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[[gradle_daemon]]
= Gradle Daemon

A daemon is a computer program that runs as a background process rather than being under the direct control of an interactive user.

Gradle runs on the Java Virtual Machine (JVM) and uses several supporting libraries with non-trivial initialization time.
Startups can be slow.
The **Gradle Daemon** solves this problem.

The Gradle Daemon is a long-lived background process that reduces the time it takes to run a build.

The Gradle Daemon reduces build times by:

* Caching project information across builds
* Running in the background so every Gradle build doesn't have to wait for JVM startup
* Benefiting from continuous runtime optimization in the JVM
* <<file_system_watching.adoc#sec:daemon_watch_fs,Watching the file system>> to calculate exactly what needs to be rebuilt before you run a build

[[understanding_daemon]]
== Understanding the Daemon

The Gradle JVM client sends the Daemon build information such as command line arguments, project directories, and environment variables so that it can run the build.
[[understanding_wrapper]]
The Wrapper is responsible for resolving dependencies, executing build scripts, creating and running tasks; when it is done, it sends the client the output.
Communication between the client and the Daemon happens via a local socket connection.

Daemons use the JVM's default minimum heap size.

If the requested build environment does not specify a maximum heap size, the Daemon uses up to 512MB of heap.
512MB is adequate for most builds.
Larger builds with hundreds of subprojects, configuration, and source code may benefit from a larger heap size.

[[sec:status]]
== Check Daemon status

To get a list of running Daemons and their statuses, use the `--status` command:

----
$ gradle --status
----

----
   PID STATUS   INFO
 28486 IDLE     7.5
 34247 BUSY     7.5
----

Currently, a given Gradle version can only connect to Daemons of the same version.
This means the status output only shows Daemons spawned running the same version of Gradle as the current project.

[[find_all_daemons]]
== Find Daemons

If you have installed the Java Development Kit (JDK), you can view live daemons with the `jps` command.

----
$ jps
----
----
33920 Jps
27171 GradleDaemon
22792
----

Live Daemons appear under the name `GradleDaemon`.
Because this command uses the JDK, you can view Daemons running any version of Gradle.

[[enable_deamon]]
== Enable Daemon

Gradle enables the Daemon by default since Gradle 3.0.
If your project doesn't use the Daemon, you can enable it for a single build with the `--daemon` flag when you run a build:

----
$ gradle <task> --daemon
----

This flag overrides any settings that disable the Daemon in your project or user `gradle.properties` files.

To enable the Daemon by default in older Gradle versions, add the following setting to the `gradle.properties` file in the project root or your Gradle User Home (`GRADLE_USER_HOME`:

====
.gradle.properties
[source,properties]
----
org.gradle.daemon=true
----
====

[[sec:disabling_the_daemon]]
== Disable Daemon

You can disable the Daemon in multiple ways but there are important considerations:

Single-use Daemon :: If the JVM args of the client process don't match what the build requires, a single-used Daemon (disposable JVM) is created.
This means the Daemon is required for the build, so it is created, used, and then stopped at the end of the build.

No Daemon :: If the `JAVA_OPTS` and `GRADLE_OPTS` match `org.gradle.jvmargs`, the Daemon will not be used at all since the build happens in the client JVM.

=== Disable for a build

To disable the Daemon for a single build, pass the `--no-daemon` flag when you run a build:

----
$ gradle <task> --no-daemon
----

This flag overrides any settings that enable the Daemon in your project including the `gradle.properties` files.

=== Disable for a project

To disable the Daemon for all builds of a project, add `org.gradle.daemon=false` to the `gradle.properties` file in the project root.

=== Disable for a user

On Windows, this command disables the Daemon for the current user:

[source,text]
----
(if not exist "%USERPROFILE%/.gradle" mkdir "%USERPROFILE%/.gradle") && (echo. >> "%USERPROFILE%/.gradle/gradle.properties" && echo org.gradle.daemon=false >> "%USERPROFILE%/.gradle/gradle.properties")
----

On UNIX-like operating systems, the following Bash shell command disables the Daemon for the current user:

[source,bash]
----
mkdir -p ~/.gradle && echo "org.gradle.daemon=false" >> ~/.gradle/gradle.properties
----

=== Disable globally

There are two recommended ways to disable the Daemon globally across an environment:

* add `org.gradle.daemon=false` to the `$<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>`/gradle.properties` file
* add the flag `-Dorg.gradle.daemon=false` to the `GRADLE_OPTS` environment variable

Don't forget to make sure your JVM arguments and `GRADLE_OPTS` / `JAVA_OPTS` match if you want to completely disable the Daemon and not simply invoke a single-use one.

