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/