通过 ASM 替换完整方法

Question

我正在尝试编写一个脚本，用基本的 throw new exception() 行替换每个方法体。我正处于学习 ASM 的起步阶段，所以任何有关查找位置的指示都将不胜感激。

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 throw new IOException() 指令插入到方法的开头，但这不会运行在运行时间，因为我收到此错误：

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.access0(URLClassLoader.java:73)
    at java.net.URLClassLoader.run(URLClassLoader.java:368)
    at java.net.URLClassLoader.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)

查看 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 更新并更新了新的错误代码

Answer 1

您将从 ClassWriter 收到的原始 MethodVisitor 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… 调用都将委托给那个 MethodVisitor，复制整个原始代码。

当然，首先，您不想复制原始代码，其次，在您对 visitMaxs 和 visitEnd 无助于创建有效代码。

当你想完全替换一个方法时，你不应该将目标编写者的MethodVisitor展示给基础class，而是仅将其用于您自己的代码生成：

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();
    }
}

通过他们的方式，您可以通过让 Class 在本地解析资源来简化您的资源访问，例如testingMethod.class.getResourceAsStream("testingMethod.class").

通过 ASM 替换完整方法

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