opentelemetry-java-instrume.../instrumentation/log4j/log4j-context-data/log4j-context-data-2.17/library-autoconfigure/README.md

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# ContextData Instrumentation for Log4j2 version 2.17 and higher
This module provides a Log4j2 `ContextDataProvider` that injects trace context from active spans
into log context.
## Quickstart
### Add these dependencies to your project
Replace `OPENTELEMETRY_VERSION` with the [latest
release](https://search.maven.org/search?q=g:io.opentelemetry.instrumentation%20AND%20a:opentelemetry-log4j-context-data-2.17-autoconfigure).
For Maven, add to your `pom.xml` dependencies:
```xml
<dependencies>
<dependency>
<groupId>io.opentelemetry.instrumentation</groupId>
<artifactId>opentelemetry-log4j-context-data-2.17-autoconfigure</artifactId>
<version>OPENTELEMETRY_VERSION</version>
<scope>runtime</scope>
</dependency>
</dependencies>
```
For Gradle, add to your dependencies:
```groovy
runtimeOnly("io.opentelemetry.instrumentation:opentelemetry-log4j-context-data-2.17-autoconfigure:OPENTELEMETRY_VERSION")
```
### Usage
`OpenTelemetryContextDataProvider` implements the Log4j2 `ContextDataProvider` SPI, and injects the
trace ID and span ID from an active span into
Log4j's [context data](https://logging.apache.org/log4j/2.x/manual/thread-context.html).
Log4j will automatically pick up the integration when you include this module. The following keys
will be added to the context when a log statement is made when a span is active:
- `trace_id`
- `span_id`
- `trace_flags`
These keys can be customized using the following system properties or environment variables:
| System property | Environment variable |
|-------------------------------------------------------|---------------------------------------------------|
| `otel.instrumentation.common.logging.trace-id` | `OTEL_INSTRUMENTATION_COMMON_LOGGING_TRACE_ID` |
| `otel.instrumentation.common.logging.span-id` | `OTEL_INSTRUMENTATION_COMMON_LOGGING_SPAN_ID` |
| `otel.instrumentation.common.logging.trace-flags` | `OTEL_INSTRUMENTATION_COMMON_LOGGING_TRACE_FLAGS` |
If the `otel.instrumentation.log4j-context-data.add-baggage` system property (or the
`OTEL_INSTRUMENTATION_LOG4J_CONTEXT_DATA_ADD_BAGGAGE` environment variable) is set to `true`,
key/value pairs in [baggage](https://opentelemetry.io/docs/concepts/signals/baggage/) will also be added to the context data.
- `baggage.<entry_name>`
You can use these keys when defining an appender in your `log4j.xml` configuration, for example:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
<Appenders>
<Console name="Console" target="SYSTEM_OUT">
<PatternLayout
pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} trace_id=%X{trace_id} span_id=%X{span_id} trace_flags=%X{trace_flags} - %msg%n"/>
</Console>
</Appenders>
<Loggers>
<Root>
<AppenderRef ref="Console" level="All"/>
</Root>
</Loggers>
</Configuration>
```