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=======================
Installing Java Modules
=======================

.. contents::

System Compatibility
====================

Java modules are regularly built and tested on macOS and Linux distributions.

Java Compatibility
==================

Java modules are compatible with JDK 11 and above. Currently, JDK versions
11, 17, 21, and latest are tested in CI.

Note that some JDK internals must be exposed by
adding ``--add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED`` to the ``java`` command:

.. code-block:: shell

   # Directly on the command line
   $ java --add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED -jar ...
   # Indirectly via environment variables
   $ env JDK_JAVA_OPTIONS="--add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED" java -jar ...

Otherwise, you may see errors like ``module java.base does not "opens
java.nio" to unnamed module`` or ``module java.base does not "opens
java.nio" to org.apache.arrow.memory.core``

Note that the command has changed from Arrow 15 and earlier. If you are still using the flags from that version
(``--add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED``) you will see the
``module java.base does not "opens java.nio" to org.apache.arrow.memory.core`` error.

If you are using flight-core or dependent modules, you will need to mark that flight-core can read unnamed modules.
Modifying the command above for Flight:

.. code-block:: shell

   # Directly on the command line
   $ java --add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED -jar ...
   # Indirectly via environment variables
   $ env JDK_JAVA_OPTIONS="--add-reads=org.apache.arrow.flight.core=ALL-UNNAMED --add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED" java -jar ...

Otherwise, you may see errors like ``java.lang.IllegalAccessError: superclass access check failed: class
org.apache.arrow.flight.ArrowMessage$ArrowBufRetainingCompositeByteBuf (in module org.apache.arrow.flight.core)
cannot access class io.netty.buffer.CompositeByteBuf (in unnamed module ...) because module
org.apache.arrow.flight.core does not read unnamed module ...``

Finally, if you are using arrow-dataset, you'll also need to report that JDK internals need to be exposed.
Modifying the command above for arrow-memory:

.. code-block:: shell

   # Directly on the command line
   $ java --add-opens=java.base/java.nio=org.apache.arrow.memory.core,ALL-UNNAMED -jar ...
   # Indirectly via environment variables
   $ env JDK_JAVA_OPTIONS="--add-opens=java.base/java.nio=org.apache.arrow.dataset,org.apache.arrow.memory.core,ALL-UNNAMED" java -jar ...

Otherwise you may see errors such as ``java.lang.RuntimeException: java.lang.reflect.InaccessibleObjectException:
Unable to make static void java.nio.Bits.reserveMemory(long,long) accessible: module
java.base does not "opens java.nio" to module org.apache.arrow.dataset``

If using Maven and Surefire for unit testing, :ref:`this argument must
be added to Surefire as well <java-install-maven-testing>`.

Installing from Maven
=====================

By default, Maven will download from the central repository: https://repo.maven.apache.org/maven2/org/apache/arrow/

Configure your pom.xml with the Java modules needed, for example:
arrow-vector, and arrow-memory-netty.

.. code-block:: xml

    <?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <modelVersion>4.0.0</modelVersion>
        <groupId>org.example</groupId>
        <artifactId>demo</artifactId>
        <version>1.0-SNAPSHOT</version>
        <properties>
            <arrow.version>9.0.0</arrow.version>
        </properties>
        <dependencies>
            <dependency>
                <groupId>org.apache.arrow</groupId>
                <artifactId>arrow-vector</artifactId>
                <version>${arrow.version}</version>
            </dependency>
            <dependency>
                <groupId>org.apache.arrow</groupId>
                <artifactId>arrow-memory-netty</artifactId>
                <version>${arrow.version}</version>
            </dependency>
        </dependencies>
    </project>

A bill of materials (BOM) module has been provided to simplify adding
Arrow modules. This eliminates the need to specify the version for
every module. An alternative to the above would be:

.. code-block:: xml

    <?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <modelVersion>4.0.0</modelVersion>
        <groupId>org.example</groupId>
        <artifactId>demo</artifactId>
        <version>1.0-SNAPSHOT</version>
        <properties>
            <arrow.version>15.0.0</arrow.version>
        </properties>
        <dependencies>
            <dependency>
                <groupId>org.apache.arrow</groupId>
                <artifactId>arrow-vector</artifactId>
            </dependency>
            <dependency>
                <groupId>org.apache.arrow</groupId>
                <artifactId>arrow-memory-netty</artifactId>
            </dependency>
        </dependencies>
        <dependencyManagement>
            <dependencies>
                <dependency>
                    <groupId>org.apache.arrow</groupId>
                    <artifactId>arrow-bom</artifactId>
                    <version>${arrow.version}</version>
                    <type>pom</type>
                    <scope>import</scope>
                </dependency>
            </dependencies>
        </dependencyManagement>
    </project>

To use the Arrow Flight dependencies, also add the ``os-maven-plugin``
plugin. This plugin generates useful platform-dependent properties
such as ``os.detected.name`` and ``os.detected.arch`` needed to resolve
transitive dependencies of Flight.

.. code-block:: xml

    <?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <modelVersion>4.0.0</modelVersion>
        <groupId>org.example</groupId>
        <artifactId>demo</artifactId>
        <version>1.0-SNAPSHOT</version>
        <properties>
            <arrow.version>9.0.0</arrow.version>
        </properties>
        <dependencies>
            <dependency>
                <groupId>org.apache.arrow</groupId>
                <artifactId>flight-core</artifactId>
                <version>${arrow.version}</version>
            </dependency>
        </dependencies>
        <build>
            <extensions>
                <extension>
                    <groupId>kr.motd.maven</groupId>
                    <artifactId>os-maven-plugin</artifactId>
                    <version>1.7.0</version>
                </extension>
            </extensions>
        </build>
    </project>

.. _java-install-maven-testing:

The ``--add-opens`` flag must be added when running unit tests through Maven:

.. code-block:: xml

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>3.0.0-M6</version>
                <configuration>
                        <argLine>--add-opens=java.base/java.nio=ALL-UNNAMED</argLine>
                </configuration>
            </plugin>
        </plugins>
    </build>

Or they can be added via environment variable, for example when executing your code:

.. code-block::

    JDK_JAVA_OPTIONS="--add-opens=java.base/java.nio=ALL-UNNAMED" mvn exec:java -Dexec.mainClass="YourMainCode"

Installing from Source
======================

See :ref:`java-development`.

IDE Configuration
=================

Generally, no additional configuration should be needed.  However,
ensure your Maven or other build configuration has the ``--add-opens``
flag as described above, so that the IDE picks it up and runs tests
with that flag as well.
