总览

先来看一下 FlatBuffers 项目已经为我们提供了什么，而我们在将 FlatBuffers 用到我们的项目中时又需要做什么的整体流程。如下图：

在使用 FlatBuffers 时，我们需要以特殊的格式定义我们的结构化数据，保存为 .fbs 文件。FlatBuffers 项目为我们提供了编译器，可用于将 .fbs 文件编译为Java文件，C++文件等，以用于我们的项目。FlatBuffers 编译器在我们的开发机，比如Ubuntu，Mac上运行。这些源代码文件是基于 FlatBuffers 提供的Java库生成的，同时我们也需要利用这个Java库的一些接口来序列化或解析数据。

我们将 FlatBuffers 编译器生成的Java文件及 FlatBuffers 的Java库导入我们的项目，就可以用 FlatBuffers 来对我们的结构化数据执行序列化和反序列化了。尽管每次手动执行 FlatBuffers 编译器生成Java文件非常麻烦，但不像 Protocol Buffers 那样，当前还没有Google官方提供的gradle插件可用。不过，我们这边开发了一个简单的 FlatBuffers gradle插件，后面会简单介绍一下，欢迎大家使用。

接下来我们更详细地看一下上面流程中的各个部分。

下载、编译 FlatBuffers 编译器

我们可以在如下位置：

https://github.com/google/flatbuffers/releases

获取官方发布的打包好的版本。针对Windows平台有编译好的可执行安装文件，对其它平台还是打包的源文件。我们也可以指向clone repo的代码，进行手动编译。这里我们从GitHub上clone代码并手动编译编译器：

$ git clone https://github.com/google/flatbuffers.git
Cloning into 'flatbuffers'...
remote: Counting objects: 7340, done.
remote: Compressing objects: 100% (46/46), done.
remote: Total 7340 (delta 16), reused 0 (delta 0), pack-reused 7290
Receiving objects: 100% (7340/7340), 3.64 MiB | 115.00 KiB/s, done.
Resolving deltas: 100% (4692/4692), done.
Checking connectivity... done.

下载代码之后，我们需要用cmake工具来为flatbuffers生成Makefile文件并编译：

$ cd flatbuffers/
$ cmake CMakeLists.txt 
-- The C compiler identification is AppleClang 7.3.0.7030031
-- The CXX compiler identification is AppleClang 7.3.0.7030031
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/netease/Projects/OpenSource/flatbuffers
$ make && make install

安装之后执行如下命令以确认已经装好：

$ flatc --version
flatc version 1.4.0 (Dec  7 2016)

flatc没有为我们提供 --help 选项，不过加了错误的参数时这个工具会为我们展示详细的用法：

$ flatc --help
flatc: unknown commandline argument: --help
usage: flatc [OPTION]... FILE... [-- FILE...]
  --binary     -b Generate wire format binaries for any data definitions.
  --json       -t Generate text output for any data definitions.
  --cpp        -c Generate C++ headers for tables/structs.
  --go         -g Generate Go files for tables/structs.
  --java       -j Generate Java classes for tables/structs.
  --js         -s Generate JavaScript code for tables/structs.
  --csharp     -n Generate C# classes for tables/structs.
  --python     -p Generate Python files for tables/structs.
  --php           Generate PHP files for tables/structs.
  -o PATH            Prefix PATH to all generated files.
  -I PATH            Search for includes in the specified path.
  -M                 Print make rules for generated files.
  --version          Print the version number of flatc and exit.
  --strict-json      Strict JSON: field names must be / will be quoted,
                     no trailing commas in tables/vectors.
  --allow-non-utf8   Pass non-UTF-8 input through parser and emit nonstandard
                     \x escapes in JSON. (Default is to raise parse error on
                     non-UTF-8 input.)
  --defaults-json    Output fields whose value is the default when
                     writing JSON
  --unknown-json     Allow fields in JSON that are not defined in the
                     schema. These fields will be discared when generating
                     binaries.
  --no-prefix        Don't prefix enum values with the enum type in C++.
  --scoped-enums     Use C++11 style scoped and strongly typed enums.
                     also implies --no-prefix.
  --gen-includes     (deprecated), this is the default behavior.
                     If the original behavior is required (no include
                     statements) use --no-includes.
  --no-includes      Don't generate include statements for included
                     schemas the generated file depends on (C++).
  --gen-mutable      Generate accessors that can mutate buffers in-place.
  --gen-onefile      Generate single output file for C#.
  --gen-name-strings Generate type name functions for C++.
  --escape-proto-ids Disable appending '_' in namespaces names.
  --gen-object-api   Generate an additional object-based API.
  --cpp-ptr-type T   Set object API pointer type (default std::unique_ptr)
  --raw-binary       Allow binaries without file_indentifier to be read.
                     This may crash flatc given a mismatched schema.
  --proto            Input is a .proto, translate to .fbs.
  --schema           Serialize schemas instead of JSON (use with -b)
  --conform FILE     Specify a schema the following schemas should be
                     an evolution of. Gives errors if not.
  --conform-includes Include path for the schema given with --conform
    PATH             
FILEs may be schemas, or JSON files (conforming to preceding schema)
FILEs after the -- must be binary flatbuffer format files.
Output files are named using the base file name of the input,
and written to the current directory or the path given by -o.
example: flatc -c -b schema1.fbs schema2.fbs data.json

