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JVM的类型签名对照表
Type Signature | Java Type |
---|---|
Z | boolean |
B | byte |
C | char |
S | short |
I | int |
J | long |
F | float |
D | double |
L | fully-qualified-class ;fully-qualified-class |
[ type | type[] |
( arg-types ) ret-type | method type |
比如,java方法是
1 |
long f (int n, String s, int[] arr); |
对应的类型签名就是
1 |
f (ILjava/lang/String;[I)J |
再比如,java方法是
1 |
private void hi(double a, List<String> b); |
那对应的类型签名就是
1 |
hi (DLjava/util/List;)V |
接下来可以利用ASM进行验证上述两个类型签名是否正确:
public class Test {
public static void main(String[] args) throws Exception {
ClassPrinter printer = new ClassPrinter();
//读取静态内部类Bazhang
ClassReader cr = new ClassReader("Test$Bazhang");
cr.accept(printer, 0);
}
//静态内部类
static class Bazhang {
public Bazhang(int a) {
}
private long f (int n, String s, int[] arr){
return 0;
}
private void hi(double a, List<String> b){
}
}
static class ClassPrinter extends ClassVisitor {
public ClassPrinter() {
super(Opcodes.ASM5);
}
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
super.visit(version, access, name, signature, superName, interfaces);
//打印出父类name和本类name
System.out.println(superName + " " + name);
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
//打印出方法名和类型签名
System.out.println(name + " " + desc);
return super.visitMethod(access, name, desc, signature, exceptions);
}
}
}
最后打印出来的内容:
1 2 3 4 |
java/lang/Object Test$Bazhang <init> ()V f (ILjava/lang/String;[I)J hi (DLjava/util/List;)V |
验证了之前的正确性,其中可以看到默认构造函数也打印出来了。
那么接下来干点有意思的事,我们往Bazhang类里新增和方法,就定为:
1 2 3 |
public void newFunc(String str){ } |
这个时候就需要用到ClassWriter了,用于拼接字节码,具体关于ClassReader、ClassVisitor、ClassWriter的文章可以查看这篇http://www.blogjava.net/DLevin/archive/2014/06/25/414292.html
public static void main(String[] args) throws Exception {
ClassReader cr = new ClassReader(Bazhang.class.getName());
ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS);
cr.accept(cw, Opcodes.ASM5);
MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, "newFunc", "(Ljava/lang/String;)V", null, null);
mv.visitInsn(Opcodes.RETURN);
mv.visitEnd();
// 获取生成的class文件对应的二进制流
byte[] code = cw.toByteArray();
//将二进制流写到out/下
FileOutputStream fos = new FileOutputStream("out/Bazhang222.class");
fos.write(code);
fos.close();
}
这样就会在out/文件夹下生成Bazhang222.class:
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
import java.util.List;
class Test$Bazhang {
Test$Bazhang() {
}
private long f(int n, String s, int[] arr) {
return 0L;
}
private void hi(double a, List<String> b) {
}
public void newFunc(String var1) {
}
}
结合之前整理的JVM指令集,使用ASM直接操作字节码也是没问题的,结尾附上ASM源码下载地址:http://forge.ow2.org/projects/asm/