docs community blog github

Java Buildpack

The Paketo Java Buildpack allows users to create an image containing a JVM application from a precompiled artifact or directly from source.

The Java Buildpack is a composite buildpack and each step in a build is handled by one of it’s components. The following docs describe common build configurations. For a full set of configuration options and capabilities see the homepages for the component buildpacks.

About the Examples

All Java Buildpack examples will use the Paketo sample applications.

Examples assume that the root of this repository is the working directory:

git clone
cd samples
copy to clipboard

The pack CLI is used throughout the examples. pack is just one of several Cloud Native Buildpack platforms than can execute builds with the Java Buildpacks. For example, Spring Boot developers may want to explore the Spring Boot Maven Plugin or Spring Boot Gradle Plugin .

Examples assume that the Paketo Base builder is the default builder:

pack config default-builder paketobuildpacks/builder:base
copy to clipboard

All java example images should return {"status":"UP"} from the actuator health endpoint.

docker run --rm --tty --publish 8080:8080 samples/java
curl -s http://localhost:8080/actuator/health | jq .
copy to clipboard

Building from Source

The Java Buildpack can build from source using any of the following build tools:

The correct build tool to use will be detected based on the contents of the application directory.

The build should produce one the of supported artifact formats. After building, the buildpack will replace provided application source code with the exploded archive. The build will proceed as described in Building from a Compiled Artifact.

Example: Building with Maven

The following command creates an image from source with maven.

pack build samples/java \
  --path java/maven
copy to clipboard

Configuring the Build Tool

Note: The following set of configuration options are not comprehensive, see the homepage for the relevant component buildpacks for a full-set of configuration options.

Selecting a Module or Artifact

For a given build <TOOL>, where <TOOL> is one of MAVEN, GRADLE, LEIN or SBT, the selected artifact can be configured with one of the following environment variable at build-time:

    • Defaults to the root module.
    • Configures the module in a multi-module build from which the buildpack will select the application artifact.
    • Example: Given BP_MAVEN_BUILT_MODULE=api, Paketo Maven Buildpack will look for the application artifact with the file pattern target/api/*.[jw]ar.
    • Defaults to a tool-specific pattern (e.g. target/*.[jw]ar for Maven, build/libs/*.[jw]ar for gradle). See component buildpack homepage for details.
    • Configures the built application artifact path, using Bash Pattern Matching.
    • Supercedes BP_<TOOL>_BUILT_MODULE if set to a non-default value.
    • Example: Given BP_MAVEN_BUILT_ARTIFACT=out/api-*.jar, the Paketo Maven Buildpack will select a file with name out/api-1.0.0.jar.

Specifying the Build Command

For a given build <TOOL>, where <TOOL> is one of MAVEN, GRADLE, LEIN or SBT, the build command can be configured with the following environment variable at build-time:

    • Defaults to a tool-specific value (e.g. -Dmaven.test.skip=true package for Maven, --no-daemon assemble for Gradle). See component buildpack homepage for details.
    • Configures the arguments to pass to the build tool.
    • Example: Given BP_GRADLE_BUILD_ARGUMENTS=war, the Paketo Gradle Buildpack will execute ./gradlew war or gradle war (depending on the presence of the gradle wrapper).

Connecting to a Private Maven Repository

A binding with type maven and key settings.xml can be used to provide custom Maven settings.

├── settings.xml
└── type

The value of settings.xml file may contain the credentials needed to connect to a private Maven repository.

Example: Providing Maven Settings

The following steps demonstrate how to use a settings.xml file from your workstation with pack.

  1. Create a directory to contain the binding.

    mkdir java/maven/binding
    copy to clipboard

  2. Indicate that the binding is of type maven with a file called type inside the binding, containing the value maven.

    echo -n "maven" > java/maven/binding/type
    copy to clipboard

  3. Copy the settings.xml file from the workstation to the binding.

    cp ~/.m2/settings.xml java/maven/binding/settings.xml
    copy to clipboard

  4. Provide the binding to pack build.

    pack build samples/java \
       --path java/maven \
       --volume $(pwd)/java/maven/binding:/platform/bindings/my-maven-settings
    copy to clipboard

Building from a Compiled Artifact

An application developer may build an image from following archive formats:

The Java Buildpack expects the application directory to contain the extracted contents of the archive (e.g. an exploded JAR). Most platforms will automatically extract any provided archives.

