Protostuff序列化分析

前言
最近项目中需要将业务对象直接序列化，然后存数据库；考虑到序列化、反序列化的时间以及生产文件的大小觉得Protobuf是一个很好的选择，但是Protobuf有的问题就是需要有一个.proto的描述文件，而且由Protobuf生成的对象用来作为业务对象并不是特别友好，往往业务对象和Protobuf对象存在一个互相转换的过程；考虑到我们仅仅是将业务对象直接序列化到数据库，发现Protobuf在这种情况下并不是特别的好；
这时候发现了Protostuff，protostuff不需要依赖.proto文件，可以直接对普通的javabean进行序列化、反序列化的操作，而效率上甚至比protobuf还快，生成的二进制数据库格式和Protobuf完全相同的，可以说是一个基于Protobuf的序列化工具。

简单测试
1.先测试一下Protostuff
提供一个简单的javabean

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public class Person {       private int id;     private String name;     private String email;                   // get/set方法省略 }

测试类PbStuff

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public class PbStuff {           public static void main(String[] args) throws FileNotFoundException,             IOException {         Schema<Person> schema = RuntimeSchema.getSchema(Person.class);         Person person1 = new Person();         person1.setId(1);         person1.setName("zhaohui");         LinkedBuffer buffer = LinkedBuffer.allocate(1024);         byte[] data = ProtobufIOUtil.toByteArray(person1, schema, buffer);         System.out.println(data.length);     } }

序列化之后二进制的大小为29字节

2.测试Protobuf
proto文件

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option java_package = "protobuf.clazz"; option java_outer_classname = "PersonX";   message Person {   required int32 id = 1;   required string name = 2;   required string email = 3; }

PBTest类

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public class PBTest {       public static void main(String[] args) {         PersonX.Person.Builder builder = PersonX.Person.newBuilder();         builder.setId(1);         builder.setName("zhaohui");         builder.setEmail("xxxxxxxx@126.com");           PersonX.Person p = builder.build();         byte[] result = p.toByteArray();         System.out.println(result.length);       } }

序列化之后二进制的大小同样也是29字节

经过简单的测试：发现Protobuf和Protostuff序列化相同的数据得到的结果是一样的
Protobuf的编码是尽其所能地将字段的元信息和字段的值压缩存储，并且字段的元信息中含有对这个字段描述的所有信息；既然Protostuff序列化之后的大小和Protobuf是一样的，那可以分析一下Protostuff的源码

源码分析
1.Schema

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/**  * Gets the schema that was either registered or lazily initialized at runtime.  * <p>  * Method overload for backwards compatibility.  */ public static <T> Schema<T> getSchema(Class<T> typeClass) {     return getSchema(typeClass, ID_STRATEGY); }   /**  * Gets the schema that was either registered or lazily initialized at runtime.  */ public static <T> Schema<T> getSchema(Class<T> typeClass,         IdStrategy strategy) {     return strategy.getSchemaWrapper(typeClass, true).getSchema(); }

getSchema方法中指定了获取Schema的默认策略类ID_STRATEGY，ID_STRATEGY在类RuntimeEnv中进行了实例化：

1	ID_STRATEGY = new DefaultIdStrategy();

可以大致看一下DefaultIdStrategy类：

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public final class DefaultIdStrategy extends IdStrategy {       final ConcurrentHashMap<String, HasSchema<?>> pojoMapping = new ConcurrentHashMap<>();       final ConcurrentHashMap<String, EnumIO<?>> enumMapping = new ConcurrentHashMap<>();       final ConcurrentHashMap<String, CollectionSchema.MessageFactory> collectionMapping = new ConcurrentHashMap<>();       final ConcurrentHashMap<String, MapSchema.MessageFactory> mapMapping = new ConcurrentHashMap<>();       final ConcurrentHashMap<String, HasDelegate<?>> delegateMapping = new ConcurrentHashMap<>();     ... }

可以发现DefaultIdStrategy内存缓存了很多Schema信息，不难理解既然要或者业务对象的类和字段信息，必然用到反射机制，这是一个很耗时的过程，进行缓存很有必要，这样下次遇到相同的类就可以不用进行反射了

