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Gwt and JSR 269's Pluggable Annotation Processing API
The document discusses the pluggable annotation processing API and its role in GWT, particularly with the transition from GWT 2.8 to GWT 3, highlighting the benefits and challenges associated with using JSR 269 for code generation in Java. The author provides insights on creating annotation processors, their integration with the Java compiler, and practical tips for using the API effectively in projects. Key limitations and historical context of annotation processing are also addressed, including notable tools and libraries that leverage this technology.
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Gwt and JSR 269's Pluggable Annotation Processing API
- 1. PLUGGABLE ANNOTATION PROCESSING API GWTCON - 2015 - LTE CONSULTING
- 2. ARNAUD TOURNIER Passionnate developper,trainer and architect at LTE Consulting. Speaker at Devoxx, GWT.create, Paris/Toulouse JUG, etc... Email : ltearno@gmail.com Twitter : @ltearno Website : www.lteconsulting.fr Full stack (x86_64 to JavaScript)
- 3. PRESENTATION AVAILABLE ON lteconsulting.fr/annotation-processing Andthe demo project is available at github.com/ltearno/gwtcon-jsr269
- 4. GWT 3 WILLDROP GENERATORS !
- 5. JSR 269 TOTHE RESCUE???
- 6. PLUGGABLE ANNOTATION PROCESSINGAPI Code generation in Java (source, byte-code and resource). Integrated with the Java compiler. Based on annotations. The developer's annotation processor receives most of the program's AST.
- 7. USED FOR ? RPCstubs, Reflection stubs, UI generation, Configuration file generation, Code checkers, Build breakers, Dependency injection, Glue code generation, Your own needs !
- 8. IN THE GWTCONTEXT GWT 3 will abandon generators because the functionality exists in standard Java : JSR 269. Even with GWT 2.8 it makes sense to use it. Causes migration problems, with most of the time quick resolution.
- 9. GOOD POINTS API iseasy to use. Generated code is visible and debuggable, Generated code is known before compilation so you can reference it directly (no GWT.create). No overhead at runtime. Does not depend on byte code : GWT compatible
- 10. BAD POINTS Only annotatedelements trigger processing. API makes it difficult to coordinate processing of multiple classes over multiple rounds (bad for incremental compilation). Dependency to external resource is not managed either.
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- 13. APT Introduced in JDK5, was removed with Java 7 because it support new language elements. Annotation Processing Tool can't Runs outside of . API includes packages.
- 14. PLUGGABLE ANNOTATION PROCESSINGAPI Fixes the sins of the past. has been included since Java 6 (2006).JSR-269 Runs inside of . API is able to welcome new language features.
- 15. HOW IT WORKS Annotationprocessors must be registered. Java source files are compiled during rounds. Each round, processors are activated and receive the program's AST. They can then generate files which will be part of the next round. When no file is generated during a round, real compilation happens.
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- 17. Let's say wewant to develop a tool that automatically generates UIs from any POJO...
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- 19. WE WILL HAVETO Write an annotation, Write an annotation processor, Register our processor through SPI, Package and use our library.
- 20. ANNOTATION CREATION This isthe annotation we use to trigger the custom annotation processing :
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- 22. REGISTERING THROUGH SPI Javacompiler searches annotation processors through SPI. Add a file named META- INF/services/javax.annotation.processing.Processor containing the annotation processors' fqn list : Other ways to register : has special flags. The also has methods to set the processors to be used.
- 23. PACKAGING The simplest wayis to have the annotation and its processor in the same package. Maven tip: dont forget to use the<compilerArgument>- proc:none</compilerArgument> options
- 24. USING THE PROCESSOR Ina project with the processor's jar in the classpath, we can use the annotation... Eclipse tips : Eclipse uses its own java compiler, JDT. Use m2e-apt to configure your project if you work with maven. Don't forget to close the processor project to have it activated.
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- 27. USING THE GENERATEDCLASS
- 28. HOW IS ITPOSSIBLE ? Using the not yet generated file is possible because the java compiler deffers processing of the NotFoundSymbolException. The error is raised at the end of the parsing and annotation processing process if the symbol has not been generated.
- 29. THE API :A BRIEF INTRODUCTION
- 30. API OVERVIEW Filer class: generate files (source, byte-code, resource) Language Model classes : browse the program's structure, Messager class : to communicate with the user, Other tools : element and type tools see javadoc of the javax.annotation.processing and javax.lang.model packages.
- 31. API : JAVASOURCE REPRESENTATION Element : representation of a language construct (class declarations, methods, ...). Ex: , ... Supports all the language structures through the and methods. Hierarchical structure : , . TypeMirror : Type representation, almost like Class<?>
- 32. API : ACCESSINGELEMENTS Elements are given as parameters in the method of the generator. Annotated elements are retrieved like this : . All the classes parsed during the current round can be obtained with : . Elements can also be retrieved with the utility methods : , and .
- 33. API : FILER Creatinga new Java source
- 34. API : MESSAGER Outputsmessages to the user. Can also generate errors and break the build. Very handy to assert things on the code. IDE integration : hints on the API for the user.
- 35. API : TOOLS Manytools can be retrieved from the field of the Other static methods and classes are helpful :
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- 38. COMPILE-TESTING github.com/google/compile-testing Annotation Processor testinglibrary developped by Google to help developping of the and projects.Auto Dagger
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- 42. LIMITATIONS Not a fullaccess to the code's AST (instructions). Processors cannot depend one on the other. Incremental compilation is difficult when having dependencies to more than one element or to external files. on Eclipse : , on maven : have to disable incremental compilation. Most of the time those limitations are not embarassing.
- 43. HACKING : based onJSR 269 and hacking both javac and jdt in order to acces to internal implementations and mutate the class AST. Technical explanations in Lombok The Hacker's guide to JavaC
- 44. NOTE ON USINGTEMPLATES Try to generate the minimal amount of code, and base it on generic implementations. This will ease debugging. Tools : Velocity, ... Java Poet, ... String.replaceAll()
- 45. LIBRARIES KNOWN USINGJSR-269 JPA meta-model generation (JSR-317), Dagger, Google Auto, Immutables, Lombok, GWT (RequestFactory), Hexa Binding...
- 46. LINKS , , , ,, , , , , , , javadoc for annotation processing javadoc for Java Language Model Hibernate Validation Lombok How Lombok works ? Lombok again... Hacking JavaC Coders Breakfast Angelika Langer presentation Dr. Macphail's trance Annotation processing history Save method parameter names
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- 48. SEE YOU ! Slides: lteconsulting.fr/annotation-processing Demo project : github.com/ltearno/gwtcon-jsr269 Twitter : LTE Consulting : lteconsulting.fr LinkedIn : fr.linkedin.com/in/lteconsulting