Avro序列化的两种实现方法：静态实现和动态实现

Avro 特点：

1. 丰富的数据结构

2. 紧凑、快速、二进制数据格式

3. 容器文件，持久化数据

4. 支持RPC

5. 代码生成是可选的，便于和动态语言进行整合

Avro 与 Thrift、Protobuf 对比：

1. Dynamic typing: Avro does not require that code be generated. Data is always accompanied by a schema that permits full processing of that data without code generation, static datatypes, etc. This facilitates construction of generic data-processing systems and languages.

2. Untagged data: Since the schema is present when data is read, considerably less type information need be encoded with data, resulting in smaller serialization size.

3. No manually-assigned field IDs: When a schema changes, both the old and new schema are always present when processing data, so differences may be resolved symbolically, using field names.

以下是下载安装步骤：

1. 下载avro-tools-1.8.1.jar，导入开发包

http://avro.apache.org/releases.html

https://mirrors.tuna.tsinghua.edu.cn/apache/avro/avro-1.8.1/java/

导入开发包：

<dependency>
  <groupId>org.apache.avro</groupId>
  <artifactId>avro</artifactId>
  <version>1.8.1</version>
</dependency>

2. 定义schema文件user.avsc

{"namespace": "example.avro",
 "type": "record",
 "name": "User",
 "fields": [
     {"name": "name", "type": "string"},
     {"name": "favorite_number",  "type": ["int", "null"]},
     {"name": "favorite_color", "type": ["string", "null"]}
 ]
}

3. 生成java代码，静态实现

语法：java -jar /path/to/avro-tools-1.8.1.jar compile schema <schema file> <destination>

java -jar /path/to/avro-tools-1.8.1.jar compile schema user.avsc .

public class SerializationByAvroSchema {
	
	
	public static void main(String[] args) throws Exception{
		readAvro() ;
	}

	static void writeAvro() throws Exception{
		Schema schema = new Schema.Parser().parse(new File("D:/tmp/user.avsc"));
		GenericRecord user1 = new GenericData.Record(schema);
		user1.put("name", "Alyssa");
		user1.put("favorite_number", 256);
		user1.put("favorite_color", "red");
		
		GenericRecord user2 = new GenericData.Record(schema);
		user2.put("name", "Alyssa2");
		user2.put("favorite_number", 258);
		user2.put("favorite_color", "green");

		DatumWriter<GenericRecord> datumWriter = new GenericDatumWriter<GenericRecord>(schema);
		DataFileWriter<GenericRecord> dataFileWriter = new DataFileWriter<GenericRecord>(datumWriter);
		
		dataFileWriter.create(schema, new File("D:/tmp/users.avro"));
		dataFileWriter.append(user1);
		dataFileWriter.append(user2);
		dataFileWriter.close();
	}
	
	static void readAvro() throws Exception{
		Schema schema = new Schema.Parser().parse(new File("D:/tmp/user.avsc"));
		
		File file = new File("D:/tmp/users.avro");
		DatumReader<GenericRecord> userDatumReader = new GenericDatumReader<GenericRecord>(schema);
		DataFileReader<GenericRecord> dataFileReader = new DataFileReader<GenericRecord>(file, userDatumReader);
		GenericRecord user = null;
		while (dataFileReader.hasNext()) {
			user = dataFileReader.next(user);
			System.out.println(user);
		}
		dataFileReader.close();
	}
}

4. 不生成java代码动态实现

public class SerializationByAvroClass {

	public static void main(String[] args) throws Exception{
		//readAvro();
		//writeJavaObject();
	}
	
	static void writeAvro() throws Exception{
		User user1 = new User();
		user1.setName("Alyssa");
		user1.setFavoriteNumber(256);

		User user2 = new User("Ben", 7, "red");

		User user3 = User.newBuilder()
		             .setName("Charlie")
		             .setFavoriteColor("blue")
		             .setFavoriteNumber(null)
		             .build();

		DatumWriter<User> userDatumWriter = new SpecificDatumWriter<User>(User.class);
		DataFileWriter<User> dataFileWriter = new DataFileWriter<User>(userDatumWriter);
		dataFileWriter.create(user1.getSchema(), new File("D:/tmp/users.avro"));
		dataFileWriter.append(user1);
		dataFileWriter.append(user2);
		dataFileWriter.append(user3);
		dataFileWriter.close();
	}
	
	static void readAvro() throws Exception{
		File file = new File("D:/tmp/users.avro");
		DatumReader<User> userDatumReader = new SpecificDatumReader<User>(User.class);
		DataFileReader<User> dataFileReader = new DataFileReader<User>(file, userDatumReader);
		User user = null;
		while (dataFileReader.hasNext()) {
			user = dataFileReader.next(user);
			System.out.println(user);
		}
		dataFileReader.close();
	}

}

Avro序列化的两种实现方法：静态实现和动态实现

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