`System.Logger` formatting issue

[mechite]

Hi,

I have not tested this issue extensively and it could be the issue of my logging implementation.
I have a System.LoggerFinder implementation, which correctly uses MessageFormat akin to the official JDK one, and experiences this issue.

In the following file/line:

httpserver/src/main/java/robaho/net/httpserver/ServerImpl.java

Line 772 in 7ed4d26

logger.log(Level.TRACE,() -> "protocol "+protocol+" uri "+uri+" headers "+headers);

the log entry seems to send {} somewhere in the log, presumably some part of the request?

[27/12/2025 23:46:10 | Tracing | avaje-jex-http-21 | robaho.net.httpserver.-190685534] closing connection: [80.57725]
[27/12/2025 23:46:10 | Tracing | avaje-jex-http-21 | robaho.net.httpserver] closing connection: remote /127.0.0.1:57725
[27/12/2025 23:46:10 | Tracing | avaje-jex-http-21 | robaho.net.httpserver.-190685534] exchange finished [80.57725]
[27/12/2025 23:46:10 | Tracing | avaje-jex-http-22 | robaho.net.httpserver.-190685534] accepted connection: Socket[addr=/127.0.0.1,port=57726,localport=80]
[27/12/2025 23:46:10 | Tracing | avaje-jex-http-22 | robaho.net.httpserver.-190685534] reading request
[27/12/2025 23:46:10 | Tracing | avaje-jex-http-22 | robaho.net.httpserver.-190685534] exchange started [80.57726]
[27/12/2025 23:46:10 | Diagnosis | avaje-jex-http-22 | robaho.net.httpserver.-190685534] Exchange request line: GET / HTTP/1.1
[27/12/2025 23:46:10 | Warning | avaje-jex-http-22 | robaho.net.httpserver.-190685534] ServerImpl unexpected exception
java.lang.IllegalArgumentException: can't parse argument number:  {}
        at java.base/java.text.MessageFormat.setFormatFromPattern(MessageFormat.java:1644)
        at java.base/java.text.MessageFormat.applyPatternImpl(MessageFormat.java:660)
        at java.base/java.text.MessageFormat.<init>(MessageFormat.java:516)
        ...
        at robaho.httpserver/robaho.net.httpserver.ServerImpl$Exchange.runPerRequest(ServerImpl.java:772)
        at robaho.httpserver/robaho.net.httpserver.ServerImpl$Exchange.run(ServerImpl.java:637)
        at robaho.httpserver/robaho.net.httpserver.ServerImpl$Dispatcher.acceptConnection(ServerImpl.java:426)
        at robaho.httpserver/robaho.net.httpserver.ServerImpl$Dispatcher.lambda$run$0(ServerImpl.java:349)
        at java.base/java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.run(ThreadPerTaskExecutor.java:291)
        at java.base/java.lang.VirtualThread.run(VirtualThread.java:462)
Caused by: java.lang.NumberFormatException: For input string: " {} "
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
        at java.base/java.lang.Integer.parseInt(Integer.java:565)
        at java.base/java.lang.Integer.parseInt(Integer.java:662)
        at java.base/java.text.MessageFormat.setFormatFromPattern(MessageFormat.java:1642)
        ... 13 more

This appears to clash with MessageFormat for me? The IllegalArgumentException appears to prevent my application from starting.
Removing robaho/httpserver from the module-path has my application working again.

---

I believe this could be solved if we leverage the formatter.
(and I don't think we need to use the Supplier lambda, if using formatter, as it won't call toString if level disabled)

- logger.log(Level.TRACE,() -> "protocol "+protocol+" uri "+uri+" headers "+headers);
+ logger.log(Level.TRACE, "protocol {0} uri {1} headers {2}", protocol, uri, headers);

^{CC @SentryMan}

[robaho]

Hi @mechite I suspect your LoggerFinder implementation is incorrect. I think you are incorrectly interpreting the log message (as String) as using formats - I expect that '{}' is in the headers - and it needs be escaped before sending to MessageFormat.

If you review the api, there is no Supplier based call that takes a format and arguments.

edit: actually, I don't think you should be passing it to message format at all if it is a pure String - there would never be any arguments to add.

[mechite]

I don't think you should be passing it to message format at all if it is a pure String

Indeed this is not the case in my implementation, it is always formatting.
Looks like it can be fixed, so I will fix it, test again, and send an update here.

Looking at jdk.internal.logger.SimpleConsoleLogger, assuming that this is the default implementation, it:

• has params == null || params.length == 0 check, preventing formatting without arguments
• has a check in #formatMessage:

               // Is it a java.text style format?
               // Ideally we could match with
               // Pattern.compile("\\{\\d").matcher(format).find())
               // However the cost is 14% higher, so we cheaply check for
               //
               boolean isJavaTestFormat = false;
               final int len = format.length();
               for (int i=0; i<len-2; i++) {
                   final char c = format.charAt(i);
                   if (c == '{') {
                       final int d = format.charAt(i+1);
                       if (d >= '0' && d <= '9') {
                           isJavaTestFormat = true;
                           break;
                       }
                   }
               }
               if (isJavaTestFormat) {
                   return java.text.MessageFormat.format(format, parameters);
               }

  ...which also is a fail-safe to prevent failed formatting.
• the above method also has a catch (Exception ex) that will always return the format string if an Exception caused the failure.

I don't think that this behaviour was specified anywhere?
I will base my implementation around theirs.

[robaho]

Since there is no way to use parameters with the Supplier format, I suspect that checking if not parameters are available is the cheapest check.