什么是字节码

java bytecode 由单字节（byte）的指令组成，理论上最多支持256个操作码，实际只使用了200左右，其他的保留给调试操作

分类

  • 栈操作指令，包括与局部变量交互的指令
  • 流程控制指令
  • 对象操作指令，包括调用方法
  • 算数运算、类型转化指令

最简单的字节码

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public class HelloByteCode {
  public static void main(String[] args) {
    HelloByteCode helloByteCode = new HelloByteCode();
  }
}

javac编译

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public class HelloByteCode {
  public HelloByteCode() {
  }

  public static void main(String[] var0) {
    new HelloByteCode();
  }
}

javap -c 查看字节码

-c：分解方法代码，即显示每个方法具体的字节码

-public | protected | package | private：用于指定显示哪种级别的类成员

-verbose：指定显示更进一步的详细信息

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Compiled from "HelloByteCode.java"
public class com.qypone.learn.bytecode.HelloByteCode {
  public com.qypone.learn.bytecode.HelloByteCode();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class com/qypone/learn/bytecode/HelloByteCode
       3: dup
       4: invokespecial #3                  // Method "<init>":()V
       7: astore_1
       8: return
}

指令：javap -c -verbose HelloByteCode.class

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Classfile /E:/github/learn/src/main/java/com/qypone/learn/bytecode/HelloByteCode.class
  Last modified 2021-12-23; size 314 bytes
  MD5 checksum 2316eef4ab6ce7a12d26b0eaa0f11709
  Compiled from "HelloByteCode.java"
public class com.qypone.learn.bytecode.HelloByteCode
  minor version: 0
  major version: 55
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#13         // java/lang/Object."<init>":()V
   #2 = Class              #14            // com/qypone/learn/bytecode/HelloByteCode
   #3 = Methodref          #2.#13         // com/qypone/learn/bytecode/HelloByteCode."<init>":()V
   #4 = Class              #15            // java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Utf8               Code
   #8 = Utf8               LineNumberTable
   #9 = Utf8               main
  #10 = Utf8               ([Ljava/lang/String;)V
  #11 = Utf8               SourceFile
  #12 = Utf8               HelloByteCode.java
  #13 = NameAndType        #5:#6          // "<init>":()V
  #14 = Utf8               com/qypone/learn/bytecode/HelloByteCode
  #15 = Utf8               java/lang/Object
{
  public com.qypone.learn.bytecode.HelloByteCode();
    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=2, args_size=1
         0: new           #2                  // class com/qypone/learn/bytecode/HelloByteCode
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: astore_1
         8: return
      LineNumberTable:
        line 5: 0
        line 6: 8
}
SourceFile: "HelloByteCode.java"

示例代码

qypone/blog (github.com)