创建 .fbs 文件

flatc支持将为 Protocol Buffers 编写的 .proto 文件转换为 .fbs 文件，如：

$ ls
addressbook.proto
$ flatc --proto addressbook.proto 
$ ls -l
total 16
-rw-r--r--  1 netease  staff  431 12  7 17:21 addressbook.fbs
-rw-r--r--@ 1 netease  staff  486 12  1 15:18 addressbook.proto

Protocol Buffers 消息文件中的一些写法，FlatBuffers 编译器还不能很好的支持，如option java_package，option java_outer_classname，和嵌套类。这里我们基于 FlatBuffers 编译器转换的 .proto 文件来获得我们的 .fbs 文件：

// Generated from addressbook.proto

namespace com.example.tutorial;

enum PhoneType : int {
  MOBILE = 0,
  HOME = 1,
  WORK = 2,
}

namespace com.example.tutorial;

table Person {
  name:string (required);
  id:int;
  email:string;
  phone:[com.example.tutorial._Person.PhoneNumber];
}

namespace com.example.tutorial._Person;

table PhoneNumber {
  number:string (required);
  type:int;
}

namespace com.example.tutorial;

table AddressBook {
  person:[com.example.tutorial.Person];
}

root_type AddressBook;

可以参考 官方的文档 来了解 .fbs 文件的详细的写法。

编译 .fbs 文件

可以通过如下命令编译 .fbs 文件：

$ flatc --java -o out addressbook.fbs

--java用于指定编译的目标编程语言。-o 参数则用于指定输出文件的路径，如过没有提供则将当前目录用作输出目录。FlatBuffers 编译器按照为不同的数据结构声明的namespace生成目录结构。对于上面的例子，会生成如下的这些文件：

$ find out
p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Menlo}span.s1 {font-variant-ligatures: no-common-ligatures}

$ find out/
out/
out//com
out//com/example
out//com/example/tutorial
out//com/example/tutorial/_Person
out//com/example/tutorial/_Person/PhoneNumber.java
out//com/example/tutorial/AddressBook.java
out//com/example/tutorial/Person.java
out//com/example/tutorial/PhoneType.java

在Android项目中使用 FlatBuffers

我们将前面由 .fbs 文件生成的Java文件拷贝到我们的项目中。我们前面提到的，FlatBuffers 的Java库比较薄，当前官方并没有发布到jcenter这样的maven仓库中，因而我们需要将这部分代码也拷贝到我们的额项目中。FlatBuffers 的Java库在其repo仓库的 java 目录下。我们有将这部分代码打包，放在公司的maven仓库中，引用的方法为，修改应用程序的 build.gradle：

repositories {
    maven {
        url "http://mvn.hz.netease.com/artifactory/libs-releases/"
    }
    maven {
        url "http://mvn.hz.netease.com/artifactory/libs-snapshots/"
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile project(':netlib')

    testCompile 'junit:junit:4.12'

    compile 'com.netease.hearttouch:ht-flatbuffers:0.0.1-SNAPSHOT'
}

添加访问 FlatBuffers 的类：

package com.netease.volleydemo;

import com.example.tutorial.AddressBook;
import com.example.tutorial.Person;
import com.example.tutorial._Person.PhoneNumber;
import com.google.flatbuffers.FlatBufferBuilder;

import java.nio.ByteBuffer;

