Metadata
TDDD05 / DF14900
Component-Based Software
Metamodeling and
Metaprogramming
Christoph Kessler, IDA, Linköpings universitet, 2010
Some slides by courtesy of U. Assmann, IDA / TU Dresden
„ Meta: means “describing”
„ Metadata: describing data (sometimes: self-describing data).
z
The language (esp., type system) for specifying metadata
is called metamodel.
„ Metalevel: the elements of the meta-level (the meta-objects)
describe the objects on the base level
„ Metamodeling: description of the model elements/concepts in the
metamodel
1. Introduction to metalevels
2. Different Ways of Metaprogramming
Metadata
3. UML Metamodel and MOF
4. Component markup
U. Assmann: Invasive Software Composition, Sect.Christoph
2.2.5Kessler,
Metamodeling;
IDA,
Linköpings universitet, 2009.
C. Szyperski: Component Software, Sect. 10.7, 14.4.1
Java Reflection
Metalevels in Programming Languages
Level 3 - Meta-Concepts in the
metameta model, the metalanguage
(language description)
Level 2 - Language concepts
(Metaclasses in the metamodel)
Method
Class
Level 1 - Software Classes
(meta-objects)
(Model)
Car
Level 0 - Software Objects
car 1
“Real World” Entities
car
3
Programming Language Concept
void
drive() {}
car1.drive()
C. Kessler, IDA, Linköpings universitet.
driving
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C. Kessler, IDA, Linköpings universitet.
Level 3 - Meta-Concepts in the
metameta model, the metalanguage
(language description)
Attribute
int
color
car1.color
car color
TDDD05 Component-Based Software
Classes in a software system
Level 2 - Language concepts
(Metaclasses in the metamodel)
Car
Level 0 - Software Objects
car 1
“Real World” Entities
car
4
Method
Attribute
void
drive() {}
car1.drive()
driving
C. Kessler, IDA, Linköpings universitet.
int
color
car1.color
car color
TDDD05 Component-Based Software
„ Reflection is computation about the metamodel in the base model.
„ The application can look at its own skeleton (metadata)
and may even change it
z
Metaclasses
C. Kessler, IDA, Linköpings universitet.
Programming Language Concept
Class
Level 1 - Software Classes
(meta-objects)
(Model)
z
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TDDD05 Component-Based Software
Reflection
(Self-Modification, Metaprogramming)
class WorkPiece { Object belongsTo; }
class RotaryTable { WorkPiece place1, place2; }
class Robot { WorkPiece piece1, piece2; }
class ConveyorBelt { WorkPiece pieces[]; }
Examples:
• Java Reflection API [Szyperski 14.4.1]
• UML metamodel (MOF)
Base
level
Metalevels in Programming Languages
Classes and Metaclasses
Concepts of a metalevel can be
represented at the base level.
This is called reification.
Data,
Code,
Information
Meta level
Concepts level
public class Class {
Attribute[] fields;
Method[] methods;
Class ( Attribute[] f, Method[] m) {
fields = f;
methods = m;
}
}
public class Attribute {..}
public class Method {..}
TDDD05 Component-Based Software
Allocating new classes, methods, fields
Removing classes, methods, fields
„ Enabled by reification of meta-objects at base level (e.g., as API)
Remark: In the literature, “reflection”
was originally introduced to denote
“computation about the own program”
[Maes'87] but has also been used in
the sense of “computing about other
programs” (e.g., components).
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C. Kessler, IDA, Linköpings universitet.
Metadata
Meta level
Data,
Data,
Code,
Code,
Information
Information
Base level
TDDD05 Component-Based Software
Example:
Creating a Class from a Metaclass
class WorkPiece { Object belongsTo; }
class RotaryTable { WorkPiece place1, place2; }
class Robot { WorkPiece piece1, piece2; }
class ConveyorBelt { WorkPiece pieces[]; }
„ Create a new class at runtime
Introspection
public class Class {
Attribute[] fields;
Method[] methods;
Class ( Attribute[] f, Method[] m) {
fields = f;
methods = m;
}
}
public class Attribute {..}
public class Method {..}
„ Read-only reflection is called introspection
z
The component can look up the metadata of itself or another
component and learn from it (but not change it!)
