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FlatBuffers 编码原理
FlatBuffers的Java库只提供了如下的4个类：
./com/google/flatbuffers/Constants.java./com/google/flatbuffers/FlatBufferBuilder.java./com/google/flatbuffers/Struct.java./com/google/flatbuffers/Table.java
Constants 类定义FlatBuffers中可用的基本原始数据类型的长度：
public class Constants { // Java doesn't seem to have these. /** The number of bytes in an `byte`. */ static final int SIZEOF_BYTE = 1; /** The number of bytes in a `short`. */ static final int SIZEOF_SHORT = 2; /** The number of bytes in an `int`. */ static final int SIZEOF_INT = 4; /** The number of bytes in an `float`. */ static final int SIZEOF_FLOAT = 4; /** The number of bytes in an `long`. */ static final int SIZEOF_LONG = 8; /** The number of bytes in an `double`. */ static final int SIZEOF_DOUBLE = 8; /** The number of bytes in a file identifier. */ static final int FILE_IDENTIFIER_LENGTH = 4;} FlatBufferBuilder 用于FlatBuffers编码，它会将我们的结构化数据序列化为字节数组。我们借助于 FlatBufferBuilder 在 ByteBuffer 中放置基本数据类型的数据、数组、字符串及对象。ByteBuffer 用于处理字节序，在序列化时，它将数据按适当的字节序进行序列化，在发序列化时，它将多个字节转换为适当的数据类型。在 .fbs 文件中定义的 table 和 struct，为它们生成的Java 类会继承 Table 和 Struct。
在反序列化时，输入的ByteBuffer数据被当作字节数组，Table提供了针对字节数组的操作，生成的Java类负责对这些数据进行解释。对于FlatBuffers编码的数据，无需进行解码，只需进行解释。在编译 .fbs 文件时，每个字段在这段数据中的位置将被确定。每个字段的类型及长度将被硬编码进生成的Java类。
Struct 类的代码也比较简洁：
package com.google.flatbuffers;import java.nio.ByteBuffer;/// @cond FLATBUFFERS_INTERNAL/** * All structs in the generated code derive from this class, and add their own accessors. */public class Struct { /** Used to hold the position of the `bb` buffer. */ protected int bb_pos; /** The underlying ByteBuffer to hold the data of the Struct. */ protected ByteBuffer bb;} 整体的结构如下图：
在序列化结构化数据时，我们首先需要创建一个 FlatBufferBuilder ，在这个对象的创建过程中会分配或从调用者那里获取 ByteBuffer，序列化的数据将保存在这个 ByteBuffer中：
/** * Start with a buffer of size `initial_size`, then grow as required. * * @param initial_size The initial size of the internal buffer to use. */ public FlatBufferBuilder(int initial_size) { if (initial_size <= 0) initial_size = 1; space = initial_size; bb = newByteBuffer(initial_size); } /** * Start with a buffer of 1KiB, then grow as required. */ public FlatBufferBuilder() { this(1024); } /** * Alternative constructor allowing reuse of {@link ByteBuffer}s. The builder * can still grow the buffer as necessary. User classes should make sure * to call {@link #dataBuffer()} to obtain the resulting encoded message. * * @param existing_bb The byte buffer to reuse. */ public FlatBufferBuilder(ByteBuffer existing_bb) { init(existing_bb); } /** * Alternative initializer that allows reusing this object on an existing * `ByteBuffer`. This method resets the builder's internal state, but keeps * objects that have been allocated for temporary storage. * * @param existing_bb The byte buffer to reuse. * @return Returns `this`. */ public FlatBufferBuilder init(ByteBuffer existing_bb){ bb = existing_bb; bb.clear(); bb.order(ByteOrder.LITTLE_ENDIAN); minalign = 1; space = bb.capacity(); vtable_in_use = 0; nested = false; finished = false; object_start = 0; num_vtables = 0; vector_num_elems = 0; return this; } static ByteBuffer newByteBuffer(int capacity) { ByteBuffer newbb = ByteBuffer.allocate(capacity); newbb.order(ByteOrder.LITTLE_ENDIAN); return newbb; } 下面我们更详细地分析基本数据类型数据、数组及对象的序列化过程。ByteBuffer 为小尾端的。
