FlatBuffers - 向後相容性

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概述

FlatBuffers schema 具有向後相容性。這意味著，如果我們稍後更改、新增或刪除 FlatBuffers schema 的屬性，現有程式碼仍然可以工作。這在維護遺留程式碼庫時非常有用。考慮一個場景，其中劇院 Schema 只包含名稱和地址，如下所示：

theater.fbs

namespace com.tutorialspoint.theater;

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

如果我們為此 schema 生成程式碼，它將支援在 FlatBuffer 二進位制檔案中儲存名稱和地址。

現在隨著時間的推移，我們需要向 schema 新增一個手機號，那麼我們需要再次生成更新的程式碼。結果，我們也需要更新寫入器和讀取器程式碼。但在生產環境中，直接更改程式碼通常並不容易，並且進行此類更改可能會破壞整個系統。FlatBuffers 在這裡確保舊的讀取器程式碼仍然可以與新的基於 schema 生成的 FlatBuffers 二進位制檔案一起正常工作，無需任何更改。

從 fbs 檔案建立 Java 類

要使用 FlatBuffers，我們現在必須使用 **flatc** 二進位制檔案從此“.fbs”檔案建立所需的類。讓我們看看如何做到這一點：

flatc  --java theater.fbs

這將在當前目錄的 **com > tutorialspoint > theater** 資料夾中建立一個 Theater.java 類。我們在應用程式中使用此類，類似於在 Flat Buffers - Schema 章節中所做的。

使用從 fbs 檔案建立的 Java 類

首先，讓我們建立一個 **寫入器** 來寫入 **劇院** 資訊：

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);

      int name = builder.createString("Silver Screener");
      int address = builder.createString("212, Maple Street, LA, California");

      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add the name and address to the Theater FlatBuffer
      Theater.addName(builder, name);
      Theater.addAddress(builder, address);

      // 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);
   }
}

接下來，我們將有一個 **讀取器** 來讀取 **劇院** 資訊：

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("Name: " + theater.name());
         System.out.println("Address: " + theater.address());
      }
   }
}

編譯專案

現在我們已經設定了 **讀取器** 和 **寫入器**，讓我們編譯專案。

mvn clean install

序列化 Java 物件

現在，編譯後，讓我們首先執行 **寫入器**：

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

Saving theater to file: theater_flatbuffers_output
Saved theater with following data to disk:
72

反序列化已序列化的物件

現在，讓我們執行 **讀取器** 從同一個檔案讀取：

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

Reading from file theater_flatbuffers_output
Name: Silver Screener
Address: 212, Maple Street, LA, California

向後相容性測試

現在讓我們向 schema 新增一個手機號，更新寫入器並執行讀取器（無需更新它）以檢查向後相容性。

theater.fbs

namespace com.tutorialspoint.theater;

table Theater {
   name:string;
   address:string;
   mobile:int;
}
root_type Theater;

從 fbs 檔案建立 Java 類

使用 **flatc** 二進位制檔案從此“.fbs”檔案建立所需的類。

flatc  --java theater.fbs

使用從 fbs 檔案建立的 Java 類

首先，讓我們建立一個 **寫入器** 來寫入 **劇院** 資訊：

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);

      int name = builder.createString("Silver Screener");
      int address = builder.createString("212, Maple Street, LA, California");

      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add the name, address and mobile to the Theater FlatBuffer
      Theater.addName(builder, name);
      Theater.addAddress(builder, address);
      Theater.addMobile(builder, 12233345);

      // 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);
   }
}

編譯專案

現在我們已經設定了 **寫入器**，讓我們編譯專案。

mvn clean install

序列化 Java 物件

現在，編譯後，讓我們首先執行 **寫入器**：

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

Saving theater to file: theater_flatbuffers_output
Saved theater with following data to disk:
76

使用舊讀取器反序列化已序列化的物件

現在，讓我們執行 **讀取器** 從同一個檔案讀取：

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

Reading from file theater_flatbuffers_output
Name: Silver Screener
Address: 212, Maple Street, LA, California

更新讀取器並再次反序列化

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("Name: " + theater.name());
         System.out.println("Address: " + theater.address());
         System.out.println("Mobile: " + theater.mobile());
      }
   }
}

編譯專案

現在我們已經設定了 **讀取器** 和 **寫入器**，讓我們編譯專案。

mvn clean install

反序列化已序列化的物件

現在，讓我們執行 **讀取器** 從同一個檔案讀取：

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

Reading from file theater_flatbuffers_output
Name: Silver Screener
Address: 212, Maple Street, LA, California
Mobile: 12233345
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