„ Typical application: find out features of components
by instantiating the metaclass:
z
Classes, methods, attributes, types
z
Very important for late (run-time) binding
Metadata
Class WorkPiece = new Class( new Attribute[]{ "Object belongsTo" }, new Method[]{});
Class RotaryTable = new Class( new Attribute[]{ "WorkPiece place1", "WorkPiece place2" },
new Method[]{});
Class Robot
Data,
Code,
Information
= new Class( new Attribute[]{ "WorkPiece piece1", "WorkPiece piece2" },
new Method[]{});
Data,
Code,
Information
Class ConveyorBelt = new Class( new Attribute[]{ "WorkPiece[] pieces" }, new Method[]{});
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C. Kessler, IDA, Linköpings universitet.
Metaprogram
at base
level
TDDD05 Component-Based
Software
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C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Reflection Example
Introcession
„ Read and Write reflection is called introcession
z
Reading Reflection
The component can look up the metadata of itself or another
component and may change it
(Introspection):
for all c in self.classes do
generate_class_start(c);
for all a in c.attributes do
generate_attribute(a);
done;
generate_class_end(c);
done;
„ Typical application: dynamic adaptation of parts of own program
z
Classes, methods, attributes, types
Full Reflection
(Introcession):
for all c in self.classes do
helpClass = makeClass( c.name + "help“ );
for all a in c.attributes do
helpClass.addAttribute(copyAttribute(a));
done;
self.addClass(helpClass);
done;
Metadata
AAreflective
reflectivesystem
systemisisaasystem
systemthat
thatuses
usesthis
thisinformation
informationabout
aboutitself
itself
ininits
itsnormal
normalcourse
courseofofexecution.
execution.
Data,
Code,
Information
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TDDD05 Component-Based Software
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Metaprogramming
on the Language Level
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Made It Simple
„ Level 0: objects
enum { Singleton, Parameterizable } BaseFeature; Metalanguage concepts
Language description concepts
public class LanguageConcept {
(Metametamodel)
String name;
BaseFeature singularity;
LanguageConcept ( String n, BaseFeature s ) {
name = n;
Good for language
singularity = s;
extension / customization,
}
e.g. with UML MOF, or for
}
compiler generation
Language concepts
(Metamodel)
„ Level 1: classes, types
„ Level 2: language elements
„ Level 3: metalanguage, language description language
LanguageConcept Class = new LanguageConcept("Class", Singleton);
LanguageConcept Attribute =
new LanguageConcept("Attribute", Singleton);
LanguageConcept Method =
new LanguageConcept("Method", Parameterizable);
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C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
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C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Use of Metamodels and
Metaprogramming
TDDD05 / DF14900
Component-Based Software
2. Different Ways of
Metaprogramming
To model, describe, introspect, and manipulate
„ Programming languages, such as Java Reflection API
„ Modeling languages, such as UML or Modelica
„ XML
- meta-level vs. base level
- static vs. dynamic
„ Compilers
„ Debuggers
„ Component systems, such as JavaBeans or CORBA DII
Metaprograms
Metaprograms are
areprograms
programs that
that compute
computeabout
about programs
programs
„ Composition systems, such as Invasive Software Composition
„ Databases
Metaprogram
„ ... many other systems ...
Program
Program’
run-time output
run-time input
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Christoph Kessler, IDA,
Linköpings universitet, 2009.
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Metaprograms can run at base level
or at meta level
Base-Level Metaprogram
Metaprogram execution at the metalevel:
„ Metaprogram is separate from base-level program
„ Direct control of the metadata as metaprogram data structures
„ Expression operators are defined directly on the metaobjects
Metalevel
Metaobjects
Repository
with Concepts/
Types/Descriptions
as Artefacts
„ Example: Compiler, program analyzer, program transformer
z
Program metadata = the internal program representation
Reflection
has
classes to create objects describing base program classes, functions, statements, variables, constants, types etc.
Metaprogram execution at the base level:
„ Metaprogram/-code embedded into the base-level program
„ All expressions etc. evaluated at base level
Base-level program
data memory:
Repository
with Objects
as Artefacts
„ Access to metadata only via special API, e.g. Java Reflection
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TDDD05 Component-Based Software
Meta-level Metaprogram
Metaobjects
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Metaprogram
Base Level
Class
Classsomeclass
someclass== foo.getClass();
foo.getClass();
C. Kessler, IDA, Linköpings universitet.
Static vs. Dynamic Metaprogramming
Metalevel
Recall: Metaprograms are programs that compute about programs.
