代码段

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

命令行

# 执行后当前目录生成HelloWorld.class
$ javac HelloWorld.java
$ java HelloWorld
Hello World
$
# 查看 JVM 反编译后的完整常量池
$ javap -verbose HelloWorld

// 字节码解析结果
Classfile /private/tmp/HelloWorld.class
  Last modified 2022-3-21; size 750 bytes
  MD5 checksum 15c6a79c2d4fd4510d4ac6e224b1a132
  Compiled from "HelloWorld.java"
public class HelloWorld
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #14.#26        // java/lang/Object."<init>":()V
   #2 = String             #27            // Tom
   #3 = Fieldref           #28.#29        // java/lang/System.out:Ljava/io/PrintStream;
   #4 = Class              #30            // java/lang/StringBuilder
   #5 = Methodref          #4.#26         // java/lang/StringBuilder."<init>":()V
   #6 = String             #31            // Hello World:
   #7 = Methodref          #4.#32         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = String             #33            // ,age:
   #9 = Fieldref           #13.#34        // HelloWorld.age:I
  #10 = Methodref          #4.#35         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #11 = Methodref          #4.#36         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #12 = Methodref          #37.#38        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #13 = Class              #39            // HelloWorld
  #14 = Class              #40            // java/lang/Object
  #15 = Utf8               age
  #16 = Utf8               I
  #17 = Utf8               <init>
  #18 = Utf8               ()V
  #19 = Utf8               Code
  #20 = Utf8               LineNumberTable
  #21 = Utf8               main
  #22 = Utf8               ([Ljava/lang/String;)V
  #23 = Utf8               <clinit>
  #24 = Utf8               SourceFile
  #25 = Utf8               HelloWorld.java
  #26 = NameAndType        #17:#18        // "<init>":()V
  #27 = Utf8               Tom
  #28 = Class              #41            // java/lang/System
  #29 = NameAndType        #42:#43        // out:Ljava/io/PrintStream;
  #30 = Utf8               java/lang/StringBuilder
  #31 = Utf8               Hello World:
  #32 = NameAndType        #44:#45        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #33 = Utf8               ,age:
  #34 = NameAndType        #15:#16        // age:I
  #35 = NameAndType        #44:#46        // append:(I)Ljava/lang/StringBuilder;
  #36 = NameAndType        #47:#48        // toString:()Ljava/lang/String;
  #37 = Class              #49            // java/io/PrintStream
  #38 = NameAndType        #50:#51        // println:(Ljava/lang/String;)V
  #39 = Utf8               HelloWorld
  #40 = Utf8               java/lang/Object
  #41 = Utf8               java/lang/System
  #42 = Utf8               out
  #43 = Utf8               Ljava/io/PrintStream;
  #44 = Utf8               append
  #45 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #46 = Utf8               (I)Ljava/lang/StringBuilder;
  #47 = Utf8               toString
  #48 = Utf8               ()Ljava/lang/String;
  #49 = Utf8               java/io/PrintStream
  #50 = Utf8               println
  #51 = Utf8               (Ljava/lang/String;)V
{
  public static int age;
    descriptor: I
    flags: ACC_PUBLIC, ACC_STATIC

  public 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 1: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=2, args_size=1
         0: ldc           #2                  // String Tom
         2: astore_1
         3: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
         6: new           #4                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
        13: ldc           #6                  // String Hello World:
        15: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        18: aload_1
        19: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        22: ldc           #8                  // String ,age:
        24: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        27: getstatic     #9                  // Field age:I
        30: invokevirtual #10                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        33: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        36: invokevirtual #12                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        39: return
      LineNumberTable:
        line 4: 0
        line 5: 3
        line 6: 39

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: bipush        10
         2: putstatic     #9                  // Field age:I
         5: return
      LineNumberTable:
        line 2: 0
}
SourceFile: "HelloWorld.java"

参考自

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