[[sec:stopping_an_existing_daemon]]
== Stop Daemon

It can be helpful to stop the Daemon when troubleshooting or debugging a failure.

Daemons automatically stop given any of the following conditions:

* Available system memory is low
* Daemon has been idle for 3 hours

To stop running Daemon processes, use the following command:

----
$ gradle --stop
----

This terminates all Daemon processes started with the same version of Gradle used to execute the command.

You can also kill Daemons manually with your operating system.
To find the PIDs for all Daemons regardless of Gradle version, see <<gradle_daemon.adoc#find_all_daemons,Find Daemons>>.

[[sec:daemon_jvm_criteria]]
== Configuring the JVM to be used

NOTE: Daemon JVM discovery and criteria are <<feature_lifecycle.adoc#sec:incubating_state,incubating>> features and are subject to change in a future release.

By default, the Gradle daemon runs with the same JVM installation that started the build.
Gradle defaults to the current shell path and `JAVA_HOME` environment variable to locate a usable JVM.

Alternatively, a different JVM installation can be specified for the build using the <<build_environment.adoc#sec:gradle_configuration_properties,`org.gradle.java.home` Gradle property>> or programmatically through the Tooling API.

If Daemon JVM criteria is available, it takes precedence over `JAVA_HOME` and `org.gradle.java.home`.

Building on the <<toolchains.adoc#toolchains,toolchain feature>>, you can now use declarative criteria to specify the JVM requirements for the build.

=== Daemon JVM criteria

The _daemon JVM criteria_ is controlled by a task, similarly to how <<gradle_wrapper.adoc#customizing_wrapper,`wrapper` task>> updates the wrapper properties.
When the task runs, it creates or updates the criteria in the `gradle/gradle-daemon-jvm.properties` file.
For more control, the task can be further configured in the build script or via command-line arguments.

As with the wrapper, the generated file should be checked into version control.
This will ensure any developer or CI server that runs the build will use the same JVM version.

With the following configuration:
====
include::sample[dir="snippets/daemon-jvm/customized-task/kotlin",files="build.gradle.kts[tags=customized-daemon-jvm-task]"]
include::sample[dir="snippets/daemon-jvm/customized-task/groovy",files="build.gradle[tags=customized-daemon-jvm-task]"]
====

When running:

----
$ ./gradlew updateDaemonJvm
----

The following file will be generated:

====
.gradle/gradle-daemon-jvm.properties
[source,properties]
----
#This file is generated by updateDaemonJvm
toolchainVersion=17
----
====

The same properties file can be produced without configuring the task in the build script. Using just a command-line argument:

----
$ ./gradlew updateDaemonJvm --jvm-version=17
----

If you run the task without any arguments, and the properties file does not exist, then the version of the current JVM used by the daemon will be used.

NOTE: Gradle only supports the major JVM version and JVM vendor as a criterion.
Support for other criteria may be added in a future release.

On the next execution of the build, the Gradle client will use this file to locate a compatible JVM installation and start the daemon with it.

==== Specifying a JVM vendor

Like the JVM version, the JVM vendor can be used as criteria to select a compatible JVM installation for the build. When no JVM vendor is specified, Gradle will consider all vendors compatible.

By default, running `updateDaemonJvm` to create the `gradle-daemon-jvm.properties` file will not generate a JVM vendor criterion. You must either explicitly specify a JVM vendor for the `updateDaemonJvm` task in the build script or pass a JVM vendor on the command-line with `--jvm-vendor=<value>`.

Gradle recognizes a small number of JVM vendor strings as special and equivalent. For example, "Adoptium" and "Temurin" are considered the same vendor. You can see the list of special vendors by running `gradle help --task updateDaemonJvm`.

