.   #14 = Utf8               Lcom/jvm/t07_bytecode/T02_ByteCommandStackConstantPoolRel;
  25.   #15 = Utf8               main
  26.   #16 = Utf8               ([Ljava/lang/String;)V
  27.   #17 = Utf8               args
  28.   #18 = Utf8               [Ljava/lang/String;
  29.   #19 = Utf8               a
  30.   #20 = Utf8               I
  31.   #21 = Utf8               b
  32.   #22 = Utf8               c
  33.   #23 = Utf8               SourceFile
  34.   #24 = Utf8               T02_ByteCommandStackConstantPoolRel.java
  35.   #25 = NameAndType        #8:#9          // "":()V
  36.   #26 = Utf8               java/lang/Short
  37.   #27 = Class              #33            // java/lang/System
  38.   #28 = NameAndType        #34:#35        // out:Ljava/io/PrintStream;
  39.   #29 = Class              #36            // java/io/PrintStream
  40.   #30 = NameAndType        #37:#38        // println:(I)V
  41.   #31 = Utf8               com/jvm/t07_bytecode/T02_ByteCommandStackConstantPoolRel
  42.   #32 = Utf8               java/lang/Object
  43.   #33 = Utf8               java/lang/System
  44.   #34 = Utf8               out
  45.   #35 = Utf8               Ljava/io/PrintStream;
  46.   #36 = Utf8               java/io/PrintStream
  47.   #37 = Utf8               println
  48.   #38 = Utf8               (I)V
  49. {
  50.   public com.jvm.t07_bytecode.T02_ByteCommandStackConstantPoolRel();
  51.     descriptor: ()V
  52.     flags: ACC_PUBLIC
  53.     Code:
  54.       stack=1, locals=1, args_size=1
  55.          0: aload_0
  56.          1: invokespecial #1                  // Method java/lang/Object."":()V
  57.          4: return
  58.       LineNumberTable:
  59.         line 15: 0
  60.       LocalVariableTable:
  61.         Start  Length  Slot  Name   Signature
  62.             0       5     0  this   Lcom/jvm/t07_bytecode/T02_ByteCommandStackConstantPoolRel;
  63.  
  64.   public static void main(java.lang.String[]);
  65.     descriptor: ([Ljava/lang/String;)V
  66.     flags: ACC_PUBLIC, ACC_STATIC
  67.     Code:
  68.       stack=2, locals=4, args_size=1
  69.          0: bipush        10
  70.          2: istore_1
  71.          3: ldc           #3                  // int 32768
  72.          5: istore_2
  73.          6: iload_1
  74.          7: iload_2
  75.          8: iadd
  76.          9: istore_3
  77.         10: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
  78.         13: iload_3
  79.         14: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
  80.         17: return
  81.       LineNumberTable:
  82.         line 17: 0
  83.         line 18: 3
  84.         line 19: 6
  85.         line 20: 10
  86.         line 21: 17
  87.       LocalVariableTable:
  88.         Start  Length  Slot  Name   Signature
  89.             0      18     0  args   [Ljava/lang/String;
  90.             3      15     1     a   I
  91.             6      12     2     b   I
  92.            10       8     3     c   I
  93. }
  94. SourceFile: "T02_ByteCommandStackConstantPoolRel.java"
  95.  
  96. Process finished with exit code 0

3）常量池载入运行时常量池

4）方法字节码载入方法区

5）main线程开始运行，分配栈帧内存

（stack=2,  locals=4）

  

6）执行引擎开始执行字节码

bipush 10

  • 将一个byte压入操作数栈（其长度会补齐4个字节），类似的指令还有
  • sipush将一个short压入操作数栈（其长度会补齐4个字节）
  • idc将一个int压入操作数栈
  • idc2_w将一个long压入操作数栈（分两次压入，因为long是8个字节）
  • 这里小的数字都是和字节码指令存在一起，超过short范围的数字存入常量池

istore_1

  • 将操作数栈顶数据弹出，存入局部变量表的槽位slot 1中，下图的args/a/b/c就是对应的槽位

idc  #3

  • idc 从常量池加载 #3 数据到操作数栈
  • 注意： Short.MAX_VALUE是32767，所以32768 = Short.MAX_VALUE + 1 超过了Short.MAX_VALUE的值，所以编译器将其放在运行时常量池中。实际是在编译期间计算好的，是一种优化，叫常量折叠优化

istore_2

  • 将操作数栈顶数据弹出，存入局部变量表的2号槽位中

iload_1

再接下来，需要执行int c = a + b；执行引擎不能直接在局部变量表进行a+b操作，需要先将a、b进行读取，然后放入操作数栈中才能进行计算分析

  • 把局部变量从1号槽位加载数据到操作数栈中

iload 2

  • 再把局部变量从2号槽位加载数据到操作数栈中

iadd

  • iadd会弹出操作数栈中的2个变量，并进行求和得到32778，最后将结果32778写回到操作数栈中


istore_3

  • 将操作数栈的数弹出，存入局部变量表3号槽位。到目前为止，局部变量a、b、c都有值了

getstatic #4

  • 去常量池找到成员变量引用，去堆中找到System.out对象。getstatic 会把对象引用放入操作数栈中

iload_3

  • 将局部变量表3号槽位的值加载到操作数栈中

invokevirtual #5

  • 找到常量池 #5项
  • 定位到方法区 java/io/PrintStream.println: (I)V方法
  • 生成新的栈帧（分配locals、stack等）
  • 传递传数，执行新栈帧中的字节码

  • 执行完毕，弹出栈帧
  • 清除main操作数栈内容

return

  • 完成main方法调用，弹出main栈帧
  • 程序结束

文章最后，给大家推荐一些受欢迎的技术博客链接：

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注：本文转载自blog.csdn.net的不埋雷的探长的文章"https://blog.csdn.net/weixin_32265569/article/details/107904061"。版权归原作者所有，此博客不拥有其著作权，亦不承担相应法律责任。如有侵权，请联系我们删除。
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