Aspect weaver
Encyclopedia
An aspect weaver is a metaprogramming
utility for aspect-oriented languages designed to take instructions specified by aspects
(isolated representations of a significant concepts in a program) and generate the final implementation code. The weaver integrates aspects into the locations specified by the software as a pre-compilation
step. By merging aspects and classes
(representations of the structure of entities in the program), the weaver generates either a woven class (which can then be compiled into bytecode) or woven bytecode
directly.
Aspect weavers take instructions known as advice specified through the use of pointcut
s and join point
s, special segments of code that indicate what methods
should be handled by aspect code. The implementation of the aspect then specifies whether the related code
should be added before, after, or throughout the related methods. By doing this, aspect weavers improve modularity
, keeping code in one place that would otherwise have been interspersed throughout various, unrelated classes.
s are already widely accepted and understood. However, the desire to create radically different programming languages to support the aspect-oriented programming
paradigm is not significant due to business-related concerns; there are risks associated with adopting new technologies. Use of an entirely new language relies on a business's ability to acquire new developers. Additionally, the existing code base of a business would need to be discarded. Finally, a business would need to acquire a new toolchain
(suite of tools) for development, which is often both an expense in both money and time. Primary concerns about roadmaps
for the adoption of new technologies tend to be the need to train new developers and adapt existing processes to the new technology.
To address these business concerns, an aspect weaver enables the use of widely adopted languages like Java
with aspect-oriented programming through minor adaptations such as AspectJ
which work with existing tools. Instead of developing an entirely new language, the aspect weaver interprets the extensions defined by AspectJ and builds "woven" Java code which can then be used by any existing Java compiler. This ensures that any existing object oriented
code will still be valid aspect-oriented code and that development will feel like a natural extension of the object-oriented language. The AspectC++
programming language extends C++
through the use of an aspect weaver, offering the additional efficiency over AspectJ
that is necessary for embedded systems while still retaining the benefits of aspect-oriented programming.
, known as advice, and distributing it throughout the various classes
in the program automatically. The result of the weaving process is a set of classes with the same names as the original classes but with additional code injected into the classes' functions automatically. The advice specifies the exact location and functionality of the injected code.
Through this weaving process, aspect weavers allow for code which would have otherwise been duplicated across classes. By eliminating this duplication, aspect weavers promote modularity
of cross-cutting concern
s. Aspects define the implementation code which would have otherwise been duplicated and then use pointcut
s and join point
s to define the advice. During weaving, the aspect weaver uses the pointcuts and join points, known as a pointcut designator, to identify the positions in candidate classes at which the implementation should be injected. The implementation is then injected into the classes at the points identified, thus permitting the code to be executed at the appropriate times without relying on manual duplication by the programmer
.
AspectJ
, pointcuts, join points, and the modularized code are defined in an aspect block similar to that of Java
classes
. Classes are defined using Java syntax. The weaving process consists of executing the aspect advice to produce only a set of generated classes that have the aspect implementation code woven into it.
The example at right shows a potential implementation of an aspect which logs the entry and exit of all methods
. Without an aspect weaver, this feature would necessitate duplication of code
in the class for every method. Instead, the entry and exit code is defined solely within the aspect.
The aspect weaver analyzes the advice specified by the pointcut in the aspect and uses that advice to distribute the implementation code into the defined class. The code differs slightly in each method due to slight variances in requirements for the method (as the method identifier
has changed). The aspect weaver determines the appropriate code to generate in each situation as defined by the implementation advice and then injects it into methods matching the specified pointcut.
, some AspectJ weavers instead weave the aspects and classes
together directly into bytecode
, acting both as the aspect weaver and compiler
. While it is expected that the performance of aspect weavers which also perform the compilation process will require more computation time due to the weaving process involved, it is also expected that this performance can be improved as aspect weavers' compiler implementations are relatively new, while traditional compilers have been thoroughly tested and optimized.
into the execution of aspect-oriented code to address performance demands. At run-time, an aspect weaver could translate aspects in a more efficient manner than traditional, static weaving approaches. Using AspectJ on a Java Virtual Machine
, dynamic weaving of aspects at run-time has been shown to improve code performance by 26%. While some implementations of just-in-time virtual machines implement this capability through a new virtual machine, some implementations can be designed to use features that already exist in current virtual machines. The requirement of a new virtual machine is contrary to one of the original design goals of AspectJ.
