Data processing: database and file management or data structures – Database design – Data structure types
Reexamination Certificate
1998-08-17
2001-06-26
Amsbury, Wayne (Department: 2818)
Data processing: database and file management or data structures
Database design
Data structure types
Reexamination Certificate
active
06253215
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention generally relates to memory management for computer systems and, more particularly, to a methodology for managing memory resources for an application program having two types of program code, native code executing directly in an operating environment and target code for execution by an abstract computing machine associated with the operating environment and responsible for memory management for both types of code.
B. Description of the Related Art
Object-oriented programming techniques have revolutionized the computer industry. For example, such techniques offer new methods for designing and implementing computer programs using an application programming interface (API) associated with a predefined set of “classes,” each of which provides a template for the creation of “objects” sharing certain attributes determined by the class. These attributes typically include a set of data fields and a set of methods for manipulating the object.
The Java™ Development Kit (JDK) from Sun Microsystems, Inc., for example, enables developers to write object-oriented programs using an API with classes defined using the Java™ programming language. The Java programming language is described, for example, in a text entitled “The Java Language Specification” by James Gosling, Bill Joy, and Guy Steele, Addison-Wesley, 1996. The class library associated with the Java API defines a hierarchy of classes with a child class (i.e., subclass) inheriting attributes (i.e., fields and methods) of its parent class. Instead of having to write all aspects of a program from scratch, programmers can simply include selected classes from the API in their programs and extend the functionality offered by such classes as required to suit the particular needs of a program. This effectively reduces the amount of effort generally required for software development.
The JDK also includes a compiler and a runtime environment with a virtual machine (VM) for executing programs. In general, software developers write programs in a programming language (in this case the Java programming language) that use classes from the API. Using the compiler, developers compile their programs into “class files” containing instructions for an abstract computing model embodied by the Java VM; these instruction are often called “bytecodes.” The runtime environment has a class loader that integrates the class files of the application with selected API classes into an executable application. The Java VM then executes the application by simulating (or “interpreting”) bytecodes on the host operating system/computer hardware. The Java VM thus acts like an abstract computing machine, receiving instructions from programs in the form of bytecodes and interpreting these bytecodes. (Another mode of execution is “just in time” compilation in which the VM dynamically compiles bytecodes into so-called native code for faster execution.) Details on the VM for the JDK can be found in a text entitled “The Java Virtual Machine Specification,” by Tim Lindholm and Frank Yellin, Addison Wesley, 1996.
The Java VM also supports multi-threaded program execution. Multi-threading is the partitioning of a computer program or application into logically independent “threads” of execution that can execute in parallel. Each thread includes a sequence of instructions to carry out a particular program task, such as a method for computing a value or for performing an input/output function. When employing a computer system with multiple processors, separate threads may execute concurrently on each processor.
Thus, object-oriented facilities like the JDK assist both development and execution of object-oriented systems. First, they enable developers to create programs in an object-oriented programming language using an API. Second, they enable developers to compile their programs, and third, they facilitate program execution by providing a virtual machine implementation.
However, object-oriented programs may not be suitable for all functions of a system or it may not be economically feasible to convert all of the programs in an existing legacy system into object-oriented programs. It may also be necessary, for a system having primarily object-oriented programs, to use features of a platform's operating system that are not available in implementations using a VM like the Java VM. Finally, the virtual machine implementation itself is generally not written in the language it executes but rather in the native code of the host machine. Thus, it is not uncommon for systems to have programs with “native” and “non-native” code.
For purposes of this description, native code includes code written in any programming language that is then compiled to run on a compatible operating system/hardware configuration. For example, native code in this context includes program code written in the C or C++ programming language and compiled by an appropriate compiler for execution on a particular platform, such as a computer having the Windows 95 operating system running on an Intel Pentium processor. Native code is distinguishable from the non-native code, which will be referred to as “target code,” because while non-native code is foreign to a platform's operating system/hardware configuration, its target for purposes of this description is an abstract computing machine, such as a VM, operating on any compatible platform configuration. For example, target code for the Java VM is generally written in the Java programming language. This combination of native and target code in the same application tends to complicate the management of memory resources (i.e., the allocation and deallocation of memory) for such systems.
In practice, when an application seeks to refer to an object, the computer must first allocate or designate memory for the object. Using a “reference” to the allocated memory, the application can then properly manipulate the object. One way to implement a reference is by means of a “pointer” or “machine address,” which uses multiple bits of information, however, other implementations are possible. Objects can themselves contain primitive data items, such as integers or floating point numbers, and/or references to other objects. In this manner, a chain of references can be created, each reference pointing to an object which, in turn, points to another object. When no chain of references in an application reaches a given object, the computer can deallocate or reclaim the corresponding memory for reuse.
Memory reclamation can be handled explicitly by the application program. This method, however, requires programmers to design programs to account for all allocated objects and to determine when the objects are available for reclamation. The alternative is to assign responsibility for memory management to a runtime system responsible for controlling program execution. The Java VM, one such system responsible for controlling program execution for example, includes a “garbage collector” to manage available memory resources used during execution of Java code.
“Garbage collection” is the term used to refer to a class of algorithms used to carry out memory management, specifically, automatic reclamation. Garbage collection algorithms generally determine reachability of objects from the references held in some set of roots. When an object is no longer reachable, the memory that the object occupies can be reclaimed and reused. There are many known garbage collection algorithms, including reference counting, mark-sweep, and generational garbage collection algorithms. These, and other garbage collection techniques, are described in detail in a book entitled “Garbage Collection, Algorithms For Automatic Dynamic Memory Management” by Richard Jones and Raphael Lins, John Wiley & Sons, 1996.
To be effective, garbage collection techniques should be able to, first, identify references that are directly accessible to the executing program, and, second, given the reference to an object, identify references contained within t
Agesen Ole
Detlefs David L.
White Derek R.
Amsbury Wayne
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Sun Microsystems
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