„ Static metaprograms
Metaprogram
for
foreach
eachclass
classcc
add
addaanew
newmethod
method int
intbar()
bar(){...}
{...}
Base Level
z
Execute before runtime
z
Metainformation removed before execution – no runtime overhead
z
Examples: Program generators, compilers, static analyzers
„ Dynamic metaprograms
z
Execute at runtime
z
Metadata stored and accessible during runtime
z
Examples:
Programs
using reflection (Introspection, Introcession);
Interpreters,
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TDDD05 Component-Based Software
TDDD05 Component-Based Software
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debuggers
TDDD05 Component-Based Software
Static Metaprogramming
Static Time
Metaprogram
Example: Static Metaprogramming (1)
Run Time
Static Time
Metalevel
Metaobjects
Base Level
Metaprogram and metaobjects
exist only at compile time.
No run-time overhead.
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Metalevel
Metaobjects
Metaprogram
Compiler
Integer
4
myType
8
type table
...malloc( N * sizeof(myType));
TDDD05 Component-Based Software
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TDDD05 Component-Based Software
Compilers Are Static Metaprograms
Example: Static Metaprogramming (2)
Metaclasses: The compiler’s own
data types to represent IR structures
C++ templates
„ Example: generic type definition
(Meta)Information about generic type
removed after compiling!
class Vector_int {
template <class E>
class Vector {
E *pelem;
int size;
E get( int index ) {...}
...
}
...
Vector<int> v1;
Vector<float> v2;
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C. Kessler, IDA, Linköpings universitet.
Base Level
...malloc( N * 8 );
Metaprogram and metaobjects
exist only at compile time.
No run-time overhead.
C. Kessler, IDA, Linköpings universitet.
z
Run Time
expanded at
compile time to
equivalent of:
int *pelem;
int size;
int get( int index ) {...}
...
}
class Vector_float {
float *pelem;
int size;
float get( int index ) {...}
...
}
...
Vector_int v1;
Vector_floatTDDD05
v2; Component-Based Software
Metaprogram
(Level 2)
Programs
Programs
ininSource
Source
Form
Parsing,
Form
IR’
Analysing
IR
Analysis,
Transformations
Programs
Programsinin
Target
TargetForm
Form
Code
Generation /
Pretty Printing
Classes etc. represented by IR objects = metaobjects
= instances of IR classes (metaclasses)
Run time objects
(Level 0)
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C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Dynamic Metaprogramming
Compilers are Static Metaprograms
char x[7];
/* array - construct the type `array 0..n-1 of ty' with alignment a or ty's */
int a[13];
Type array( Type ty, int n, int a )
{
...
if (ty && isfunc(ty)) {
chartype
1
error( "illegal type `array of %t'\n", ty );
return array ( inttype, n, 0 );
inttype
4
}
voidtype
0
if (a == 0)
a = ty->align;
ARRAY(7,chartype)
7
if (level > GLOBAL && isarray(ty) && ty->size == 0)
ARRAY(13,inttype)
52
error( "missing array size\n“ );
if (ty->size == 0) {
type table excerpt
if (unqual(ty) == voidtype)
error( "illegal type `array of %t'\n", ty );
else if (Aflag >= 2)
warning( "declaring type `array of %t' is undefined\n", ty );
} else if (n > INT_MAX / ty->size) {
error( "size of `array of %t' exceeds %d bytes\n", ty, INT_MAX );
n = 1;
Source: lcc C compiler,
}
excerpt of file ”types.c”
return tynode ( ARRAY, ty, n * ty->size, a, (Generic)0 );
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} C. Kessler, IDA, Linköpings universitet.
(typeTDDD05
tableComponent-Based
management)
Software
Repository
with Concepts/
Types/Descriptions
as Artefacts
Metaobjects
Metalevel
Reflection
Base-level program
data memory:
Repository
with Objects
Metaas Artefacts
program
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Class
Classsomeclass
someclass== foo.getClass();
foo.getClass();
C. Kessler, IDA, Linköpings universitet.
Base Level
TDDD05 Component-Based Software
Summary: Ways of Metaprogramming
Reflective Architecture
„ A system with a reflective architecture maintains metadata and a
causal connection between meta- and base level.
