extending-logging.md

Logging

The Java buildpack logs all messages, regardless of severity to <app dir>/.java-buildpack.log. It also logs messages to $stderr, filtered by a configured severity.

If the buildpack fails with an exception, the exception message is logged with a log level of ERROR whereas the exception stack trace is logged with a log level of DEBUG to prevent users from seeing stack traces by default.

Sensitive Information in Logs

The Java buildpack logs sensitive information, such as environment variables which may contain security credentials.

You should be careful not to expose this information inadvertently, for example by posting standard error stream contents or the contents of <app dir>/.java-buildpack.log to a public discussion list.

Logger Usage

The JavaBuildpack::Logging::LoggerFactory class manages instances that meet the contract of the standard Ruby Logger. In normal usage, the Buildpack class configures the LoggerFactory. Logger instances are then retrieved for classes that require them:

@logger = JavaBuildpack::Logging::LoggerFactory.get_logger DownloadCache

This logger is used like the standard Ruby logger and supports both parameter and block forms:

logger.info('success')
logger.debug { "#{costly_method}" }

Configuration

For general information on configuring the buildpack, refer to Configuration and Extension.

The console logging severity filter is set to DEBUG, INFO, WARN, ERROR, FATAL using the following strategies in descending priority:

1. $JBP_LOG_LEVEL environment variable. This can be set using the cf set-env <app name> JBP_LOG_LEVEL DEBUG command.
2. Ruby --verbose and --debug flags. Setting either of these is the equivalent of setting the log severity level to DEBUG.
3. default_log_level value in config/logging.yml.
4. Fallback to INFO if none of the above are set.