[hadoop2.7.1]I/O之序列化（serializer）

先来看下org.apache.hadoop.io.serializer的类图（hadoop2.7.1）：

由类图看：

接口三个：

1、Deserializer：定义反序列化接口；

2、Serializer：定义序列化接口；
3、Serialization：定义了一系列和序列化相关并相互依赖对象的接口。

依据这三个接口，分别实现了2个类，分别是支持Writable机制的WritableSerialization和支持Java序列化的JavaSerialization，这样一共是6个实现类。

SerilizationFactory：维护一个Serilization的ArrayList。它具有参数为Configuration的构造函数，把parameter io.serializations中逗号隔开的serialization都添加进来。

Deserializer：将字节流转为一个对象。这个接口的方法有：打开流，反序列化，关闭流

源码：

package org.apache.hadoop.io.serializer;

import java.io.IOException;
import java.io.InputStream;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * <p>
 * Provides a facility for deserializing objects of type <T> from an
 * {@link InputStream}.
 * </p>
 * 
 * <p>
 * Deserializers are stateful, but must not buffer the input since
 * other producers may read from the input between calls to
 * {@link #deserialize(Object)}.
 * </p>
 * @param <T>
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Evolving
public interface Deserializer<T> {
  /**
   * <p>Prepare the deserializer for reading.</p>
   */
  void open(InputStream in) throws IOException;
  
  /**
   * <p>
   * Deserialize the next object from the underlying input stream.
   * If the object <code>t</code> is non-null then this deserializer
   * <i>may</i> set its internal state to the next object read from the input
   * stream. Otherwise, if the object <code>t</code> is null a new
   * deserialized object will be created.
   * </p>
   * @return the deserialized object
   */
  T deserialize(T t) throws IOException;
  
  /**
   * <p>Close the underlying input stream and clear up any resources.</p>
   */
  void close() throws IOException;
}

Serializer：将一个对象转换为一个字节流的实现实例，该接口的方法有：打开流，序列化，关闭流

源码：

package org.apache.hadoop.io.serializer;

import java.io.IOException;
import java.io.OutputStream;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * <p>
 * Provides a facility for serializing objects of type <T> to an
 * {@link OutputStream}.
 * </p>
 * 
 * <p>
 * Serializers are stateful, but must not buffer the output since
 * other producers may write to the output between calls to
 * {@link #serialize(Object)}.
 * </p>
 * @param <T>
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Evolving
public interface Serializer<T> {
  /**
   * <p>Prepare the serializer for writing.</p>
   */
  void open(OutputStream out) throws IOException;
  
  /**
   * <p>Serialize <code>t</code> to the underlying output stream.</p>
   */
  void serialize(T t) throws IOException;
  
  /**
   * <p>Close the underlying output stream and clear up any resources.</p>
   */  
  void close() throws IOException;
}

Serialization：使用抽象工厂的设计模式，封装了一对Serializer/Deserializer，判断是否支持输入的类，根据输入的类给出序列化接口和反序列化接口。

源码：

package org.apache.hadoop.io.serializer;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * <p>
 * Encapsulates a {@link Serializer}/{@link Deserializer} pair.
 * </p>
 * @param <T>
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Evolving
public interface Serialization<T> {
  
  /**
   * Allows clients to test whether this {@link Serialization}
   * supports the given class.
   */
  boolean accept(Class<?> c);
  
  /**
   * @return a {@link Serializer} for the given class.
   */
  Serializer<T> getSerializer(Class<T> c);

  /**
   * @return a {@link Deserializer} for the given class.
   */
  Deserializer<T> getDeserializer(Class<T> c);
}

SerializationFactory ：序列化工厂，初始化时从配置项io.serializations中获取序列化工具，默认使用org.apache.hadoop.io.serializer.WritableSerialization作为序列化工具。通过调用getSerializer和getDeserializer来获取序列化与反序列化工具。

源码：

package org.apache.hadoop.io.serializer;

import java.util.ArrayList;
import java.util.List;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.CommonConfigurationKeys;
import org.apache.hadoop.io.serializer.avro.AvroReflectSerialization;
import org.apache.hadoop.io.serializer.avro.AvroSpecificSerialization;
import org.apache.hadoop.util.ReflectionUtils;

/**
 * <p>
 * A factory for {@link Serialization}s.
 * </p>
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Evolving
public class SerializationFactory extends Configured {
  
  private static final Log LOG =
    LogFactory.getLog(SerializationFactory.class.getName());

  private List<Serialization<?>> serializations = new ArrayList<Serialization<?>>();
  
  /**
   * <p>
   * Serializations are found by reading the <code>io.serializations</code>
   * property from <code>conf</code>, which is a comma-delimited list of
   * classnames.
   * </p>
   */
  public SerializationFactory(Configuration conf) {
    super(conf);
    for (String serializerName : conf.getTrimmedStrings(
      CommonConfigurationKeys.IO_SERIALIZATIONS_KEY,
      new String[]{WritableSerialization.class.getName(),
        AvroSpecificSerialization.class.getName(),
        AvroReflectSerialization.class.getName()})) {
      add(conf, serializerName);
    }
  }
  
