docs/source/cdata.rst

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================
C Data Interface
================

Arrow supports exchanging data without copying or serialization within the same process
through :external+arrow:ref:`c-data-interface`, even between different language runtimes.

.. note::

   The Arrow Java C Data Interface implementation does not support importing arrays with
   a non-zero offset.

Java to Python
--------------

See :external+arrow:doc:`python/integration/python_java` to implement Java to
Python communication using the C Data Interface.

Java to C++
-----------

See :external+arrow:doc:`developers/cpp/building` to build the Arrow C++ libraries:

.. code-block:: shell

   $ git clone https://github.com/apache/arrow.git
   $ cd arrow/cpp
   $ mkdir build   # from inside the `cpp` subdirectory
   $ cd build
   $ cmake .. --preset ninja-debug-minimal
   $ cmake --build .
   $ tree debug/
   debug/
   ├── libarrow.800.0.0.dylib
   ├── libarrow.800.dylib -> libarrow.800.0.0.dylib
   └── libarrow.dylib -> libarrow.800.dylib

Share an Int64 array from C++ to Java
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**C++ Side**

Implement a function in CDataCppBridge.h that exports an array via the C Data Interface:

.. code-block:: cpp

   #include <iostream>
   #include <arrow/api.h>
   #include <arrow/c/bridge.h>

   void FillInt64Array(const uintptr_t c_schema_ptr, const uintptr_t c_array_ptr) {
       arrow::Int64Builder builder;
       builder.Append(1);
       builder.Append(2);
       builder.Append(3);
       builder.AppendNull();
       builder.Append(5);
       builder.Append(6);
       builder.Append(7);
       builder.Append(8);
       builder.Append(9);
       builder.Append(10);
       std::shared_ptr<arrow::Array> array = *builder.Finish();

       struct ArrowSchema* c_schema = reinterpret_cast<struct ArrowSchema*>(c_schema_ptr);
       auto c_schema_status = arrow::ExportType(*array->type(), c_schema);
       if (!c_schema_status.ok()) c_schema_status.Abort();

       struct ArrowArray* c_array = reinterpret_cast<struct ArrowArray*>(c_array_ptr);
       auto c_array_status = arrow::ExportArray(*array, c_array);
       if (!c_array_status.ok()) c_array_status.Abort();
   }

**Java Side**

For this example, we will use `JavaCPP`_ to call our C++ function from Java,
without writing JNI bindings ourselves.

.. 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>java-cdata-example</artifactId>
       <version>1.0-SNAPSHOT</version>

       <properties>
           <maven.compiler.source>17</maven.compiler.source>
           <maven.compiler.target>17</maven.compiler.target>
           <arrow.version>9.0.0</arrow.version>
       </properties>
       <dependencies>
           <dependency>
               <groupId>org.bytedeco</groupId>
               <artifactId>javacpp</artifactId>
               <version>1.5.7</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-c-data</artifactId>
               <version>${arrow.version}</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-vector</artifactId>
               <version>${arrow.version}</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-memory-core</artifactId>
               <version>${arrow.version}</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-memory-netty</artifactId>
               <version>${arrow.version}</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-format</artifactId>
               <version>${arrow.version}</version>
           </dependency>
       </dependencies>
   </project>

.. code-block:: java

   import org.bytedeco.javacpp.annotation.Platform;
   import org.bytedeco.javacpp.annotation.Properties;
   import org.bytedeco.javacpp.tools.InfoMap;
   import org.bytedeco.javacpp.tools.InfoMapper;

   @Properties(
           target = "CDataJavaToCppExample",
           value = @Platform(
                   include = {
                           "CDataCppBridge.h"
                   },
                   compiler = {"cpp17"},
                   linkpath = {"/arrow/cpp/build/debug/"},
                   link = {"arrow"}
           )
   )
   public class CDataJavaConfig implements InfoMapper {

       @Override
       public void map(InfoMap infoMap) {
       }
   }

.. code-block:: shell

   # Compile our Java code
   $ javac -cp javacpp-1.5.7.jar CDataJavaConfig.java

   # Generate CDataInterfaceLibrary
   $ java -jar javacpp-1.5.7.jar CDataJavaConfig.java

   # Generate libjniCDataInterfaceLibrary.dylib
   $ java -jar javacpp-1.5.7.jar CDataJavaToCppExample.java

   # Validate libjniCDataInterfaceLibrary.dylib created
   $ otool -L macosx-x86_64/libjniCDataJavaToCppExample.dylib
   macosx-x86_64/libjniCDataJavaToCppExample.dylib:
       libjniCDataJavaToCppExample.dylib (compatibility version 0.0.0, current version 0.0.0)
       @rpath/libarrow.800.dylib (compatibility version 800.0.0, current version 800.0.0)
       /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1200.3.0)
       /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1311.0.0)

**Java Test**

Let's create a Java class to test our bridge:

