Using Instrumentation to record unhandled exception

Question

I was trying to debug java application using instrumentation. The problem with current system are

• Hardly written any log statements
• Poor exception handling

This made very difficult to trace root cause of broken functionality.

To handle the situation I have developed tool,java agent using Instrumentation API , and I was able to inject log statements and half of the problem solved.

But the next problem is to recording the Exception. I want to extend my tool record every exception thrown during the execution of the application. I tried injecting 'try-catch' block using javaassist API for methods (using addCatch, insertBefore and insertAfter), and it is effective certain extent.

public byte[] transform(ClassLoader    loader,
        String              className,
        Class<?>            classBeingRedefined,
        ProtectionDomain    protectionDomain,
        byte[]              classfileBuffer)
        throws IllegalClassFormatException {
     if (className.startsWith("com/alu/")) {
          return insertLog(className, classBeingRedefined, classfileBuffer);
     }

     if(className.endsWith("Exception")){
         System.out.println("============= exception occured "+className);
     }

Here inserLog(..) method will inject necessary log statement and works fine,but when there is any Exception it doesn't come to transformer.

But the problem is some of the method handles exception inside ( even with out log/sysout).

eg:

try {
            if(search.equals("Category")){
                //do operation
            }
        } catch (Exception e) {
        }

This code eats NullPointerException when value of search is null, I never know this exception and application fail for some thing else.

Ultimately what I want is a mechanism to record any exception thrown by application. following details are to be captured

• Exception Type
• Excpetion Stacktrace
• Method and class name

I know there is API Thread.setDefaultUncaughtExceptionHandler, but not sure how it use with java instrumentation. I don't have any source access the application.

[update 1]

I found below link tells to use retransformation , I will give a try and update

How to instrument java system classes?

Any guidance would be greatly helpful.

Stans

> but when there is any Exception it doesn't come to transformer. Please, clarify this statement. How do you know that Exception does not come to transformer? Do you wait for ============= exception occured in stdout? Of course, it won't, because you instrumenting you code, that throws exception, and you log all exceptions being retransformed itself. – Typebar

@AlexeyAndreev, I am not quite clear with the statement you log all exceptions being retransformed itself , could you rephrase it pls – Stans

OK, I will extend my answer, as it is not so easy to write complex comments here – Typebar

Answer 1

I think you should use ASM to manipulate bytecode directly. Here is algoritms:

1. Visit all try/catch blocks (see visitTryCatchBlock) and save all handler labels
2. Visit instructions until one of the handler labels met.

3. After handler label insert logging code

   GETSTATIC java/lang/System out
   LDC "exception X occured"
   INVOKEVIRTUAL java/io/PrintStream println (java/lang/String)V
   

And ensure that your javaagent works fine. Checkt that your MANIFEST.MF file contains proper premain declaration and enables class transformation.

---

About your current code. Here

 if (className.startsWith("com/alu/")) {
      return insertLog(className, classBeingRedefined, classfileBuffer);
 }

you transforming classes inside particular package. That classes contain code that, in particular, throw exceptions.

And here

 if(className.endsWith("Exception")){
     System.out.println("============= exception occured "+className);
 }

you log of class being retransfomed when it is first loaded by JVM, when its name ends with "Exception". Not when exception occured. But transforming exception is useless itself. So I guess you should proceed like this:

if (className.startsWith("com/alu/")) {
    System.out.println("============= class transformed "+ className);
    return insertLog(className, classBeingRedefined, classfileBuffer);
} 

So you could know that your agent at least works.

---

You have to deal with code like this

    try {
        if(search.equals("Category")){
            //do operation
        }
    } catch (Exception e) {
    }

where exceptions are swallowed. You transform methods that they will be like this:

try {
    try {
        if(search.equals("Category")){
            //do operation
        }
    } catch (Exception e) {
    }
} catch (Exception e) {
    e.printStackTrace();
}

Of course, when exception was swallowed by the first catch, the second one never cathes it. Instead, you should transform existing catch blocks themself, to get the following code:

try {
    if(search.equals("Category")){
        //do operation
    }
} catch (Exception e) {
    e.printStackTrace();
}

Above I shown you how to achieve this with ASM.

Typebar

hmmm, I got your point, I need to visit all catch block and the transform it. Good idea! thanks man – Stans

thanks for directions. I am able to record the exception with Instrumentation. Later I will look in to asm to fine tune my solution. Posted my solution below. – Stans

I understand modifying System classes(adding new field/method) is some thing not possible with javaassist , hence I vote for your solution. – Stans

Answer 2

After more research, I found a starting point(thanks to @jarnbjo), which I believe take me to complete solution

There is an option to retransform System classes,

SimpleClassTransformer transformer = new SimpleClassTransformer();
    instrumentation.addTransformer(transformer,true);
    try {
        instrumentation.retransformClasses(java.lang.NullPointerException.class);
    } catch (UnmodifiableClassException e) {
        e.printStackTrace();
    }

I applied this code in my premain class and able to reload the NullPointerException class. I still would need to work on Transformer implementation to enable instant recording of exception.

