The Art of Metaprogramming in Java

The Art of Metaprogramming
in Java

Abdelmonaim Remani
@PolymathicCoder

About Me
 Software Architect at Just.me Inc.
 Interested in technology evangelism and enterprise software development and
architecture
 Frequent speaker (JavaOne, JAX, OSCON, OREDEV, etc…)
 Open-source advocate
 President and founder of a number of user groups
- NorCal Java User Group
- The Silicon Valley Spring User Group
- The Silicon Valley Dart Meetup
 Bio: http://about.me/PolymathicCoder
 Twitter: @PolymathicCoder
 Email: abdelmonaim.remani@gmail.com

About Me
 Today is my birthday!

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License

 Creative Commons Attribution Non-Commercial 3.0 Unported
- http://creativecommons.org/licenses/by-nc/3.0

 Disclaimer: The graphics, logos, and trademarks used this presentation belong to
their rightful owners.

What Is Metadata?
 The term “Metadata” was coined by Philip Bagley in 1986 in “Extension of
Programming Language Concepts”
 Data about data or data that describes other data
- Structural (Before there is any data)
- Its type
- Its valid values
- How it relates to other data
- Its purpose
- Etc…
- Descriptive (After data is there)
- How it was created
- The context within which it exists
- Etc…

How Is Metadata Expressed?
 Implicit
- Adhering to a certain convention
 Explicit
- External to the code
- DSL (Domain-Specific Language)
- Markup: XML, RDF, etc…
- Etc…
- Internal to the code
- As comments on code
- Javadoc: @author, @version, @since, etc…
- XDoclet
- Etc…
- As code itself
- HTML meta tags: <meta name=“author” content=“Abdelmonaim Remani”>
- Java Annotations: @Override, @Deprecated, etc…
- Embedded DSL
- Etc…

How Is Metadata Being Used?
 Schema
- Data Dictionary in RDBMS
- Check Constraints (JSR 303 Validation, Etc…)
- Etc…
 Semantics
- WSDL (Web Services Description Language)
- RDF (Resource Description Framework)
- Etc…
 Data Management
- Build/deployment instructions (.svn files, .git, etc…)
- Etc…
 Configurations
 Etc…

Java Annotations
 JSR 175 (A Metadata Facility for the Java Programming Language)
- Introduced in J2SE 5.0 (September 30, 2004)
- Standardized how annotations are declared in Java code
 An alternative way to Javadoc comments, externally as XML, etc…
- More readable
- Closer to the code
- Statically-typed
- Can be retained until runtime

JDK Annotations
 Java Language Spec Annotations
- @Override
- @Deprecated
- @SuppressWarning
 JSR 250 (Common Annotations for the Java Platform)
- @PostConstruct
- @Resource
- @DenyAll
- Etc…

Write Your Own

 You need to tell the compiler how the annotation is to be treated
 In java.lang.annotation package
- @Target
- The element type the annotation can be applied to
- ElememtType.ANNOTATION_TYPE
- ElememtType.CONSTRUCTOR
- ElememtType.FIELD
- ElememtType.LOCAL_VARIABLE
- ElememtType.METHOD
- ElememtType.PACKAGE
- ElememtType.PARAMETER
- ElememtType.TYPE

Write Your Own
 In java.lang.annotation package
- @Retention
- RetentionPolicy.SOURCE
- Discarded by the compiler
- RetentionPolicy.CLASS
- Included in the class file but ignored the JVM. This is the default
- RetentionPolicy.RUNTINE
- Included in the class file and read by the JVM.
- @Documented
- Whether it should be shown in the javadoc or not
- @Inherited
- Allowed to be inherited by subclasses

Write Your Own
 Attributes
- Carry additional metadata details
- May only be
- Primitives
- Enums
- Java.lang.String
- Java.lang.Class
- One-dimensional arrays of the above
- May have a default value
- No modifiers or parameters

Notes
 Annotating a package
- Must create a file named package-info.java in the target package
- Note that “package-info” is not a valid Java identifier
- Most IDEs you will prevent you from creating a class name “package-info”
- Make sure you create it as a regular file with .java extension instead

What is Metaprogramming?
 Writing programs that write or manipulate other programs or themselves based on
some metadata
 Metaprogramming
-!= Generative/Automatic Programming
 Ralph Johnson
- “It is about programming at the meta level, that is about changing your
interpreter or changing how code is compiled”
 Black art and a Big-boy’s toy
 An underused feature
 Supported in many languages and across several platforms
 The backbone of many of the most successful frameworks

