Annotation processing and code gen

Annotation Processing
& Code Gen
@kojilin
2015/05/30@TWJUG

Outline
•My requirements
•Annotation Processing
•Code gen

Kaif Android Client
•Using REST client to fetch article and
debates
•Need stable connection

Kaif Android Client
•Using REST client to fetch article and
debates
•Need stable connection
•If not, stale data is better than no data
•Local cache Database
•Http cache response

Rest Http Client
•Retroﬁt from Square
•A type-safe REST client for Android
and Java
•http://square.github.io/retroﬁt/

public interface GitHubService {
@GET("/users/{user}/repos")
List<Repo> listRepos(@Path("user") String user);
}

RestAdapter restAdapter = new RestAdapter.Builder()
.setEndpoint("https://api.github.com")
.build();
GitHubService service = 
restAdapter.create(GitHubService.class);

List<Repo> repos = service.listRepos("octocat");

Header
@Headers("Cache-Control: max-stale=86400")
@GET("/users/{user}/repos")
List<Repo> listRepos$$RetryStale(@Path("user")  
String user);

How to retry ?
try {
// normal access
} catch (Exception e) {
// force local cache if network error
}

How to retry ?
try {
service.listRepos("koji");
if (e instanceof RetrofitError
&& isNetworkError(e)) {
service.listRepos$$RetryStale("koji");
}
}

If we do it manually
•Every service access need previous try
& catch code.
try {
if (e instanceof RetrofitError
&& isNetworkError(e)) {
service.listRepos$$RetryStale("koji");
} 
}

& catch code.
•Using Proxy pattern

service =  
Proxy.newProxyInstance(serviceClass.getClassLoader(), 
new Class[] { serviceClass }, 
new RetryStaleHandler(restAdapter.create( 
Class.forName(serviceClass.getName() 
+ "$$RetryStale"))));
@Override  
public Object invoke(Object proxy, Method  
method, Object[] args) throws Throwable { 
... 
if(method isGet and canRetry) { 
try & catch block
} 
... 
}
RetryStaleHandler.java

& catch code.
•Using Proxy pattern
•Almost every GET method need  
$$RetryStale version.

@GET("/users/{user}/repos") 
List<Repo> listRepos(@Path("user") String user); 
 
@Headers("Cache-Control: max-stale=86400") 
List<Repo> listRepos$$RetryStale(@Path("user") String user); 
 
@GET("/repos/{owner}/{repo}/contributors") 
List<Contributor> contributors(@Path("owner") String owner,  
@Path("repo") String repo); 
 
List<Contributor> contributors$$RetryStale(@Path("owner") String  
owner, @Path("repo") String repo);
}

& catch code.
•Using Proxy pattern.
•Almost every GET method need  
$$RetryStale.
•Generating boilerplate code during
build.

How ?
•Scan all interface has @GET method.
•Generate new interface has both non-
cache and cache method.

When scan?
•Compile time.
•Annotation Processing.
•Run time.
•Reﬂection.
•Slow and feedback at runtime
•Do we know the concrete class we want
to generate when we are writing ?

Generate code
•bytecode
•.java

Generate bytecode
•Can do more than what java source can.
•Bytecode is diﬃcult to read and write.
•ASM
•JarJar
•Dexmaker for android

Generate .java
•Readable and maintainable.
•JavaPoet from Square
•Successor of JavaWriter
•https://github.com/square/javapoet

 
}

public interface GitHubService$$RetryStale {
 
List<Repo> listRepos$$RetryStale(@Path("user") String user); 
 
 
List<Contributor> contributors$$RetryStale(@Path("owner") String  
owner, @Path("repo") String repo);
}

AutoValue
@AutoValue 
public abstract class Foo {
public abstract String name();
public abstract int age();
}

final class AutoValue_Foo extends Foo { 
private final String name; 
private final int age; 
 
AutoValue_Foo( 
String name, 
int age) { 
if (name == null) { 
throw new  
NullPointerException("Null name"); 
} 
this.name = name; 
this.age = age; 
} 
 
@Override 
public String name() { 
return name; 
} 
 
@Override 
public int age() { 
return age; 
}
 
@Override 
public String toString() { 
return "Foo{" 
+ "name=" + name 
+ ", age=" + age 
+ "}"; 
} 
 
@Override 
public boolean equals(Object o) { 
if (o == this) { 
return true; 
} 
if (o instanceof Foo) { 
Foo that = (Foo) o; 
return  
(this.name.equals(that.name())) 
&& (this.age == that.age()); 
} 
return false; 
} 
 
@Override 
public int hashCode() { 
int h = 1; 
h *= 1000003; 
h ^= name.hashCode(); 
h *= 1000003; 
h ^= age; 
return h; 
} 
}

Butter Knife
 
@InjectView(R.id.content) 
public TextView content; 
@InjectView(R.id.last_update_time) 
public TextView lastUpdateTime; 
@InjectView(R.id.vote_score) 
public TextView voteScore; 
@InjectView(R.id.debater_name) 
public TextView debaterName; 
@InjectView(R.id.debate_actions) 
public DebateActions debateActions; 
 
public DebateViewHolder(View itemView) { 
super(itemView); 
ButterKnife.inject(this, itemView); 
content.setOnTouchListener(new  
ClickableSpanTouchListener()); 
}

@Override public void inject(final Finder finder, final T target, Object
source) { 
View view; 
view = finder.findRequiredView(source, 2131296343, "field 'content'"); 
target.content = finder.castView(view, 2131296343, "field 'content'"); 
view =  
finder.findRequiredView(source, 2131296375, "field 'lastUpdateTime'"); 
target.lastUpdateTime =  
finder.castView(view, 2131296375, "field 'lastUpdateTime'"); 
view = finder.findRequiredView(source, 2131296374, "field 'voteScore'"); 
target.voteScore = finder.castView(view, 2131296374, "field
'voteScore'"); 
view = finder.findRequiredView(source, 2131296373, "field
'debaterName'"); 
target.debaterName =
finder.castView(view, 2131296373, "field 'debaterName'"); 
view = finder.findRequiredView(source, 2131296370, "field
'debateActions'"); 
target.debateActions =  
finder.castView(view, 2131296370, "field 'debateActions'"); 
} 
 