[update 2] Finally I got a break through!!

After re transforming the required Exception class, I injected my code right inside the constructor of the Exception class.

Here is my transformer class,

public class SimpleClassTransformer implements ClassFileTransformer{

public byte[] transform(ClassLoader    loader,
        String              className,
        Class<?>            classBeingRedefined,
        ProtectionDomain    protectionDomain,
        byte[]              classfileBuffer)
                throws IllegalClassFormatException {
    System.out.println("----------------- "+className);
    if (className.startsWith("com/alu/")) {
        return insertLog(className, classBeingRedefined, classfileBuffer);
    }

    if(className.endsWith("Exception")){
        System.out.println("============= exception occured");
        return recordError(className, classBeingRedefined, classfileBuffer);
    }

    return classfileBuffer;
}

private byte[] recordError(String name, Class clazz, byte[] b){
    ClassPool pool = ClassPool.getDefault();
    CtClass cl = null;
    try {
        cl = pool.makeClass(new java.io.ByteArrayInputStream(b));
        if (cl.isInterface() == false) {
            CtConstructor[] constr=cl.getDeclaredConstructors();
            for(CtConstructor con:constr){
                    con.insertAfter("System.out.println(\"Hey hurrray I got you mannnnnnn  -------\");  ");
            }
            b = cl.toBytecode();
        }
    } catch (Exception e) {
        System.out.println(e);
    } finally {
        if (cl != null) {
            cl.detach();
        }
    }
    return b;
}

Stans

I have got an exception "java.lang.UnsupportedOperationException: retransformClasses is not supported in this environment" when retransformClasses. – Camber

For anyone using this approach: Make sure you have the line Can-Retransform-Classes: true in your MANIFEST.MF of your agent-jar – Playtime

Answer 3

I solved this challenge this way. I used javaassist method addCatch.

Short demo

 CtClass exceptionCtClass = ClassPool.getDefault().makeClass("java.lang.Exception");
 method.addCatch("{ System.out.println(\"Caught exception\");\n$_e.printStackTrace();\nthrow $_e; }", exceptionCtClass, "$_e");

Full example

public class ExceptionReporterAgent {

    public static void premain(final String agentArgs, final Instrumentation inst) {
        inst.addTransformer(new ExceptionReporterTransformer());
    }
}

public class ExceptionReporterTransformer implements ClassFileTransformer {

    private CtClass exceptionCtClass;

    public byte[] transform(ClassLoader loader, String className, Class redefiningClass, ProtectionDomain domain, byte[] bytes) {
        return transformClass(redefiningClass, bytes);
    }

    private byte[] transformClass(Class classToTransform, byte[] b) {
        ClassPool pool = ClassPool.getDefault();
        CtClass cl = null;
        try {
            cl = pool.makeClass(new java.io.ByteArrayInputStream(b));
            if (cl.getName().endsWith("ClassCException")) {
                exceptionCtClass = cl; //Or any exception, you can move this logic into constructor.
            } else {
                modifyClass(cl);
            }

            b = cl.toBytecode();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (cl != null) {
                cl.detach();
            }
        }

        return b;
    }

    private byte[] modifyClass(CtClass cl) throws Exception {
        CtBehavior[] methods = cl.getDeclaredBehaviors();
        for (CtBehavior method : methods) {
            changeMethod(method);
        }
        return cl.toBytecode();
    }

    private void changeMethod(CtBehavior method) throws CannotCompileException {
        method.addLocalVariable("$_start", CtClass.longType);
        method.addLocalVariable("$_end", CtClass.longType);
        method.addLocalVariable("$_total", CtClass.longType);
        method.insertBefore("{ $_start = System.currentTimeMillis(); }");

        //For methods returning String
        // method.insertAfter("{ $_end = System.currentTimeMillis();\n$_total = $_end - $_start;\nSystem.out.println(\"Total: \" + $_total);return \"AAA\"; }");


        method.addCatch("{ System.out.println(\"Caught exception\");\n$_e.printStackTrace();\nthrow $_e; }", exceptionCtClass, "$_e");

        //For methods returning String
        // method.insertAfter("{ System.out.println(\"Finally\");\nreturn \"ABC\"; }", true);

        method.insertBefore("{ System.out.println(\"Before\"); }");

        System.out.println("Modifying method: " + method.getName());
    }
}

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