RFR (L): 8046148: JEP 158 Unified JVM Logging

Marcus Larsson marcus.larsson at oracle.com
Mon Sep 7 13:33:04 UTC 2015


Please review the following patch adding the unified logging framework 
to hotspot.



See the JEP description for a general overview of the new feature. Below 
are some notes on the implementation.

The patch adds the new 'share/vm/logging' subdirectory containing the 
unified logging framework. The main entry point is log.hpp, which 
contains the necessary macros and definitions to use the framework.

Log tags are defined/listed in logTag.hpp, and are passed as template 
arguments to the Log class. Every combination of tags used in a log call 
has a corresponding LogTagSet instance, which keeps a track of all the 
outputs it should write the log message to (and their levels). Having 
tags as template arguments allows mapping directly from a set of tags to 
the LogTagSet instance, which means that the overhead for disabled 
logging should be low. Currently each log message can be tagged with up 
to 5 tags, but this can be increased if ever required (and with C++11's 
variadic templates the limit can be removed completely).

The LogConfiguration class keeps track of configured outputs (stdout, 
stderr, and possible file outputs). Configuration is done either by 
command line arguments (-Xlog) or by DCMD. Both methods will in turn use 
the LogConfiguration class to perform the parsing & configuration. This 
configuration includes iterating over all LogTagSet instances and 
updating them accordingly. The LogTagLevelExpression class is used to 
represent the selection of tags and levels for a given configuration 
request (the "what"-expression).

The LogDecorators class contains a selection of decorators. Instances of 
this class is kept in LogTagSet to track what decorators to use (this is 
the union of all decorators used by its outputs). Each log call will 
create a LogDecorations instance (note: different classes), which will 
contain the actual decoration strings for the log message. These 
decorations are used for each output the tagset is set to log on, and 
are then discarded.

The LogPrefix class allows messages of specific sets of tags to be 
prefixed. The prefix should supply a printf-style format with argument. 
(This allows GC logging to prefix messages of certain tagsets with 
GCId.) Prefixes are implemented using template specializations based on 
the specified tags, with the general/unspecialized case giving an empty 

The LogOutput class defines the interface for all types of log outputs. 
LogFileStreamOutput corresponds to FILE* stream based log outputs. 
LogFileOutput builds on this and adds the file management and log 
rotation support.

A simple jtreg test is included in this patch. Additional tests will be 
added at a later stage.


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