Write Yourself an Annotation Processor

Annotation processors were standardized in Java 6, but it wasn't until recently that they began gaining some popularity. In this talk, I will argue that they are a pretty nice form of compile-time metaprogramming, a facility which a lot of languages are sorely lacking. I will explain why it makes sense to write an annotation processor, show some real-world examples, both small and big, and then implement a simple yet useful annotation processor from scratch.

Transcript

1. Write Yourself an Annotation Processor ladicek@gmail.com
3. Write Yourself an Annotation Processor ladicek@gmail.com
4. Why?
5. First... What?
7. | | v .-------------. | Code | '-------------' | .-------------. | | Code | | | Generator | | '-------------' | | v | .-------------. '------>| Compiler | '-------------' | | Compiled v Code
8. | | v .-------------. .------>| Code *.java | | '-------------' | | .-------------. | | Annotation | | | Processor | | '-------------' | ^ v | .-------------. '-------| Compiler | '-------------' | | Compiled v Code
9. So... Why?
11. got a problem?
12. We can solve any problem by introducing an extra level of indirection. fundamental theorem of software engineering
13. compile-time metaprogramming
17. Examples
18. google.github.io/auto Save time. Save code. Save sanity.
19. @AutoService(Plugin.class) public class MyPlugin implements Plugin { ... }
20. @AutoValue public abstract class Person { public static Person of(String name, int age) { return new AutoValue_Person(name, age); } public abstract String name(); public abstract int age(); }
21. square.github.io/dagger javax.inject
22. @Module class DripCoffeeModule { @Provides Heater provideHeater() { return new ElectricHeater(); } @Provides Pump providePump(Thermosiphon pump) { return pump; } }
23. jakewharton.github.io/ butterknife view injection for Android
24. public class ExampleActivity extends Activity { @InjectView(R.id.title) TextView title; @InjectView(R.id.subtitle) TextView subtitle; @InjectView(R.id.footer) TextView footer; @Override public void onCreate(Bundle state) { super.onCreate(state); setContentView(R.layout.simple_activity); ButterKnife.inject(this); ... use injected views ... } }
25. JBoss Logging Tools logging internationalization
26. @MessageLogger(projectCode = "EXMPL") public interface ExampleLogger { ExampleLogger LOGGER = Logger.getMessageLogger( ExampleLogger.class, "com.example"); @LogMessage(level = Level.INFO) @Message(id = 1, value = "Version: %s") void version(String version); @LogMessage(level = Level.FATAL) @Message(id = 2, value = "Too bad, %s is melting") void meltDown(@Cause Throwable cause, String value); }
27. openjdk.java.net/projects/ code-tools/jmh Java Microbenchmark Harness
28. @State(Scope.Thread) @BenchmarkMode(Mode.AverageTime) @OutputTimeUnit(TimeUnit.NANOSECONDS) public class JMHSample_08_DeadCode { private double x = Math.PI; @Benchmark public void wrong() { // the entire computation can be optimized out Math.log(x); } @Benchmark public double correct() { return Math.log(x); } }
29. openjdk.java.net/projects/ code-tools/jcstress Java Concurrency Stress tests
30. @JCStressTest @Outcome(id = "[0]", expect = Expect.ACCEPTABLE) @Outcome(id = "[-1]", expect = Expect.ACCEPTABLE) @Outcome( expect = Expect.ACCEPTABLE_SPEC) @State public class LongAtomicityTest { long x; @Actor public void actor1() { x = -1; } @Actor public void actor2(LongResult1 r) { r.r1 = x; } }
31. etc. etc. etc. etc.
32. I’m in! How?
34. | | v .-------------. .------>| Code *.java | | '-------------' | | .-------------. | | Annotation | | | Processor | | '-------------' | ^ v | .-------------. '-------| Compiler | '-------------' | | Compiled v Code
35. Collecting data
36. 1) Beware of exceptions Catch everything in process and print a stacktrace
37. 2) Beware of class loading Don’t read Class-typed annotation values
38. 3) Build a typed model
39. 4) Tie errors to the elements
40. 5) Don’t claim the annotations Unless it makes sense
41. Generating code
42. 1) Mustache (+ others) https://github.com/spullara/mustache.java
43. 2) JavaPoet https://github.com/square/javapoet http://corner.squareup.com/2015/01/javapoet.html
44. 3) JDeparser https://github.com/jdeparser/jdeparser2 http://dmlloyd.github.io/presentations/jdeparser
45. Testing is possible https://github.com/google/compile-testing
46. Resources https://www.youtube.com/watch?v=dOcs-NKK-RA http://jamezp.github.io/talks/annotation-processor
47. Q’s? A’s http://devconf.cz/f/99