Notes

Outline

Java Virtual Machine (JVM) Reasons for learning JVM Resource The Java TM Virtual Machine Specification (2 nd Ed), by Tim Lindholm & Frank Yellin, Addison-Wesley,1999 http://java.sun.com/docs/books/vmspec/

JVM Types and Words byte, char, short, int, float, reference, and returnAddress in one word boolean, byte, char, and short are converted to int Special representations for byte, char, and short arrays are possible long and double in two words

JVM Architecture

Data Areas of JVM Method area (shared by all threads) One record for each loaded class that contains: Constant pool Code for methods and constructors Heap (shared by all threads) One record for each class instance or array Stack (one per thread) Hold frames associated with method invocations

Stack Frame Structure

The class File Format

Names and Descriptors Class, method, and field names are all stored symbolically as strings A descriptor is a string representing the type of a field or method Class names Class names are always fully qualified Use forward slash rather than dot as the delimeter Ex: Java.util.Vector => java/util/Vector

Descriptors Primitive types B, C, D, F, I,J(long), S, Z(boolean), V(void) Arrays Use [ Classes Lclassname Ex: Type: String valueOf(char[] , int offset, int count) Descriptor:([CII)Ljava/lang/String;

Constants CONSTANT_Class CONSTANT_Fieldref CONSTANT_Methodref CONSTANT_InterfaceMthodref CONSTANT_String, CONSTANT_Integer CONSTANT_Float CONSTANT_Long CONSTANT_Double CONSTANT_NameAndType CONSTANT_Utf8

field_info and method_info field_info Access flags Name Descriptor Static values method_info Access flags Name Descriptor Code

Instruction Set Load and store (e.g., iload, istore ldc, iconst_<i>) Arithmetic (e.g., iadd, isub, imul, idiv, irem) Type conversion (e.g., i2b, i2f, i2d) Object creation and manipulation (e.g, new, newarray, iaload, iastore , getfield,  putfield) Operand stack manipulation (e.g., pop, dup, dup_x1, swap) Control transfer (e.g., goto, ifeq, tableswitch) Method invocation and return (invokevirtual, invokeinterface, invokespecial, invokestatic, ireturn) Exception handling and synchronization (e.g.,athrow)

Compiling for JVM Constants, Local Variables, and Control constructs

Compiling for JVM Receiving Arguments and Invoking Methods

Compiling for JVM Working with Class Instances

Compiling for JVM Arrays

Compiling for JVM Switches

Compiling for JVM Manipulation of the Operand Stack

Compiling for JVM Exception Handling

Review Questions Does the efficiency of a program change after an int variable is changed to byte? When does dynamic loading take place? When does dynamic linking take place? Why are run-time byte-code verification and type checking necessary? How are exceptions handled in JVM? How different is the JVM from a stack machine for Pascal?