To accomplish just-in-time weaving, a change to the virtual machine
that executes the compiled bytecode
is necessary. A proposed solution for AspectJ uses a layered
approach which builds upon the existing Java Virtual Machine to add support for join point management and callbacks to a Dynamic Aspect-Oriented Programming Engine. An alternative implementation uses a weaving engine that uses breakpoints to halt execution at the pointcut, select an appropriate method, embed it into the application, and continue. The use of breakpoints in this manner has been shown to reduce performance due to a very large number of context switch
es.
from aspect code can be done at compile-time, some optimizations can only be performed by the aspect weaver. For example, AspectJ contains two similar but distinct keywords,
keyword
on every execution. Studies have shown that the unnecessary creation of join point objects in AspectJ can lead to a performance overhead of 5% at run-time, while performance degradation is only approximately 1% when this object is not created.
Compile-time performance is generally worse in aspect weavers than their traditional compiler counterparts due to the additional work necessary for locating methods which match the specified pointcuts. A study done showed that the AspectJ compiler ajc is about 34% slower than the Sun Microsystems
Java 1.3 compiler and about 62% slower than the Java 1.4 compiler.
Metaprogramming
Metaprogramming is the writing of computer programs that write or manipulate other programs as their data, or that do part of the work at compile time that would otherwise be done at runtime...
utility for aspect-oriented languages designed to take instructions specified by aspects
Aspect (computer science)
In computer science, an aspect of a program is a feature linked to many other parts of the program, but which is not related to the program's primary function. An aspect crosscuts the program's core concerns, therefore violating its separation of concerns that tries to encapsulate unrelated functions...
(isolated representations of a significant concepts in a program) and generate the final implementation code. The weaver integrates aspects into the locations specified by the software as a pre-compilation
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
step. By merging aspects and classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
(representations of the structure of entities in the program), the weaver generates either a woven class (which can then be compiled into bytecode) or woven bytecode
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...
directly.
Aspect weavers take instructions known as advice specified through the use of pointcut
Pointcut
In aspect-oriented computer programming, a pointcut is a set of join points. Whenever the program execution reaches one of the join points described in the pointcut, a piece of code associated with the pointcut is executed. This allows a programmer to describe where and when additional code...
s and join point
Join point
In computer science, a join point is a point in the control flow of a program. In aspect-oriented programming a set of join points is described as a pointcut...
s, special segments of code that indicate what methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
should be handled by aspect code. The implementation of the aspect then specifies whether the related code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
should be added before, after, or throughout the related methods. By doing this, aspect weavers improve modularity
Modularity (programming)
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
, keeping code in one place that would otherwise have been interspersed throughout various, unrelated classes.
Motivation
Many programming languageProgramming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s are already widely accepted and understood. However, the desire to create radically different programming languages to support the aspect-oriented programming
Aspect-oriented programming
In computing, aspect-oriented programming is a programming paradigm which aims to increase modularity by allowing the separation of cross-cutting concerns...
paradigm is not significant due to business-related concerns; there are risks associated with adopting new technologies. Use of an entirely new language relies on a business's ability to acquire new developers. Additionally, the existing code base of a business would need to be discarded. Finally, a business would need to acquire a new toolchain
Toolchain
In software, a toolchain is the set of programming tools that are used to create a product...
(suite of tools) for development, which is often both an expense in both money and time. Primary concerns about roadmaps
Plan
A plan is typically any diagram or list of steps with timing and resources, used to achieve an objective. See also strategy. It is commonly understood as a temporal set of intended actions, through which one expects to achieve a goal...
for the adoption of new technologies tend to be the need to train new developers and adapt existing processes to the new technology.
