【原】Kryo序列化篇

Kryo是一个快速有效的对象图序列化Java库。它的目标是快速、高效、易使用。该项目适用于对象持久化到文件或数据库中或通过网络传输。Kryo还可以自动实现深浅的拷贝/克隆。 就是直接复制一个对象对象到另一个对象，而不是对象转换为字节然后转化为对象。

目前已经被用在下列项目中：
KryoNet (NIO networking)
Twitter's Scalding (Scala API for Cascading)
Twitter's Chill (Kryo serializers for Scala)
Apache Fluo (Kryo is default serialization for Fluo Recipes)
Apache Hive (query plan serialization)
Apache Spark (shuffled/cached data serialization)
DataNucleus (JDO/JPA persistence framework)
CloudPelican
Yahoo's S4 (distributed stream computing)
Storm (distributed realtime computation system, in turn used by many others)
Cascalog (Clojure/Java data processing and querying details)
memcached-session-manager (Tomcat high-availability sessions)
Mobility-RPC (RPC enabling distributed applications)
akka-kryo-serialization (Kryo serializers for Akka)
Groupon
Jive
DestroyAllHumans (controls a robot!)
kryo-serializers (additional serializers)

How

Kryo序列化实例：

Kryo kryo = new Kryo();
    // ...
    Output output = new Output(new FileOutputStream("file.bin"));
    SomeClass someObject = ...
    kryo.writeObject(output, someObject);
    output.close();
    // ...
    Input input = new Input(new FileInputStream("file.bin"));
    SomeClass someObject = kryo.readObject(input, SomeClass.class);
    input.close();

Kryo默认为不同的数据类型定义了不同的序列化器，其具体类型如下所示：

col1	col2	col3	col4	col5
boolean	Boolean	byte	Byte	char
Character	short	Short	int	Integer
long	Long	float	Float	double
Double	byte[]	String	BigInteger	BigDecimal
Collection	Date	Collections.emptyList	Collections.singleton	Map
StringBuilder	TreeMap	Collections.emptyMap	Collections.emptySet	KryoSerializable
StringBuffer	Class	Collections.singletonList	Collections.singletonMap	Currency
Calendar	TimeZone	Enum	EnumSet

Kryo类在构造参数中会为上述表中的类型初始化对应的序列化类。

其它类会使用默认序列化器FieldSerializer，当然你也可以通过下面的代码更改默认序列化器：

Kryo kryo = new Kryo();
kryo.setDefaultSerializer(AnotherGenericSerializer.class);

也可以为每个类指定序列化类，如下

Kryo kryo = new Kryo();
kryo.register(SomeClass.class, new SomeSerializer());
kryo.register(AnotherClass.class, new AnotherSerializer());

Why

1.下面通过以下Kryo4.0源代码看一下
Kryo类的构造函数为：

public Kryo (ClassResolver classResolver, ReferenceResolver referenceResolver, StreamFactory streamFactory) {
		if (classResolver == null) throw new IllegalArgumentException("classResolver cannot be null.");

		this.classResolver = classResolver;
		classResolver.setKryo(this);

		this.streamFactory = streamFactory;
		streamFactory.setKryo(this);

