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Try-finally clauses defined and demonstrated

Through code samples, tables, and a Java virtual machine simulation, this article examines the bytecodes of the Java virtual machine that deal with finally clauses

By Bill Venners, JavaWorld.com, 02/01/97

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Note that the bytecodes that implement the return statement store a copy of the return value into local variable 3 before jumping to the miniature subroutine that represents the second finally clause. Then, after the miniature subroutine returns (in this case it never does, because the continue is always executed), the return value is retrieved from local variable 3 and returned. This highlights the way the JVM returns values when finally clauses are also executed. Rather than returning the value of i after the finally clause is executed, the JVM will return the value that i had just before the finally clause was executed. This means that even if the finally clause changes the value of i, the method will still return the value that i had when the return statement was first reached, before the finally clause was invoked. If you wanted the finally clause to be able to change the return value of the method, you would have to put an actual return statement with the new return value into the finally clause itself.

To drive the simulation, just press the Step button. Each press of the Step button will cause the Java virtual machine to execute one bytecode instruction. To start the simulation over, press the Reset button. To cause the Java virtual machine to repeatedly execute bytecodes with no further coaxing on your part, press the Run button. The Java virtual machine will then execute the bytecodes until the Stop button is pressed. The text area at the bottom of the applet describes the next instruction to be executed. Happy clicking.

About the author

Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill.
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Resources
  • The book The Java Virtual Machine Specification (http://www.aw.com/cp/lindholm-yellin.html), by Tim Lindholm and Frank Yellin (ISBN 0-201-63452-X), part of The Java Series (http://www.aw.com/cp/javaseries.html), from Addison-Wesley, is the definitive Java virtual machine reference.
  • Previous Under The Hood articles
  • The lean, mean virtual machine -- Gives an introduction to the Java virtual machine. Look here to see how the garbage collected heap fits in with the other parts of the Java virtual machine.
  • The Java class file lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.
  • Java's garbage-collected heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the Java virtual machine in particular.
  • Bytecode basics -- Introduces the bytecodes of the Java virtual machine, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating Point Arithmetic -- Describes the Java virtual machine's floating-point support and the bytecodes that perform floating-point operations.
  • Logic and Arithmetic -- Describes the Java virtual machine's support for logical and integer arithmetic, and the related bytecodes.
  • Objects and Arrays -- Describes how the Java virtual machine deals with objects and arrays, and discusses the relevant bytecodes.
  • Exceptions -- Describes how the Java virtual machine deals with exceptions, and discusses the relevant bytecodes.