How is Metaprogramming Supported?
 Exposing the internals of the runtime/compiler as an API
 Dynamic execution of expressions containing programming commands
- Code as Strings
- Code as a series if method calls
 A program transformation system
- A description gets transformed it to a target language
- The compiler itself
- YACC takes in grammar, and produces C/C++ code containing yyparse()
- ANTLR (ANother Tool for Language Recognition)

Concepts
 Metalanguage
- The language in which the metaprogram is written in
 Object Language
- The language in which the target (produced) program is written in
 Reflection or Reflexivity
- When metalanguage == object language
- No translation necessary

Usage in code
 Static data that can be pre-computed or pre-generated at compile time
 Eliminate boiler-plate
- Code that cannot be abstracted in functions for DRYness sake
- Think Aspects in AOP
- Stereotypes in Spring
- Etc…
- Code of common methods
- Getters/setters, toString(), hashCode(), equals(), etc…
 Etc…

Benefits
 In code
- Performance gain
- Flexibility
- Simplicity
- Etc…
 Development
- Minimize the LOC to express a solution
- Productivity gain
- Reduced development time/efficiency
- Etc…

How to?
 Many techniques focused on specific aspects of metaprogramming
 No well-defined best practices

 This presentation is an attempt to bring in some structure through defining a
process
- Defining the metadata
- Processing the metadata
- Metamodel construction
- Validating the metamodel
- Metamodel interpretation

Metadata Processing
 Programmatically reading/accessing metadata
- Parsing raw metadata
- Call to an API
- Reflection
- Query
- Etc…
- Tools

Metadata Processing in Java: Annotation Processing

At Runtime: JSR 175
 JSR 175: A Metadata Facility for the Java Programming Language
- Defined the core reflection API for reading/accessing annotations on annotated
elements at runtime as long as their retention policy extends to the runtime
- Reflection
- Reading annotations is done in reference to the structure of the program
 Libraries
- Reflections: http://code.google.com/p/reflections/
- FEST-Reflect: http://fest.easytesting.org/
- ReflectASM: http://code.google.com/p/reflectasm/
- Etc…

At Build Time: JSR 269
 Mirror-Based Reflection
- Reflective capabilities are encapsulated in intermediary objects called mirrors
- Annotations are accessible through a processor API
 In J2SE 5.0 didn’t standardize a processing API
- We used apt, a stand-alone tool, along with the Mirror API (com.sun.mirror)

 In Java SE 6 (December 11, 2006)
- JSR 269: Pluggable Annotation Processing API
-http://docs.oracle.com/javase/6/docs/technotes/guides/javac/index.htm

- Leverages JSR 199 (Java Compiler API)
- Javax.tools
- Programmatically invoke javac
- Implements ServiceLoader interface of SPI (JSR 24 - ServiceProvider API))
- Provides the DiagnosticListener interface to allow listening for warnings and
error by the compiler
- Extends javac as a plug-in allowing to write custom annotation processors
- Seamless integration with javac
- Finds if there is an annotation registered to process a particular annotation
- Plugs it into the compiler

 JSR 269 Defines 2 APIs
- Writing annotation processors and interacting with the processing environment
- Javax.annotation.processing
- Modeling the Java Programming Language
- Javax.lang.model
- Type declarations and types (Accommodates Generics)

Code
 Implement javax.annotation.processing.Processor or extend
javax.annotation.processing.AbstractProcessor
- Process method
- Returns whether or not the annotations processed are claimed by the
processor. This determines whether other processors can further process
them or not
 Configure the processor using annotations
- @SupportedAnnotationTypes
- Register the annotations to be processed with their full-qualified name.
Wildcats are supported as well
- @SupportedSourceVersion
- The Java version supported
- @SupportedOptions
- Register supported command-line options

Code
 Annotation processor registration
- Javac command-line options
- -processor <processor classes>
- -proc:none or -proc:only. It is enabled by default
- -processorpath <annotation path>
- Leverage JSR 24 (ServiceProvider API) to automatically register processors
- Compile the processor with –proc:none and package it in a jar
- Include in META-INF/services a file named
javax.annotation.processing.Processor containing a text entry of the full-
qualified name of the processor class
- Optional: Multiple processor can be registered or ordered

Metadata Processing
 Annotated elements might have nested annotated elements and so forth
 Visitor Design Pattern
- Separation of annotation processing code from the object structure

What is a Metamodel?
 Metadata is processed into a model that can be accessed programmatically
- Static
- Dynamic
 A association of the data and its metadata

Static Metamodel
- A metamodel based on
- One all-knowing “god object” encapsulating all possible metadata values that
could be associated with the one annotated element
- Advantages
- Simple
- Statically typed
- Disadvantages
- Nulls all over

Dynamic Metamodel
 Ravioli Code
- Metamodel is structured in small and loosely-coupled components
 Decorator Design Pattern
- Annotated elements are decorated with annotations that bring in metadata
 Advantages
- Flexible
 Disadvantages
- Complex

