It would be truly awesome to get this to work! Triple yay! I would keep it as simple as possible though, and not worry too much about being super user friendly. Asking folks to recompile to get this feature is not a big ask, and avoiding complexity like weaving is a big advantage.
Not working for Log4j 1 etc is also not a problem, and may even further our goal to get people to migrate. On Sat, Jul 9, 2022 at 19:39 Gary Gregory <garydgreg...@gmail.com> wrote: > There might be something to reuse from https://projectlombok.org/ > > Gary > > On Sat, Jul 9, 2022, 06:05 Volkan Yazıcı <vol...@yazi.ci> wrote: > > > Inspired by this SO post <https://stackoverflow.com/a/72437386/1278899> > > and > > with some help from Piotr <https://stackoverflow.com/a/72916795/1278899 > >, > > I > > have drafted an example where I redefine a class such that every logger > > call is preceded with a static source location capture. The experiment > aims > > to replace the current source location capture that uses an > exception-based > > expensive mechanism with inlining the source location using bytecode > > weaving. The sources are publicly available on GitHub. > > < > https://github.com/vy/asm-playground/tree/master/src/main/java/com/vlkan> > > In a nutshell, the magic is as follows: > > > > I have a logger library (Log4j.java > > < > > > https://github.com/vy/asm-playground/blob/master/src/main/java/com/vlkan/Log4j.java > > >) > > as follows: > > > > public static final ThreadLocal<SourceLocation> LOCATION_REF = > > ThreadLocal.withInitial(SourceLocation::new); > > > > public static void log() { > > SourceLocation location = LOCATION_REF.get(); > > boolean locationProvided = location.lineNumber > 0; > > if (!locationProvided) { > > StackTraceElement[] stackTraceElements = new > > Throwable().getStackTrace(); > > // Skip the first element pointing to this method. > > StackTraceElement stackTraceElement = stackTraceElements[1]; > > location.init( > > stackTraceElement.getFileName(), > > stackTraceElement.getClassName(), > > stackTraceElement.getMethodName(), > > stackTraceElement.getLineNumber()); > > } > > System.out.format( > > "[%s] %s%n", > > location, > > locationProvided ? "provided location" : "populated > > location"); > > } > > > > Here note how `log()` uses a thread-local to see if there is already a > > `SourceLocation` provided. If so, it leverages that, otherwise it > populates > > the source location using the stack trace of an exception. > > > > Below is my actual application (AppActual.java > > < > > > https://github.com/vy/asm-playground/blob/master/src/main/java/com/vlkan/AppActual.java > > >), > > that is, what the actual/existing user code looks like: > > > > public static void main(String[] args) { > > System.out.println("should log at line 9"); > > log(); > > System.out.println("nothing to see here"); > > System.out.println("should log at line 12"); > > log(); > > f(); > > } > > > > private static void f() { > > System.out.println("adding some indirection"); > > System.out.println("should log at line 19"); > > log(); > > } > > > > I want to transform this into the following expected form > (AppExpected.java > > < > > > https://github.com/vy/asm-playground/blob/master/src/main/java/com/vlkan/AppExpected.java > > >) > > that exploits the `LOCATION_REF` thread-local to inline the source > location > > information: > > > > public static void main(String[] args) { > > System.out.println("should log at line 9"); > > LOCATION_REF.get().init("AppExpected.java", > > "com.vlkan.AppExpected", "main", 9); > > log(); > > System.out.println("nothing to see here"); > > System.out.println("should log at line 12"); > > LOCATION_REF.get().init("AppExpected.java", > > "com.vlkan.AppExpected", "main", 12); > > log(); > > f(); > > } > > > > private static void f() { > > System.out.println("adding some indirection"); > > System.out.println("should log at line 19"); > > LOCATION_REF.get().init("AppExpected.java", > > "com.vlkan.AppExpected", "main", 19); > > log(); > > } > > > > And... 👉 AppTransforming.java > > < > > > https://github.com/vy/asm-playground/blob/master/src/main/java/com/vlkan/AppTransforming.java > > > > > 👈, my dear friends, performs exactly this transformation: converts > > `AppActual` bytecode into `AppExpected`. 😍 > > > > I think we can extend this experiment to implement zero-cost source > > location capture for Log4j. Though I will appreciate your help on some > > loose ends. Assuming we have a bullet-proof mechanism to inline source > > location capture given a class, what is the right way to ship this? As a > > Maven plugin that kicks in at compile time? Wouldn't that make this > feature > > impossible to use without recompiling user sources? As a runtime utility? > > If so, what about the cost of classpath scanning & weaving? If the > bytecode > > weaving only intercepts at Log4j API calls, this won't work for Log4j 1 > > bridge, SLF4J, or any other indirect access to the Log4j API. What do you > > think? I have used a thread-local to pass the source location to the > > caller, is there a better alternative? (Putting aside the dynamic-scoped > > variables to be shipped with Loom.) > > >