所以可以看到DefaultIdStrategy中有很多这种模式的方法：

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public <T> HasSchema<T> getSchemaWrapper(Class<T> typeClass, boolean create)    {        HasSchema<T> hs = (HasSchema<T>) pojoMapping.get(typeClass.getName());        if (hs == null && create)        {            hs = new Lazy<>(typeClass, this);            final HasSchema<T> last = (HasSchema<T>) pojoMapping.putIfAbsent(                    typeClass.getName(), hs);            if (last != null)                hs = last;        }          return hs;    }

先get，如果为null，就putIfAbsent

当业务对象的Schema还没被缓存，这时候就会去create，RuntimeSchema提供了createFrom方法：

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public static <T> RuntimeSchema<T> createFrom(Class<T> typeClass,         Set<String> exclusions, IdStrategy strategy) {     final Map<String, java.lang.reflect.Field> fieldMap = findInstanceFields(typeClass);     ...省略     final Field<T> field = RuntimeFieldFactory.getFieldFactory(                     f.getType(), strategy).create(fieldMapping, name, f,                     strategy);             fields.add(field);         }     }       return new RuntimeSchema<>(typeClass, fields, RuntimeEnv.newInstantiator(typeClass));  }

主要就是对typeClass进行反射，然后进行封装；将字段类型封装成了RuntimeFieldFactory，最后通过RuntimeFieldFactory的create方法封装进入Field类中，RuntimeFieldFactory列举了所有支持的类型：

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static final RuntimeFieldFactory<BigDecimal> BIGDECIMAL;    static final RuntimeFieldFactory<BigInteger> BIGINTEGER;    static final RuntimeFieldFactory<Boolean> BOOL;    static final RuntimeFieldFactory<Byte> BYTE;    static final RuntimeFieldFactory<ByteString> BYTES;    static final RuntimeFieldFactory<byte[]> BYTE_ARRAY;    static final RuntimeFieldFactory<Character> CHAR;    static final RuntimeFieldFactory<Date> DATE;    static final RuntimeFieldFactory<Double> DOUBLE;    static final RuntimeFieldFactory<Float> FLOAT;    static final RuntimeFieldFactory<Integer> INT32;    static final RuntimeFieldFactory<Long> INT64;    static final RuntimeFieldFactory<Short> SHORT;    static final RuntimeFieldFactory<String> STRING;      static final RuntimeFieldFactory<Integer> ENUM;    static final RuntimeFieldFactory<Object> OBJECT;    static final RuntimeFieldFactory<Object> POJO;    static final RuntimeFieldFactory<Object> POLYMORPHIC_POJO;      static final RuntimeFieldFactory<Collection<?>> COLLECTION =            new RuntimeFieldFactory<Collection<?>>(ID_COLLECTION)

当然还有常用的Map类型，在RuntimeMapFieldFactory中定义了

2.LinkedBuffer buffer = LinkedBuffer.allocate(1024);
开辟了1024字节缓存，用来存放业务对象序列化之后存放的地方，当然你可能会担心这个大小如果不够怎么办，后面的代码中可以看到，如果空间不足，会自动扩展的，所有这个大小要设置一个合适的值，设置大了浪费空间，设置小了会自动扩展浪费时间。

3.byte[] data = ProtobufIOUtil.toByteArray(person1, schema, buffer);
ProtobufIOUtil提供的就是以Protobuf编码的格式来序列化业务对象

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public static <T> byte[] toByteArray(T message, Schema<T> schema, LinkedBuffer buffer) {     if (buffer.start != buffer.offset)         throw new IllegalArgumentException("Buffer previously used and had not been reset.");       final ProtobufOutput output = new ProtobufOutput(buffer);     try     {         schema.writeTo(output, message);     }     catch (IOException e)     {     }       return output.toByteArray(); }

schema中调用writeTo方法，将message中的消息保存到ProtobufOutput中

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public final void writeTo(Output output, T message) throws IOException {     for (Field<T> f : getFields())         f.writeTo(output, message); }

第一步中将业务对象的字段信息都封装到了Field中了，可以看一下Field类提供的几个方法：

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/**  * Writes the value of a field to the {@code output}.  */ protected abstract void writeTo(Output output, T message)         throws IOException;   /**  * Reads the field value into the {@code message}.  */ protected abstract void mergeFrom(Input input, T message)         throws IOException;   /**  * Transfer the input field to the output field.  */ protected abstract void transfer(Pipe pipe, Input input, Output output,         boolean repeated) throws IOException;

