![]() ![]() ![]() Unless wildcard (* ) is specified, only log messages tagged withĮxactly the tags specified will be matched. Where 'what' is a combination of tags and levels on the form Here's how to put all debug messages in the file application.log: file = : Defines a file to pipe all messages into.On the console that is the terminal window unless redirected, in IDEs it is usually shown in the same tab/view as stdout but printed red. On the console that is the terminal window unless redirected, in IDEs it is often shown in a separate tab or view. It comes after the selectors (separated by a colon) and has three possible locations: selectively switch to lower log levels with -Xlog:tag_1* =debugĬompared to the non-trivial selectors, the output configuration is really simple.use them with -Xlog:tag_1*,tag_2*,tag_n* to display all info messages that were tagged with any of them.find interesting tags in the output of java -Xlog:help.Using selectors, there are three easy steps to get to know a subsystem of the JVM: # truncated a few messages showing final GC statistics Maximum heap 4192206848 # truncated about two dozen message > Heap $ java -Xlog:gc* =debug -version > Heap region size: 1M Hence, using gc (for garbage collection) versus gc+heap, for example, should select different messages. Still, a message has to contain exactly those to be selected. Yes, a selector can name several tags by concatenating them with +. You don't have to take that route, though, -Xlog:help shows the same information but more beautifully formatted (see below).Ī little surprising (at least at first) is the detail that messages only match a selector if their tags exactly match the given ones. I had to truncate the output but trust me, there's a lot of helpful information in those messages. > logging: Logging for the log framework itself > Available log decorators: > Available log tags: > Described tag combinations: $ java -Xlog:logging =debug -version > Log configuration fully initialized. One of the first messages tells us that the HotSpot virtual machine begins its work: Simply execute java -Xlog (maybe append -version to get rid of the helpful but long display of command line options) and have a look at the output - of which there is a lot. ![]() We'll look at each of them in turn, but before doing that, we can have a look at the kind of messages -Xlog produces. which output to use (for example a file).which information to include (for example time stamps and process IDs).which messages to log (by tag and/or by log level).This is the only flag regarding this mechanism - any further configuration is immediately appended to that option. Logging can be activated with the java option -Xlog. ![]() The unified logging infrastructure is very similar to known logging frameworks The logging output can be configured according to your needs. It generates textual messages, attaches some meta information like tags (describing the originating subsystem), a log level (describing the importance of the message), and time stamps before printing them somewhere. The JVM-internal, unified logging infrastructure is very similar to known logging frameworks like Log4j or Logback that you might have used for your application. The -Xlog option can be a bit intimidating, so in this post we will master it step by step, learning how to use it to select which messages and information to show. This gives uniform access to log messages from different subsystems such as class loading, threading, the garbage collector, the module system, or the interaction with the underlying operating system. Java 9 comes with a unified logging architecture ( JEP 158) that pipes a lot of messages that the JVM generates through the same mechanism, which can be configured with the -Xlog option. ![]()
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