Validation?
 Ensuring the validity or correctness of semantics of the metamodel
- Verification that a set of constraints are satisfied
 Compliance additional consistency constraints outside the Java language
specification
- Java only has @Target
 Example
- Assuming that you are writing your own JSR 318 (Enterprise javabeans 3.1)
implementation
- You should not allow a POJO to be annotated with both @Statefull and
@Stateless

Constraint Satisfaction
 This is anything but simple
- A Constraint Satisfaction Problem
- Can be resolved by multiple algorithms (Backtracking, Constrain reparation,
and local search)
- These algorithms are out of the scope of this presentation

Validating the Metamodel
 The imperative way
- In Java
- A jungle of conditional statements

 The logical way (Logic Programming)
- Semantics are represented declaratively as predicates (Facts and Rules)
- Procedurally interpreted query resolution
- Constraint Logic Programming (An extension of Logic Programming)
- Used for digital circuit verification
• Prolog is King
- tuProlog: Implementation of the Prolog interpreter in Java
- http://alice.unibo.it/xwiki/bin/view/Tuprolog/
- Jlog: Implementation of the Prolog interpreter in Java
- http://jlogic.sourceforge.net/
- JPL: Java Interface to SWI Prolog
- http://www.swi-prolog.org/FAQ/Java.html
 Yeah you’re gonna have to learn that

 Clojure
- Lisp-like language on the JVM
- core.logic supports logic and constraint programming
 Other
- Prova Rule Language
- http://www.prova.ws/
- Mercury
- http://www.mercury.csse.unimelb.edu.au/

 Rules Engine
- Drools Expert
- A highly performance optimized rules engine
- Rules are written in mvel (A powerful expression language) or XML
- Integrates with Java collaborator POJOs

Transformations
 Changing the structure of existing code
- AST (Abstract Syntax Tree) rewriting
- Adding/removing behavior
- Automatic generation of cookie-cutter code
- Create complex class hierarchies
 Notable byte code manipulation libraries
- ASM
- GCLIB
- Javassist
- Etc…

Transformations
 ASM
- Small
- Fast
- Low-level
- Require a good understanding of byte-code and Java Language Spec
- Good documentation
- http://asm.ow2.org/
 GCLIB
- Built on top of ASM
- High performance
- Issues with signed jars
- Dropped by Hibernate in favor of Javassist for lack of support and active
- http://cglib.sourceforge.net/

Transformations
 Javassist
- Very easy to use
- Works with signed jars
- Good documentation
- Excellent Javadoc
- Adopted by hibernate
- Slower than GCLIB
- http://www.jboss.org/javassist/

Weaving in Cross-Cutting Concerns
 OOP creates a hierarchical object model by nature
 Certain things are not necessary a part of the application logic
- Logging
- Transactions
- Security
- Etc…
 AOP (Aspect Oriented Programming)
- Code weaving
- Dynamic Proxies

Mapping
 Map one object to another based on the metamodel
 Example
- Dozer
- http://dozer.sourceforge.net/
- ORM Frameworks

Mapping

Metadata Row
Class

Metamodel

POJO
Class Mapper Creates Row
Instance

POJO
Instance

Other
 Inversion of Control
- Dependency Injection (JSR 330)
 Instrumentation
- Monitor and measure performance
- Diagnostics
- Trace info
- Etc…
 Etc…

Metaprogramming in the Wild
 Spring
- Lightweight POJO-based frameworks
- Non-invasiveness
- Stereotyping a class in lieu of inheritance to acquire behavior
- Arbitrary methods signature instead of overriding inherited ones
- Declarative configuration
- Dependency Injection
 Rails
- Rapid Development
- Convention over configuration
- Active Record Pattern
 Hibernate/JPA (JSR 317)
- ORM
 Guice
 Spring AOP
 Etc…

Model-Driven Engineering
 Philosophy
- In an application
- Artifacts that are good candidates for auto-generation
- Why?
- The architecture is defined by
- Making abstractions guided by the separation of concerns
- Most deal with the same concerns
- Avoid rediscovering the best approach every time
- Code generation, Scaffolding, etc…

Averroes
 A metaprogramming framework
- Auto-discovery of annotations
- @Discoverable
- Annotation processing
- Metamodel construction
- Metamodel validation
- Annotations of annotations (meta-metadata)
- @AccompaniedWith
- @AccompaniedWithAtLeastOne
- @ForType
- @ForTypeAnnotatedWith
- @ForTypeIncluding
- @NotAccompaniedWith
 A work in progress…
- https://github.com/PolymathicCoder/Averroes

The Art of Metaprogramming in Java

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