  @SuppressWarnings("unchecked")
  private void add(Configuration conf, String serializationName) {
    try {
      Class<? extends Serialization> serializionClass =
        (Class<? extends Serialization>) conf.getClassByName(serializationName);
      serializations.add((Serialization)
      ReflectionUtils.newInstance(serializionClass, getConf()));
    } catch (ClassNotFoundException e) {
      LOG.warn("Serialization class not found: ", e);
    }
  }

  public <T> Serializer<T> getSerializer(Class<T> c) {
    Serialization<T> serializer = getSerialization(c);
    if (serializer != null) {
      return serializer.getSerializer(c);
    }
    return null;
  }

  public <T> Deserializer<T> getDeserializer(Class<T> c) {
    Serialization<T> serializer = getSerialization(c);
    if (serializer != null) {
      return serializer.getDeserializer(c);
    }
    return null;
  }

  @SuppressWarnings("unchecked")
  public <T> Serialization<T> getSerialization(Class<T> c) {
    for (Serialization serialization : serializations) {
      if (serialization.accept(c)) {
        return (Serialization<T>) serialization;
      }
    }
    return null;
  }
  
}

下面对SerializationFactory生产Serializations做个简单的解析说明：

首先来看其构造函数里的一个全局参数：CommonConfigurationKeys.IO_SERIALIZATIONS_KEY，它的值定义如下：

/** See <a href="{@docRoot}/../core-default.html">core-default.xml</a> */
  public static final String  IO_SERIALIZATIONS_KEY = "io.serializations";

而使用SerializationFactory的构造函数：public SerializationFactory(Configuration conf) 时，使用配置文件：Configuration: core-default.xml, core-site.xml。如：

SerializationFactory factory = new SerializationFactory(conf);

而在hadoop2.7.1中默认配置文件core-default.xml的io.serializations的属性如下：

<property>
  <name>io.serializations</name>
  <value>org.apache.hadoop.io.serializer.WritableSerialization,org.apache.hadoop.io.serializer.avro.AvroSpecificSerialization,org.apache.hadoop.io.serializer.avro.AvroReflectSerialization<alue>
  <description>A list of serialization classes that can be used for
  obtaining serializers and deserializers.</description>
</property>

由此，通过SerializationFactory生产的Serializations有三个：

org.apache.hadoop.io.serializer.WritableSerialization,
org.apache.hadoop.io.serializer.avro.AvroSpecificSerialization,
org.apache.hadoop.io.serializer.avro.AvroReflectSerialization

通过其方法public <T> Serializer<T> getSerializer(Class<T> c)， public <T> Serialization<T> getSerialization(Class<T> c)便能得到相应的Serialization：

public <T> Serializer<T> getSerializer(Class<T> c) {
    Serialization<T> serializer = getSerialization(c);
    if (serializer != null) {
      return serializer.getSerializer(c);
    }
    return null;
  }

<div>  @SuppressWarnings("unchecked")
  public <T> Serialization<T> getSerialization(Class<T> c) {
    for (Serialization serialization : serializations) {
      if (serialization.accept(c))<strong> </strong>{                          //注1
        return (Serialization<T>) serialization;
      }
    }
    return null;
  }</div>

注1： if (serialization.accept(c))将会调用相应类的accept函数，例如：如果serialization的值为：org.apache.hadoop.io.serializer.WritableSerialization,则将调用：

@InterfaceAudience.Private
  @Override
  public boolean accept(Class<?> c) {
    return Writable.class.isAssignableFrom(c);
  }

如果serialization的值为：org.apache.hadoop.io.serializer.avro.AvroSpecificSerialization，则将调用：

@InterfaceAudience.Private
  @Override
  public boolean accept(Class<?> c) {
    return SpecificRecord.class.isAssignableFrom(c);          //注2
  }

注2：

public boolean isAssignableFrom(Class<?> cls)

判定此 Class 对象所表示的类或接口与指定的 Class 参数所表示的类或接口是否相同，或是否是其超类或超接口。如果是则返回 true；否则返回 false。如果该 Class 表示一个基本类型，且指定的 Class 参数正是该 Class 对象，则该方法返回 true；否则返回 false。

特别地，通过身份转换或扩展引用转换，此方法能测试指定 Class 参数所表示的类型能否转换为此 Class 对象所表示的类型。有关详细信息，请参阅 Java Language Specification 的第 5.1.1 和 5.1.4 节。

参数：

cls - 要检查的 Class 对象

返回：

表明 cls 类型的对象能否赋予此类对象的 boolean 值

抛出：

NullPointerException - 如果指定的 Class 参数为 null。