To address these business concerns, an aspect weaver enables the use of widely adopted languages like Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
with aspect-oriented programming through minor adaptations such as AspectJ
AspectJ
AspectJ is an aspect-oriented extension created at PARC for the Java programming language. It is available in Eclipse Foundation open-source projects, both stand-alone and integrated into Eclipse. AspectJ has become the widely-used de-facto standard for AOP by emphasizing simplicity and usability...
which work with existing tools. Instead of developing an entirely new language, the aspect weaver interprets the extensions defined by AspectJ and builds "woven" Java code which can then be used by any existing Java compiler. This ensures that any existing object oriented
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
code will still be valid aspect-oriented code and that development will feel like a natural extension of the object-oriented language. The AspectC++
AspectC++
AspectC++ is an aspect-oriented extension of C and C++ languages. It is based on source-to-source translation, translating AspectC++ source code to C++...
programming language extends C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
through the use of an aspect weaver, offering the additional efficiency over AspectJ
AspectJ
AspectJ is an aspect-oriented extension created at PARC for the Java programming language. It is available in Eclipse Foundation open-source projects, both stand-alone and integrated into Eclipse. AspectJ has become the widely-used de-facto standard for AOP by emphasizing simplicity and usability...
that is necessary for embedded systems while still retaining the benefits of aspect-oriented programming.
Implementation
Aspect weavers operate by taking instructions specified by aspectsAspect (computer science)
In computer science, an aspect of a program is a feature linked to many other parts of the program, but which is not related to the program's primary function. An aspect crosscuts the program's core concerns, therefore violating its separation of concerns that tries to encapsulate unrelated functions...
, known as advice, and distributing it throughout the various classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
in the program automatically. The result of the weaving process is a set of classes with the same names as the original classes but with additional code injected into the classes' functions automatically. The advice specifies the exact location and functionality of the injected code.
Through this weaving process, aspect weavers allow for code which would have otherwise been duplicated across classes. By eliminating this duplication, aspect weavers promote modularity
Modular programming
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
of cross-cutting concern
Cross-cutting concern
In computer science, cross-cutting concerns are aspects of a program which affect other concerns.These concerns often cannot be cleanly decomposed from the rest of the system in both the design and implementation, and can result in either scattering , tangling , or both.For instance, if writing an...
s. Aspects define the implementation code which would have otherwise been duplicated and then use pointcut
Pointcut
In aspect-oriented computer programming, a pointcut is a set of join points. Whenever the program execution reaches one of the join points described in the pointcut, a piece of code associated with the pointcut is executed. This allows a programmer to describe where and when additional code...
s and join point
Join point
In computer science, a join point is a point in the control flow of a program. In aspect-oriented programming a set of join points is described as a pointcut...
s to define the advice. During weaving, the aspect weaver uses the pointcuts and join points, known as a pointcut designator, to identify the positions in candidate classes at which the implementation should be injected. The implementation is then injected into the classes at the points identified, thus permitting the code to be executed at the appropriate times without relying on manual duplication by the programmer
Programmer
A programmer, computer programmer or coder is someone who writes computer software. The term computer programmer can refer to a specialist in one area of computer programming or to a generalist who writes code for many kinds of software. One who practices or professes a formal approach to...
.
aspect Logger { pointcut method : execution(* *(..)); before : method { System.out.println("Entering " + thisJoinPoint.getSignature.toString); } after : method { System.out.println("Leaving " + thisJoinPoint.getSignature.toString); } } public class Foo { public void bar { System.out.println("Executing Foo.bar"); } public void baz { System.out.println("Executing Foo.baz"); } } |
| A sample aspect Aspect (computer science) In computer science, an aspect of a program is a feature linked to many other parts of the program, but which is not related to the program's primary function. An aspect crosscuts the program's core concerns, therefore violating its separation of concerns that tries to encapsulate unrelated functions... and class Class (computer science) In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior... defined in the AspectJ AspectJ AspectJ is an aspect-oriented extension created at PARC for the Java programming language. It is available in Eclipse Foundation open-source projects, both stand-alone and integrated into Eclipse. AspectJ has become the widely-used de-facto standard for AOP by emphasizing simplicity and usability... programming language Programming language A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely.... |
public class Foo { public void bar { System.out.println("Entering Foo.bar"); System.out.println("Executing Foo.bar"); System.out.println("Leaving Foo.bar"); } public void baz { System.out.println("Entering Foo.baz"); System.out.println("Executing Foo.baz"); System.out.println("Leaving Foo.baz"); } } |
| The woven class that results from executing an aspect weaver on the above sample |
Weaving in AspectJ
In the programming languageProgramming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
AspectJ
AspectJ
AspectJ is an aspect-oriented extension created at PARC for the Java programming language. It is available in Eclipse Foundation open-source projects, both stand-alone and integrated into Eclipse. AspectJ has become the widely-used de-facto standard for AOP by emphasizing simplicity and usability...