If the JVM vendor you specify is not treated as a special value, Gradle considers the value as an exact match. For example, to match the vendor "My Custom JVM", the vendor criterion must be "My Custom JVM".

=== Daemon JVM discovery

To locate a compatible JVM installation, Gradle re-uses the mechanism provided by the <<toolchains.adoc#toolchains,Java Toolchains>> feature.
This feature is used to locate a JVM installation that matches the criteria specified in the `gradle/gradle-daemon-jvm.properties` file.

NOTE: The _daemon JVM discovery process_ does not support auto-provisioning of new JVM installations.
This will be added in a future release.

[[sec:tools_and_ides]]
== Tools & IDEs

The <<third_party_integration.adoc#embedding,Gradle Tooling API>> used by IDEs and other tools to integrate with Gradle _always_ uses the Gradle Daemon to execute builds.
If you execute Gradle builds from within your IDE, you already use the Gradle Daemon.
There is no need to enable it for your environment.

== Continuous Integration

We recommend using the Daemon for developer machines and Continuous Integration (CI) servers.

[[deamon_compatibility]]
== Compatibility

Gradle starts a new Daemon if no idle or compatible Daemons exist.

The following values determine compatibility:

* *Requested build environment*, including the following:
** Java version
** JVM attributes
** JVM properties
* Gradle version

Compatibility is based on exact matches of these values.
For example:

* If a Daemon is available with a Java 8 runtime, but the requested build environment calls for Java 10,
then the Daemon is not compatible.

* If a Daemon is available running Gradle 7.0, but the current build uses Gradle 7.4, then the
Daemon is not compatible.

Certain properties of a Java runtime are _immutable_: they cannot be changed once the JVM has started.
The following JVM system properties are immutable:

* `file.encoding`
* `user.language`
* `user.country`
* `user.variant`
* `java.io.tmpdir`
* `javax.net.ssl.keyStore`
* `javax.net.ssl.keyStorePassword`
* `javax.net.ssl.keyStoreType`
* `javax.net.ssl.trustStore`
* `javax.net.ssl.trustStorePassword`
* `javax.net.ssl.trustStoreType`
* `com.sun.management.jmxremote`

The following JVM attributes controlled by startup arguments are also immutable:

* The maximum heap size (the `-Xmx` JVM argument)
* The minimum heap size (the `-Xms` JVM argument)
* The boot classpath (the `-Xbootclasspath` argument)
* The "assertion" status (the `-ea` argument)

If the requested build environment requirements for any of these properties and attributes
differ from the Daemon's JVM requirements, the Daemon is not compatible.

NOTE: For more information about build environments, see <<build_environment.adoc#build_environment,the build environment documentation>>.

[[sec:why_the_daemon]]
== Performance Impact

The Daemon can reduce build times by 15-75% when you build the same project repeatedly.

In between builds, the Daemon waits idly for the next build.
As a result, your machine only loads Gradle into memory once for multiple builds instead of once per build.
This is a significant performance optimization.

=== Runtime Code Optimizations

The JVM gains significant performance from **runtime code optimization**: optimizations applied to code while it runs.

JVM implementations like OpenJDK's Hotspot progressively optimize code during execution.
Consequently, subsequent builds can be faster purely due to this optimization process.

With the Daemon, perceived build times can drop dramatically between a project's 1^st^ and 10^th^ builds.

=== Memory Caching

The Daemon enables in-memory caching across builds.
This includes classes for plugins and build scripts.

Similarly, the Daemon maintains in-memory caches of build data, such as the hashes of task inputs and outputs for incremental builds.

== Performance Monitoring

Gradle actively monitors heap usage to detect memory leaks in the Daemon.

When a memory leak exhausts available heap space, the Daemon:

. Finishes the currently running build.
. Restarts before running the next build.

Gradle enables this monitoring by default.

To disable this monitoring, set the `org.gradle.daemon.performance.enable-monitoring` Daemon option to `false`.

You can do this on the command line with the following command:

----
$ gradle <task> -Dorg.gradle.daemon.performance.enable-monitoring=false
----

Or you can configure the property in the `gradle.properties` file in the project root or your GRADLE_USER_HOME (Gradle User Home):

====
.gradle.properties
[source,properties]
----
org.gradle.daemon.performance.enable-monitoring=false
----
====
