• class文件格式概述

  我们先看下classFile的结构(查阅java 虚拟机规范)

ClassFile{    u4  magic;CA为一组，1111    u2  minor_version;    u2  major_version;    u2  constant_pool_count;    cp_info  constant_pool[constant_pool_count-1];    u2  access_flags;    u2  this_class;    u2  super_class;    u2  infterfaces_count;    u2  infterfaces[interfaces_count];    u2  fields_count;    field_info  fields[fields_count];    u2  methods_count;    method_info methods[methods_couunt];   u2  attributes_count;   attribute_info  attributes[attributes_count];  }在ClassFile结构中，各项的含义描述如下：magic(魔数)minor_version(副版本号)、major_version(主版本号)constant_pool_count(常量池计数器)constant_pool[](常量池)access_flag(访问标识)this_class(类索引)super_class(父类索引)infterfaces_count(接口计数器)infterfaces[](接口表)fields_count(字段计数器)fields[](字段表)methods_count(方法计数器)methods[](方法表)attributes_count(属性计数器)attributes[](属性表)

二进制字节码如下：

• Class文件是一组以8字节为单位的字节流，各个数据项目按顺序紧凑排列(如上图开头的字符CA为一组，CA用二进制表示为1100 1010，共8字节)

• 对于占用空间大于8字节的数据项，按照高位在前的方式分割成多个8字节进行存储

• Class文件格式里面只有两种类型：无符号数、表

  • 无符号数：基本数据类型，以u1、u2、u4、u8来代表几个字节的无符号数(如classfile第一行的u4  magic;对应二进制字节码的CA FE BA BE)

  • 表：由多个无符号数和其他表构成的复合数据类型，通常以"_info"结尾

• 阅读class文件

创建如下代码：

public class Test {    private static String params = "Hello World";    public static void main(String[] args) {        System.out.println("params = " + params);    }}

通过反编译(javap)查看生成的class文件：

Classfile /F:/crazy/freedom/ifd-jvm/target/classes/cn/crazy/dreamer/Test.class  Last modified 2020-10-12; size 861 bytes  MD5 checksum 03940161a851df384ccc9ab26db9046c  Compiled from "Test.java"public class cn.crazy.dreamer.Test  minor version: 0  major version: 52  // 与jdk有关  flags: ACC_PUBLIC, ACC_SUPER  // 访问标识public，superConstant pool:  // 常量池   #1 = Methodref          #12.#30        // java/lang/Object."":()V   #2 = Fieldref           #31.#32        // java/lang/System.out:Ljava/io/PrintStream;   #3 = Class              #33            // java/lang/StringBuilder   #4 = Methodref          #3.#30         // java/lang/StringBuilder."":()V   #5 = String             #34            // params =   #6 = Methodref          #3.#35         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;   #7 = Fieldref           #11.#36        // cn/crazy/dreamer/Test.params:Ljava/lang/String;   #8 = Methodref          #3.#37         // java/lang/StringBuilder.toString:()Ljava/lang/String;   #9 = Methodref          #38.#39        // java/io/PrintStream.println:(Ljava/lang/String;)V  #10 = String             #40            // Hello World  #11 = Class              #41            // cn/crazy/dreamer/Test  #12 = Class              #42            // java/lang/Object  #13 = Utf8               params  #14 = Utf8               Ljava/lang/String;  #15 = Utf8                 #16 = Utf8               ()V  #17 = Utf8               Code  #18 = Utf8               LineNumberTable  #19 = Utf8               LocalVariableTable  #20 = Utf8               this  #21 = Utf8               Lcn/crazy/dreamer/Test;  #22 = Utf8               main  #23 = Utf8               ([Ljava/lang/String;)V  #24 = Utf8               args  #25 = Utf8               [Ljava/lang/String;  #26 = Utf8               MethodParameters  #27 = Utf8                 #28 = Utf8               SourceFile  #29 = Utf8               Test.java  #30 = NameAndType        #15:#16        // "":()V  #31 = Class              #43            // java/lang/System  #32 = NameAndType        #44:#45        // out:Ljava/io/PrintStream;  #33 = Utf8               java/lang/StringBuilder  #34 = Utf8               params =  #35 = NameAndType        #46:#47        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  #36 = NameAndType        #13:#14        // params:Ljava/lang/String;  #37 = NameAndType        #48:#49        // toString:()Ljava/lang/String;  #38 = Class              #50            // java/io/PrintStream  #39 = NameAndType        #51:#52        // println:(Ljava/lang/String;)V  #40 = Utf8               Hello World  #41 = Utf8               cn/crazy/dreamer/Test  #42 = Utf8               java/lang/Object  #43 = Utf8               java/lang/System  #44 = Utf8               out  #45 = Utf8               Ljava/io/PrintStream;  #46 = Utf8               append  #47 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;  #48 = Utf8               toString  #49 = Utf8               ()Ljava/lang/String;  #50 = Utf8               java/io/PrintStream  #51 = Utf8               println  #52 = Utf8               (Ljava/lang/String;)V{  public cn.crazy.dreamer.Test();  // 无参构造    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=1, locals=1, args_size=1         0: aload_0         1: invokespecial #1                  // Method java/lang/Object."":()V         4: return      LineNumberTable:        line 11: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   Lcn/crazy/dreamer/Test;  public static void main(java.lang.String[]); // main方法    descriptor: ([Ljava/lang/String;)V    flags: ACC_PUBLIC, ACC_STATIC    Code:      stack=3, locals=1, args_size=1         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         3: new           #3                  // class java/lang/StringBuilder         6: dup         7: invokespecial #4                  // Method java/lang/StringBuilder."":()V        10: ldc           #5                  // String params =        12: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        15: getstatic     #7                  // Field params:Ljava/lang/String;        18: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        21: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;        24: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V        27: return      LineNumberTable:        line 16: 0        line 17: 27      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      28     0  args   [Ljava/lang/String;    MethodParameters:      Name                           Flags      args  static {};  // 静态变量初始化，在jvm静态变量初始化都是通过静态块实现    descriptor: ()V    flags: ACC_STATIC     Code:      stack=1, locals=0, args_size=0         0: ldc           #10                 // String Hello World         2: putstatic     #7                  // Field params:Ljava/lang/String;         5: return      LineNumberTable:        line 13: 0}SourceFile: "Test.java"

