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diff --git a/src/arrow/java/README.md b/src/arrow/java/README.md new file mode 100644 index 000000000..29d1fcf4c --- /dev/null +++ b/src/arrow/java/README.md @@ -0,0 +1,164 @@ +<!--- + Licensed to the Apache Software Foundation (ASF) under one + or more contributor license agreements. See the NOTICE file + distributed with this work for additional information + regarding copyright ownership. The ASF licenses this file + to you under the Apache License, Version 2.0 (the + "License"); you may not use this file except in compliance + with the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, + software distributed under the License is distributed on an + "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + KIND, either express or implied. See the License for the + specific language governing permissions and limitations + under the License. +--> + +# Arrow Java + +## Getting Started + +The following guides explain the fundamental data structures used in the Java implementation of Apache Arrow. + +- [ValueVector](https://arrow.apache.org/docs/java/vector.html) is an abstraction that is used to store a sequence of values having the same type in an individual column. +- [VectorSchemaRoot](https://arrow.apache.org/docs/java/vector_schema_root.html) is a container that can hold multiple vectors based on a schema. +- The [Reading/Writing IPC formats](https://arrow.apache.org/docs/java/ipc.html) guide explains how to stream record batches as well as serializing record batches to files. + +Generated javadoc documentation is available [here](https://arrow.apache.org/docs/java/). + +## Setup Build Environment + +install: + - Java 8 or later + - Maven 3.3 or later + +## Building and running tests + +``` +git submodule update --init --recursive # Needed for flight +cd java +mvn install +``` +## Building and running tests for arrow jni modules like gandiva and orc (optional) + +[Arrow Cpp][2] must be built before this step. The cpp build directory must +be provided as the value for argument arrow.cpp.build.dir. eg. + +``` +cd java +mvn install -P arrow-jni -am -Darrow.cpp.build.dir=../../release +``` + +The gandiva library is still in Alpha stages, and subject to API changes without +deprecation warnings. + +## Flatbuffers dependency + +Arrow uses Google's Flatbuffers to transport metadata. The java version of the library +requires the generated flatbuffer classes can only be used with the same version that +generated them. Arrow packages a version of the arrow-vector module that shades flatbuffers +and arrow-format into a single JAR. Using the classifier "shade-format-flatbuffers" in your +pom.xml will make use of this JAR, you can then exclude/resolve the original dependency to +a version of your choosing. + +### Updating the flatbuffers generated code + +1. Verify that your version of flatc matches the declared dependency: + +```bash +$ flatc --version +flatc version 1.12.0 + +$ grep "dep.fbs.version" java/pom.xml + <dep.fbs.version>1.12.0</dep.fbs.version> +``` + +2. Generate the flatbuffer java files by performing the following: + +```bash +cd $ARROW_HOME + +# remove the existing files +rm -rf java/format/src + +# regenerate from the .fbs files +flatc --java -o java/format/src/main/java format/*.fbs + +# prepend license header +find java/format/src -type f | while read file; do + (cat header | while read line; do echo "// $line"; done; cat $file) > $file.tmp + mv $file.tmp $file +done +``` + +## Performance Tuning + +There are several system/environmental variables that users can configure. These trade off safety (they turn off checking) for speed. Typically they are only used in production settings after the code has been thoroughly tested without using them. + +* Bounds Checking for memory accesses: Bounds checking is on by default. You can disable it by setting either the +system property("arrow.enable_unsafe_memory_access") or the environmental variable +("ARROW_ENABLE_UNSAFE_MEMORY_ACCESS") to "true". When both the system property and the environmental +variable are set, the system property takes precedence. + +* null checking for gets: ValueVector get methods (not getObject) methods by default verify the slot is not null. You can disable it by setting either the +system property("arrow.enable_null_check_for_get") or the environmental variable +("ARROW_ENABLE_NULL_CHECK_FOR_GET") to "false". When both the system property and the environmental +variable are set, the system property takes precedence. + +## Java Properties + + * For java 9 or later, should set "-Dio.netty.tryReflectionSetAccessible=true". +This fixes `java.lang.UnsupportedOperationException: sun.misc.Unsafe or java.nio.DirectByteBuffer.(long, int) not available`. thrown by netty. + * To support duplicate fields in a `StructVector` enable "-Darrow.struct.conflict.policy=CONFLICT_APPEND". +Duplicate fields are ignored (`CONFLICT_REPLACE`) by default and overwritten. To support different policies for +conflicting or duplicate fields set this JVM flag or use the correct static constructor methods for `StructVector`s. + +## Java Code Style Guide + +Arrow Java follows the Google style guide [here][3] with the following +differences: + +* Imports are grouped, from top to bottom, in this order: static imports, +standard Java, org.\*, com.\* +* Line length can be up to 120 characters +* Operators for line wrapping are at end-of-line +* Naming rules for methods, parameters, etc. have been relaxed +* Disabled `NoFinalizer`, `OverloadMethodsDeclarationOrder`, and +`VariableDeclarationUsageDistance` due to the existing code base. These rules +should be followed when possible. + +Refer to `java/dev/checkstyle/checkstyle.xml for rule specifics. + +## Test Logging Configuration + +When running tests, Arrow Java uses the Logback logger with SLF4J. By default, +it uses the logback.xml present in the corresponding module's src/test/resources +directory, which has the default log level set to INFO. +Arrow Java can be built with an alternate logback configuration file using the +following command run in the project root directory: + +```bash +mvn -Dlogback.configurationFile=file:<path-of-logback-file> +``` + +See [Logback Configuration][1] for more details. + +## Integration Tests + +Integration tests which require more time or more memory can be run by activating +the `integration-tests` profile. This activates the [maven failsafe][4] plugin +and any class prefixed with `IT` will be run during the testing phase. The integration +tests currently require a larger amount of memory (>4GB) and time to complete. To activate +the profile: + +```bash +mvn -Pintegration-tests <rest of mvn arguments> +``` + +[1]: https://logback.qos.ch/manual/configuration.html +[2]: https://github.com/apache/arrow/blob/master/cpp/README.md +[3]: http://google.github.io/styleguide/javaguide.html +[4]: https://maven.apache.org/surefire/maven-failsafe-plugin/ |