Java字节码文件结构剖析

今天起开启JVM的新的知识学习篇章----Java的字节码，那学习Java字节码有啥用呢？我们知道Java是跨平台的一门语言，编写一次到处运行，而支撑着这个特性的根基为两点：JVM和.class字节码文件，java在不同的平台上编译生成的.class文件是一模一样的，而JVM针对不同的操作系统会有不同的JVM，也就是JVM其实并非是跨平台的，而生成的.class文件其实是一个二进制文件，该文件并非是给人看的，而是给机器看的，所以肯定是看不懂的，但是针对字节码JVM是有规范定义的，而如今基于JVM的语言也越来越多，如：scala,groovy,kotlin等，虽说语言不同但是编译出来的字节码文件都是符合JVM规范的，也就是对于JVM来说不管咱们用的是哪种语言编写的，只关心字节码文件是否符合字节码的规范，只要符合了就可以加载运行。另外在了解了字节码文件之后，对于很多深层次的东东【如synchronized关键字、volatile关键字】就会了解得更加透彻了，当然啦，学习它当然是非常之枯燥的，但是这个枯燥是为了让自己学得更加的通透，一切都是值得的，所以下面硬着头皮开始：

先编写一个简单的代码作为字节码学习的开端，如下：

然后编译生成字节码文件：

然后双击打开它：

【提示】：当我们完全搞懂了字节码的东东之后，也就搞明白了这些反编译工具的运行机制了，为什么从.class文件能过翻译成源代码呢？

可以看到该反编译还是挺强大的，几乎跟源代码看到的是一样的，这个对于咱们来观察字节码文件不太方便，所以换用java命令来查看一下，这里用javap命令，如下：

貌似用这个命令执行之后看到的信息没有啥价值，所以这里加一个参数再来看：

xiongweideMacBook-Pro:classes xiongwei$ pwd
/Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes
xiongweideMacBook-Pro:classes xiongwei$ javap -c com/jvm/bytecode/MyTest1
Compiled from "MyTest1.java"
public class com.jvm.bytecode.MyTest1 {
  public com.jvm.bytecode.MyTest1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_1
       6: putfield      #2                  // Field a:I
       9: return

  public int getA();
    Code:
       0: aload_0
       1: getfield      #2                  // Field a:I
       4: ireturn

  public void setA(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #2                  // Field a:I
       5: return
}

另外还有一个参数可以看到更加详细的字节码信息，如下：

xiongweideMacBook-Pro:classes xiongwei$ pwd
/Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes
xiongweideMacBook-Pro:classes xiongwei$ javap -c com/jvm/bytecode/MyTest1
Compiled from "MyTest1.java"
public class com.jvm.bytecode.MyTest1 {
  public com.jvm.bytecode.MyTest1();
xiongweideMacBook-Pro:classes xiongwei$ pwd
/Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes
xiongweideMacBook-Pro:classes xiongwei$ javap -verbose com/jvm/bytecode/MyTest1
Classfile /Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes/com/jvm/bytecode/MyTest1.class
  Last modified Aug 2, 2018; size 479 bytes
  MD5 checksum 4616561f95c24d6b04ea48a360437b8d
  Compiled from "MyTest1.java"
public class com.jvm.bytecode.MyTest1
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #3.#21         // com/jvm/bytecode/MyTest1.a:I
   #3 = Class              #22            // com/jvm/bytecode/MyTest1
   #4 = Class              #23            // 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/jvm/bytecode/MyTest1;
  #14 = Utf8               getA
  #15 = Utf8               ()I
  #16 = Utf8               setA
  #17 = Utf8               (I)V
  #18 = Utf8               SourceFile
  #19 = Utf8               MyTest1.java
  #20 = NameAndType        #7:#8          // "<init>":()V
  #21 = NameAndType        #5:#6          // a:I
  #22 = Utf8               com/jvm/bytecode/MyTest1
  #23 = Utf8               java/lang/Object
{
  public com.jvm.bytecode.MyTest1();
    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: iconst_1
         6: putfield      #2                  // Field a:I
         9: return
      LineNumberTable:
        line 3: 0
        line 4: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  this   Lcom/jvm/bytecode/MyTest1;

  public int getA();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #2                  // Field a:I
         4: ireturn
      LineNumberTable:
        line 7: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/jvm/bytecode/MyTest1;

  public void setA(int);
    descriptor: (I)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: iload_1
         2: putfield      #2                  // Field a:I
         5: return
      LineNumberTable:
        line 11: 0
        line 12: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/jvm/bytecode/MyTest1;
            0       6     1     a   I
}
SourceFile: "MyTest1.java"