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《深入理解Java虚拟机》-（实战）练习修改class文件

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这是一篇修改class文件的文章。注释并不完全，要抓住这次练习的目的：

boolean在虚拟机中是以何种方式解读的

好的，开始我的表演

1.安装asmtools.jar（本文尾部有步骤）

2.编写一个java文件，并编译，执行
 2.1 Foo.java
 public class Foo {

   public static void main(String[] args) {

     boolean flag = true;

     if (flag)   {

         System.out.println("Hello, Java!");

     }

     if (flag == true) {

         System.out.println("Hello, JVM!");

     }

   }

 }

2.2 编译并运行

[root@localhost tmp]# javac Foo.java

[root@localhost tmp]# java Foo

Hello, Java!

Hello, JVM!

3.查看编译后的java文件,class （注意看黄色部分的变化）

[root@localhost tmp]# javap -verbose Foo

Classfile /usr/local/asmtools-7.0-build/binaries/lib/tmp/Foo.class

  Last modified Aug 12, 2019; size 493 bytes

  MD5 checksum d51944604c5b4e45cb895501910347ea

  Compiled from "Foo.java"

public class Foo

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = Methodref          #7.#17         // java/lang/Object."<init>":()V

   #2 = Fieldref           #18.#19        // java/lang/System.out:Ljava/io/PrintStream;

   #3 = String             #20            // Hello, Java!

   #4 = Methodref          #21.#22        // java/io/PrintStream.println:(Ljava/lang/String;)V

   #5 = String             #23            // Hello, JVM!

   #6 = Class              #24            // Foo

   #7 = Class              #25            // java/lang/Object

   #8 = Utf8               <init>

   #9 = Utf8               ()V

  #10 = Utf8               Code

  #11 = Utf8               LineNumberTable

  #12 = Utf8               main

  #13 = Utf8               ([Ljava/lang/String;)V

  #14 = Utf8               StackMapTable

  #15 = Utf8               SourceFile

  #16 = Utf8               Foo.java

  #17 = NameAndType        #8:#9          // "<init>":()V

  #18 = Class              #26            // java/lang/System

  #19 = NameAndType        #27:#28        // out:Ljava/io/PrintStream;

  #20 = Utf8               Hello, Java!

  #21 = Class              #29            // java/io/PrintStream

  #22 = NameAndType        #30:#31        // println:(Ljava/lang/String;)V

  #23 = Utf8               Hello, JVM!

  #24 = Utf8               Foo

  #25 = Utf8               java/lang/Object

  #26 = Utf8               java/lang/System

  #27 = Utf8               out

  #28 = Utf8               Ljava/io/PrintStream;

  #29 = Utf8               java/io/PrintStream

  #30 = Utf8               println

  #31 = Utf8               (Ljava/lang/String;)V

{

  public Foo();

    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=2, locals=2, args_size=1

         0: iconst_1                    //常量1入栈

         1: istore_1                    //把栈顶值存储到局部变量表下标为1的位置，即flag =1;

         2: iload_1                     //取局部变量表中下标为1的变量压栈

         3: ifeq          14            //（jump if i == 0） 将栈顶值与0比较，如果相等，则跳入14步骤。

         6: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;

         9: ldc           #3                  // String Hello, Java!

        11: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

        14: iload_1                     //取局部变量表中下标为1的变量压栈

        15: iconst_1                    //常量1入栈

        16: if_icmpne     27            //比较栈顶两个值，如果不相等，则跳转到27

        19: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;

        22: ldc           #5                  // String Hello, JVM!

        24: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

        27: return

      LineNumberTable:

        line 3: 0

        line 4: 2

        line 5: 6

        line 7: 14

        line 8: 19

        line 10: 27

      StackMapTable: number_of_entries = 2

        frame_type = 252 /* append */

          offset_delta = 14

          locals = [ int ]

        frame_type = 12 /* same */

}

SourceFile: "Foo.java"

4.使用asmtools.jar修改class文件

[root@localhost tmp]# java -jar ../asmtools.jar jdis Foo.class > Foo.jasm.1

[root@localhost tmp]# ls

Foo.class  Foo.jasm.1  Foo.java

[root@localhost tmp]# java -cp ../asmtools.jar org.openjdk.asmtools.jdis.Main Foo.class > Foo.jasm.1

[root@localhost tmp]# awk 'NR==1,/iconst_1/{sub(/iconst_1/,"iconst_2")} 1' Foo.jasm.1 > Foo.jasm

[root@localhost tmp]# ls

Foo.class  Foo.jasm  Foo.jasm.1  Foo.java

[root@localhost tmp]# java Foo

Hello, Java!

