我在 jar 里有一个类的方法，我想用我自己的方法交换它的主体。在这种情况下，我只想让方法在控制台上打印出“GOT IT”并返回 true；

我正在使用系统加载器来加载 jar 的类。我正在使用反射使系统类加载器能够通过字节码加载类。这部分似乎工作正常。

我正在按照此处找到的方法替换示例进行操作:asm.ow2.org/current/asm-transformations.pdf。

我的代码如下:

public class Main 
{
    public static void main(String[] args) 
    {
        URL[] url = new URL[1];
        try
        {
            url[0] = new URL("file:////C://Users//emist//workspace//tmloader//bin//runtime//tmgames.jar");
            verifyValidPath(url[0]);
        }
        catch (Exception ex)
        {
            System.out.println("URL error");
        }
        Loader l = new Loader();
        l.loadobjection(url);
    }

    public static void verifyValidPath(URL url) throws FileNotFoundException
    {
        File filePath = new File(url.getFile());
        if (!filePath.exists()) 
        {
          throw new FileNotFoundException(filePath.getPath());
        }
    }
}

class Loader
{
    private static final Class[] parameters = new Class[] {URL.class};

    public static void addURL(URL u) throws IOException
    {
        URLClassLoader sysloader = (URLClassLoader)  ClassLoader.getSystemClassLoader();
        Class sysclass = URLClassLoader.class;

        try 
        {
            Method method = sysclass.getDeclaredMethod("addURL", parameters);
            method.setAccessible(true);
            method.invoke(sysloader, new Object[] {u});
        }
        catch (Throwable t) 
        {
            t.printStackTrace();
            throw new IOException("Error, could not add URL to system classloader");
        }

    }

    private Class loadClass(byte[] b, String name) 
    {
        //override classDefine (as it is protected) and define the class.
        Class clazz = null;
        try 
        {
            ClassLoader loader = ClassLoader.getSystemClassLoader();
            Class cls = Class.forName("java.lang.ClassLoader");
            java.lang.reflect.Method method =
                cls.getDeclaredMethod("defineClass", new Class[] { String.class, byte[].class, int.class, int.class });

            // protected method invocaton
            method.setAccessible(true);
            try 
            {
                Object[] args = new Object[] {name, b, new Integer(0), new Integer(b.length)};
                clazz = (Class) method.invoke(loader, args);
            }
            finally 
            {
                method.setAccessible(false);
            }
        }
        catch (Exception e) 
        {
            e.printStackTrace();
            System.exit(1);
        }
        return clazz;
    }

    public void loadobjection(URL[] myJar)
    {
        try 
        {
            Loader.addURL(myJar[0]);            
            //tmcore.game is the class that holds the main method in the jar
            /*
            Class<?> classToLoad = Class.forName("tmcore.game", true, this.getClass().getClassLoader());
            if(classToLoad == null)
            {
                System.out.println("No tmcore.game");
                return;
            }
            */
            MethodReplacer mr = null;

            ClassReader cr = new ClassReader("tmcore.objwin");
            ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
            MethodVisitor mv = null;
            try
            {
                mr = new MethodReplacer(cw, "Test", "(Ljava/lang/String;ZLjava/lang/String;)Z");
            }
            catch (Exception e)
            {
                System.out.println("Method Replacer Exception");
            }
            cr.accept(mr, ClassReader.EXPAND_FRAMES);

            PrintWriter pw = new PrintWriter(System.out);
            loadClass(cw.toByteArray(), "tmcore.objwin");
            Class<?> classToLoad = Class.forName("tmcore.game", true, this.getClass().getClassLoader());
            if(classToLoad == null)
            {
                System.out.println("No tmcore.game");
                return;
            }

            //game doesn't have a default constructor, so we need to get the reference to public game(String[] args)
            Constructor ctor = classToLoad.getDeclaredConstructor(String[].class);
            if(ctor == null)
            {
                System.out.println("can't find constructor");
                return;
            }

            //Instantiate the class by calling the constructor
            String[] args = {"tmgames.jar"};
            Object instance = ctor.newInstance(new Object[]{args});
            if(instance == null)
            {
                System.out.println("Can't instantiate constructor");
            }

            //get reference to main(String[] args)
            Method method = classToLoad.getDeclaredMethod("main", String[].class);
            //call the main method
            method.invoke(instance);

        }   
        catch (Exception ex)
        {
            System.out.println(ex.getMessage());
            ex.printStackTrace();
        }
    }
}


public class MethodReplacer extends ClassVisitor implements Opcodes
{
    private String mname;
    private String mdesc;
    private String cname;

    public MethodReplacer(ClassVisitor cv, String mname, String mdesc)
    {
        super(Opcodes.ASM4, cv);
        this.mname = mname;
        this.mdesc = mdesc;
    }

    public void visit(int version, int access, String name, String signature, 
                      String superName, String[] interfaces)
    {
        this.cname = name;
        cv.visit(version, access, name, signature, superName, interfaces);
    }

    public MethodVisitor visitMethod(int access, String name, String desc, String signature,
                                     String[] exceptions)
    {
        String newName = name;
        if(name.equals(mname) && desc.equals(mdesc))
        {
            newName = "orig$" + name;
            generateNewBody(access, desc, signature, exceptions, name, newName);
            System.out.println("Replacing");
        }
        return super.visitMethod(access,  newName,  desc,  signature,  exceptions);
    }

