java - 在运行时向方法添加 if stmt 时出现 VerifyException

Question

假设我有一个简单的

class C$Manipulatables{
    public Wrapper get(long v, String f){
        if(v == 0){
            return new Wrapper(0);
        }else{
            throw new RuntimeException("unacceptable: v not found");
        }
    }
}

我现在想在 C$Manipulatables 中重新定义 get，s.t.上面写着

class C$Manipulatables{
    public Wrapper get(long v, String f){
        if(v == 1){ return null; }
        if(v == 0){
            return new Wrapper(0);
        }else{
            throw new RuntimeException("unacceptable: v not found");
        }
    }
}

如果我尝试使用新值，这将触发空指针异常，但没关系 - 现在我只想确认新代码已加载。

我们使用 ASM 添加代码(我将在这篇文章的底部添加完整的可复制粘贴代码，所以我只是放大相关部分，在这里):

    class AddingMethodVisitor extends MethodVisitor implements Opcodes{
        int v;
        public AddingMethodVisitor(int v, int api, MethodVisitor mv) {
            super(api, mv);
            this.v = v;
        }

        @Override
        public void visitCode() {
            super.visitCode();

            mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number

            /*if arg1 == the new version*/
            mv.visitLdcInsn(v);
            Label lSkip = new Label();

            mv.visitInsn(LCMP);
            mv.visitJumpInsn(IFNE, lSkip);

            mv.visitInsn(ACONST_NULL);
            mv.visitInsn(ARETURN);

            /*else*/
            mv.visitLabel(lSkip);
            mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);

        }
    }

并使用 ByteBuddy 重新加载类(同样，帖子底部的完整代码):

        ClassReader cr;
        try {
            /*note to self: don't forget the ``.getClassLoader()``*/
            cr = new ClassReader(manipsClass.getClassLoader().getResourceAsStream( manipsClass.getName().replace('.','/') + ".class"));
        }catch(IOException e){
            throw new RuntimeException(e);
        }

        ClassWriter cw = new ClassWriter(cr, 0);

        VersionAdder adder = new VersionAdder(C.latest + 1,Opcodes.ASM5,cw);

        cr.accept(adder,0);

        System.out.println("reloading C$Manipulatables class");
        byte[] bytes = cw.toByteArray();

        ClassFileLocator classFileLocator = ClassFileLocator.Simple.of(manipsClass.getName(), bytes);

        new ByteBuddy()
                .redefine(manipsClass,classFileLocator)
                .name(manipsClass.getName())
                .make()
                .load(C.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
        ;

        C.latest++;
        System.out.println("RELOADED");
    }
}

这失败了。

got 0
reloading C$Manipulatables class
Exception in thread "main" java.lang.VerifyError
    at sun.instrument.InstrumentationImpl.redefineClasses0(Native Method)
    at sun.instrument.InstrumentationImpl.redefineClasses(InstrumentationImpl.java:170)
    at net.bytebuddy.dynamic.loading.ClassReloadingStrategy$Strategy$1.apply(ClassReloadingStrategy.java:261)
    at net.bytebuddy.dynamic.loading.ClassReloadingStrategy.load(ClassReloadingStrategy.java:171)
    at net.bytebuddy.dynamic.TypeResolutionStrategy$Passive.initialize(TypeResolutionStrategy.java:79)
    at net.bytebuddy.dynamic.DynamicType$Default$Unloaded.load(DynamicType.java:4456)
    at redefineconcept.CInserter.addNext(CInserter.java:60)
    at redefineconcept.CInserter.run(CInserter.java:22)
    at redefineconcept.CInserter.main(CInserter.java:16)

Process finished with exit code 1

事实上，当我评论 return null stmt 生成时(正如我在下面提供的完整代码中所做的那样)，这甚至会失败。

显然，java 只是不喜欢我构造我的 IF 的方式，即使它本质上是我在

上使用 asmifier 时得到的代码

public class B {

    public Object run(long version, String field){
        if(version == 2) {
            return null;
        }
        return null;
    }
}

产生了

{
mv = cw.visitMethod(ACC_PUBLIC, "run", "(JLjava/lang/String;)Ljava/lang/Object;", null, null);
mv.visitCode();
mv.visitVarInsn(LLOAD, 1);
mv.visitLdcInsn(new Long(2L));
mv.visitInsn(LCMP);
Label l0 = new Label();
mv.visitJumpInsn(IFNE, l0);
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
mv.visitLabel(l0);
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
mv.visitMaxs(4, 4);
mv.visitEnd();
}

我只是简单地离开了之后的一切

mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);

因为在那之后，方法的已经存在的部分紧随其后。

我确实相信 Java 不喜欢 visitFrame 的某些方面。

假设我更改了我的 visitCode s.t.上面写着

        public void visitCode() {
            super.visitCode();

            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitLdcInsn("Work, you ..!");
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

//            mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number
//
//            /*if arg1 == the new version*/
//            mv.visitLdcInsn(v);
//            Label lSkip = new Label();
//
//            mv.visitInsn(LCMP);
//            mv.visitJumpInsn(IFNE, lSkip);
//
////            mv.visitInsn(ACONST_NULL);
////            mv.visitInsn(ARETURN);
//
//            /*else*/
//            mv.visitLabel(lSkip);
//            mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);