If a WAR is detect the Java Buildpack will install Apache Tomcat. For exact set of supported Tomcat versions can be found in the Java Buildpack releases notes. For tomcat configuration options see the Apache Tomcat Buildpack.

The component buildpack for the provided artifact format will contribute a start command to the image.

Note: All three of the Apache Tomcat Buildpack, Executable Jar Buildpack, and DistZip Buildpack may opt-in during detection. However, only one of these buildpacks will actually contribute to the final image. This happens because the artifact type may be unknown during detection, if for example a previous buildpack compiles the artifact.

Example: Building from an Executable JAR

The following command uses Maven to compile an executable JAR and then uses pack to build an image from the JAR.

cd java/maven
./mvnw package
pack build samples/java \
   --path /target/demo-0.0.1-SNAPSHOT.jar
copy to clipboard

The resulting application image will be identical to that built in the Building with Maven example.

About the JVM

The Java Buildpack uses the BellSoft Liberica implementations of the JRE and JDK. JVM installation is handled by the BellSoft Liberica Buildpack. The JDK will be installed in the build container but only the JRE will be contributed to the application image.

See the homepage for the Bellsoft Liberica Buildpack for a full set of configuration options.

The JVM Version

The Bellsoft Liberica Buildpack provides support for the latest patch release of all version lines supported at the time of buildpack release. The exact set of JDK/JRE versions support by a given buildpack version can be found in the Java Buildpack release notes.

Inspecting the JVM Version

The exact JRE version that was contributed to a given image can be read from the Bill-of-Materials.

Example Inspecting the JRE Version

Given an image named samples/java built from one of examples above, the following command should print the exact version of the installed JRE.

pack inspect-image samples/app --bom | jq '.local[] | select(.name=="jre") | .metadata.version'
copy to clipboard

Configuring the JVM Version

The following environment variable configures the JVM version at build-time.

    • Defaults to the latest LTS version at the time of release.
    • Configures a specific JDK or JRE version.
    • Example: Given BP_JVM_VERSION=8 or BP_JVM_VERSION=8.* the buildpack will install the latest patch releases of the Java 8 JDK and JRE.

Runtime JVM Configuration

The Java Buildpack configures the JVM by setting JAVA_TOOL_OPTIONS in the JVM environment.

Memory Calculator

The Java Buildpack installs a component called the Memory Calculator which will configure JVM memory based on the resources available to the container at runtime. The calculated flags will be appended to JAVA_TOOL_OPTIONS.

Configuring JVM at Runtime

The runtime JVM can be configured in two ways:

  1. Buildpack-provided runtime components including the Memory Calculator accept semantically named environment variables which are then used to derive JAVA_TOOL_OPTIONS flags. Examples include:
  2. Flags can be set directly at runtime with the JAVA_TOOL_OPTIONS environment variable. User-provided flags will be appended to buildpack-provided flags. If the user and a buildpack set the same flag, user-provided flags take precedence.

See the homepage for the Bellsoft Liberica Buildpack for a full set of configuration options.

Using Alternative JVMs

By default, the Paketo Java buildpack will use the Liberica JVM. The following Paketo JVM buildpacks may be used to substitute alternate JVM implemenations in place of Liberica’s JVM.

JVM Buildpack
Alibaba Dragonwell Paketo Alibaba Dragonwell Buildpack
Amazon Corretto Paketo Amazon Corretto Buildpack
Azul Zulu Paketo Azul Zulu Buildpack
BellSoft Liberica Paketo BellSoft Liberica Buildpack - Default
Eclipse OpenJ9 Paketo Eclipse OpenJ9 Buildpack
GraalVM Paketo GraalVM Buildpack
Microsoft OpenJDK Paketo Microsoft OpenJDK Buildpack
SapMachine Paketo SapMachine Buildpack

To use an alternative JVM, you will need to set two --buildpack arguments to pack build, one for the alternative JVM buildpack you’d like to use and one for the Paketo Java buildpack (in that order). This works because while you end up with two JVM buildpacks, the first one, the one you’re specifying will claim the build plan entries so the second one will end up being a noop and doing nothing.

