我正在尝试编写一个脚本，用一个基本的throw new exception（）行替换每个方法体。我正在学习ASM的开始步骤，所以任何关于在哪里寻找的指针都会受到赞赏。
到目前为止我做了什么：
package methodtester;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import static org.objectweb.asm.Opcodes.ACC_INTERFACE;
import static org.objectweb.asm.Opcodes.ASM4;
import static org.objectweb.asm.Opcodes.ATHROW;
import static org.objectweb.asm.Opcodes.DUP;
import static org.objectweb.asm.Opcodes.INVOKESPECIAL;
import static org.objectweb.asm.Opcodes.NEW;
public class MethodTransformer {
public static void main(String[] args) throws IOException {
InputStream in = MethodTester.class.getResourceAsStream("/methodtester/testingMethod.class");
ClassReader classReader = new ClassReader(in);
ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
ExceptionThrower exceptionThrower = new ExceptionThrower(classWriter);
classReader.accept(exceptionThrower, 0);
File outputDir = new File("build/classes/methodtester/");
outputDir.mkdirs();
DataOutputStream dataOutputStream =
new DataOutputStream(
new FileOutputStream(
new File(outputDir,"testingMethod-postASM.class")));
dataOutputStream.write(classWriter.toByteArray());
}
public static class ExceptionThrower extends ClassVisitor {
private String _className;
private boolean _isInterface;
public ExceptionThrower(ClassVisitor classVisitor) {
super(ASM4, classVisitor);
}
@Override
public void visit(
int version, int access, String name, String signature,
String superName, String[] interfaces) {
cv.visit(version, access, name, signature, superName, interfaces);
_className = name;
_isInterface = (access & ACC_INTERFACE) != 0;
}
@Override
public MethodVisitor visitMethod(
int access, String name, String desc, String signature,
String[] exceptions) {
MethodVisitor mv = cv.visitMethod(access, name, desc, signature,
exceptions);
if (!_isInterface && mv != null && !name.equals("<init>")) {
ExceptionThrowerMethod exceptionThrowerMethod =
new ExceptionThrowerMethod(mv);
return exceptionThrowerMethod;
}
return mv;
}
public static class ExceptionThrowerMethod extends MethodVisitor {
public ExceptionThrowerMethod(MethodVisitor methodVisitor) {
super(ASM4, methodVisitor);
}
@Override
public void visitCode() {
mv.visitCode();
mv.visitTypeInsn(NEW, "java/io/IOException");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/io/IOException", "<init>", "()V", false);
mv.visitInsn(ATHROW);
mv.visitMaxs(2, 0);
mv.visitEnd();
}
}
}
}
到目前为止，我能够将新的IOException（）指令插入到方法的开头，但是这不会在运行时运行，因为我收到了这个错误：
Exception in thread "main" java.lang.ClassFormatError: Invalid start_pc 8 in LocalVariableTable in class file methodtester/testingMethod
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at methodtester.MethodTester.main(MethodTester.java:27)
看着javap -c-v我得到：
public void testing() throws java.io.IOException;
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: new #14 // class java/io/IOException
3: dup
4: invokespecial #15 // Method java/io/IOException."<init>":()V
7: athrow
LocalVariableTable:
Start Length Slot Name Signature
8 0 0 this Lmethodtester/testingMethod;
LineNumberTable:
line 24: 8
line 25: 8
Exceptions:
throws java.io.IOException
}
使用ASMifier查看类我发现方法是：
mv = cw.visitMethod(ACC_PUBLIC, "testing", "()V", null, null);
mv.visitCode();
mv.visitTypeInsn(NEW, "java/io/IOException");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/io/IOException", "<init>", "()V", false);
mv.visitInsn(ATHROW);
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn("testing method");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(2, 1);
mv.visitEnd();
}
看起来我需要更改LocalVariableTable以使其从0开始，长度为0。
谢谢！
编辑：使用COMPUTE_FRAMES更新并更新新的错误代码
答案 0 :(得分：1)
您将从MethodVisitor收到的原始ClassWriter传递给自定义MethodVisitor的超级构造函数。这意味着，您未覆盖的每个visit…调用都将委派给MethodVisitor，复制整个原始代码。
当然，首先，您不希望复制原始代码，其次，在调用visit…和{后，在作家上接收相互矛盾的visitMaxs调用{1}}无助于创建有效的代码。
如果您想完全替换方法，则不应将目标编写者的visitEnd展示给基类，而是仅将其用于您自己的代码生成：
MethodVisitor按照他们的方式，您可以通过让public static class ExceptionThrowerMethod extends MethodVisitor {
private final MethodVisitor target;
public ExceptionThrowerMethod(MethodVisitor methodVisitor) {
super(ASM4, null);
this.target=methodVisitor;
}
@Override
public void visitCode() {
target.visitCode();
target.visitTypeInsn(NEW, "java/io/IOException");
target.visitInsn(DUP);
target.visitMethodInsn(INVOKESPECIAL,"java/io/IOException","<init>","()V",false);
target.visitInsn(ATHROW);
target.visitMaxs(2, 0);
target.visitEnd();
}
}
在本地解析资源来简化资源访问，例如： Class。