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In this guide, you can learn how to set up and configure a logger in the MongoDB Java driver.

You will learn how to:

This guide shows how to record events in the driver. If you would like to learn how to use information about the activity of the driver in code, consider reading ourguide on monitoring.

This section gives background on the dependencies necessary to set up a logger and provides an example logger setup.

The MongoDB Java driver uses the Simple Logging Facade For Java (SLF4J). SLF4J allows you to specify your logging framework of choice at deployment time. For more information about SLF4J,see the SLF4J documentation.

Setting up a logger is optional. When you start your application the MongoDB Java driver looks for the slf4j-api artifact in your classpath. If the driver can't find the slf4j-api artifact, the driver logs the following warning withjava.util.logging and disables all further logging:


WARNING: SLF4J not found on the classpath.  Logging is disabled for the 'org.mongodb.driver' component

To set up a logger, you must include the following in your project.

Note

For the most popular logging frameworks, there is often a single binding artifact that lists the slf4j-api and the logging framework as dependencies. This means that you can set up a logger by adding one artifact to your project's dependency list. You will see this in the example below.

A binding is a piece of code that connects the slf4j-api artifact with a logging framework. The following example shows how to bind the slf4j-api artifact to the two most popular logging frameworks, Log4j2 and Logback.

This example shows how to set up your logger. Click the tab corresponding to the logging framework you would like to use in your project.

Tip

Dependency Versions

Select the build tool you are using in your project.

Add the following dependency to your pom.xml file.


<dependencies>

  <dependency>

    <groupId>ch.qos.logback</groupId>

    <artifactId>logback-classic</artifactId>

    <version>1.2.11</version>

  </dependency>

</dependencies>

Add the following dependency to your build.gradle file.


dependencies {

   implementation 'ch.qos.logback:logback-classic:1.2.11'

}

Once you have included the preceding dependency, connect to your MongoDB instance and retrieve a document with the following code.


MongoClient mongoClient = MongoClients.create(<connection uri>);

MongoDatabase database = mongoClient.getDatabase(<database>);

MongoCollection<Document> collection = database.getCollection(<collection>);

collection.find().first();

You should see output that resembles the following:


...

12:14:55.853 [main] DEBUG org.mongodb.driver.connection - Opened connection [connectionId{localValue:3, serverValue:3}] to <MongoDB hostname>

12:14:55.861 [main] DEBUG org.mongodb.driver.protocol.command - Command "find" started on database <database> using a connection with driver-generated ID 3 and server-generated ID 3 to <MongoDB hostname>. The request ID is 5. Command: {"find": "<collection>", "filter": {}, "limit": 1, "singleBatch": true, "$db": "<database>", "lsid": {"id": {"$binary": {"base64": "<_id>", "subType": "04"}}}, "$readPreference": {"mode": "primaryPreferred"}}

12:14:55.864 [main] DEBUG org.mongodb.driver.protocol.command - Command "find" succeeded in 4.34 ms using a connection with driver-generated ID 3 and server-generated ID 3 to <MongoDB hostname. The request ID is 5. Command reply: {"cursor": {"id": 0, "ns": "<database>.<collection>", "firstBatch": []}, "ok": 1.0, "$clusterTime": {"clusterTime": {"$timestamp": {"t": 1673778535, "i": 1}}, "signature": {"hash": {"$binary": {"base64": "<_id>", "subType": "00"}}, "keyId": 0}}, "operationTime": {"$timestamp": {"t": 1673778535, "i": 1}}}

Note

Default Log Level

The default log level of Logback is DEBUG. To learn how to change your Logback logger's log level, see theexample in the Configure Your Logger section of this page.

For more information about Logback, see theLogback manual.

Select the build tool you are using in your project.

Add the following dependency to your pom.xml file.


<dependencies>

  <dependency>

    <groupId>org.apache.logging.log4j</groupId>

    <artifactId>log4j-slf4j-impl</artifactId>

    <version>2.17.1</version>

  </dependency>

</dependencies>

Add the following dependency to your build.gradle file.


dependencies {

  implementation 'org.apache.logging.log4j:log4j-slf4j-impl:2.17.1'

}

Once you have included the preceding dependency, log an error using the following code.


import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

...

Logger logger = LoggerFactory.getLogger("MyApp");

logger.error("Logging an Error");

You should see output that resembles the following:


12:35:00.438 [main] ERROR <my package path> - Logging an Error

Note

Default Log Level

The default log level of Log4J2 is ERROR. This means that running standard operations in the MongoDB Java driver will not produce output from Log4J2 without configuration. To learn how to change your Log4J2 logger's log level, see theexample in the Configure Your Logger section of this page.

For more information about Log4j2, see theLog4j2 manual.

To configure your logger, you must use the configuration system of the logging framework bound to SLF4J.

In the following example we show how you can use your logging framework's configuration system to set your logger's log level.

A logger's log level specifies a lower bound for how urgent a message must be for the logger to output that message.

This example shows how to configure your logger's log level to INFO. Select the tab corresponding to the logging framework you are using in your project.

Specify Logback configurations in a file named logback.xml. You don't need to create your logback.xml file in a specific location, but you must be able to access it from your classpath.

The Logback framework defines the following log levels. The following lists the log levels, ordered from most urgent to least urgent:

Set your logback.xml file to the following.


