【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经过网络传输。Kryo还能够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外一个对象，而不是对象转换为字节而后转化为对象。

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默认为不一样的数据类型定义了不一样的序列化器，其具体类型以下所示：

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，固然你也能够经过下面的代码更改默认序列化器：

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

也能够为每一个类指定序列化类，以下

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

Why

1.下面经过如下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：可看作类与序列化类的Map集合
• ClassResolver ：负责类的注册、根据类标识序列化、根据字节码获得类标识
• ReferenceResolver接口 ：当容许引用时，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.为何在使用一个类以前先要将Kryo注册一下？
在序列化时写一个类名是低效的，因此用一个ID来替代类名，只须要使用前完成注册便可，代码以下：

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

//和上面的基本同样，只不过是本身定义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
• 为何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.