.. code-block:: java

   import org.apache.arrow.c.ArrowArray;
   import org.apache.arrow.c.ArrowSchema;
   import org.apache.arrow.c.Data;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.BigIntVector;

   public class TestCDataInterface {
       public static void main(String[] args) {
           try(
               BufferAllocator allocator = new RootAllocator();
               ArrowSchema arrowSchema = ArrowSchema.allocateNew(allocator);
               ArrowArray arrowArray = ArrowArray.allocateNew(allocator)
           ){
               CDataJavaToCppExample.FillInt64Array(
                       arrowSchema.memoryAddress(), arrowArray.memoryAddress());
               try(
                   BigIntVector bigIntVector = (BigIntVector) Data.importVector(
                           allocator, arrowArray, arrowSchema, null)
               ){
                   System.out.println("C++-allocated array: " + bigIntVector);
               }
           }
       }
   }

.. code-block:: shell

   C++-allocated array: [1, 2, 3, null, 5, 6, 7, 8, 9, 10]

Share an Int32 array from Java to C++
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Java Side**

For this example, we will build a JAR with all dependencies bundled.

.. 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>cpptojava</artifactId>
       <version>1.0-SNAPSHOT</version>
       <properties>
           <maven.compiler.source>17</maven.compiler.source>
           <maven.compiler.target>17</maven.compiler.target>
           <arrow.version>9.0.0</arrow.version>
       </properties>
       <dependencies>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-c-data</artifactId>
               <version>${arrow.version}</version>
           </dependency>
           <dependency>
               <groupId>org.apache.arrow</groupId>
               <artifactId>arrow-memory-netty</artifactId>
               <version>${arrow.version}</version>
           </dependency>
       </dependencies>
       <build>
           <plugins>
               <plugin>
                   <groupId>org.apache.maven.plugins</groupId>
                   <artifactId>maven-assembly-plugin</artifactId>
                   <executions>
                       <execution>
                           <phase>package</phase>
                           <goals>
                               <goal>single</goal>
                           </goals>
                           <configuration>
                               <descriptorRefs>
                                   <descriptorRef>jar-with-dependencies</descriptorRef>
                               </descriptorRefs>
                           </configuration>
                       </execution>
                   </executions>
               </plugin>
           </plugins>
       </build>
   </project>

.. code-block:: java

   import org.apache.arrow.c.ArrowArray;
   import org.apache.arrow.c.ArrowSchema;
   import org.apache.arrow.c.Data;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.FieldVector;
   import org.apache.arrow.vector.IntVector;
   import org.apache.arrow.vector.VectorSchemaRoot;

   import java.util.Arrays;

   public class ToBeCalledByCpp {
       final static BufferAllocator allocator = new RootAllocator();

       /**
        * Create a {@link FieldVector} and export it via the C Data Interface
        * @param schemaAddress Schema memory address to wrap
        * @param arrayAddress Array memory address to wrap
        */
       public static void fillVector(long schemaAddress, long arrayAddress){
           try (ArrowArray arrow_array = ArrowArray.wrap(arrayAddress);
                ArrowSchema arrow_schema = ArrowSchema.wrap(schemaAddress) ) {
               Data.exportVector(allocator, populateFieldVectorToExport(), null, arrow_array, arrow_schema);
           }
       }

       /**
        * Create a {@link VectorSchemaRoot} and export it via the C Data Interface
        * @param schemaAddress Schema memory address to wrap
        * @param arrayAddress Array memory address to wrap
        */
       public static void fillVectorSchemaRoot(long schemaAddress, long arrayAddress){
           try (ArrowArray arrow_array = ArrowArray.wrap(arrayAddress);
                ArrowSchema arrow_schema = ArrowSchema.wrap(schemaAddress) ) {
               Data.exportVectorSchemaRoot(allocator, populateVectorSchemaRootToExport(), null, arrow_array, arrow_schema);
           }
       }

       private static FieldVector populateFieldVectorToExport(){
           IntVector intVector = new IntVector("int-to-export", allocator);
           intVector.allocateNew(3);
           intVector.setSafe(0, 1);
           intVector.setSafe(1, 2);
           intVector.setSafe(2, 3);
           intVector.setValueCount(3);
           System.out.println("[Java] FieldVector: \n" + intVector);
           return intVector;
       }

       private static VectorSchemaRoot populateVectorSchemaRootToExport(){
           IntVector intVector = new IntVector("age-to-export", allocator);
           intVector.setSafe(0, 10);
           intVector.setSafe(1, 20);
           intVector.setSafe(2, 30);
           VectorSchemaRoot root = new VectorSchemaRoot(Arrays.asList(intVector));
           root.setRowCount(3);
           System.out.println("[Java] VectorSchemaRoot: \n" + root.contentToTSVString());
           return root;
       }
   }

Build the JAR and copy it to the C++ project.

.. code-block:: shell

   $ mvn clean install
   $ cp target/cpptojava-1.0-SNAPSHOT-jar-with-dependencies.jar <c++ project path>/cpptojava.jar

**C++ Side**

This application uses JNI to call Java code, but transfers data (zero-copy) via the C Data Interface instead.

