After digging into the Java SE 8 Final Specification I’m able to answer my questions.
(1) In response to my first question
Is there any way to annotate a lambda expression similar to annotating
a corresponding anonymous class, so one gets the expected “Hello
World” output in the example above?
No.
When annotating the Class Instance Creation Expression (ยง15.9) of an anonymous type, then the annotation will be stored in the class file either for the extending interface or the extending class of the anonymous type.
For the following anonymous interface annotation
Consumer<String> c = new @MyTypeAnnotation("Hello ") Consumer<String>() {
@Override
public void accept(String str) {
System.out.println(str);
}
};
the type annotation can then be accessed at runtime by calling Class#getAnnotatedInterfaces():
MyTypeAnnotation a = c.getClass().getAnnotatedInterfaces()[0].getAnnotation(MyTypeAnnotation.class);
If creating an anonymous class with an empty body like this:
class MyClass implements Consumer<String>{
@Override
public void accept(String str) {
System.out.println(str);
}
}
Consumer<String> c = new @MyTypeAnnotation("Hello ") MyClass(){/*empty body!*/};
the type annotation can also be accessed at runtime by calling Class#getAnnotatedSuperclass():
MyTypeAnnotation a = c.getClass().getAnnotatedSuperclass().getAnnotation(MyTypeAnnotation.class);
This kind of type annotation is not possible for lambda expressions.
On a side note, this kind of annotation is also not possible for normal class instance creation expressions like this:
Consumer<String> c = new @MyTypeAnnotation("Hello ") MyClass();
In this case, the type annotation will be stored in the method_info structure of the method, where the expression occurred and not as an annotation of the type itself (or any of its super types).
This difference is important, because annotations stored in the method_info will not be accessible at runtime by the Java reflection API. When looking at the generated byte code with ASM, the difference looks like this:
Type Annotation on an anonymous interface instance creation:
@Java8Example$MyTypeAnnotation(value="Hello ") : CLASS_EXTENDS 0, null
// access flags 0x0
INNERCLASS Java8Example$1
Type Annotation on a normal class instance creation:
NEW Java8Example$MyClass
@Java8Example$MyTypeAnnotation(value="Hello ") : NEW, null
While in the first case, the annotation is associated with the inner class, in the second case, the annotation is associated with the instance creation expression inside the methods byte code.
(2) In response to the comment from @assylias
You can also try (@MyTypeAnnotation(“Hello “) String s) ->
System.out.println(s) although I have not managed to access the
annotation value…
Yes, this is actually possible according to the Java 8 specification. But it is not currently possible to receive the type annotations of the formal parameters of lambda expressions through the Java reflection API, which is most likely related to this JDK bug: Type Annotations Cleanup. Also the Eclipse Compiler does not yet store the relevant Runtime[In]VisibleTypeAnnotations attribute in the class file – the corresponding bug is found here: Lambda parameter names and annotations don’t make it to class files.
(3) In response to my second question
In the example, where I did cast the lambda expression and annotated
the casted type: Is there any way to receive this annotation instance
at runtime, or is such an annotation always implicitly restricted to
RetentionPolicy.SOURCE?
When annotating the type of a cast expression, this information also gets stored in the method_info structure of the class file. The same is true for other possible locations of type annotations inside the code of a method like e.g. if(c instanceof @MyTypeAnnotation Consumer). There is currently no public Java reflection API to access these code annotations. But since they are stored in the class file, it is at least potentially possible to access them at runtime – e.g. by reading the byte code of a class with an external library like ASM.
