Jasper Ji

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学习Java字节码

发表于

Java字节码只是简单的了解，感觉缺乏深入的了解，特写此篇记录下。

准备一个HelloWorld的测试类。

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public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

编译生成HelloWorld.class文件

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javac HelloWorld.java

我们使用javap命令来查看class文件

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javap -v HelloWorld.class

输出结果

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Classfile /Users/jasperji/eclipse-workspace/maventest/src/main/java/com/jasper/ji/maven/test/HelloWorld.class
  Last modified 2021-9-8; size 452 bytes
  MD5 checksum 1f346aeb85559b06180507b8838406dd
  Compiled from "HelloWorld.java"
public class com.jasper.ji.maven.test.HelloWorld
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#17        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #18            // Hello, World!
   #4 = Methodref          #19.#20        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #21            // com/jasper/ji/maven/test/HelloWorld
   #6 = Class              #22            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               SourceFile
  #14 = Utf8               HelloWorld.java
  #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Class              #23            // java/lang/System
  #17 = NameAndType        #24:#25        // out:Ljava/io/PrintStream;
  #18 = Utf8               Hello, World!
  #19 = Class              #26            // java/io/PrintStream
  #20 = NameAndType        #27:#28        // println:(Ljava/lang/String;)V
  #21 = Utf8               com/jasper/ji/maven/test/HelloWorld
  #22 = Utf8               java/lang/Object
  #23 = Utf8               java/lang/System
  #24 = Utf8               out
  #25 = Utf8               Ljava/io/PrintStream;
  #26 = Utf8               java/io/PrintStream
  #27 = Utf8               println
  #28 = Utf8               (Ljava/lang/String;)V
{
  public com.jasper.ji.maven.test.HelloWorld();
    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 3: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Hello, World!
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 5: 0
        line 6: 8
}
SourceFile: "HelloWorld.java"

另外也可以直接查看二进制，这里使用hexdump。

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hexdump HelloWorld.class

不过感觉还是javap提供的这个比较人性化，更好的阅读。

总结

字节码作为一种中间形式，Java可以编译成字节码，Scala也可以编译成字节码，但是从语言层面他们又是不一样的。这个也让我重新再读了下《Java程序员修炼之道》这本书，原来对于书中第三部分JVM上的多语言编程，不是很感冒。但字节码并不是机器码，所以在理解某些问题时最终还是要落到汇编语言级别的。

参考

字节码增强技术探索