Hello, JVM!

5.再次编译,执行

[root@localhost tmp]# java -jar  ../asmtools.jar jasm Foo.jasm

[root@localhost tmp]# java Foo

Hello, Java!

[root@localhost tmp]# ls

Foo.class  Foo.jasm  Foo.jasm.1  Foo.java

6.查看修改后的class文件

[root@localhost tmp]# javap -verbose Foo

Classfile /usr/local/asmtools-7.0-build/binaries/lib/tmp/Foo.class

  Last modified Aug 12, 2019; size 431 bytes

  MD5 checksum 18cfb8b8b7d9d49e9ffce213e70c8898

  Compiled from "Foo.jasm"

public class Foo

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = String             #10            // Hello, Java!

   #2 = String             #11            // Hello, JVM!

   #3 = Fieldref           #27.#12        // java/lang/System.out:Ljava/io/PrintStream;

   #4 = Methodref          #8.#17         // java/lang/Object."<init>":()V

   #5 = Methodref          #13.#30        // java/io/PrintStream.println:(Ljava/lang/String;)V

   #6 = Utf8               (Ljava/lang/String;)V

   #7 = Utf8               out

   #8 = Class              #9             // java/lang/Object

   #9 = Utf8               java/lang/Object

  #10 = Utf8               Hello, Java!

  #11 = Utf8               Hello, JVM!

  #12 = NameAndType        #7:#23         // out:Ljava/io/PrintStream;

  #13 = Class              #15            // java/io/PrintStream

  #14 = Utf8               ([Ljava/lang/String;)V

  #15 = Utf8               java/io/PrintStream

  #16 = Utf8               main

  #17 = NameAndType        #29:#20        // "<init>":()V

  #18 = Utf8               SourceFile

  #19 = Utf8               println

  #20 = Utf8               ()V

  #21 = Utf8               StackMapTable

  #22 = Utf8               Foo.jasm

  #23 = Utf8               Ljava/io/PrintStream;

  #24 = Utf8               Code

  #25 = Class              #26            // Foo

  #26 = Utf8               Foo

  #27 = Class              #28            // java/lang/System

  #28 = Utf8               java/lang/System

  #29 = Utf8               <init>

  #30 = NameAndType        #19:#6         // println:(Ljava/lang/String;)V

{

  public Foo();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=1, locals=1, args_size=1

         0: aload_0

         1: invokespecial #4                  // Method java/lang/Object."<init>":()V

         4: return

  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: iconst_2                     //载入常量2入栈

         1: istore_1                     //把栈顶值存储到局部变量表下标为1的位置，即flag =2;

         2: iload_1                        //取局部变量表中下标为1的变量压栈

         3: ifeq          14

         6: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;

         9: ldc           #1                  // String Hello, Java!

        11: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

        14: iload_1                    //取局部变量表中下标为1的变量压栈

        15: iconst_1                   //常量1入栈

        16: if_icmpne     27           //比较栈顶两个值，如果不相等，则跳转到27

        19: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;

        22: ldc           #2                  // String Hello, JVM!

        24: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

        27: return

      StackMapTable: number_of_entries = 2

        frame_type = 252 /* append */

          offset_delta = 14

          locals = [ int ]

        frame_type = 12 /* same */

}

SourceFile: "Foo.jasm"

　　

　

附：asmtools.jar的安装(centos 6/7)

0.先cd进入需要安装到的目录地址
 1. Mercurial是一种轻量级分布式版本控制系统，采用Python语言实现。
yum install hg

2. 版本是2.6.2，发现不是最新版，去官方下载centos 7最新版3.9.2

wget https://www.mercurial-scm.org/release/centos7/RPMS/x86_64/mercurial-3.9.2-1.x86_64.rpm

3.升级2.6.2 到3.9.2 （centos 6 我就没有用这一步。。）

rpm -Uvh mercurial-3.9.2-1.x86_64.rpm

4.安装ASMTOOLS.jar

hg clone http://hg.openjdk.java.net/code-tools/asmtools/ asmtools

cd asmtools/build/

yum install ant

ant //编译生成asmtools.jar

5.生成的jar位置：

[root@localhost lib]# ls

asmtools.jar

[root@localhost lib]# pwd

/usr/local/asmtools-7.0-build/binaries/lib

　　

　　

　　

　　

 

04-26 16:31
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