FlatBuffers - 联合体

概述

联合体数据类型是 FlatBuffers 的复合数据类型之一。它用于创建灵活的数据结构，可以采用任何所需类型。

继续我们从Flat Buffers - 字符串章节的剧院示例，以下是我们需要用来指示 FlatBuffers 将要创建一个联合体的语法：

theater.fbs

namespace com.tutorialspoint.theater;

union People { Employee, Viewer }

table Theater {
   people: People;
}

table Employee {
   name:string;
   address:string;
   id: int;
}

table Viewer {
   name: string;
   address: string;
}
root_type Theater;

现在我们的表包含定义为两个表 Employee 和 Viewer 的 People 的联合体属性。在 Theater 表中，我们定义了联合类型的 people，这意味着我们可以将 Employee 或 Viewer 中的任何一个存储在 people 变量中。

从 fbs 文件创建 Java 类

要使用 FlatBuffers，我们现在必须使用flatc二进制文件从这个“.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

这将在当前目录中的com > tutorialspoint > theater文件夹中创建Theater、People、Employee和Viewer类。我们在应用程序中使用此类，与Flat Buffers - 模式章节中所做的一样。

使用从 fbs 文件创建的 Java 类

创建和写入联合体

为了创建一个联合体，我们需要首先准备所需类型的偏移量，例如 Viewer，然后我们可以将 viewer 及其类型添加到 flat buffer。

// create offset for Viewer
int viewerName = builder.createString("Mery");
int viewerAddress = builder.createString("Avenue 4");
int viewer = Viewer.createViewer(builder, viewerName, viewerAddress);

//add union tyoe
Theater.addPeopleType(builder, People.Viewer);
// add details to the Theater FlatBuffer
Theater.addPeople(builder, viewer);

下面的示例代码显示了创建联合体的过程。

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 offset for Viewer
      int viewerName = builder.createString("Mery");
      int viewerAddress = builder.createString("Avenue 4");
      int viewer = Viewer.createViewer(builder, viewerName, viewerAddress);
      
      // create offset for vector
      //int people = Theater.createPeople
      
      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);

      //add union type
      Theater.addPeopleType(builder, People.Viewer);

      // add details to the Theater FlatBuffer
      Theater.addPeople(builder, viewer);
      
      // 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);
   }
}

读取联合体

为了读取联合体，我们可以检查联合体对象的类型，然后相应地检索值。

// get the saved union type
int unionType = theater.peopleType();

// if union is of type Viewer
if(unionType == People.Viewer) {
   Viewer viewer = (Viewer)theater.people(new Viewer());
   System.out.println("Name: " + viewer.name());
   System.out.println("Address: " + viewer.address());       	 
} 
// if union is of type Employee
else if(unionType == People.Employee) {
   Employee employee = (Employee)theater.people(new Employee());
   System.out.println("Name: " + employee.name());
   System.out.println("Address: " + employee.address());  
   System.out.println("Id: " + employee.id()); 
}

下面的示例代码显示了读取联合体的过程。

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("People: ");
         // get the type of union
		 int unionType = theater.peopleType();
         // if union is of Viewer type  
         if(unionType == People.Viewer) {
            Viewer viewer = (Viewer)theater.people(new Viewer());
            System.out.println("Name: " + viewer.name());
            System.out.println("Address: " + viewer.address());       	 
         } else if(unionType == People.Employee) {
            Employee employee = (Employee)theater.people(new Employee());
            System.out.println("Name: " + employee.name());
            System.out.println("Address: " + employee.address());  
            System.out.println("Id: " + employee.id()); 
         }       
      }
   }
}

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:
60

反序列化序列化的对象

现在，让我们执行读取器以从同一文件读取：

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

Reading from file theater_flatbuffers_output
People:
Name: Mery
Address: Avenue 4

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结构体。在下一章Flat Buffers - 嵌套表中，我们将了解复合类型的嵌套表。

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