# Chapter 18. Logging

The log is the main 'UI' of a build tool. If it is too verbose, real warnings and problems are easily hidden by this. On the other hand you need the relevant information for figuring out if things have gone wrong. Gradle defines 6 log levels, as shown in Table 18.1, “Log levels”. There are two Gradle-specific log levels, in addition to the ones you might normally see. Those levels are QUIET and LIFECYCLE. The latter is the default, and is used to report build progress.

Table 18.1. Log levels

Level | Used for
ERROR | Error messages
QUIET | Important information messages
WARNING | Warning messages
LIFECYCLE | Progress information messages
INFO | Information messages
DEBUG | Debug messages

## 18.1. Choosing a log level

You can use the command line switches shown in Table 18.2, “Log level command-line options” to choose different log levels. In Table 18.3, “Stacktrace command-line options” you find the command line switches which affect stacktrace logging.

Table 18.2. Log level command-line options

Option | Outputs Log Levels
no logging options | LIFECYCLE and higher
`-q` | QUIET and higher
`-i` | INFO and higher
`-d` | DEBUG and higher (that is, all log messages)

Table 18.3. Stacktrace command-line options

Option | Meaning
No stacktrace options | No stacktraces are printed to the console in case of a build error (e.g. a compile error). Only in case of internal exceptions will stacktraces be printed. If the loglevel option `-d` is chosen, truncated stacktraces are always printed.
`-s` | Truncated stacktraces are printed. We recommend this over full stacktraces. Groovy full stacktraces are extremely verbose (Due to the underlying dynamic invocation mechanisms. Yet they usually do not contain relevant information for what has gone wrong in your code.)
`-S` | The full stacktraces are printed out.

## 18.2. Writing your own log messages

A simple option for logging in your build file is to write messages to standard output. Gradle redirects anything written to standard output to it's logging system at the `QUIET` log level.

Example 18.1. Using stdout to write log messages

`build.gradle`

    println 'A message which is logged at QUIET level'

Gradle also provides a `logger` property to a build script, which is an instance of `Logger`. This interface extends the SLF4J `Logger` interface and adds a few Gradle specific methods to it. Below is an example of how this is used in the build script:

Example 18.2. Writing your own log messages

`build.gradle`

    logger.quiet('An info log message which is always logged.')
    logger.error('An error log message.')
    logger.warn('A warning log message.')
    logger.lifecycle('A lifecycle info log message.')
    logger.info('An info log message.')
    logger.debug('A debug log message.')
    logger.trace('A trace log message.')

You can also hook into Gradle's logging system from within other classes used in the build (classes from the `buildSrc` directory for example). Simply use an SLF4J logger. You can use this logger the same way as you use the provided logger in the build script.

Example 18.3. Using SLF4J to write log messages

`build.gradle`

    import org.slf4j.Logger
    import org.slf4j.LoggerFactory

    Logger slf4jLogger = LoggerFactory.getLogger('some-logger')
    slf4jLogger.info('An info log message logged using SLF4j')

## 18.3. Logging from external tools and libraries

Internally, Gradle uses Ant and Ivy. Both have their own logging system. Gradle redirects their logging output into the Gradle logging system. There is a 1:1 mapping from the Ant/Ivy log levels to the Gradle log levels, except the Ant/Ivy `TRACE` log level, which is mapped to Gradle `DEBUG` log level. This means the default Gradle log level will not show any Ant/Ivy output unless it is an error or a warning.

There are many tools out there which still use standard output for logging. By default, Gradle redirects standard output to the `QUIET` log level and standard error to the `ERROR` level. This behavior is configurable. The project object provides a `LoggingManager`, which allows you to change the log levels that standard out or error are redirected to when your build script is evaluated.

Example 18.4. Configuring standard output capture

`build.gradle`

    logging.captureStandardOutput LogLevel.INFO
    println 'A message which is logged at INFO level'

To change the log level for standard out or error during task execution, tasks also provide a `LoggingManager`.

Example 18.5. Configuring standard output capture for a task

`build.gradle`

    task logInfo {
        logging.captureStandardOutput LogLevel.INFO
        doFirst {
            println 'A task message which is logged at INFO level'
        }
    }

Gradle also provides integration with the Java Util Logging, Jakarta Commons Logging and Log4j logging toolkits. Any log messages which your build classes write using these logging toolkits will be redirected to Gradle's logging system.

## 18.4. Changing what Gradle logs

You can replace much of Gradle's logging UI with your own. You might do this, for example, if you want to customize the UI in some way - to log more or less information, or to change the formatting. You replace the logging using the `Gradle.useLogger()` method. This is accessible from a build script, or an init script, or via the embedding API. Below is an example init script which changes how task execution and build completion is logged.

Example 18.6. Customizing what Gradle logs

`init.gradle`

    useLogger(new CustomEventLogger())

    class CustomEventLogger extends BuildAdapter implements TaskExecutionListener {

        public void beforeExecute(Task task) {
            println "[$task.name]"
        }

        public void afterExecute(Task task, TaskState state) {
            println()
        }

        public void buildFinished(BuildResult result) {
            println 'build completed'
        }
    }

Output of `gradle -I init.gradle build`

    > gradle -I init.gradle build
    [compile]
    compiling source

    [testCompile]
    compiling test source

    [test]
    running unit tests

    [build]

    build completed

Your logger can implement any of the listener interfaces listed below. When you register a logger, only the logging for the interfaces that it implements is replaced. Logging for the other interfaces is left untouched. You can find out more about the listener interfaces in Section 48.6, “Responding to the lifecycle in the build script”.

- `BuildListener`

- `ProjectEvaluationListener`

- `TaskExecutionGraphListener`

- `TaskExecutionListener`

- `TaskActionListener`