Actually, I managed to get my “Hello World” example working with a cast expression like
testTypeAnnotation(list,(@MyTypeAnnotation("Hello ") Consumer<String>) (p -> System.out.println(p)));
by parsing the calling methods byte code using ASM. But the code is very hacky and inefficient, and one should probably never do something like this in production code. Anyway, just for completeness, here is the complete working “Hello World” example:
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import org.objectweb.asm.AnnotationVisitor;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.TypePath;
import org.objectweb.asm.TypeReference;
public class Java8Example {
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE_USE)
public @interface MyTypeAnnotation {
public String value();
}
public static void main(String[] args) {
List<String> list = Arrays.asList("World!", "Type Annotations!");
testTypeAnnotation(list, new @MyTypeAnnotation("Hello ") Consumer<String>() {
@Override
public void accept(String str) {
System.out.println(str);
}
});
list = Arrays.asList("Type-Cast Annotations!");
testTypeAnnotation(list,(@MyTypeAnnotation("Hello ") Consumer<String>) (p -> System.out.println(p)));
}
public static void testTypeAnnotation(List<String> list, Consumer<String> consumer){
MyTypeAnnotation annotation = null;
for (AnnotatedType t : consumer.getClass().getAnnotatedInterfaces()) {
annotation = t.getAnnotation(MyTypeAnnotation.class);
if (annotation != null) {
break;
}
}
if (annotation == null) {
// search for annotated parameter instead
loop: for (Method method : consumer.getClass().getMethods()) {
for (AnnotatedType t : method.getAnnotatedParameterTypes()) {
annotation = t.getAnnotation(MyTypeAnnotation.class);
if (annotation != null) {
break loop;
}
}
}
}
if (annotation == null) {
annotation = findCastAnnotation();
}
for (String str : list) {
if (annotation != null) {
System.out.print(annotation.value());
}
consumer.accept(str);
}
}
private static MyTypeAnnotation findCastAnnotation() {
// foundException gets thrown, when the cast annotation is found or the search ends.
// The found annotation will then be stored at foundAnnotation[0]
final RuntimeException foundException = new RuntimeException();
MyTypeAnnotation[] foundAnnotation = new MyTypeAnnotation[1];
try {
// (1) find the calling method
StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace();
StackTraceElement previous = null;
for (int i = 0; i < stackTraceElements.length; i++) {
if (stackTraceElements[i].getMethodName().equals("testTypeAnnotation")) {
previous = stackTraceElements[i+1];
}
}
if (previous == null) {
// shouldn't happen
return null;
}
final String callingClassName = previous.getClassName();
final String callingMethodName = previous.getMethodName();
final int callingLineNumber = previous.getLineNumber();
// (2) read and visit the calling class
ClassReader cr = new ClassReader(callingClassName);
cr.accept(new ClassVisitor(Opcodes.ASM5) {
@Override
public MethodVisitor visitMethod(int access, String name,String desc, String signature, String[] exceptions) {
if (name.equals(callingMethodName)) {
// (3) visit the calling method
return new MethodVisitor(Opcodes.ASM5) {
int lineNumber;
String type;
public void visitLineNumber(int line, Label start) {
this.lineNumber = line;
};
public void visitTypeInsn(int opcode, String type) {
if (opcode == Opcodes.CHECKCAST) {
this.type = type;
} else{
this.type = null;
}
};
public AnnotationVisitor visitInsnAnnotation(int typeRef, TypePath typePath, String desc, boolean visible) {
if (lineNumber == callingLineNumber) {
// (4) visit the annotation, if this is the calling line number AND the annotation is
// of type MyTypeAnnotation AND it was a cast expression to "java.util.function.Consumer"
if (desc.endsWith("Java8Example$MyTypeAnnotation;") && this.type != null && this.type.equals("java/util/function/Consumer")) {
TypeReference reference = new TypeReference(typeRef);
if (reference.getSort() == TypeReference.CAST) {
return new AnnotationVisitor(Opcodes.ASM5) {
public void visit(String name, final Object value) {
if (name.equals("value")) {
// Heureka! - we found the Cast Annotation
foundAnnotation[0] = new MyTypeAnnotation() {
@Override
public Class<? extends Annotation> annotationType() {
return MyTypeAnnotation.class;
}
@Override
public String value() {
return value.toString();
}
};
// stop search (Annotation found)
throw foundException;
}
};
};
}
}
} else if (lineNumber > callingLineNumber) {
// stop search (Annotation not found)
throw foundException;
}
return null;
};
};
}
return null;
}
}, 0);
} catch (Exception e) {
if (foundException == e) {
return foundAnnotation[0];
} else{
e.printStackTrace();
}
}
return null;
}
}