This AppTransforming class
<https://github.com/vy/asm-playground/blob/master/src/main/java/com/vlkan/v2/AppTransforming.java>,
transforms the following
private static final Logger LOGGER = LogManager.getLogger();
public static void main(String[] args) {
LOGGER.info("should log at line 11");
System.out.println("nothing to see here");
LOGGER.info("should log at line 13");
f();
}
private static void f() {
System.out.println("adding some indirection");
LOGGER.info("should log at line 19");
}
to this:
private static final Logger LOGGER = LogManager.getLogger();
public static void main(String[] args) {
LOGGER.atInfo().withLocation(Log4jLocationRegistry.get(0)).log("should log
at line 11");
System.out.println("nothing to see here");
LOGGER.atInfo().withLocation(Log4jLocationRegistry.get(1)).log("should log
at line 13");
f();
}
private static void f() {
System.out.println("adding some indirection");
LOGGER.atInfo().withLocation(Log4jLocationRegistry.get(2)).log("should log
at line 19");
}
My next step will be to run this at compile time. (Will keep you posted.)
Please note the specialization on `Logger#info(String)` call in
`AppTransforming`. Since Ralph did not like thread-locals, we need to
implement every such possible specialization of the public API.
On Sat, Jul 9, 2022 at 12:04 PM Volkan Yazıcı <[email protected]> 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.)
>
