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

Question

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 1

JEP 358: Helpful NullPointerExceptions adds such a feature to OpenJDK 14. It is disabled by default; you have to specify -XX:+ShowCodeDetailsInExceptionMessages to enable it. With it, your example results in:

Exception in thread "main"
java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because "st" is null
    at TestNPE.main(TestNPE.java:7)

Classes do not need to be recompiled to take advantage of this feature. It was originally developed for the SAP JVM.