        }

然后重新定义工作。我得到了预期的异常，因为代码落入了无法处理新版本号的原始 if-else block ，但至少我确实得到了输出。

got 0
reloading C$Manipulatables class
RELOADED
Work, you ..!
Exception in thread "main" java.lang.RuntimeException: unacceptable: v not found
    at redefineconcept.C$Manipulatables.get(C.java:27)
    at redefineconcept.C.get(C.java:10)
    at redefineconcept.CInserter.run(CInserter.java:23)
    at redefineconcept.CInserter.main(CInserter.java:16)

如果您能帮我解决这个问题，我将不胜感激。插入 java 将接受的新 if stmt 的正确方法是什么？

完整代码

C.java

(注意 C$Manipulatables 类是必需的，因为 ByteBuddy 不能重新定义具有静态初始化器的类。)

package redefineconcept;

public class C {
    public static volatile int latest = 0;

    public static final C$Manipulatables manips = new C$Manipulatables();

    public int get(){
        int v = latest;
        return manips.get(v,"").value;
    }
}

class Wrapper{
    int value;

    public Wrapper(int value){
        this.value = value;
    }
}

class C$Manipulatables{
    public Wrapper get(long v, String f){
        if(v == 0){
            return new Wrapper(0);
        }else{
            throw new RuntimeException("unacceptable: v not found");
        }
    }
}

CInserter.java

package redefineconcept;

import net.bytebuddy.ByteBuddy;
import net.bytebuddy.agent.ByteBuddyAgent;
import net.bytebuddy.dynamic.ClassFileLocator;
import net.bytebuddy.dynamic.loading.ClassReloadingStrategy;
import net.bytebuddy.jar.asm.*;

import java.io.IOException;

    public class CInserter {
        public static void main(String[] args) {

            ByteBuddyAgent.install();

            new CInserter().run();
        }

        private void run(){
            C c = new C();
            System.out.println("got " + c.get());
            addNext();
            System.out.println("got " + c.get()); //should trigger nullptr exception
        }

        private void addNext(){

            Object manips;
            String manipsFld = "manips";

            try {
                manips = C.class.getDeclaredField(manipsFld).get(null);
            }catch(NoSuchFieldException | IllegalAccessException e){
                throw new RuntimeException(e);
            }

            Class<?> manipsClass = manips.getClass();
            assert(manipsClass.getName().equals("redefineconcept.C$Manipulatables"));


            ClassReader cr;
            try {
                /*note to self: don't forget the ``.getClassLoader()``*/
                cr = new ClassReader(manipsClass.getClassLoader().getResourceAsStream( manipsClass.getName().replace('.','/') + ".class"));
            }catch(IOException e){
                throw new RuntimeException(e);
            }

            ClassWriter cw = new ClassWriter(cr, 0);

            VersionAdder adder = new VersionAdder(C.latest + 1,Opcodes.ASM5,cw);

            cr.accept(adder,0);

            System.out.println("reloading C$Manipulatables class");
            byte[] bytes = cw.toByteArray();

            ClassFileLocator classFileLocator = ClassFileLocator.Simple.of(manipsClass.getName(), bytes);

            new ByteBuddy()
                    .redefine(manipsClass,classFileLocator)
                    .name(manipsClass.getName())
                    .make()
                    .load(C.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
            ;

            C.latest++;
            System.out.println("RELOADED");
        }
    }

class VersionAdder extends ClassVisitor{
    private int v;
    public VersionAdder(int v, int api, ClassVisitor cv) {
        super(api, cv);
        this.v = v;
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
        MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);

        if(mv != null && name.equals("get")){
            return new AddingMethodVisitor(v,Opcodes.ASM5,mv);
        }

        return mv;
    }

    class AddingMethodVisitor extends MethodVisitor implements Opcodes{
        int v;
        public AddingMethodVisitor(int v, int api, MethodVisitor mv) {
            super(api, mv);
            this.v = v;
        }

        @Override
        public void visitCode() {
            super.visitCode();

            mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number

            /*if arg1 == the new version*/
            mv.visitLdcInsn(v);
            Label lSkip = new Label();

            mv.visitInsn(LCMP);
            mv.visitJumpInsn(IFNE, lSkip);

//            mv.visitInsn(ACONST_NULL);
//            mv.visitInsn(ARETURN);

            /*else*/
            mv.visitLabel(lSkip);
            mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);

        }
    }
}

Answer 1

我在您的代码中注意到的一个错误是以下行

mv.visitLdcInsn(v);

这段代码的目的是创建并加载一个 long 常量，但是 v 的类型是 int，因此将创建一个整型常量，从而创建一个类型将字节码与下一行的 lcmp 进行比较时出现字节码错误。 visitLdcInsn 将根据您传入的对象类型创建不同的常量类型，因此参数必须是您想要的确切类型。

附带说明，您首先不需要 LDC 来创建值为 1 的长整型常量，因为有专门的字节码指令用于此，lconst_1。在 ASM 中，这应该类似于 visitInsn(LCONST_1);