    private void generateNewBody(int access, String desc, String signature, String[] exceptions,
                                String name, String newName)
    {
        MethodVisitor mv = cv.visitMethod(access,  name,  desc,  signature,  exceptions);
        mv.visitCode();
        mv.visitVarInsn(Opcodes.ALOAD, 0);
        mv.visitMethodInsn(access, cname, newName, desc);
        mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        mv.visitLdcInsn("GOTit!");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V");
        mv.visitInsn(ICONST_0);
        mv.visitInsn(IRETURN);
        mv.visitMaxs(0, 0);
        mv.visitEnd();
    }
}

问题似乎出在 MethodReplacer 内的 generateMethodBody 中的 mv.visitMethodInsn(access, cname, newName, desc);。

我收到“常量池中的非法类型”错误。

我不确定自己遗漏了什么……但在阅读和测试了大约 3 天之后，我仍然一无所获。

[编辑]

如果您想知道，tmcore 是一款针对律师的单人“异议”游戏。我这样做是为了好玩。该程序成功启动游戏并且一切正常，从 MethodReplacer 中删除修改使游戏按设计运行。所以这个问题似乎与我在方法替换器中的错误字节码/修改无关。

[EDIT2]

CheckClassAdapter.verify(cr, true, pw); 返回函数在编辑之前应该具有的完全相同的字节码。就好像没有完成更改一样。

[EDIT3]

classtoload 的副本根据评论被注释掉了

最佳答案

如果您使用的是 Eclipse，则应安装 Bytecode Outline - 它是必不可少的。

我为你想要实现的目标构建了一个小测试(这应该与你的测试方法的签名匹配，你将不得不更改包和类名):

package checkASM;

public class MethodCall {

    public boolean Test(String a, boolean b, String c) {
        System.out.println("GOTit");
        return false;
    }
}

需要以下字节码来构建方法:

{
mv = cw.visitMethod(ACC_PUBLIC, "Test",
    "(Ljava/lang/String;ZLjava/lang/String;)Z", null, null);
mv.visitCode();
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitFieldInsn(GETSTATIC, "java/lang/System",
   "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn("GOTit");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream",
   "println", "(Ljava/lang/String;)V");
Label l2 = new Label();
mv.visitLabel(l2);
mv.visitInsn(ICONST_0);
mv.visitInsn(IRETURN);
Label l3 = new Label();
mv.visitLabel(l3);
mv.visitLocalVariable("this", "LcheckASM/MethodCall;", null, l1, l3, 0);
mv.visitLocalVariable("a", "Ljava/lang/String;", null, l1, l3, 1);
mv.visitLocalVariable("b", "Z", null, l1, l3, 2);
mv.visitLocalVariable("c", "Ljava/lang/String;", null, l1, l3, 3);
mv.visitMaxs(4, 4);
mv.visitEnd();
}

可以省略对 visitLineNumber 的调用。很明显，您缺少所有标签，忘记加载方法参数，没有忽略返回值，为 visitMaxs 设置了错误的值(这不一定需要，这取决于您的 ClassWriter 标志，如果我没有记错)并且没有访问局部变量(或本例中的参数)。

此外，您的类加载似乎有点困惑/困惑。 我没有 jar (所以我不能说这些是否有效)，但也许你可以替换 Main 和 Loader:

主要:

import java.io.File;
import java.io.FileNotFoundException;
import java.net.URL;

public class Main {
    public static void main(String[] args) {
        try {
            Loader.instrumentTmcore(args);
        } catch (Exception e) {
            System.err.println("Ooops");
            e.printStackTrace();
        }
    }
}

加载程序:

import java.io.IOException;
import java.io.PrintWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;

public class Loader {

    public static ClassReader fetchReader(String binaryName) throws Exception {
        return new ClassReader(
                Loader.class.getClassLoader().getSystemResourceAsStream(
                    binaryName.replace('.', '/') + ".class"
                )
            )
        ;
    }

    public static synchronized Class<?> loadClass(byte[] bytecode)
                throws Exception {
        ClassLoader scl = ClassLoader.getSystemClassLoader();
        Class<?>[] types = new Class<?>[] {
                String.class, byte[].class, int.class, int.class
        };
        Object[] args = new Object[] {
                null, bytecode, 0, bytecode.length
        };
        Method m = ClassLoader.class.getMethod("defineClass", types);
        m.setAccessible(true);
        return (Class<?>) m.invoke(scl, args);
    }

    public static void instrumentTmcore(String[] args) throws Exception {
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        MethodReplacer mr = new MethodReplacer(cw, "Test",
                    "(Ljava/lang/String;ZLjava/lang/String;)Z");
        fetchReader("tmcore.objwin").accept(mr, ClassReader.EXPAND_FRAMES);
        loadClass(cw.toByteArray());
        Class.forName("tmcore.game")
            .getMethod("main", new Class<?>[] {args.getClass()})
            .invoke(null, new Object[] { args });
    }
}

关于Java ASM 字节码修改——改变方法体，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/11770353/