This example will switch in the Azul Zulu buildpack:

pack build samples/jar --buildpack --buildpack paketo-buildpacks/java`
copy to clipboard

There is one drawback to this approach. When using the method above to specify an alternative JVM vendor buildpack, this alternate buildpack ends up running before the CA certs buildpack and therefore traffic from the alternate JVM vendor buildpack won’t trust any additional CA certs. This is not expected to impact many users because JVM buildpacks should reach out to URLs that have a cert signed by a known authority with a CA in the default system truststore.

If you have customized your JVM buildpack to download the JVM from a URL that uses a certificate not signed by a well-known CA, you can workaround this by specifying the CA certs buildpack to run first. This works because while you will end up with the CA certificates buildpack specified twice, the lifecycle is smart enough to drop the second one.

For example:

pack build samples/jar --buildpack paketo-buildpacks/ca-certificates --buildpack --buildpack paketo-buildpacks/java`
copy to clipboard

It does not hurt to use this command for all situations, it is just more verbose and most users can get away without specifying the CA certificates buildpack to be first.

Spring Boot Applications

If the application uses Spring Boot the Spring Boot Buildpack will enhance the resulting image by adding additional metadata to the image config, applying Boot-specific performance optimizations, and enabling runtime auto-configuration.

Additional Metadata

The Spring Boot Buildpack adds the following additional image labels:

  • org.opencontainers.image.title - set to the value of Implementation-Title from MANIFEST.MF.
  • org.opencontainers.image.version - set to the values of Implementation-Version from MANIFEST.MF.
  • org.springframework.boot.version - set to the value of Spring-Boot-Version from MANIFEST.MF.
  • - containing configuration metadata.
  • - containing, if present.

In addition, the buildpack will add an entry with name dependencies to the Bill-of-Materials listing the application dependencies.

Example: Inspecting Application Dependencies

The following command uses pack to list every dependency of a sample application.

pack inspect-image samples/java --bom | jq '.local[] | select(.name=="dependencies") | .metadata.dependencies[].name'
copy to clipboard

Runtime Auto-Configuration

The Spring Boot Buildpack adds Spring Cloud Bindings to the application class path. Spring Cloud Bindings will auto-configure the application to connect to an external service when a binding of a supported type provides credentials and connection information at runtime. Runtime auto-configuration is enabled by default but can be disabled with the BPL_SPRING_CLOUD_BINDINGS_ENABLED environment variable.


The Spring Boot Buildpack can apply domain-specific knowledge to optimize the performance of Spring Boot applications. For example, if the buildpack detects that the application is a reactive web application the thread count will be reduced to 50 from a default of 250.

Connecting to an APM

The Java Buildpack supports the following APM integrations:

APM integration are enabled with bindings. If a binding of the correct type is provided at build-time the corresponding java agent will be contributed to the application image. Connection credentials will be read from the binding at runtime.

Example: Connecting to Azure Application Insights

The following command builds an image with the Azure Application Insights Java Agent

pack build samples/java --volume "$(pwd)/java/application-insights/binding:/platform/bindings/application-insights"
copy to clipboard

To connect to Azure Applicaiton Insights at runtime a valid Instrumentation Key is required.

echo "<Instrumentation Key>" > java/application-insights/binding/InstrumentationKey
docker run --rm --tty \
  --env SERVICE_BINDING_ROOT=/bindings \
  --volume "$(pwd)/java/application-insights/binding:/bindings/app-insights" \
copy to clipboard


If BP_DEBUG_ENABLED is set at build-time and BPL_DEBUG_ENABLED is set at runtime the Debug Buildpack will configure the application to accept debugger connections. The debug port defaults to 8000 and can be configured with BPL_DEBUG_PORT at runtime. If BPL_DEBUG_SUSPEND is set at runtime, the JVM will suspend execution until a debugger has attached.

Example: Remote Debugging

The following commands builds a debug-enabled image.

pack build samples/java \
  --path java/jar \
  --env BP_DEBUG_ENABLED=true
copy to clipboard

To run the image with the debug port published:

docker run --env BPL_DEBUG_ENABLED=true --publish 8000:8000 samples/java
copy to clipboard

Connect your IDE debugger to connect to the published port. Eclipse Remote Debug Configuration

Enabling JMX

If BP_JMX_ENABLED is set at build-time and BPL_JMX_ENABLED is set at runtime, the JMX Buildpack will enable JMX. The JMX connector will listen on port 5000 by default. The port can be configured with the BPL_JMX_PORT environment variable at runtime.