/**
 * Created by hanpfei0306 on 16-12-5.
 */

public class AddressBookFlatBuffers {
    public static byte[] encodeTest(String[] names) {
        FlatBufferBuilder builder = new FlatBufferBuilder(0);

        int[] personOffsets = new int[names.length];

        for (int i = 0; i < names.length; ++ i) {
            int name = builder.createString(names[i]);
            int email = builder.createString("zhangsan@gmail.com");

            int number1 = builder.createString("0157-23443276");
            int type1 = 1;
            int phoneNumber1 = PhoneNumber.createPhoneNumber(builder, number1, type1);

            int number2 = builder.createString("136183667387");
            int type2 = 0;
            int phoneNumber2 = PhoneNumber.createPhoneNumber(builder, number2, type2);

            int[] phoneNubers = new int[2];
            phoneNubers[0] = phoneNumber1;
            phoneNubers[1] = phoneNumber2;

            int phoneNumbersPos = Person.createPhoneVector(builder, phoneNubers);

            int person = Person.createPerson(builder, name, 13958235, email, phoneNumbersPos);

            personOffsets[i] = person;
        }
        int persons = AddressBook.createPersonVector(builder, personOffsets);

        AddressBook.startAddressBook(builder);
        AddressBook.addPerson(builder, persons);
        int eab = AddressBook.endAddressBook(builder);
        builder.finish(eab);
        byte[] data = builder.sizedByteArray();
        return data;
    }

    public static byte[] encodeTest(String[] names, int times) {
        for (int i = 0; i < times - 1; ++ i) {
            encodeTest(names);
        }
        return encodeTest(names);
    }

    public static AddressBook decodeTest(byte[] data) {
        AddressBook addressBook = null;
        ByteBuffer byteBuffer = ByteBuffer.wrap(data);
        addressBook = AddressBook.getRootAsAddressBook(byteBuffer);
        return addressBook;
    }

    public static AddressBook decodeTest(byte[] data, int times) {
        AddressBook addressBook = null;
        for (int i = 0; i < times; ++ i) {
            addressBook = decodeTest(data);
        }
        return addressBook;
    }
}

使用 flatbuf-gradle-plugin

我们有开发一个 FlatBuffers 的gradle插件，以方便开发，项目位置。这个插件的设计有参考Google的protobuf-gradle-plugin，功能及用法也与protobuf-gradle-plugin类似。

应用flatbuf-gradle-plugin

修改应用程序的 build.gradle 以应用flatbuf-gradle-plugin。

1. 为buildscript添加对flatbuf-gradle-plugin的依赖：

   buildscript {
    repositories {
        maven {
            url "http://mvn.hz.netease.com/artifactory/libs-releases/"
        }
        maven {
            url "http://mvn.hz.netease.com/artifactory/libs-snapshots/"
        }
    }
    dependencies {
        classpath 'com.netease.hearttouch:ht-flatbuf-gradle-plugin:0.0.1-SNAPSHOT'
    }
   }
   

2. 在apply plugin: 'com.android.application'后面应用flatbuf的plugin：

   apply plugin: 'com.android.application'
   apply plugin: 'com.netease.flatbuf'
   

3. 添加flatbuf块，对flatbuf-gradle-plugin的执行做配置：

   flatbuf {
    flatc {
        path = '/usr/local/bin/flatc'
    }
   
    generateFlatTasks {
        all().each { task ->
            task.builtins {
                remove java
            }
            task.builtins {
                java { }
            }
        }
    }
   }
   

   flatc块用于配置 FlatBuffers 编译器，这里我们指定用我们之前手动编译的编译器。 task.builtins的块必不可少，这个块用于指定我们要为那些编程语言生成代码，这里我们为Java生成代码。

4. 指定 .fbs 文件的路径

    sourceSets {
        main {
            flat {
                srcDir 'src/main/flat'
            }
        }
    }
   

   我们将 FlatBuffers 的IDL文件放在src/main/flat目录下。

这样我们就不用再那么麻烦每次手动执行flatc了。

相关阅读：

在Android中使用FlatBuffers（上篇）

在Android中使用FlatBuffers（中篇）

在Android中使用FlatBuffers（下篇）

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