字节码（一）

一、Java代码编译执行

1.1 示意图

+---------------+			+---------------+			+---------------+         
|	.java       |---------->| JAVA Compiler |---------->|	.class		|-------+
|				|			|	 (javac)    |			|				|      	|
+---------------+			+---------------+			+---------------+      	|
																				|
																				|
+-----------------------------------------------------------------------+	   	|
|								 JVM									|		|	
|  +----------+		+---------------+     +---------+     +---------+   |	 	|
|  |    OS	  |		| JAVA Runtime  |     |	ByteCode|	  |	 Class	|	|		|
|  |  (Linux) |<----|	 System     |<----|	Verifier|<----|	 Loader	|	|<------+
|  +----------+		+---------------+     +---------+	  +---------+	|
|																		|	
+-----------------------------------------------------------------------+

1.2 字节码查看

• 用UE或者其它16进制工具(winhex)或者Linux命令xxd打开字节码文件，如下图所示：其中
  （1）CA FE BA BE为魔数。魔数值固定不变，且放在文件开头，JVM可以根据文件的开头来判断这个文件是否可能是一个.class文件，如果是，才会继续进行之后的操作。
  （2）次版本号（minor version）：0
  （3）主版本号（major version）：61 （16* 3 + 13 = 61），意味着编译该文件的Java版本号为17
  

• 字节码查看命令： javap -c 类名 或者 javap -v 类名来反汇编字节码文件
  

D:\>javap -v ByteCode.class
Classfile /D:/ByteCode.class
  Last modified 2024-12-12; size 513 bytes
  MD5 checksum 9e3103bd209e0d6546d6f7c99075e5f9
  Compiled from "ByteCode.java"
public class ByteCode
  minor version: 0
  major version: 61
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #2.#3          // java/lang/Object."<init>":()V
   #2 = Class              #4             // java/lang/Object
   #3 = NameAndType        #5:#6          // "<init>":()V
   #4 = Utf8               java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Fieldref           #8.#9          // java/lang/System.out:Ljava/io/PrintStream;
   #8 = Class              #10            // java/lang/System
   #9 = NameAndType        #11:#12        // out:Ljava/io/PrintStream;
  #10 = Utf8               java/lang/System
  #11 = Utf8               out
  #12 = Utf8               Ljava/io/PrintStream;
  #13 = String             #14            // ByteCode
  #14 = Utf8               ByteCode
  #15 = Methodref          #16.#17        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #16 = Class              #18            // java/io/PrintStream
  #17 = NameAndType        #19:#20        // println:(Ljava/lang/String;)V
  #18 = Utf8               java/io/PrintStream
  #19 = Utf8               println
  #20 = Utf8               (Ljava/lang/String;)V
  #21 = Class              #14            // ByteCode
  #22 = Utf8               Code
  #23 = Utf8               LineNumberTable
  #24 = Utf8               LocalVariableTable
  #25 = Utf8               this
  #26 = Utf8               LByteCode;
  #27 = Utf8               main
  #28 = Utf8               ([Ljava/lang/String;)V
  #29 = Utf8               args
  #30 = Utf8               [Ljava/lang/String;
  #31 = Utf8               SourceFile
  #32 = Utf8               ByteCode.java
{
  public ByteCode();
    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
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   LByteCode;

  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     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #13                 // String ByteCode
         5: invokevirtual #15                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 3: 0
        line 4: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  args   [Ljava/lang/String;
}
SourceFile: "ByteCode.java"

• JDK version和class file version(Class编译版本号)对应关系

JDK 19 = 63,
JDK 18 = 62,
JDK 17 = 61,
JDK 16 = 60,
JDK 15 = 59,
JDK 14 = 58,
JDK 13 = 57,
JDK 12 = 56,
JDK 11 = 55,
JDK 10 = 54,
JDK 9 = 53,
JDK 8 = 52,
JDK 7 = 51,
JDK 6.0 = 50,
JDK 5.0 = 49,
JDK 1.4 = 48,
JDK 1.3 = 47,
JDK 1.2 = 46,
JDK 1.1 = 45.0-45.6