Metaprogram runs at:
Base level
Compile/Deployment time
(static metaprogramming)
C++ template programs Compiler
C sizeof(...) operator
transformations;
C preprocessor
COMPOST
Run time
Java Reflection
(dynamic metaprogramming) JavaBeans
introspection
z
The metaobjects describe
structure, features, semantics of domain objects
z
This connection is kept consistent
Meta level
„ Reflection is thinking about oneself (or others) at the base level
with the help of metadata
Debugger
„ Metaprogramming is programming with metaobjects,
either at base level or meta level
Reflection
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TDDD05 Component-Based Software
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TDDD05 / DF14900
Component-Based Software
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
UML Metamodel and MOF
UML metamodel
„ specifies UML semantics
„ in the form of a (UML) class model (= reification)
„ specified in UML Superstructure document (OMG 2006)
using only elements provided in MOF
3. UML Metamodel and MOF
UML metametamodel: MOF (”Meta-Object Facility”)
„ self-describing
„ subset of UML (= reification)
„ for bootstrapping the UML specification
Christoph Kessler, IDA,
Linköpings universitet, 2009.
UML metamodel hierarchy
UML Extension possibility 1: Stereotypes
„ e.g., <<metaclass>> is a stereotype (specialization) of a class
z by subclassing metaclass ”Class” of the UML metamodel
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TDDD05 Component-Based Software
UML Metamodel
(Simplified Excerpt)
ModelElement
Metametalevel (L3)
UML metametamodel: MOF
name: Name
ModelElement : ModelElement
name: Name
UML metamodel
1
subtype
1
supertype
Generalizable
Element
is instance of
Metalevel (L2)
Class : ModelElement
isRoot: Boolean
isLeaf: Boolean
isAbstract: Boolean
isActive: Boolean
*
*
is instance of
Base level (L1)
UML base-level
class diagrams
owner
Cat : Class
Classifier
name: String
color: Color
age: Integer
1
type
*
is instance of
Object level (L0)
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name: ”Tom”
color: White
age: 7
C. Kessler, IDA, Linköpings universitet.
discriminator: Name
BehavioralFeature
isQuery: Boolean
Operation
<<metaclass>>
Class
Interface
C. Kessler, Boolean
IDA, Linköpings universitet.
30 isActive:
ownerScope: ScopeKind
visibility: VisibilityKind
multiplicity: Multiplicity
changeable: ChangeableKind
targetScope: ScopeKind
initialValue: Expression
TDDD05 Component-Based Software
*
StructuralFeature
Attribute
tom : Cat
Feature
Generalization
isAbstract: Boolean
concurrency: CallConcurKind
Datatype
TDDD05 Component-Based Software
Example: Reading the UML Metamodel
UML vs. programming language
metamodel hierarchies
Some semantics rules expressed in the UML metamodel above:
Metametalevel (L3)
„ Each model element must have a name.
„ A class can be a root, leaf, or abstract
z
Metalevel (L2)
UML metamodel
Class : ModelElement
Base level (L1)
UML base-level
class diagrams
(via Classifier)
Cat : Class
C. Kessler, IDA, Linköpings universitet.
is instance of
class Cat {
String name;
Color color;
Integer age;
...
}
models
is instance of
Object level (L0)
(1:N relation to Classifier),
e.g. classes, interfaces, datatypes
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java.lang.Class
name: String
color: Color
age: Integer
„ Each attribute has a type
z
models
isActive: Boolean
is instance of
(1:N relations to class ”Generalization”)
„ A class can have many features, e.g. attributes, operations
z
--
name: Name
is instance of
(inherited from GenerizableElement)
„ A class can have many subclasses and many superclasses
z
ModelElement : ModelElement
UML metametamodel: MOF
TDDD05 Component-Based Software
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tom : Cat
is instance of
models
name: ”Tom”
color: White
age: 7
C. Kessler, IDA, Linköpings universitet.
Cat tom = new
Cat(”Tom”, White, 7);
TDDD05 Component-Based Software
TDDD05 / DF14900
Component-Based Software
Caution
„ A metamodel is not a model of a model
but a model of a modeling language of models.