提供了三个抽象方法，分别是写数据，读数据和转移数据
下面已int类型为实例，看看实现：

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public static final RuntimeFieldFactory<Integer> INT32 = new RuntimeFieldFactory<Integer>(            ID_INT32)    {        @Override        public <T> Field<T> create(int number, java.lang.String name,                final java.lang.reflect.Field f, IdStrategy strategy)        {            final boolean primitive = f.getType().isPrimitive();            final long offset = us.objectFieldOffset(f);            return new Field<T>(FieldType.INT32, number, name,                    f.getAnnotation(Tag.class))            {                @Override                public void mergeFrom(Input input, T message)                        throws IOException                {                    if (primitive)                        us.putInt(message, offset, input.readInt32());                    else                        us.putObject(message, offset,                                Integer.valueOf(input.readInt32()));                }                  @Override                public void writeTo(Output output, T message)                        throws IOException                {                    if (primitive)                        output.writeInt32(number, us.getInt(message, offset),                                false);                    else                    {                        Integer value = (Integer) us.getObject(message, offset);                        if (value != null)                            output.writeInt32(number, value.intValue(), false);                    }                }                ...            };        }

上面这段代码可以在RuntimeUnsafeFieldFactory中找到，基本的数据类型都在此类中能找到，collection和map分别在RuntimeRepeatedFieldFactory和RuntimeMapFieldFactory中，writeTo方法调用了ProtobufOutput中的writeInt32方法：

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public void writeInt32(int fieldNumber, int value, boolean repeated) throws IOException     {          ...          tail = writeTagAndRawVarInt32(                   makeTag(fieldNumber, WIRETYPE_VARINT),                   value,                   this,                   tail);           ...     }

写入field的Tag已经Value，Protobuf也是这种形式存放的，如下图所示：

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public static LinkedBuffer writeTagAndRawVarInt32(int tag, int value,             final WriteSession session, LinkedBuffer lb)     {         final int tagSize = computeRawVarint32Size(tag);         final int size = computeRawVarint32Size(value);         final int totalSize = tagSize + size;           if (lb.offset + totalSize > lb.buffer.length)             lb = new LinkedBuffer(session.nextBufferSize, lb);           final byte[] buffer = lb.buffer;         int offset = lb.offset;         lb.offset += totalSize;         session.size += totalSize;           if (tagSize == 1)             buffer[offset++] = (byte) tag;         else         {             for (int i = 0, last = tagSize - 1; i < last; i++, tag >>>= 7)                 buffer[offset++] = (byte) ((tag & 0x7F) | 0x80);               buffer[offset++] = (byte) tag;         }           if (size == 1)             buffer[offset] = (byte) value;         else         {             for (int i = 0, last = size - 1; i < last; i++, value >>>= 7)                 buffer[offset++] = (byte) ((value & 0x7F) | 0x80);               buffer[offset] = (byte) value;         }           return lb;     }

tag是通过makeTag方法创建的：

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public static int makeTag(final int fieldNumber, final int wireType) {     return (fieldNumber << TAG_TYPE_BITS) | wireType; }

fieldNumber每个字段的标号，wire_type是该字段的数据类型，所有如果我们改变了业务对象类中字段的顺序，或者改变了字段的类型，都会出现反序列化失败；
前面提到的数据压缩在方法computeRawVarint32Size中体现出来了：

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public static int computeRawVarint32Size(final int value) {     if ((value & (0xffffffff << 7)) == 0)         return 1;     if ((value & (0xffffffff << 14)) == 0)         return 2;     if ((value & (0xffffffff << 21)) == 0)         return 3;     if ((value & (0xffffffff << 28)) == 0)         return 4;     return 5; }

根据value值的范围，返回不同的字节数；接下来的代码也可以看到检查LinkedBuffer的空间是否足够，不够进行扩充；接下来的代码就是用压缩的方式将tag和Value存入缓存中。

总结
大致了解了Protostuff对业务对象序列化的过程，不管是简单的测试还是通过查看源码，都可以发现Protostuff的序列化方式是完全借鉴Protobuf来实现的。

来自为知笔记(Wiz)