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  • ONIP-3: Allow User Supplied Logging Capabilities
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Document statusDRAFT
Document owner

Jeff Zemerick


OpenNLP uses System.out() and System.err() by default for logging. This is fine for instances in which OpenNLP's CLI tools are being used but may not be ideal when OpenNLP is used as a library. Previous work in this area includes  OPENNLP-675 - Absence of logging and usage of System.out Closed . This proposal presents a method for allowing the user to customize the default logging behavior when OpenNLP is used as a library.

Current State

As discussed above, OpenNLP currently (as of 1.7.2) defaults to using System.out() and System.err() for logging messages and errors, respectively.

The Problem

When using OpenNLP as a library the output of logs to standard out and standard error is not ideal as these logs often need to be captured by the application for external storage and reporting (and also just to keep from cluttering up standard out).

Proposed Solution

The proposed solution is to create an OpenNLP Logger interface that developers can implement to customize the logging. The user can provide their own implementation of this interface to control OpenNLP's logging. This interface will have to exist in a new project (perhaps opennlp-model?) in order to avoid circular dependencies. (The opennlp-tools project will have a dependency on this project. The user's project can either have an explicit dependency on opennlp-model or a transitive dependency based on the project's requirements.)

The naming of objects in this proposal is mainly for illustrative purposes. I'm not proposing any specific naming and am open to naming suggestions.

An example of the logging interface is:

The default implementation used when the user has not provided an implementation of Logger will be:

The DefaultLogger class mimics the current behavior of OpenNLP of using System.out().


A new project contains the Logger interface to avoid circular dependencies.


A new class in OpenNLP would be created that stores a static reference to the Logger implementation. By default this static variable would reference the DefaultLogger. The user can set their own Logger implementation at any time through the setLogger() function.

Existing logging statements in OpenNLP would be modified to perform the logging via the LoggingConfiguration class. For example, from NameSampleCountersStream:



This proposal:

  • Presents a way to let users of OpenNLP as a library to control logging.
  • Requires:
    • A new project that contains a Logger interface.
    • Modifying current System.out() and System.err() calls to reference the new LoggerConfiguration class.

Future Work

A similar approach could be taken for functions that expect a PrintStream object.