		this.referenceResolver = referenceResolver;
		if (referenceResolver != null) {
			referenceResolver.setKryo(this);
			references = true;
		}
//private final ArrayList<DefaultSerializerEntry> defaultSerializers = new ArrayList(33);用一个大小为33的Entry来存放类及序列化器
		addDefaultSerializer(byte[].class, ByteArraySerializer.class);
		addDefaultSerializer(char[].class, CharArraySerializer.class);
		addDefaultSerializer(short[].class, ShortArraySerializer.class);
		addDefaultSerializer(int[].class, IntArraySerializer.class);
		addDefaultSerializer(long[].class, LongArraySerializer.class);
		addDefaultSerializer(float[].class, FloatArraySerializer.class);
		addDefaultSerializer(double[].class, DoubleArraySerializer.class);
		addDefaultSerializer(boolean[].class, BooleanArraySerializer.class);
		addDefaultSerializer(String[].class, StringArraySerializer.class);
		addDefaultSerializer(Object[].class, ObjectArraySerializer.class);
		addDefaultSerializer(KryoSerializable.class, KryoSerializableSerializer.class);
		addDefaultSerializer(BigInteger.class, BigIntegerSerializer.class);
		addDefaultSerializer(BigDecimal.class, BigDecimalSerializer.class);
		addDefaultSerializer(Class.class, ClassSerializer.class);
		addDefaultSerializer(Date.class, DateSerializer.class);
		addDefaultSerializer(Enum.class, EnumSerializer.class);
		addDefaultSerializer(EnumSet.class, EnumSetSerializer.class);
		addDefaultSerializer(Currency.class, CurrencySerializer.class);
		addDefaultSerializer(StringBuffer.class, StringBufferSerializer.class);
		addDefaultSerializer(StringBuilder.class, StringBuilderSerializer.class);
		addDefaultSerializer(Collections.EMPTY_LIST.getClass(), CollectionsEmptyListSerializer.class);
		addDefaultSerializer(Collections.EMPTY_MAP.getClass(), CollectionsEmptyMapSerializer.class);
		addDefaultSerializer(Collections.EMPTY_SET.getClass(), CollectionsEmptySetSerializer.class);
		addDefaultSerializer(Collections.singletonList(null).getClass(), CollectionsSingletonListSerializer.class);
		addDefaultSerializer(Collections.singletonMap(null, null).getClass(), CollectionsSingletonMapSerializer.class);
		addDefaultSerializer(Collections.singleton(null).getClass(), CollectionsSingletonSetSerializer.class);
		addDefaultSerializer(TreeSet.class, TreeSetSerializer.class);
		addDefaultSerializer(Collection.class, CollectionSerializer.class);
		addDefaultSerializer(TreeMap.class, TreeMapSerializer.class);
		addDefaultSerializer(Map.class, MapSerializer.class);
		addDefaultSerializer(TimeZone.class, TimeZoneSerializer.class);
		addDefaultSerializer(Calendar.class, CalendarSerializer.class);
		addDefaultSerializer(Locale.class, LocaleSerializer.class);
		addDefaultSerializer(Charset.class, CharsetSerializer.class);
		addDefaultSerializer(URL.class, URLSerializer.class);
		OptionalSerializers.addDefaultSerializers(this);
		TimeSerializers.addDefaultSerializers(this);
		lowPriorityDefaultSerializerCount = defaultSerializers.size();

		// Primitives and string. Primitive wrappers automatically use the same registration as primitives.
		register(int.class, new IntSerializer());
		register(String.class, new StringSerializer());
		register(float.class, new FloatSerializer());
		register(boolean.class, new BooleanSerializer());
		register(byte.class, new ByteSerializer());
		register(char.class, new CharSerializer());
		register(short.class, new ShortSerializer());
		register(long.class, new LongSerializer());
		register(double.class, new DoubleSerializer());
		register(void.class, new VoidSerializer());
	}

• Kryo：可看做类与序列化类的Map集合
• ClassResolver ：负责类的注册、根据类标识序列化、根据字节码得到类标识
• ReferenceResolver接口 ：当允许引用时，该类用于跟踪已经读写的对象，为写对象提供ID，根据ID读取对象。
  它有两个实现子类：ListReferenceResolver和MapReferenceResolver
• StreamFactory接口 ：基于系统配置提供输入和输出流。
  它有两个实现子类：DefaultStreamFactory和FastestStreamFactory

2.为什么在使用一个类之前先要将Kryo注册一下？
在序列化时写一个类名是低效的，所以用一个ID来替代类名，只需要使用前完成注册即可，代码如下：

//用先一个可用的整型ID来注册该类，如果类已经被注册，那么就返回之前的注册信息类
public Registration register (Class type) {
		Registration registration = classResolver.getRegistration(type);
		if (registration != null) return registration;
		return register(type, getDefaultSerializer(type));
	}

//和上面的基本一样，只不过是自己定义ID
public Registration register (Class type, int id) {
		Registration registration = classResolver.getRegistration(type);
		if (registration != null) return registration;
		return register(type, getDefaultSerializer(type), id);
	}

参考资料

• 官网文档
• 官网文档中文翻译版
• 为什么Java序列化慢
• Unsafe

QA

官方文档中有一段话：

Using Unsafe-based IO may result in a quite significant performance boost (sometimes up-to an order of magnitude), depending on your application. In particular, it helps a lot when serializing large primitive arrays as part of your object graphs.

下面是答案：

Unsafe-based IO uses methods from sun.misc.Unsafe for reading and writing from/to memory. Many of those methods map almost 1:1 to processor instructions. Moreover, reading/writing of arrays of native types is executed in bulk instead of doing it element-by-element as it is usually done by Kryo. Together, these features often provide a significant performance boost.