FULL PRODUCT VERSION :
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) Client VM (build 25.20-b23, mixed mode, sharing)

javac 1.8.0_20

ADDITIONAL OS VERSION INFORMATION :
Windows 7 Home Premium 64-bit
Microsoft Windows [Version 6.1.7601]

EXTRA RELEVANT SYSTEM CONFIGURATION :
Using 32-bit JDK

A DESCRIPTION OF THE PROBLEM :
The API Javadoc states:

javax.annotation.processing.Messager

printMessage(Diagnostic.Kind kind, CharSequence msg, Element e, AnnotationMirror a, AnnotationValue v)
Prints a message of the specified kind at the location of the annotation value inside the annotation mirror of the annotated element.

However the location reported is the entire annotation, not just one value within it. If run from the command line, javac positions the caret (^) pointing at the start of the annotation. If run through ToolProvider, the position indexes in the resulting Diagnostic object encompass the whole annotation, not just one value.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Call javax.annotation.processing.Messager.printMessage(Diagnostic.Kind kind, CharSequence msg, Element e, AnnotationMirror a, AnnotationValue v) and observe the location associated with the resulting Diagnostic object or console output.

Run with compiler interface:
javac *.java
java Run

Run from command line:
javac *.java
javac -processor Proc Test.java


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
From compiler interface Run.java expected:
21, 21, 34

From command line expected (requires fixed width font):

Found annotated element: i
  Found annotation mirror: @Anno(something="x")
    Found value: "x"
Test.java:2: error: Here's the value
        @Anno(something="x")
              ^
1 error


ACTUAL -
From compiler interface:
15, 15, 35

From command line:

Found annotated element: i
  Found annotation mirror: @Anno(something="x")
    Found value: "x"
Test.java:2: error: Here's the value
        @Anno(something="x")
        ^
1 error

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
----- Anno.java -----
public @interface Anno {
String something();
}

----- Test.java -----
class Test {
@Anno(something="x")
int i;
}

----- Proc.java -----
import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.tools.Diagnostic.Kind;
import javax.lang.model.element.*;
import java.util.*;

@SupportedAnnotationTypes("Anno")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class Proc extends AbstractProcessor {
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(Anno.class);
for (Element el : elements) {
System.out.println("Found annotated element: " + el);
for (AnnotationMirror mir : el.getAnnotationMirrors()) {
System.out.println(" Found annotation mirror: " + mir);
AnnotationValue val = mir.getElementValues().values().iterator().next();
System.out.println(" Found value: " + val);
processingEnv.getMessager().printMessage(Kind.ERROR, "Here's the value",
el, mir, val);
}
}
return true;
}
}

----- Run.java -----
import java.util.*;
import javax.tools.*;

public class Run {
public static void main(String[] args) {
JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMan = javac.getStandardFileManager(null, null, null);
DiagnosticCollector<JavaFileObject> diag = new DiagnosticCollector<>();
Iterable<? extends JavaFileObject> files = fileMan.getJavaFileObjects("Test.java");
JavaCompiler.CompilationTask task = javac.getTask(null, fileMan, diag, null, null, files);
task.setProcessors(Arrays.asList(new Proc()));
task.call();
Diagnostic<?> d = diag.getDiagnostics().get(0);
System.out.println(d.getStartPosition() + ", " + d.getPosition() + ", " + d.getEndPosition());
}
}
---------- END SOURCE ----------