Using Javassist, how do I add code to an empty loop? - javassist

Question

Using Javassist 3.20.0.GA, I am trying to inject code in to an empty loop.
Everything I try, I keep running in to a java.lang.VerifyError error at the point I attempt to create a new instance of the modified class.
I've attempted to isolate the issue to a small program that fails to run successfully.
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.bytecode.Bytecode;
import javassist.bytecode.CodeAttribute;
import javassist.bytecode.CodeIterator;
import javassist.bytecode.InstructionPrinter;
import javassist.bytecode.MethodInfo;
import javassist.compiler.Javac;
public class EmptyLoopTest {
private void testEmptyLoopModification() throws Exception {
ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.get(EmptyLoopTest.class.getName() + "$EmptyLoopClass");
CtMethod m = cc.getDeclaredMethod("emptyLoopMethod");
printMethod("Before modifications", m);
MethodInfo methodInfo = m.getMethodInfo();
CodeAttribute ca = methodInfo.getCodeAttribute();
CodeIterator ci = ca.iterator();
Javac jv = new Javac(cc);
jv.compileStmnt("System.out.println(\"injected into loop\");");
Bytecode b = jv.getBytecode();
adjustCodeAttributeIfNeeded(b, ca);
ci.insertAt(0, b.get());
printMethod("After modifications", m);
Class c = cc.toClass();
logInfo("Attempting to create instance of modified class");
c.newInstance();
}
private void adjustCodeAttributeIfNeeded(Bytecode b, CodeAttribute ca){
int locals = b.getMaxLocals();
int stack = b.getMaxStack();
if(stack > ca.getMaxStack()) {
ca.setMaxStack(stack);
}
if(locals > ca.getMaxLocals()) {
ca.setMaxLocals(locals);
}
}
private void printMethod(String title, CtMethod m){
logInfo(title);
InstructionPrinter instructionPrinter = new InstructionPrinter(System.out);
instructionPrinter.print(m);
}
private void logInfo(String message){
System.out.println("");
System.out.println("------" + message);
}
public class EmptyLoopClass {
public void emptyLoopMethod() {
for(;;){
}
}
}
public static void main(String[] args) throws Exception {
// have errors written to sysout so all output from this program is in order
System.setErr(System.out);
new EmptyLoopTest().testEmptyLoopModification();
}
}
When I run this program, the following is written to the console..
------Before modifications
0: goto 0
------After modifications
0: getstatic #32 = Field java.lang.System.out(Ljava/io/PrintStream;)
3: ldc #34 = "injected into loop"
5: invokevirtual #40 = Method java.io.PrintStream.println((Ljava/lang/String;)V)
8: goto 8
------Attempting to create instance of modified class
Exception in thread "main" java.lang.VerifyError: Expecting a stackmap frame at branch target 8
Exception Details:
Location:
EmptyLoopTest$EmptyLoopClass.emptyLoopMethod()V #8: goto
Reason:
Expected stackmap frame at this location.
Bytecode:
0x0000000: b200 2012 22b6 0028 a700 00
Stackmap Table:
same_frame(#0)
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Class.java:2671)
at java.lang.Class.getConstructor0(Class.java:3075)
at java.lang.Class.newInstance(Class.java:412)
at EmptyLoopTest.testEmptyLoopModification(EmptyLoopTest.java:35)
at EmptyLoopTest.main(EmptyLoopTest.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Process finished with exit code 1
If everything was working as expected, I'd expect the goto instruction after modifications to goto 0 instead of what it's currently showing, goto 8. It's as if the StackMapTable wasn't adjusted appropriately when I invoked javassist.bytecode.CodeIterator#insertAt.
Can anyone spot what I'm doing wrong here?
Thanks,
Eric

Answer 1

you create class like this c.newInstance(); so it supposed to have 0-parameters constructor - to be static inner class
public static class EmptyLoopClass {
public void emptyLoopMethod() {
for(;;){
}
}
you should call methodInfo.rebuildStackMap(cp); after code modifications. JVM uses StackMap to validate class file.
Anyway you are ending up with bytecode
3: ldc #34 = "injected into loop"
5: invokevirtual #40 = Method java.io.PrintStream.println((Ljava/lang/String;)V)
8: goto 8
This code is not inside the loop, it is before loop. Actual loop is single instruction 8: goto 8 and you have to generate some additional loop code if you want to inject instructions inside it.