《深入理解Java虚拟机》-（实战）练习修改class文件
2019-08-12 23:19
1016 查看
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.javapublic 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!");
}
}
}
View Code
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"
View Code
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
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