Strange exception table entry produced by Sun's javac

Question

Given this program:

class Test {
    public static void main(String[] args) {
        try {
            throw new NullPointerException();
        } catch (NullPointerException npe) {
            System.out.println("In catch");
        } finally {
            System.out.println("In finally");
        }
    }
}

Sun's javac (v 1.6.0_24) produces the following bytecode:

public static void main(java.lang.String[]);

        // Instantiate / throw NPE
   0:   new     #2;         // class NullPointerException
   3:   dup
   4:   invokespecial   #3; // Method NullPointerException."<init>":()V
   7:   athrow

        // Start of catch clause
   8:   astore_1
   9:   getstatic       #4; // Field System.out
   12:  ldc     #5;         // "In catch"
   14:  invokevirtual   #6; // Method PrintStream.println
   17:  getstatic       #4; // Field System.out

        // Inlined finally block
   20:  ldc     #7;         // String In finally
   22:  invokevirtual   #6; // Method PrintStream.println
   25:  goto    39

        // Finally block
        // store "incomming" exception(?)
   28:  astore_2
   29:  getstatic       #4; // Field System.out
   32:  ldc     #7;         // "In finally"
   34:  invokevirtual   #6; // Method PrintStream.println

        // rethrow "incomming" exception
   37:  aload_2
   38:  athrow

   39:  return

With the following exception table:

  Exception table:
   from   to  target type
     0     8     8   Class NullPointerException
     0    17    28   any
    28    29    28   any

My question is: Why on earth does it include that last entry in the exception table?!

As I understand it, it basically says "if the astore_2 throws an exception, catch it, and retry the same instruction".

Such entry is produced even with empty try / catch / finally clauses such as

try {} catch (NullPointerException npe) {} finally {}

---

Some observations

• Eclipse compiler does not produce any such exception table entry
• The JVM spec does not document any runtime exceptions for the astore instruction.
• I know that it is legal for the JVM to throw VirtualMachineError for any instruction. I guess the peculiar entry prevents any such errors from propagating out from that instruction.

Answer 1

Looking at the OpenJDK 7 source code, I would venture to guess the reason for that last 28 29 28 any exception table entry is because the code that handles the astore bytecode (see code starting at line 1871) can throw an java.lang.LinkageError exception if the popped value from the operand stack is not a returnAddress or reference type (see the Java Virtual Machine Specification for astore) and they want this error condition to show up on the stack trace.

In the event that there is a bad operand type on the operand stack, the JVM will clear the operand stack (getting rid of that bad operand), put a LinkageError on the operand stack, and execute the astore bytecode again, this time successfully executing the astore bytecode using a JVM provided LinkageError object reference. See the athrow documentation for more information.

I would greatly suspect the root cause of throwing a LinkageError during astore processing is due to the complexities JSR/RET subroutines introduce into bytecode verification (OpenJDK changes 6878713, 6932496 and 7020373 are recent evidence of JSR's continued complexity; I'm sure Sun/Oracle has other closed source tests that we're not seeing in OpenJDK). The OpenJDK 7020373 change uses LinkageError to validate/invalidate test results.

Answer 2

There are only two possible explanations: the compiler contains a bug or it's placing a kind of watermark for obscure reasons.

That entry is certainly bogus because any exception thrown by a finally block itself must send execution flow to outer exception handler or finally block, but never "run again" the same finally block.

Also, a good evidence that it's a bug/watermark, is the fact that Eclipse (and perhaps other Java compilers) are not generating such entry, and even so Eclipse-generated classes work fine on Sun's JVM.

That said, this post is interesting because it seems that the class file is valid and verified. If I were a JVM implementor, I would ignore that entry and fill a bug for Sun/Oracle!

Answer 3

My understanding is that the second exception table entry is the implicit catch everything clause added by the compiler to cover any exceptions/errors thrown in the body or the catch handlers and the third entry is the guard on that implicit catch to force the flow through the finally execution.