Starting with v2.2, flattr4j logs the communication with the Flattr server. The java.util.logging API is used for logging.

By default, an API call is logged with INFO level. If the call should fail, an error is logged with WARNING level. These levels are enabled by default, so you should see the respective logging messages on your logging device.

Details of the API call (like URLs, parameters and JSON responses) are also logged with the FINE level. It is usually not necessary for you to see them, unless you want to debug flattr4j or want to find out more about why an API call actually failed.

By default, java.util.logging does not write FINE level to the logging device. There are several ways to enable FINE level logging, but actually none of them are really fine. For example, you can create a file named logging.properties with the following content:

org.shredzone.flattr4j.level = ALL

Now invoke your Java VM like this:

java -Djava.util.logging.config.file=path/to/logging.properties ...

Sadly, if you just place a logging.properties file into your classpath, it is ignored by java.util.logging.

Logging on Android

Since flattr4j v2.3, all log output is logged via android.util.Log on Android environments. Depending on your configuration, you will see some or all of the log levels on logcat.

For debugging purposes, you can enable verbose logging by executing this command:

adb shell setprop log.tag.flattr4j VERBOSE

At flattr4j v2.2, java.util.logging was also used on Android, which means that log messages at FINE level were filtered out and not shown on logcat. If you need FINE level output on Android, make sure you’re using at least flattr4j v2.3.

Why java.util.logging instead of log4j or slf4j?

I know that even though the API of java.util.logging is quite fine, configuring it can be a real PITA. Other logging frameworks like log4j and slf4j are widespread and easier to use.

However, I decided to use java.util.logging because it is the only API that is common both to Java and Android. The alternative was to use android.util.Log on Android, and a log framework on Java. The implementation would be too complex just for the simple purpose of logging a few lines.

If you use slf4j in your project, there is a jul to slf4j bridge that routes the java.util.logging output to slf4j.