, pointcuts, join points, and the modularized code are defined in an aspect block similar to that of Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
. Classes are defined using Java syntax. The weaving process consists of executing the aspect advice to produce only a set of generated classes that have the aspect implementation code woven into it.
The example at right shows a potential implementation of an aspect which logs the entry and exit of all methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
. Without an aspect weaver, this feature would necessitate duplication of code
Duplicate code
Duplicate code is a computer programming term for a sequence of source code that occurs more than once, either within a program or across different programs owned or maintained by the same entity. Duplicate code is generally considered undesirable for a number of reasons...
in the class for every method. Instead, the entry and exit code is defined solely within the aspect.
The aspect weaver analyzes the advice specified by the pointcut in the aspect and uses that advice to distribute the implementation code into the defined class. The code differs slightly in each method due to slight variances in requirements for the method (as the method identifier
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
has changed). The aspect weaver determines the appropriate code to generate in each situation as defined by the implementation advice and then injects it into methods matching the specified pointcut.
Weaving to bytecode
Instead of generating a set of woven source codeSource code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
, some AspectJ weavers instead weave the aspects and classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
together directly into bytecode
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...
, acting both as the aspect weaver and compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
. While it is expected that the performance of aspect weavers which also perform the compilation process will require more computation time due to the weaving process involved, it is also expected that this performance can be improved as aspect weavers' compiler implementations are relatively new, while traditional compilers have been thoroughly tested and optimized.
Run-time weaving
Developments in AspectJ have revealed the potential to incorporate just-in-time compilationJust-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...
into the execution of aspect-oriented code to address performance demands. At run-time, an aspect weaver could translate aspects in a more efficient manner than traditional, static weaving approaches. Using AspectJ on a Java Virtual Machine
Java Virtual Machine
A Java virtual machine is a virtual machine capable of executing Java bytecode. It is the code execution component of the Java software platform. Sun Microsystems stated that there are over 4.5 billion JVM-enabled devices.-Overview:...
, dynamic weaving of aspects at run-time has been shown to improve code performance by 26%. While some implementations of just-in-time virtual machines implement this capability through a new virtual machine, some implementations can be designed to use features that already exist in current virtual machines. The requirement of a new virtual machine is contrary to one of the original design goals of AspectJ.
To accomplish just-in-time weaving, a change to the virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
that executes the compiled bytecode
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...
is necessary. A proposed solution for AspectJ uses a layered
Abstraction layer
An abstraction layer is a way of hiding the implementation details of a particular set of functionality...
approach which builds upon the existing Java Virtual Machine to add support for join point management and callbacks to a Dynamic Aspect-Oriented Programming Engine. An alternative implementation uses a weaving engine that uses breakpoints to halt execution at the pointcut, select an appropriate method, embed it into the application, and continue. The use of breakpoints in this manner has been shown to reduce performance due to a very large number of context switch
Context switch
A context switch is the computing process of storing and restoring the state of a CPU so that execution can be resumed from the same point at a later time. This enables multiple processes to share a single CPU. The context switch is an essential feature of a multitasking operating system...
es.
Performance
Aspect weavers' performance, as well as the performance of the code that they produce, has been a subject of analysis. It is preferable that the improvement in modularity supplied by aspect weaving does not impact run-time performance. Aspect weavers are able to perform aspect-specific optimizations. While traditional optimizations such as the elimination of unused special variablesVariable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
from aspect code can be done at compile-time, some optimizations can only be performed by the aspect weaver. For example, AspectJ contains two similar but distinct keywords,
thisJoinPoint, which contains information about this particular instance of woven code, and thisJoinPointStaticPart, which contains information common to all instances of code relevant to that set of advice. The optimization of replacing thisJoinPoint with the more efficient and staticStatic variable
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
keyword
thisJoinPointStaticPart can only be done by the aspect weaver. By performing this replacement, the woven program avoids the creation of a join point objectObject (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
on every execution. Studies have shown that the unnecessary creation of join point objects in AspectJ can lead to a performance overhead of 5% at run-time, while performance degradation is only approximately 1% when this object is not created.
Compile-time performance is generally worse in aspect weavers than their traditional compiler counterparts due to the additional work necessary for locating methods which match the specified pointcuts. A study done showed that the AspectJ compiler ajc is about 34% slower than the Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
Java 1.3 compiler and about 62% slower than the Java 1.4 compiler.