FlatBuffers - 向量

概述

向量数据类型是FlatBuffers的复合数据类型之一。它等同于我们使用的语言（例如Java等）中的数组或列表。

继续我们从FlatBuffers - 字符串章节的剧院示例，以下是我们需要使用的语法，以指示FlatBuffers我们将创建一个向量：

theater.fbs

namespace com.tutorialspoint.theater;

table Theater {
   snacks:[string];  // vector of strings
   tickets:[float];    // vector of floats 	
}
root_type Theater;

现在我们的表格包含字符串和浮点数的向量属性。

从fbs文件创建Java类

要使用FlatBuffers，我们现在必须使用flatc二进制文件从此“.fbs”文件创建所需的类。让我们看看如何做到这一点：

flatc  --java theater.fbs

这将在当前目录的com > tutorialspoint > theater文件夹中创建一个Theater.java类。我们在应用程序中使用此类，类似于FlatBuffers - Schema章节中所做的。

使用从fbs文件创建的Java类

创建和写入向量

为了创建一个向量，我们需要首先准备标量类型数组的偏移量，然后我们可以将向量添加到flat buffer中。

// create data for an array of strings
int popcorn = builder.createString("Popcorn");
int coke = builder.createString("Coke");
int chips = builder.createString("Chips");
int soda = builder.createString("Soda");

// create array for snacks
int[] snacks = {popcorn, coke, chips, soda};

// create offset for snacks vector
int snacksVector = Theater.createSnacksVector(builder, snacks);

// add details to the Theater FlatBuffer
Theater.addSnacks(builder, snacksVector);

以下示例代码显示了创建字符串和整数向量的过程。

TheaterWriter.java

package com.tutorialspoint.theater;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import com.google.flatbuffers.FlatBufferBuilder;

public class TheaterWriter {
   public static void main(String[] args) throws FileNotFoundException, IOException {
      // create a flat buffer builder
      // it will be used to create Theater FlatBuffer
      FlatBufferBuilder builder = new FlatBufferBuilder(1024);

      // create data for an array of strings
      int popcorn = builder.createString("Popcorn");
      int coke = builder.createString("Coke");
      int chips = builder.createString("Chips");
      int soda = builder.createString("Soda");
      
      // create array for snacks
      int[] snacks = {popcorn, coke, chips, soda};
      
      // create array for tickets
      float[] tickets = {100.0f, 100.f, 200.f};
      
      // create offset for snacks vector
      int snacksVector = Theater.createSnacksVector(builder, snacks);
      
      // create offset for tickets vector
      int ticketsVector = Theater.createTicketsVector(builder, tickets);
      
      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add details to the Theater FlatBuffer
      Theater.addSnacks(builder, snacksVector);
      Theater.addTickets(builder, ticketsVector);       

      // mark end of data being entered in Greet FlatBuffer
      int theater = Theater.endTheater(builder);

      // finish the builder
      builder.finish(theater);

      // get the bytes to be stored
      byte[] data = builder.sizedByteArray();

      String filename = "theater_flatbuffers_output";
      System.out.println("Saving theater to file: " + filename);
      // write the builder content to the file named theater_flatbuffers_output
      try(FileOutputStream output = new FileOutputStream(filename)){
         output.write(data);
      }
      System.out.println("Saved theater with following data to disk: \n" + theater);
   }
}

读取向量

为了读取向量，我们有方法来获取向量的长度和通过索引获取条目，如下所示。

// iterate snacks vector of length determined by snacksLength() method
for(int i = 0; i < theater.snacksLength(); i++ ) {
   // get a snack by its index
   System.out.print(" " + theater.snacks(i));
}

以下示例代码显示了读取字符串和整数向量的过程。

TheaterReader.java

package com.tutorialspoint.theater;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;

public class TheaterReader {
   public static void main(String[] args) throws FileNotFoundException, IOException {

      String filename = "theater_flatbuffers_output";
      System.out.println("Reading from file " + filename);
      try(FileInputStream input = new FileInputStream(filename)) {
         // get the serialized data
         byte[] data = input.readAllBytes();
         ByteBuffer buf = ByteBuffer.wrap(data);
         // read the root object in serialized data
         Theater theater = Theater.getRootAsTheater(buf);
         // print theater values 
         System.out.println("Snacks: ");
         for(int i = 0; i < theater.snacksLength(); i++ ) {
            System.out.print(" " + theater.snacks(i));
         }
         System.out.println("\nTickets: ");
         for(int i = 0; i < theater.ticketsLength(); i++ ) {
            System.out.print(" " + theater.tickets(i));
         }         
      }
   }
}

编译项目

现在我们已经设置了读取器和写入器，让我们编译项目。

mvn clean install

序列化Java对象

现在，编译后，让我们首先执行写入器：

> java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterWriter

Saving theater information to file: theater_flatbuffers_output
Saved theater information with following data to disk:
96

反序列化序列化的对象

现在，让我们执行读取器从同一文件读取：

java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterReader

Reading from file theater_flatbuffers_output
Snacks:
 Popcorn Coke Chips Soda
Tickets:
 100.0 100.0 200.0

因此，正如我们所看到的，我们能够通过将二进制数据反序列化为Theater对象来读取序列化的向量。在下一章FlatBuffers - 结构体中，我们将了解结构体，一种复合类型。

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