《深入理解Java虚拟机》-（实战）练习修改class文件

[root@localhost tmp]# javac Foo.java
[root@localhost tmp]# java Foo
Hello, Java!
Hello, JVM!

[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!

[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

[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"