FlatBuffers编码基本数据类型
FlatBuffer 的基本数据类型主要包括如下这些：
BooleanByteShortIntLongFloatDouble
FlatBufferBuilder 提供了三组方法用于操作这些数据：
public void putBoolean(boolean x); public void putByte (byte x); public void putShort (short x); public void putInt (int x); public void putLong (long x); public void putFloat (float x); public void putDouble (double x); public void addBoolean(boolean x); public void addByte (byte x); public void addShort (short x); public void addInt (int x); public void addLong (long x); public void addFloat (float x); public void addDouble (double x); public void addBoolean(int o, boolean x, boolean d); public void addByte(int o, byte x, int d); public void addShort(int o, short x, int d); public void addInt (int o, int x, int d); public void addLong (int o, long x, long d); public void addFloat (int o, float x, double d); public void addDouble (int o, double x, double d);
putXXX 那一组，直接地将一个数据放入 ByteBuffer 中，它们的实现基本如下面这样：
public void putBoolean(boolean x) { bb.put(space -= Constants.SIZEOF_BYTE, (byte) (x ? 1 : 0)); } public void putByte(byte x) { bb.put(space -= Constants.SIZEOF_BYTE, x); } public void putShort(short x) { bb.putShort(space -= Constants.SIZEOF_SHORT, x); } Boolean值会被先转为byte类型再放入 ByteBuffer。另外一点值得注意的是，数据是从 ByteBuffer 的结尾处开始放置的，space用于记录最近放入的数据的位置及剩余的空间。
addXXX(XXX x) 那一组在放入数据之前会先做对齐处理，并在需要时扩展 ByteBuffer 的容量：
static ByteBuffer growByteBuffer(ByteBuffer bb) { int old_buf_size = bb.capacity(); if ((old_buf_size & 0xC0000000) != 0) // Ensure we don't grow beyond what fits in an int. throw new AssertionError("FlatBuffers: cannot grow buffer beyond 2 gigabytes."); int new_buf_size = old_buf_size << 1; bb.position(0); ByteBuffer nbb = newByteBuffer(new_buf_size); nbb.position(new_buf_size - old_buf_size); nbb.put(bb); return nbb; } public void pad(int byte_size) { for (int i = 0; i < byte_size; i++) bb.put(--space, (byte) 0); } public void prep(int size, int additional_bytes) { // Track the biggest thing we've ever aligned to. if (size > minalign) minalign = size; // Find the amount of alignment needed such that `size` is properly // aligned after `additional_bytes` int align_size = ((~(bb.capacity() - space + additional_bytes)) + 1) & (size - 1); // Reallocate the buffer if needed. while (space < align_size + size + additional_bytes) { int old_buf_size = bb.capacity(); bb = growByteBuffer(bb); space += bb.capacity() - old_buf_size; } pad(align_size); } public void addBoolean(boolean x) { prep(Constants.SIZEOF_BYTE, 0); putBoolean(x); } public void addInt(int x) { prep(Constants.SIZEOF_INT, 0); putInt(x); } 对齐是数据存放的起始位置相对于ByteBuffer的结束位置的对齐，additional bytes被认为是不需要对齐的，且在必要的时候会在ByteBuffer可用空间的结尾处填充值为0的字节。在扩展 ByteBuffer 的空间时，老的ByteBuffer被放在新ByteBuffer的结尾处。
addXXX(int o, XXX x, YYY y) 这一组方法在放入数据之后，会将 vtable 中对应位置的值更新为最近放入的数据的offset。
public void addShort(int o, short x, int d) { if (force_defaults || x != d) { addShort(x); slot(o); } } public void slot(int voffset) { vtable[voffset] = offset(); } 后面我们在分析编码对象时再来详细地了解vtable。
基本上，在我们的应用程序代码中不要直接调用这些方法，它们主要在构造对象时用于存储对象的基本数据类型字段。
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