ASM 操作字节码初探

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

long f (int n, String s, int[] arr);
        

对应的类型签名就是

f (ILjava/lang/String;[I)J
        

再比如，java方法是

private void hi(double a, List<String> b);
        

那对应的类型签名就是

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：

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:

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www.blogjava.net/DLevin/arch…

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：forge.ow2.org/projects/as…