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How the Java Virtual Machine Handles Exceptions
A Detailed Study with Examples of Classes and Methods
by Bill Venners
First Published in JavaWorld, January 1997

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Summary
All Java programs are compiled into class files that contain bytecodes, the machine language of the Java virtual machine. This article takes a look at the way exceptions are handled by the Java virtual machine, including the exception table and the bytecodes related to exceptions.


Welcome to another installment of Under The Hood. This column aims to give Java developers a glimpse of the mysterious mechanisms clicking and whirring beneath their running Java programs. This month's article continues the discussion of the bytecode instruction set of the Java virtual machine by examining the manner in which the Java virtual machine handles exception throwing and catching, including the relevant bytecodes. This article does not discuss finally clauses -- that's next month's topic. Subsequent articles will discuss other members of the bytecode family.

Exceptions
Exceptions allow you to smoothly handle unexpected conditions that occur as your programs run. To demonstrate the way the Java virtual machine handles exceptions, consider a class named NitPickyMath that provides methods that perform addition, subtraction, multiplication, division, and remainder on integers. NitPickyMath performs these mathematical operations the same as the normal operations offered by Java's "+", "-", "*", "/", and "%" operators, except the methods in NitPickyMath throw checked exceptions on overflow, underflow, and divide-by-zero conditions. The Java virtual machine will throw an ArithmeticException on an integer divide-by-zero, but will not throw any exceptions on overflow and underflow. The exceptions thrown by the methods of NitPickyMath are defined as follows:

class OverflowException extends Exception {
}

class UnderflowException extends Exception {
}

class DivideByZeroException extends Exception {
}

A simple method that catches and throws exceptions is the remainder method of class NitPickyMath:

static int remainder(int dividend, int divisor)
    throws DivideByZeroException {

    try {
        return dividend % divisor;
    }
    catch (ArithmeticException e) {
        throw new DivideByZeroException();
    }
}

The remainder method simply performs the remainder operation upon the two ints passed as arguments. The remainder operation throws an ArithmeticException if the divisor of the remainder operation is a zero. This method catches this ArithmeticException and throws a DivideByZeroException.

The difference between a DivideByZero and an ArithmeticException exception is that the DivideByZeroException is a checked exception and the ArithmeticException is unchecked. Because the ArithmeticException is unchecked, a method need not declare this exception in a throws clause even though it might throw it. Any exceptions that are subclasses of either Error or RuntimeException are unchecked. (ArithmeticException is a subclass of RuntimeException.) By catching ArithmeticException and then throwing DivideByZeroException, the remainder method forces its clients to deal with the possibility of a divide-by-zero exception, either by catching it or declaring DivideByZeroException in their own throws clauses. This is because checked exceptions, such as DivideByZeroException, thrown within a method must be either caught by the method or declared in the method's throws clause. Unchecked exceptions, such as ArithmeticException, need not be caught or declared in the throws clause.

javac generates the following bytecode sequence for the remainder method:

The main bytecode sequence for remainder:

   0 iload_0 // Push local variable 0 (arg passed as divisor)
   1 iload_1 // Push local variable 1 (arg passed as dividend)
   2 irem // Pop divisor, pop dividend, push remainder
   3 ireturn // Return int on top of stack (the remainder)

The bytecode sequence for the catch (ArithmeticException) clause:

   4 pop // Pop the reference to the ArithmeticException
                           // because it isn't used by this catch clause.

   5 new #5 <Class DivideByZeroException>
                           // Create and push reference to new object of class
                           // DivideByZeroException.

DivideByZeroException

   8 dup // Duplicate the reference to the new
                           // object on the top of the stack because it
                           // must be both initialized
                           // and thrown. The initialization will consume
                           // the copy of the reference created by the dup.

   9 invokenonvirtual #9 <Method DivideByZeroException.<init>()V>
                           // Call the constructor for the DivideByZeroException
                           // to initialize it. This instruction
                           // will pop the top reference to the object.

  12 athrow // Pop the reference to a Throwable object, in this
                           // case the DivideByZeroException,
                           // and throw the exception.

The bytecode sequence of the remainder method has two separate parts. The first part is the normal path of execution for the method. This part goes from pc offset zero through three. The second part is the catch clause, which goes from pc offset four through twelve.

The irem instruction in the main bytecode sequence may throw an ArithmeticException. If this occurs, the Java virtual machine knows to jump to the bytecode sequence that implements the catch clause by looking up and finding the exception in a table. Each method that catches exceptions is associated with an exception table that is delivered in the class file along with the bytecode sequence of the method. The exception table has one entry for each exception that is caught by each try block. Each entry has four pieces of information: the start and end points, the pc offset within the bytecode sequence to jump to, and a constant pool index of the exception class that is being caught. The exception table for the remainder method of class NitPickyMath is shown below:

Exception table:
   from to target type
     0 4 4 <Class java.lang.ArithmeticException>

The above exception table indicates that from pc offset zero through three, inclusive, ArithmeticException is caught. The try block's endpoint value, listed in the table under the label "to", is always one more than the last pc offset for which the exception is caught. In this case the endpoint value is listed as four, but the last pc offset for which the exception is caught is three. This range, zero to three inclusive, corresponds to the bytecode sequence that implements the code inside the try block of remainder. The target listed in the table is the pc offset to jump to if an ArithmeticException is thrown between the pc offsets zero and three, inclusive.

If an exception is thrown during the execution of a method, the Java virtual machine searches through the exception table for a matching entry. An exception table entry matches if the current program counter is within the range specified by the entry, and if the exception class thrown is the exception class specified by the entry (or is a subclass of the specified exception class). The Java virtual machine searches through the exception table in the order in which the entries appear in the table. When the first match is found, the Java Virtual Machine sets the program counter to the new pc offset location and continues execution there. If no match is found, the Java virtual machine pops the current stack frame and rethrows the same exception. When the Java virtual machine pops the current stack frame, it effectively aborts execution of the current method and returns to the method that called this method. But instead of continuing execution normally in the previous method, it throws the same exception in that method, which causes the Java virtual machine to go through the same process of searching through the exception table of that method.

A Java programmer can throw an exception with a throw statement such as the one in the catch (ArithmeticException) clause of remainder, where a DivideByZeroException is created and thrown. The bytecode that does the throwing is shown in the following table:

Throwing Exceptions
Opcode Operand(s) Description
athrow (none) pops Throwable object reference, throws the exception

The athrow instruction pops the top word from the stack and expects it to be a reference to an object that is a subclass of Throwable (or Throwable itself). The exception thrown is of the type defined by the popped object reference.

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