我们通过反编译生成的classFile是jdk工具将二进制的.class文件根据java虚拟机规范解析成我们开发人员比较易于阅读的格式。javap工具生成非正式的"虚拟机汇编语言"，格式如下：[[...]][]

• 是指令操作码在数组中的下标，该数组以字节形式来存储当前方法的java虚拟机代码；也可以是相对于方法起始处的字节偏移量(对应上面代码中的 0)

• 是指令的助记码(对应上面代码中的 ldc)

• 是操作数(对应上面代码中的 #10)

• 是行尾的注释(对应上面代码中的 // String Hello World)

阅读class文件时需要注意：

• constant_pool_count：是从1开始的

• 不同的常量类型，用tag来区分的，它后面对应的info结构是不一样的

• L表示对象，[表示数组、V表示void

• stack:方法执行时操作栈的深度

• locals:局部变量所需的存储空间，单位是slot(slot是虚拟机为局部变量分配内存所使用的最小单位)

• args_size:参数个数，为1的话，因实例方法默认会传入this,local也会预留一个slot来存放

以如下classfile为例，我们大概来阅读下：

static {};    descriptor: ()V    flags: ACC_STATIC    Code:      stack=1, locals=0, args_size=0         0: ldc           #10                 // String Hello World         2: putstatic     #7                  // Field params:Ljava/lang/String;         5: return      LineNumberTable:        line 13: 0        ldc #10:装载常量hello world(#10表示在常量池中的编号)putstatic #7:赋值给params

我们粗略阅读下main方法反编译生成的代码：

public static void main(java.lang.String[]);    descriptor: ([Ljava/lang/String;)V    flags: ACC_PUBLIC, ACC_STATIC    Code:      stack=3, locals=1, args_size=1         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         3: new           #3                  // class java/lang/StringBuilder         6: dup         7: invokespecial #4                  // Method java/lang/StringBuilder."":()V        10: ldc           #5                  // String params =        12: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        15: getstatic     #7                  // Field params:Ljava/lang/String;        18: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        21: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;        24: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V        27: return      LineNumberTable:        line 16: 0        line 17: 27      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      28     0  args   [Ljava/lang/String;    MethodParameters:      Name                           Flags      args      getstatic     #2：拿到PrintStreamnew           #3：创建stringbuild对象dup：复制invokespecial #4：调用初始化方法初始化stringbuild对象ldc           #5：装载常量invokevirtual #6: 调用方法getstatic     #7:获取静态变量

• 使用IDEA执行javap

每次执行javap反编译命令时，都要进入.class文件的目录，相对来说比较麻烦，我们可以通过idea来运行javap异常点击"Settrings"->"Tools"->"External Tools"，点击"+"添加一个外部工具

进入编辑页面，按如图所示进行配置：

在java文件上点击右键(前提是java文件已经编译完成)，按如图所示进行操作，即可在idea命令窗口输出class信息。

class字节码规范在java虚拟机规范中有详细描述，建议到官网中下载官方文档进行阅读(https://docs.oracle.com/javase/specs/index.html)，也可以购买JVM方面的书籍(Java 虚拟机规范[爱飞翔 周志明 等译])