Jasper Ji

始以竹篦子久知痛痒,后因一滴水渐至澎冲。

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学习Java字节码

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