4. Component Markup
„ A model (e.g. in UML) describes a language-specific software item
at the same level of the metalevel hierarchy.
z
In contrast, metadata describes it from the next higher level,
from which it can be instantiated.
„ MOF is a subset of UML able to describe itself
– no higher metalevels required for UML.
... A simple aid for introspection and reflection...
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Christoph Kessler, IDA,
Linköpings universitet, 2009.
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Markup Languages
Hungarian Notation
„ Hungarian notation is a markup method that defines
„ Convey more semantics for the artifact they markup
naming conventions for identifiers in languages
„ HTML, XML, SGML are markup languages
„ Remember: a component is a container
„ Markup can make contents of the component accessible
for the external world, i.e., for composition
z
It can offer the content for introspection
z
Or even introcession
z
to convey more semantics for composition in a component
system
z
but still, to be compatible with the syntax of the component
language
z
so that standard tools can still be used
„ The composition environment can ask about the
names in the interfaces of a component
(introspection)
z
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TDDD05 Component-Based Software
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and can deduce more semantics from naming conventions
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Generic Types in COMPOST
Java Beans Naming Schemes
„ Metainformation for JavaBeans is identified by markup
in the form of Hungarian Notation.
z
<< ClassBox >>
<< ClassBox >>
T
„ Property access
class SimpleList {
genericTType elem;
SimpleList next;
genericTType getNext() {
return next.elem;
}
}
37
This metainformation is needed, e.g., by the JavaBeans Assembly tools
to find out which classes are beans and what properties and events they
have.
class SimpleList {
WorkPiece elem;
SimpleList next;
WorkPiece getNext() {
return next.elem;
}
}
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
setField(Object value);
z
Object getField();
„ Event firing
38
z
fire<Event>
z
register<Event>Listener
z
unregister<Event>Listener
C. Kessler, IDA, Linköpings universitet.
TDDD05 Component-Based Software
Markup is Essential
for Component Composition
Markup by Comments
„ Javadoc tags, XDoclet
z
„ C# attributes
z
@author
z
//@author
z
@date
z
//@date
z
@deprecated
z
//selfDefinedData
„ because it identifies metadata,
which in turn supports introspection and introcession
„ Components that are not marked-up cannot be composed
„ Java 1.5 attributes
z
Can annotate any declaration
e.g. class, method, interface,
field, enum, parameter, ...
„ C# /.NET attributes
z
[author(Uwe Assmann)]
z
predefined and user-defined
z
[date Feb 24]
z
class C extends B {
@Overrides
public int foo() { ... }
z
[selfDefinedData(...)]
„ Every component model has to introduce
a strategy for component markup
„ Insight:
A component system that supports composition techniques
must be a reflective architecture!
...
}
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C. Kessler, IDA, Linköpings universitet.
What Have We Learned?
TDDD05 Component-Based Software
(1)
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C. Kessler, IDA, Linköpings universitet.
What Have We Learned?
TDDD05 Component-Based Software
(2)
„ Reflection is a program’s ability to reason about and possibly modify
itself or other programs with the help of metadata.
z
Reflection is enabled by reification of the metamodel.
z
Introspection is thinking about a program, but not modifying.
„ The UML metamodel is a description of UML
specified in terms of the UML metametamodel, MOF
z
UML models describe program objects on the same level of the
meta-hierarchy level.
„ A metaprogram is a program that computes about programs
z
Metaprograms can execute at the base level or at the metalevel.
z
Metacode can execute statically or at run time.
„ Component and composition systems are reflective architectures
z
Markup marks the variation and extension points of components
e.g., using Hungarian notation, Comments/Annotations,
external markup (separate files referencing the contents)
z Composition introspects the markup
z Look up type information, interface information, property
information
z or full reflection
Static
metaprogramming at base level
e.g. C++ templates, AOP
Static
metaprogramming at meta level
e.g. Compiler analysis / transformations
Dynamic
metaprogramming at base level
e.g. Java Reflection
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TDDD05 Component-Based Software
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TDDD05 Component-Based Software