Status: In Progress
Affects Version/s: None
Fix Version/s: 3.0.0
The existing plugin system revolves around @Plugin annotations which group together configurable interfaces into categories. The category of a plugin typically determines the configuration and instantiation strategy for the plugin classes involved which leads to some ad hoc methods for different plugins to obtain or configure different aspects of the system. All classes that are included in the plugin system can be queried at runtime by constructing a PluginManager with the category name as its constructor argument. To date, there are a few plugin categories (case-insensitive):
- Core: used for configuration elements in config files for general core plugins like appenders, filters, layouts, etc. These are provided values from parsed configuration nodes and can also obtain the Configuration instance to access other services.
- Level: used for specifying custom log levels so that they can be instantiated early enough for configurations to make use of it.
- ConfigurationFactory: used for parsing and validating configuration sources in some format into a configuration node tree which is transformed into a Configuration. Also useful for programmatic configuration.
- Lookup: used for variable interpolation in strings via StrLookup classes.
- TypeConverter: used for converting strings into other objects. Mostly useful for conversion of plugin attributes into common types along with use in certain core plugins such as database column mappers.
- Converter: used for pattern layout conversion keys. These are used for formatting log event info into a layout.
- Watcher: used for watching configuration sources for changes in order to allow for reconfiguration.
Then there are some system-wide plugin-like classes that can be specified via system properties including:
- ClockFactory: controls Clock instances for obtaining the current time. Mostly useful in testing and scenarios where approximate time is more useful than exact time.
- LogEventFactory: constructs LogEvent instances for a given log message and parameters. Useful for implementing different allocation strategies for log events.
- MergeStrategy: used for composite configurations.
- ContextSelector: used for obtaining a LoggerContext for an invocation context when initializing or obtaining Logger instances. By default, this maps contexts to class loaders, and additional strategies are available including async loggers, a singleton context, a JNDI-lookup-based context, and an OSGi bundle context.
- LoggerContextFactory: used for binding a logging implementation to the Logging API.
- ShutdownCallbackRegistry: used for registering a logging system cleanup shutdown callback in various systems. The default strategy hooks in to the Runtime shutdown threads API.
- ContextDataInjector: used for adding initial context data into the ThreadContext of a LogEvent.
This system grew organically over time, and after identifying various duplicated plugin functionality around the codebase, the plugin system should be overhauled along the lines of a more standard @Inject-style dependency injection framework. By adopting more conventional DI patterns, this will remove the need for ad hoc plugin mechanisms in different subsystems, allow for more flexible declarations of plugins by specifying direct classes needed by plugins rather than doing manual lookups on the Configuration instance, allow for smarter initialization strategies to minimize startup time and resource usage based on configurations rather than eagerly loading plugins, make it simpler to write unit tests, and make it generally simpler to write and maintain plugins without requiring deep expertise of esoteric subsystems.
- Beans: beans are analogous to Java classes with additional metadata to enable lifecycle management and dependency injection. Unlike a normal Java class, a bean has injection points (like constructor arguments, methods, and fields) where other managed instances can be injected at runtime based on the currently configured beans. As these are managed, they can have post-construct and pre-destruct callbacks for further lifecycle customization.
- Scopes: scopes control where the instances are stored or to what lifecycle they're tied to. At minimum, this includes dependent-scoped (creates new instances for every injection point) and singleton-scoped (creates and maintains a single instance for all injection points). In general DI frameworks, other scopes include things like HTTP sessions and HTTP requests. For this framework, the ContextSelector configuration API corresponds to a Configuration scope.
- Qualifiers: qualifiers add metadata to a bean besides its type to match on for injection. Typical qualifiers are names. Plugin configuration attribute names would likely make a good match here as another example.
- Injection points: in order to obtain injectable values into a bean instance, injection points control how these instances are bound to the bean. This is essentially the strategy for binding data to fields, constructors, and methods.
- Providers: providers are analogous to plugin builders in that they specify how to create an instance of type T given some plugin builder class Builder<T> implements Provider<T>.
- Producer and disposer methods: beans can specify producer and disposer methods as alternative ways to specify how to construct a bean instance. These are analogous to plugin factory methods.
Adopting such a plugin system based on beans, all the plugins that currently require either a Configuration or Node instance to do anything should be refactored to rely on proper dependency injection. This should be used to replace as many disparate dependency injection and configuration systems as possible to reduce the complexity required to implement plugins that rely on shared components.
Beyond the dependency injection API itself, this epic is motivated by the following goals:
- Simplify plugin classes that currently require manual method calls to Configuration or Node.
- Unify the various ad hoc dependency injection for configuration-scoped or singleton-scoped classes (the latter which are overridden via system properties) including:
- ConfigurationScheduler (or scheduled tasks in general)
- Make dependencies between classes more explicit via inversion of control which allows for easier testing and modularity.
- Avoid the use of manual configuration handling at the API level of any plugins including basic string parsing (as currently supported through the TypeConverter API), class loading from strings, and ad hoc reflection. This relates to the changes made in
- Provide smart initialization logic to continue (or maintain) improvements to startup time and avoid loading plugins that aren't referenced in the loaded configuration.
- Provide scopes for other ad hoc scopes/contexts used in various plugins such as HTTP request, HTTP session, servlet context, etc.