.. code-block:: cpp

   #include <iostream>
   #include <jni.h>

   #include <arrow/api.h>
   #include <arrow/c/bridge.h>

   JNIEnv *CreateVM(JavaVM **jvm) {
       JNIEnv *env;
       JavaVMInitArgs vm_args;
       JavaVMOption options[2];
       options[0].optionString = "-Djava.class.path=cpptojava.jar";
       options[1].optionString = "-DXcheck:jni:pedantic";
       vm_args.version = JNI_VERSION_10;
       vm_args.nOptions = 2;
       vm_args.options = options;
       int status = JNI_CreateJavaVM(jvm, (void **) &env, &vm_args);
       if (status < 0) {
           std::cerr << "\n<<<<< Unable to Launch JVM >>>>>\n" << std::endl;
           return nullptr;
       }
       return env;
   }

   int main() {
       JNIEnv *env;
       JavaVM *jvm;
       env = CreateVM(&jvm);
       if (env == nullptr) return EXIT_FAILURE;
       jclass javaClassToBeCalledByCpp = env->FindClass("ToBeCalledByCpp");
       if (javaClassToBeCalledByCpp != nullptr) {
           jmethodID fillVector = env->GetStaticMethodID(javaClassToBeCalledByCpp,
                                                         "fillVector",
                                                         "(JJ)V");
           if (fillVector != nullptr) {
               struct ArrowSchema arrowSchema;
               struct ArrowArray arrowArray;
               std::cout << "\n<<<<< C++ to Java for Arrays >>>>>\n" << std::endl;
               env->CallStaticVoidMethod(javaClassToBeCalledByCpp, fillVector,
                                         static_cast<jlong>(reinterpret_cast<uintptr_t>(&arrowSchema)),
                                         static_cast<jlong>(reinterpret_cast<uintptr_t>(&arrowArray)));
               auto resultImportArray = arrow::ImportArray(&arrowArray, &arrowSchema);
               std::shared_ptr<arrow::Array> array = resultImportArray.ValueOrDie();
               std::cout << "[C++] Array: " << array->ToString() << std::endl;
           } else {
               std::cerr << "Could not find fillVector method\n" << std::endl;
               return EXIT_FAILURE;
           }
           jmethodID fillVectorSchemaRoot = env->GetStaticMethodID(javaClassToBeCalledByCpp,
                                                                   "fillVectorSchemaRoot",
                                                                   "(JJ)V");
           if (fillVectorSchemaRoot != nullptr) {
               struct ArrowSchema arrowSchema;
               struct ArrowArray arrowArray;
               std::cout << "\n<<<<< C++ to Java for RecordBatch >>>>>\n" << std::endl;
               env->CallStaticVoidMethod(javaClassToBeCalledByCpp, fillVectorSchemaRoot,
                                         static_cast<jlong>(reinterpret_cast<uintptr_t>(&arrowSchema)),
                                         static_cast<jlong>(reinterpret_cast<uintptr_t>(&arrowArray)));
               auto resultImportVectorSchemaRoot = arrow::ImportRecordBatch(&arrowArray, &arrowSchema);
               std::shared_ptr<arrow::RecordBatch> recordBatch = resultImportVectorSchemaRoot.ValueOrDie();
               std::cout << "[C++] RecordBatch: " << recordBatch->ToString() << std::endl;
           } else {
               std::cerr << "Could not find fillVectorSchemaRoot method\n" << std::endl;
               return EXIT_FAILURE;
           }
       } else {
           std::cout << "Could not find ToBeCalledByCpp class\n" << std::endl;
           return EXIT_FAILURE;
       }
       jvm->DestroyJavaVM();
       return EXIT_SUCCESS;
   }

CMakeLists.txt definition file:

.. code-block:: cmake

   cmake_minimum_required(VERSION 3.19)
   project(cdatacpptojava)
   find_package(JNI REQUIRED)
   find_package(Arrow REQUIRED)
   message(STATUS "Arrow version: ${ARROW_VERSION}")
   include_directories(${JNI_INCLUDE_DIRS})
   set(CMAKE_CXX_STANDARD 17)
   add_executable(${PROJECT_NAME} main.cpp)
   target_link_libraries(cdatacpptojava PRIVATE Arrow::arrow_shared)
   target_link_libraries(cdatacpptojava PRIVATE ${JNI_LIBRARIES})

**Result**

.. code-block:: text

   <<<<< C++ to Java for Arrays >>>>>
   [Java] FieldVector:
   [1, 2, 3]
   [C++] Array: [
     1,
     2,
     3
   ]

   <<<<< C++ to Java for RecordBatch >>>>>
   [Java] VectorSchemaRoot:
   age-to-export
   10
   20
   30

   [C++] RecordBatch: age-to-export:   [
     10,
     20,
     30
   ]

.. _`JavaCPP`: https://github.com/bytedeco/javacpp