One Two Three One Two Three - 1 month ago 11
Java Question

Is it possible to track down which expression caused an NPE?

When I get an NPE, I'll get a stack trace with line number.
That's helpful, but if the line is very dense and/or contains nested expression, it's still impossible to figure out which reference was null.

Surely, this information must've been available somewhere.
Is there a way to figure this out? (If not java expression, then at least the bytecode instruction that caused NPE would be helpful as well)

Edit #1: I've seen a few comments suggesting breaking up the line, etc, which, no offence, is really non-constructive and irrelevant. If I could do that, I would have ! Let just say this modifying the source is out of the question.

Edit #2: apangin has posted an excellent answer below, which I accepted. But it's SOOO COOL that I had to include the output here for anyone who doesn't want to try it out themselves! ;)

So suppose I have this driver program TestNPE.java

1 public class TestNPE {
2 public static void main(String[] args) {
3 int n = 0;
4 String st = null;
5
6 System.out.println("about to throw NPE");
7 if (n >= 0 && st.isEmpty()){
8 System.out.println("empty");
9 }
10 else {
11 System.out.println("othereise");
12 }
13 }
14
15 }


The bytecode looks like this (showing only the main() method and omitting other irrelevant parts)

Code:
stack=2, locals=3, args_size=1
0: iconst_0
1: istore_1
2: aconst_null
3: astore_2
4: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
7: ldc #3 // String about to throw NPE
9: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
12: iload_1
13: iflt 34
16: aload_2
17: invokevirtual #5 // Method java/lang/String.isEmpty:()Z
20: ifeq 34
23: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
26: ldc #6 // String empty
28: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
31: goto 42
34: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
37: ldc #7 // String othereise
39: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
42: return


Now when you run the TestNPE driver with the agent, you'll get this

$ java -agentpath:libRichNPE.o TestNPE
about to throw NPE
Exception in thread "main" java.lang.NullPointerException: location=17
at TestNPE.main(TestNPE.java:7)


So that points to the invokevirtual #5 at offset 17! Just HOW COOL IS THAT?

Answer

When an exception happens, JVM knows the original bytecode that caused the exception. However, StackTraceElement does not track bytecode indices.

The solution is to capture bytecode index using JVMTI whenever exception occurs.

The following sample JVMTI agent will intercept all exceptions, and if exception type is NullPointerException, the agent will replace its detailMessage with the bytecode location information.

#include <jvmti.h>
#include <stdio.h>

static jclass NullPointerException;
static jfieldID detailMessage;

void JNICALL VMInit(jvmtiEnv* jvmti, JNIEnv* env, jthread thread) {
    jclass localNPE = env->FindClass("java/lang/NullPointerException");
    NullPointerException = (jclass) env->NewGlobalRef(localNPE);

    jclass Throwable = env->FindClass("java/lang/Throwable");
    detailMessage = env->GetFieldID(Throwable, "detailMessage", "Ljava/lang/String;");
}

void JNICALL ExceptionCallback(jvmtiEnv* jvmti, JNIEnv* env, jthread thread,
                               jmethodID method, jlocation location, jobject exception,
                               jmethodID catch_method, jlocation catch_location) {
    if (env->IsInstanceOf(exception, NullPointerException)) {
        char buf[32];
        sprintf(buf, "location=%ld", (long)location);
        env->SetObjectField(exception, detailMessage, env->NewStringUTF(buf));
    }
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM* vm, char* options, void* reserved) {
    jvmtiEnv* jvmti;
    vm->GetEnv((void**)&jvmti, JVMTI_VERSION_1_0);

    jvmtiCapabilities capabilities = {0};
    capabilities.can_generate_exception_events = 1;
    jvmti->AddCapabilities(&capabilities);

    jvmtiEventCallbacks callbacks = {0};
    callbacks.VMInit = VMInit;
    callbacks.Exception = ExceptionCallback;
    jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks));
    jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_VM_INIT, NULL);
    jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_EXCEPTION, NULL);

    return 0;
}

Compile this into a shared library and run java with -agentpath option:

java -agentpath:/pato/to/libRichNPE.so Main
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