<configuration>

   <appender name="CONSOLE"

             class="ch.qos.logback.core.ConsoleAppender">

      <encoder>

         <pattern>

               %-4relative [%thread] %-5level %logger{30} - %msg%n

         </pattern>

      </encoder>

   </appender>

   <root level="INFO">

      <appender-ref ref="CONSOLE" />

   </root>

</configuration>

To test that your logger configuration was successful, run the following code.


MongoClient mongoClient = MongoClients.create(<connection uri>);

MongoDatabase database = mongoClient.getDatabase(<database>);

MongoCollection<Document> collection = database.getCollection(<collection>);

collection.find().first();

You should see output that resembles the following.


...

1317 [cluster-ClusterId{value='<your cluster id>', description='null'}-<your connection uri>] INFO  org.mongodb.driver.cluster - Discovered replica set primary <your connection uri>

1568 [main] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:7, serverValue:<server value>}] to <your connection uri>

For more information about configuring Logback, see thethe Logback Manual.

Specify Log4j2 configurations in a file named log4j2.xml. You don't need to create your log4j2.xml file in a specific location, but you must be able to access it from your classpath.

The Log4j2 framework defines the following log levels. The following lists the log levels, ordered from most urgent to least urgent:

Set your log4j2.xml file to the following.


<?xml version="1.0" encoding="UTF-8"?>

<Configuration status="INFO">

   <Appenders>

      <Console name="Console" target="SYSTEM_OUT">

            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>

      </Console>

   </Appenders>

   <Loggers>

      <Root level="INFO">

            <AppenderRef ref="Console"/>

      </Root>

   </Loggers>

</Configuration>

To test that your logger configuration was successful, run the following code.


MongoClient mongoClient = MongoClients.create(<connection uri>);

MongoDatabase database = mongoClient.getDatabase(<database>);

MongoCollection<Document> collection = database.getCollection(<collection>);

collection.find().first();

You should see output that resembles the following.


...

10:14:57.633 [cluster-ClusterId{value=<your cluster id>, description='null'}-<your connection uri>] INFO  org.mongodb.driver.cluster - Discovered replica set primary <your connection uri>

10:14:57.790 [main] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:7, serverValue:<your server value>}] to <your connection uri>

For more information about configuring Log4j2, see the officialLog4j2 configuration guide.

Your logger uses logger names to help organize different logging events. Logger names are strings that form a hierarchy. A logger is an ancestor of another logger if its name followed by a "." is a prefix of the other logger's name. For example,"grandparent" is an ancestor of "grandparent.parent" which is an ancestor of "grandparent.parent.child".

For a concrete example, this is what a logger hierarchy looks like in code.


import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

...

Logger logger_parent = LoggerFactory.getLogger("parent");

Logger logger_child = LoggerFactory.getLogger("parent.child");

A logger inherits the properties of its ancestor logger and can define its own. You can think of this as similar to class inheritance in Java.

The MongoDB Java driver defines the following logger names to organize different logging events in the driver. Here are the logger names defined in the driver and the logging events they correspond to.

This example shows how to change the log level for a specific driver logger. We set the root logger to OFF and the org.mongodb.driver.connection logger to INFO. This will cause the application to only log messages related to connecting to a MongoDB instance.

Select the tab corresponding to the logging framework you are using in your project.

Set your logback.xml file to the following.


<configuration>

   <appender name="CONSOLE"

             class="ch.qos.logback.core.ConsoleAppender">

      <encoder>

         <pattern>

               %-4relative [%thread] %-5level %logger{30} - %msg%n

         </pattern>

      </encoder>

   </appender>

   <logger name="org.mongodb.driver.connection" level="INFO" additivity="true"/>

   <root level="OFF">

      <appender-ref ref="CONSOLE" />

   </root>

</configuration>

To test that your logger configuration was successful, run the following code.


MongoClient mongoClient = MongoClients.create(<connection uri>);

MongoDatabase database = mongoClient.getDatabase(<database>);

MongoCollection<Document> collection = database.getCollection(<collection>);

collection.find().first();

You should see output that resembles the following.


...

829  [cluster-rtt-ClusterId{value='<some value>', description='null'}-<your connection URI>] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:2, serverValue:<your server value>}] to <your connection uri>

977  [main] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:7, serverValue:<your server value>}] to <your connection uri>

For more information about configuring Logback, see theofficial Logback configuration guide.

Set your log4j2.xml file to the following.


<?xml version="1.0" encoding="UTF-8"?>

<Configuration status="INFO">

   <Appenders>

      <Console name="Console" target="SYSTEM_OUT">

            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>

      </Console>

   </Appenders>

   <Loggers>

      <Logger name="org.mongodb.driver.connection" level="INFO"/>

      <Root level="OFF">

            <AppenderRef ref="Console"/>

      </Root>

   </Loggers>

</Configuration>

To test that your logger configuration was successful, run the following code.


MongoClient mongoClient = MongoClients.create(<connection uri>);

MongoDatabase database = mongoClient.getDatabase(<database>);

MongoCollection<Document> collection = database.getCollection(<collection>);

collection.find().first();

You should see output that resembles the following.


...

15:40:23.005 [cluster-ClusterId{value='<some value>', description='null'}-<your connection uri>] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:3, serverValue:<your server value>}] to <your connection uri>

15:40:23.159 [main] INFO  org.mongodb.driver.connection - Opened connection [connectionId{localValue:7, serverValue:<your server value>}] to <your connection uri>

For more information about configuring Log4j2, see theofficial Log4J2 configuration guide.