一、内存结构图

二、案例解读

我们使用javap对字节码进行反编译，来看下程序计数器怎么体现的。

IDEA写一个简单的Java代码

反编译命令
javap -verbose InitTest.class

$ javap -verbose InitTest.class
Classfile /E:/workspace/JVMDemo/out/production/chapter01/com/rh/java/InitTest.class
  Last modified 2024-9-23; size 491 bytes
  MD5 checksum 56fd724ef01f37a20b2872002edd43cd
  Compiled from "InitTest.java"
public class com.rh.java.InitTest
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#23         // java/lang/Object."<init>":()V
   #2 = Fieldref           #3.#24         // com/rh/java/InitTest.a:I
   #3 = Class              #25            // com/rh/java/InitTest
   #4 = Class              #26            // java/lang/Object
   #5 = Utf8               a
   #6 = Utf8               I
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               Lcom/rh/java/InitTest;
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               args
  #17 = Utf8               [Ljava/lang/String;
  #18 = Utf8               i
  #19 = Utf8               j
  #20 = Utf8               k
  #21 = Utf8               SourceFile
  #22 = Utf8               InitTest.java
  #23 = NameAndType        #7:#8          // "<init>":()V
  #24 = NameAndType        #5:#6          // a:I
  #25 = Utf8               com/rh/java/InitTest
  #26 = Utf8               java/lang/Object
{
  public int a;
    descriptor: I
    flags: ACC_PUBLIC

  public com.rh.java.InitTest();
    descriptor: ()V
    flags: 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: bipush        8
         7: putfield      #2                  // Field a:I
        10: return
      LineNumberTable:
        line 3: 0
        line 4: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/rh/java/InitTest;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=4, args_size=1
         0: bipush        10
         2: istore_1
         3: bipush        20
         5: istore_2
         6: iload_1
         7: iload_2
         8: iadd
         9: istore_3
        10: return
      LineNumberTable:
        line 7: 0
        line 8: 3
        line 9: 6
        line 10: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  args   [Ljava/lang/String;
            3       8     1     i   I
            6       5     2     j   I
           10       1     3     k   I
}
SourceFile: "InitTest.java"

类似下图，最左边的序号，我们可以简单的理解为jvm指令的内存地址。

三、工作流程

Java程序的运行流程大概如下
解释器，通过程序计数器，拿到指令的内存地址，从而获取到需要执行的指令
然后，解释器把指令翻译成机器码，CPU才能执行对应的机器码。
简单的理解为，当前程序运行到哪一行代码，就看程序计数器记录的指令地址。

而在物理硬件上，程序计数器是通过什么实现的了？
是通过CPU中的寄存器部分实现的效果，寄存器是CPU中运行速度最快的部分。

四、特点

1、线程私有的
每个线程都会有个属于自己的程序计数器。

2、不会存在内存溢出异常