Run the following program with JDK 6:

---%<---
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Writer;
import java.util.Collections;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic.Kind;
import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.StandardLocation;
import javax.tools.ToolProvider;
public class Demo {
    public static void main(String[] args) {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        System.err.println("using " + compiler.getClass() + " from " + compiler.getClass().getProtectionDomain().getCodeSource());
        CompilationTask task = compiler.getTask(null, null, null, Collections.singleton("-proc:only"), Collections.singleton("java.lang.Object"), null);
        task.setProcessors(Collections.singleton(new Proc()));
        System.err.println("success? " + task.call());
        task = compiler.getTask(null, null, null, Collections.singleton("-proc:only"), Collections.singleton("java.lang.Object"), null);
        task.setProcessors(Collections.singleton(new Proc()));
        System.err.println("success? " + task.call());
    }
    @SupportedAnnotationTypes("*")
    @SupportedSourceVersion(SourceVersion.RELEASE_6)
    private static class Proc extends AbstractProcessor {
        int count;
        @Override public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
            if (roundEnv.processingOver() || count++ > 0) {
                return false;
            }
            System.err.println("running Proc");
            try {
                processingEnv.getMessager().printMessage(Kind.NOTE, "found previous content of length " +
                        processingEnv.getFiler().getResource(StandardLocation.SOURCE_OUTPUT, "p", "C.java").getCharContent(false).length());
            } catch (FileNotFoundException x) {
                processingEnv.getMessager().printMessage(Kind.NOTE, "not previously there");
            } catch (IOException x) {
                processingEnv.getMessager().printMessage(Kind.ERROR, "while reading: " + x);
            }
            try {
                Writer w = processingEnv.getFiler().createSourceFile("p.C").openWriter();
                w.write("/* hello! */ package p; class C {}");
                w.close();
                processingEnv.getMessager().printMessage(Kind.NOTE, "wrote new content");
            } catch (IOException x) {
                processingEnv.getMessager().printMessage(Kind.ERROR, "while writing: " + x);
            }
            return true;
        }
    }
    private Demo() {}
}
---%<---

I get:

---%<---
using class com.sun.tools.javac.api.JavacTool from (file:/.../jdk1.6.0_26/lib/tools.jar <no signer certificates>)
running Proc
Note: not previously there
Note: wrote new content
success? true
running Proc
Note: found previous content of length 34
Note: wrote new content
success? true
---%<---

as expected. Now running the same program on JDK 7:

---%<---
using class com.sun.tools.javac.api.JavacTool from (file:/.../jdk1.7.0-b146/lib/tools.jar <no signer certificates>)
running Proc
warning: Supported source version 'RELEASE_6' from annotation processor 'Demo$Proc' less than -source '1.7'
Note: not previously there
Note: wrote new content
1 warning
success? true
running Proc
warning: Supported source version 'RELEASE_6' from annotation processor 'Demo$Proc' less than -source '1.7'
Note: not previously there
Note: wrote new content
1 warning
success? true
---%<---

Note that Filer.getResource(SOURCE_OUTPUT, ...) is throwing FileNotFoundException even though the source file is in fact present.