@Override public void reset(T target) { 
target.content = null; 
target.lastUpdateTime = null; 
target.voteScore = null; 
target.debaterName = null; 
target.debateActions = null; 
}

JPA metamodel
@Entity 
public class Pet {
@Id 
Long id;
String name;
}

@StaticMetaModel(Pet.class) 
public class Pet_ {
public static volatile SingularAttribute<Person,  
Long> id;
public static volatile SingularAttribute<Person,  
String> name;
}
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Order> cq = cb.createQuery(Pet.class);
cq.where(cb.equal(itemNode.get(Pet_.id), 5)) 
.distinct(true);

Annotation Processing
•JSR 269
•Pluggable Annotation Processing API
•Run automatically when javac runs
•Using ServiceLoader to ﬁnd
•META-INF/services/
javax.annotation.processing.Processor
•Running inside JVM

@SupportedSourceVersion(...) 
@SupportedAnnotationTypes(  
"retrofit.http.GET") 
public class MyProcessor extends  
AbstractProcessor {
@Override public synchronized void  
init(ProcessingEnvironment env){ }
@Override public boolean process( 
Set<? extends TypeElement> annotations,  
RoundEnvironment env) { } 
}

@SupportedSourceVersion(...) 
@SupportedAnnotationTypes(...) 
public class MyProcessor extends  
AbstractProcessor {
@Override public synchronized void  
init(ProcessingEnvironment env){ }
RoundEnvironment env) { } 
}

ProcessingEnvironment
•Types
•ElementUtils
•Filer
•Messager

Elements
package foo.bar; 
 
public class Foo { // Type Element
String bar; // Variable Element
public void hoge() { // Executable Element 
System.out.println("Hello!"); 
} 
}

Types
package foo.bar; 
 
public class Foo {
String bar;
public void hoge() {  
System.out.println("Hello!"); 
} 
}

RoundEnvironment env) {
Set<? extends Element> elements =  
env.getElementsAnnotatedWith(GET.cl ass);
// generating code for elements
}

JavaPoet
package foo.bar; 
 
public final class Foo { 
public static void main(String[] args) { 
System.out.println("Hello, JavaPoet!"); 
} 
}

MethodSpec main = MethodSpec.methodBuilder("main") 
.addModifiers(Modifier.PUBLIC, Modifier.STATIC) 
.returns(Void.class) 
.addParameter(String[].class, "args") 
.addStatement("$T.out.println($S)", System.class,  
"Hello, JavaPoet!") 
.build(); 
 
TypeSpec helloWorld =  
TypeSpec.classBuilder("HelloWorld") 
.addModifiers(Modifier.PUBLIC, Modifier.FINAL) 
.addMethod(main) 
.build(); 
 
JavaFile javaFile =  
JavaFile.builder("foo.bar", helloWorld).build(); 
 
javaFile.writeTo(System.out);

Element to JavaPoet
•AnnotationSpec#get
•AnnotationMirror to AnnotationSpec
•MethodSpect#overriding
•Override MethodElement

public interface Foo$$RetryStale {
@GET("/foo/{user}") 
List<Foo> foo(@Path("user") String user); 
 
@GET("/foo/{user}") 
List<Foo> foo$$RetryStale(@Path("user") String user); 
 
}

TypeSpec.Builder typeSpecBuilder = TypeSpec.interfaceBuilder( 
classElement.getSimpleName() + "$RetryStale") 
.addModifiers(Modifier.PUBLIC);
Interface
public interface Foo$$RetryStale

Method
 
MethodSpec.Builder builder =  
MethodSpec.methodBuilder("foo$RetryStale"); 
builder.addModifiers(Modifier.ABSTRACT) 
.addModifiers(Modifier.PUBLIC); 
 
methodElement.getParameters().stream().map(variableElement -> { 
 
ParameterSpec.Builder parameterBuilder =  
ParameterSpec.builder(TypeName.get(variableElement.asType()), 
variableElement.getSimpleName().toString()); 
 
variableElement.getAnnotationMirrors().stream()
.map(AnnotationSpecUtil::generate) 
.forEach(parameterBuilder::addAnnotation); 
 
return parameterBuilder.build(); 
 
}).forEach(builder::addParameter);
List<Foo> foo(@Path("user") String user)

Method
methodElement.getAnnotationMirrors() 
.stream() 
.map(AnnotationSpecUtil::generate) 
.forEach(builder::addAnnotation); 
@GET("/foo/{user}")

Convenient Libraries
•Google Auto-Service
•Truth
•Making your tests and their error
messages more readable and
discoverable
•Google Compile testing

AutoService
@AutoService(Processor.class) 
public class MyProcessor extends AbstracProcessor {
...
}
•Generating META-INF/services/
javax.annotation.processing.Processor

Truth and Compile testing
ASSERT.about(javaSources()) 
.that(ImmutableList.of(inputFile)) 
.processedWith(new MyProcessor()) 
.compilesWithoutError() 
.and() 
.generatesSources(outputFile);

References
•ANNOTATION PROCESSING 101 
http://hannesdorfmann.com/annotation-processing/
annotationprocessing101/
•Annotation Processing Boilerplate
Destruction 
https://speakerdeck.com/jakewharton/annotation-
processing-boilerplate-destruction-square-waterloo-2014

Annotation processing and code gen

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