JEP 158: Unified JVM Logging

AuthorStaffan Larsen
Discussionserviceability dash dev at openjdk dot java dot net
Endorsed-byMikael Vidstedt


Introduce a common logging system for all components of the JVM.


Stretch goals:


It is outside the scope of this JEP to add the actual logging calls from all JVM components. This JEP will only provide the infrastructure to do the logging.

It is also outside the scope of the JEP to enforce a logging format, apart from the format of the decorations and the use of human-readable plain text.

This JEP will not add logging to Java code in the JDK.


The JVM is complex system-level component where root-cause analysis is often a difficult and time-consuming task. Without extensive serviceability features it is often close to impossible to find the root cause of intermittent crashes or performance quirks in a production environment. Fine-grained, easy-to-configure JVM logging available for use by support and sustaining engineering is one such feature.

JRockit has this feature and it has been instrumental in providing support to customers. The proposed design is heavily influenced by what exists in JRockit.



The logging framework defines a set of components in the JVM. Each component is identified by its name (for example: gc, compiler, threads, etc). The set of components can be changed in the source code as required. A component does not have to correspond to a particular part of the JVM, but could also identify a topic of interest (for example gcpause or inlining). Logging can be enabled or disabled for a specific component, and logging can be redirected to file based on the component.


Each log message has a logging level associated with it. The available levels are error, warning, info, debug and trace in increasing order of verbosity. For each component, a logging level can be selected to control the amount of information from that component.


Logging messages can be decorated with information about the message. Here is a list of the possible decorations:

The active set of decorations can be controlled by the user. The same active set is used for all components. The default set of decorations is: component, level, and timestamp since start of VM. Decorations will be prepended to the log message:

[gc][info][6.456s] Old collection complete

Command-line options

There will be single set of command-line options to control logging from all components of the JVM. The following command-line options exists:


This will set the logging level of the specified components. By default, each enabled component has a logging level of info. There is a meta-component called all to simplify the enabling of all components.


This will redirect the output from a component to the specified filename.


This sets the active decorations for all log messages.

File rotation

When redirecting logging to a file, file rotation will be implemented in a fashion similar to the current GC log files.

Enabling at runtime

Logging can be enabled at runtime through Diagnostic Commands (the jcmd utility). Everything that can be specified on the command line can also be specified dynamically with Diagnostic Commands.

JVM interface

In the JVM a new Log class will be created with an API with functionality similar to:

Log::warning(LogComponent comp, const char* format_string, ...)
Log::info(LogComponent comp, const char* format_string, ...)
Log::debug(LogComponent comp, const char* format_string, ...)

Log::message(LogComponent comp, LogLevel level,
             const char* format_string, ...)

bool Log::enabled(LogLevel level, LogComponent comp)
LogLevel Log::getLevel(LogComponent comp)


The different log levels will have guidelines that define the expected performance overhead for the level. For example: “error and warning level shouldn’t affect performance; info level should be acceptable for production; debug and trace levels do not have performance requirements.”

Backward compatibility

The JVM already has a number of different flags to control logging. It is a goal to continue to support these as much as possible (at least for some time). The current flags should be mapped to the new flags as much as possible. In some cases this may not be possible and we will have to consider them on a case-by-case basis.

Future extensions

In the future, it may make sense to add a Java API for writing log messages to this infrastructure, for use from classes in the JDK.

Initially, only two backends will be developed: stderr and file. Future projects could add other backends. For example: syslog, Windows Event Viewer, socket, etc.

Open issues


It is extremely important that logging in itself does not cause any instabilities, thus extensive testing is required.

Functional testing will have to be done by enabling certain log messages and checking for their presence on stderr or files.

Because it will be possible to dynamically enable logging, we need to stress test this by continuously enabling and disabling logging while running applications.

Risks and Assumptions

The design outlined above may not satisfy all uses of logging in the JVM today. If that is the case, the design will have to be revisited.