Question

我想在特定行上更新现有类中的if语句而不更改整个方法。

这是目标代码（类，方法和一些代码的名称已更改，因为它们不相关）：

public class Target extends Something {
   public Target(){
        super();
        //some code...
   }

    public Result targetMethod(Data firstPar, Data secondPar){
        if(someStatement()) {
            return Result.FAIL;
        } else {
            if(firstPar.check()){ //here is the line I want to change
                 firstPar.doSomething()
            }
            secondPar.doSomething();
            return Result.SUCCESS;
        }

    }
}

在此代码中，我想将if(firstPar.check())更改为以下内容：

if(firstPar.check() && !Utilities.someOtherCheck(firstPar, secondPar))

以下是我尝试解决此问题的方法：

ClassNode node = new ClassNode();
ClassReader reader = new ClassReader(bytes); //bytes - original target class
reader.accept(node, 0);
ClassWriter cw = new ClassWriter(0);
node.accept(cw);
MethodVisitor visitor = cw.visitMethod(ACC_PUBLIC, "targetMethod", "<it's signature>", null, null);
visitor.visitCode();
//No idea what to do next

我不明白的事情：

如何在MethodVisitor中正确选择一行？
如何仅更改if语句的一半？
如何获取新类的字节码（将生成该字节码）在我们改变目标类之后）？

如果您能提供某种示例代码，那就太好了。

谢谢！

Answer 1

问题1可能是最困难的。您需要通过识别某些模式来找出插入指令的位置。如果您假设firstPar.check()仅被调用一次，那么您可以查找if(firstPar.check())的以下字节码说明：

ALOAD 1
INVOKEVIRTUAL Data.check ()Z
IFEQ L3

如果L3返回check，则false是跳转标签。

对于问题2，请注意if(firstPar.check() && !Utilities.someOtherCheck(firstPar, secondPar))的字节码指令是：

ALOAD 1
INVOKEVIRTUAL Data.check ()Z
IFEQ L3
ALOAD 1
ALOAD 2
INVOKESTATIC Utilities.someOtherCheck (LData;LData;)Z
IFNE L3

因此，您需要在IFEQ L3之后插入4条新说明。

您可以使用Tree API来创建此项，您可以通过继承targetMethod和ClassVisitor来为MethodNode创建适配器：

private static class ClassAdapter extends ClassVisitor {
    public ClassAdapter(ClassVisitor cv) {
        super(Opcodes.ASM5, cv);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc,
            String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);
        if (name.equals("targetMethod"))
            return new MethodAdapter(access, name, desc, signature, exceptions, mv);
        else
            return mv;
    }
}

private static class MethodAdapter extends MethodNode {
    public MethodAdapter(int access, String name, String desc,
            String signature, String[] exceptions, MethodVisitor mv) {
        super(Opcodes.ASM5, access, name, desc, signature, exceptions);
        this.mv = mv;
    }
    // More to come ...
}

在MethodAdapter内，您可以覆盖visitEnd以迭代方法内的所有instructions，并尝试检测上述3条指令并在其后插入4条新指令：

@Override
public void visitEnd() {
    // Iterates all instructions in the method
    ListIterator<AbstractInsnNode> itr = instructions.iterator();
    while (itr.hasNext()) {
        // Checks whether the instruction is ALOAD 1
        AbstractInsnNode node = itr.next();
        if (node.getOpcode() != Opcodes.ALOAD
                || ((VarInsnNode) node).var != 1)
            continue;

        // Checks whether the next instruction is INVOKEVIRTUAL
        if (node.getNext() == null
                || node.getNext().getOpcode() != Opcodes.INVOKEVIRTUAL)
            continue;

        // Checks the invoked method name and signature
        MethodInsnNode next = (MethodInsnNode) node.getNext();
        if (!next.owner.equals("Data")
                || !next.name.equals("check")
                || !next.desc.equals("()Z"))
            continue;

        // Checks whether the next of the next instruction is IFEQ
        AbstractInsnNode next2 = next.getNext();
        if (next2 == null
                || next2.getOpcode() != Opcodes.IFEQ)
            continue;

        // Creates a list instructions to be inserted
        InsnList list = new InsnList();
        list.add(new VarInsnNode(Opcodes.ALOAD, 1));
        list.add(new VarInsnNode(Opcodes.ALOAD, 2));
        list.add(new MethodInsnNode(Opcodes.INVOKESTATIC, 
            "Utilities", "someOtherCheck", 
            "(LData;LData;)Z", false));
        list.add(new JumpInsnNode(Opcodes.IFNE, ((JumpInsnNode) next2).label));

        // Inserts the list, updates maxStack to at least 2, and we are done
        instructions.insert(next2, list);
        maxStack = Math.max(2, maxStack);
        break;
    }
    accept(mv);
}

要使用适配器，您可以使用ClassReader和ClassWriter对其进行链接。下面，我还链接一个TraceClassVisitor来打印tmp目录中的日志文件：

ClassReader reader = new ClassReader("Target");
ClassWriter writer = new ClassWriter(reader, 0);
TraceClassVisitor printer = new TraceClassVisitor(writer, 
        new PrintWriter(System.getProperty("java.io.tmpdir") + File.separator + "Target.log"));
ClassAdapter adapter = new ClassAdapter(printer);
reader.accept(adapter, 0);
byte[] b = writer.toByteArray(); // The modified bytecode

使用ASM选择和修改`if`语句

1 个答案: