使用 ASM 结果"Exceeded max stack size during"修改方法主体

Question

我想用新的内容(sample.class:sayHello方法)替换方法体，然后执行sample.class。原来的sayHelo声明是:

    public int sayHello(String args){
}

我想把它的主体修改为:

    System.out.println("sxu says: hello world!");
    return 1;

但是示例执行结果抛出异常:

Exception in thread "main" java.lang.VerifyError: Operand stack overflow
Exception Details:
  Location:
    code/sxu/demo/data/sample.sayHello(Ljava/lang/String;)I @4: ldc
  Reason:
    Exceeded max stack size.
  Current Frame:
    bci: @4
    flags: { }
    locals: { 'code/sxu/demo/data/sample', 'java/lang/String' }
    stack: { 'code/sxu/demo/data/sample', 'java/io/PrintStream' }
  Bytecode:
    0000000: 2ab2 0028 122a b600 1704 acb2 0028 2bb6
    0000010: 0017 04ac    

我使用asm工具，在我的代码中有三个java类:

public class Adapt extends ClassLoader {
    @Override
    protected synchronized Class<?> loadClass(final String name,
            final boolean resolve) throws ClassNotFoundException {
        if (name.startsWith("java.")) {
            return super.loadClass(name, resolve);
        } else {
            System.err.println("Adapt: loading class '" + name
                    + "' with on the fly adaptation");
        }
        // gets an input stream to read the bytecode of the class
        String resource = name.replace('.', '/') + ".class";
        InputStream is = getResourceAsStream(resource);
        byte[] b;
        // adapts the class on the fly
        try {
            ClassReader cr = new ClassReader(is);
            ClassWriter cw = new ClassWriter(0);
            ClassVisitor cv = new ClassAdapter(cw);
            cr.accept(cv, 0);
            b = cw.toByteArray();
        } catch (Exception e) {
            throw new ClassNotFoundException(name, e);
        }
        FileOutputStream fos = new FileOutputStream("/tmp/"+resource.substring(resource.lastIndexOf("/")) );
            fos.write(b);
            fos.close();

        // returns the adapted class
        return defineClass(name, b, 0, b.length);
    }
    public static void main(String[] args) {
          // loads the application class (in args[0]) with an Adapt class loader
        ClassLoader loader = new Adapt();
        Class<?> c;
        try {
            c = loader.loadClass(args[0]);
            // calls the 'main' static method of this class with the
            // application arguments (in args[1] ... args[n]) as parameter
            Method m = c.getMethod("main", new Class<?>[] { String[].class });
            String[] applicationArgs = new String[args.length - 1];
            System.arraycopy(args, 1, applicationArgs, 0, applicationArgs.length);
            m.invoke(null, new Object[] { applicationArgs });
    }
}
public class ClassAdapter extends ClassVisitor {
    private String owner;
    public ClassAdapter(ClassVisitor cv) {
        super(Opcodes.ASM4, cv);
    }
    @Override
    public void visit(final int version, final int access, final String name,
            final String signature, final String superName,
            final String[] interfaces) {
        owner = name;
        super.visit(version, access, name, signature, superName, interfaces);
    }
    @Override
    public FieldVisitor visitField(final int access, final String name,
            final String desc, final String signature, final Object value) {
        FieldVisitor fv = super
                .visitField(access, name, desc, signature, value);
        if ((access & Opcodes.ACC_STATIC) == 0) { ...
        }
        return fv;
    }
    @Override
    public MethodVisitor visitMethod(final int access, final String name,
            final String desc, final String signature, final String[] exceptions) {
        System.out.println("calling method name: "+ name);
        MethodVisitor mv = cv.visitMethod(access, name, desc, signature,
                exceptions);
        //Rewrite sayHello method 
        if(name.equals("sayHello")){
            mv = new MethodModifierAdapter(mv);
            return mv;
            /*
            mv.visitCode();
            mv.visitVarInsn(Opcodes.ALOAD, 0);
            mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out",
                    "Ljava/io/PrintStream;");
            // pushes the "Hello World!" String constant
            mv.visitLdcInsn("Sxu says: Hello world!");
            // invokes the 'println' method (defined in the PrintStream class)
            mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
            mv.visitInsn(Opcodes.ICONST_1);
            mv.visitInsn(Opcodes.IRETURN);
            // this code uses a maximum of two stack elements and two local
            // variables
            mv.visitMaxs(5, 5);
            mv.visitEnd();
            */
        }
        return mv == null ? null : new TraceFieldCodeAdapter(Opcodes.ASM4, mv, owner);
    }
}
public class MethodModifierAdapter extends MethodVisitor implements Opcodes {
    boolean _modified ;
    public MethodModifierAdapter(MethodVisitor mv) {
        super(Opcodes.ASM4, mv);
        _modified = false;
    }
    @Override
    public void visitCode() {
        if(!_modified){
            _modified = true;
            mv.visitCode();
            mv.visitVarInsn(Opcodes.ALOAD, 0);
            mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out",
                    "Ljava/io/PrintStream;");
            // pushes the "Hello World!" String constant
            mv.visitLdcInsn("Sxu says: Hello world!");
            // invokes the 'println' method (defined in the PrintStream class)
            mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
            mv.visitInsn(Opcodes.ICONST_1);
            mv.visitInsn(Opcodes.IRETURN);
            // this code uses a maximum of two stack elements and two local
            // variables
            mv.visitMaxs(5, 5);
            mv.visitEnd();
        }
    }
}

在classadapter::visitMethod中，我创建了一个新的MethodModifierAdapter。MethodModifierAdapter的visitCode会重写方法体。

新sample.class的javap结果是:

 public int sayHello(java.lang.String);
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0       
         1: getstatic     #40                 // Field java/lang/System.out:Ljava/io/PrintStream;
         4: ldc           #42                 // String Sxu says: Hello world!
         6: invokevirtual #23                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         9: iconst_1      
        10: ireturn       
        11: getstatic     #40                 // Field java/lang/System.out:Ljava/io/PrintStream;
        14: aload_1       
        15: invokevirtual #23                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        18: iconst_1      
        19: ireturn       
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
              11       9     0  this   Lcode/sxu/demo/data/sample;
              11       9     1  args   Ljava/lang/String;
      LineNumberTable:
        line 12: 11
        line 13: 18

根据其内容，MethodModifierAdapter::visitcode do更新方法内容。

我猜我使用ASM API的方式有一些错误，但不清楚是哪一个…

Answer 1

您插入的代码需要比原始代码更多的堆栈空间。因此，您必须手动更新最大堆栈声明，或者告诉ASM自动计算所需的堆栈空间。用new ClassWriter(ClassWriter.COMPUTE_MAXS)替换你的建筑new ClassWriter(0)。标志ClassWriter.COMPUTE_MAXS将告诉ASM(重新)计算堆栈需求。