Example: Enabling JMX

The following commands builds a JMX enabled image.

pack build samples/java \
  --path java/jar \
  --env BP_JMX_ENABLED=true
copy to clipboard

To run the image with the JMX port published:

docker run --env BPL_JMX_ENABLED=true --publish 5000:5000 samples/java
copy to clipboard

Connect JConsole to the published port. JConsole

Selecting a Process

The Java buildpack will contribute a default process type that starts the application.

Example: Running the default process

Execute the following commands to start the default process type using a samples/java image built from any previous example command.

docker run  --rm --publish 8080:8080 samples/java
curl -s http://localhost:8080/actuator/health
copy to clipboard

Providing Additional Arguments

Additional arguments can be provided to the application using the container CMD. In Kubernetes set CMD using the args field on the container resource.

Example: Setting the Server Port

Execute the following command passes an additional argument to application start command, setting the port to 8081.

docker run --rm --publish 8081:8081 samples/java --server.port=8081
curl -s http://localhost:8081/actuator/health
copy to clipboard

Executing a Custom Command

To override the buildpack-provided start command with a custom command, set the container ENTRYPOINT

Example: Starting an Interactive Shell

The following command runs Bash interactively:

docker run --rm --entrypoint bash samples/java
copy to clipboard

Executing a Custom Command in the Buildpack-Provided Environment

Every buildpack-generated image contains an executable called the launcher which can be used to execute a custom command in an environment containing buildpack-provided environment variables. The launcher will execute any buildpack provided profile scripts before running to provided command, in order to set environment variables with values that should be calculated dynamically at runtime.

To run a custom start command in the buildpack-provided environment set the ENTRYPOINT to launcher and provide the command using the container CMD.

Example: Inspecting the Buildpack-Provided JAVA_TOOL_OPTIONS The following command will print value of $JAVA_TOOL_OPTIONS set by the buildpack:

docker run --rm --entrypoint launcher samples/java echo 'JAVA_TOOL_OPTIONS: $JAVA_TOOL_OPTIONS'
copy to clipboard

Each argument provided to the launcher will be evaluated by the shell prior to execution and the original tokenization will be preserved. Note that, in the example above 'JAVA_TOOL_OPTIONS: $JAVA_TOOL_OPTIONS' is single quoted so that $JAVA_TOOL_OPTIONS is evaluated in the container, rather than by the host shell.


The following component buildpacks compose the Java Buildpack. Buildpacks are listed in the order they are executed.

Buildpack Required/Optional Responsibility
Paketo CA Certificates Buildpack Optional Adds CA certificates to the system truststore at build and runtime.
Paketo BellSoft Liberica Buildpack Required Provides the JDK and/or JRE.
Paketo Gradle Buildpack Optional Builds Gradle-based applications from source.
Paketo Leiningen Buildpack Optional Builds Leiningen-based applications from source.
Paketo Maven Buildpack Optional Builds Maven-based applications from source.
Paketo SBT Buildpack Optional Builds SBT-based applications from source.
Paketo Executable JAR Buildpack Optional Contributes a process Type that launches an executable JAR.
Paketo Apache Tomcat Buildpack Optional Contributes Apache Tomcat and a process type that launches a WAR with Tomcat.
Paketo DistZip Buildpack Optional Contributes a process type that launches a DistZip-style application.
Paketo Spring Boot Buildpack Optional Contributes configuration and metadata to Spring Boot applications.
Paketo Procfile Buildpack Optional Allows the application to define or redefine process types with a Procfile
Paketo Azure Application Insights Buildpack Optional Contributes the Application Insights Agent and configures it to connect to the service.
Paketo Debug Buildpack Optional Configures debugging for JVM applications.
Paketo Google Stackdriver Buildpack Optional Contributes Stackdriver agents and configures them to connect to the service.
Paketo JMX Buildpack Optional Configures JMX for JVM applications.
Paketo Encrypt At Rest Buildpack Optional Encrypts an application layer and contributes a profile script that decrypts it at launch time.
Paketo Environment Variables Buildpack Optional Contributes arbitrary user-provided environment variables to the image.
Paketo Image Labels Buildpack Optional Contributes OCI-specific and arbitrary user-provided labels to the image.

Last modified: July 23, 2021