协议和编解码是一个网络应用程序的核心问题之一，客户端和服务器通过约定的协议来传输消息(数据)，通过特定的格式来编解码字节流，并转化成业务消息，提供给上层框架调用。

Thrift的协议比较简单，它把协议和编解码整合在了一起。抽象类TProtocol定义了协议和编解码的顶层接口。个人感觉采用抽象类而不是接口的方式来定义顶层接口并不好，TProtocol关联了一个TTransport传输对象，而不是提供一个类似getTransport()的接口，导致抽象类的扩展性比接口差。

TProtocol主要做了两个事情:

1. 关联TTransport对象

2.定义一系列读写消息的编解码接口，包括两类，一类是复杂数据结构比如readMessageBegin, readMessageEnd,  writeMessageBegin, writMessageEnd.还有一类是基本数据结构，比如readI32, writeI32, readString, writeString

public abstract class TProtocol {

  /**
   * Transport
   */
  protected TTransport trans_;

　public abstract void writeMessageBegin(TMessage message) throws TException;

  public abstract void writeMessageEnd() throws TException;

  public abstract void writeStructBegin(TStruct struct) throws TException;

  public abstract void writeStructEnd() throws TException;

  public abstract void writeFieldBegin(TField field) throws TException;

  public abstract void writeFieldEnd() throws TException;

  public abstract void writeFieldStop() throws TException;

  public abstract void writeMapBegin(TMap map) throws TException;

  public abstract void writeMapEnd() throws TException;

  public abstract void writeListBegin(TList list) throws TException;

  public abstract void writeListEnd() throws TException;

  public abstract void writeSetBegin(TSet set) throws TException;

  public abstract void writeSetEnd() throws TException;

  public abstract void writeBool(boolean b) throws TException;

  public abstract void writeByte(byte b) throws TException;

  public abstract void writeI16(short i16) throws TException;

  public abstract void writeI32(int i32) throws TException;

  public abstract void writeI64(long i64) throws TException;

  public abstract void writeDouble(double dub) throws TException;

  public abstract void writeString(String str) throws TException;

  public abstract void writeBinary(ByteBuffer buf) throws TException;

  /**
   * Reading methods.
   */

  public abstract TMessage readMessageBegin() throws TException;

  public abstract void readMessageEnd() throws TException;

  public abstract TStruct readStructBegin() throws TException;

  public abstract void readStructEnd() throws TException;

  public abstract TField readFieldBegin() throws TException;

  public abstract void readFieldEnd() throws TException;

  public abstract TMap readMapBegin() throws TException;

  public abstract void readMapEnd() throws TException;

  public abstract TList readListBegin() throws TException;

  public abstract void readListEnd() throws TException;

  public abstract TSet readSetBegin() throws TException;

  public abstract void readSetEnd() throws TException;

  public abstract boolean readBool() throws TException;

  public abstract byte readByte() throws TException;

  public abstract short readI16() throws TException;

  public abstract int readI32() throws TException;

  public abstract long readI64() throws TException;

  public abstract double readDouble() throws TException;

  public abstract String readString() throws TException;

  public abstract ByteBuffer readBinary() throws TException;

  /**
   * Reset any internal state back to a blank slate. This method only needs to
   * be implemented for stateful protocols.
   */
  public void reset() {}
  
  /**
   * Scheme accessor
   */
  public Class<? extends IScheme> getScheme() {
    return StandardScheme.class;
  }

}

调用方

1. 方法的名称，包括类的名称和方法的名称

2. 方法的参数，包括类型和参数值

3.一些附加的数据，比如附件，超时事件，自定义的控制信息等等

返回方

1. 调用的返回码

2. 返回值

3.异常信息

从TProtocol的定义我们可以看出Thrift的协议约定如下事情:

1. 先writeMessageBegin表示开始传输消息了，写消息头。Message里面定义了方法名，调用的类型，版本号，消息seqId

2. 接下来是写方法的参数，实际就是写消息体。如果参数是一个类，就writeStructBegin

3. 接下来写字段，writeFieldBegin, 这个方法会写接下来的字段的数据类型和顺序号。这个顺序号是Thrfit对要传输的字段的一个编码，从１开始

4. 如果是一个集合就writeListBegin/writeMapBegin，如果是一个基本数据类型，比如int, 就直接writeI32

5. 每个复杂数据类型写完都调用writeXXXEnd，直到writeMessageEnd结束

6. 读消息时根据数据类型读取相应的长度

每个writeXXX都是采用消息头+消息体的方式。我们来看TBinaryProtocol的实现。

1. writeMessgeBegin方法写了消息头，包括4字节的版本号和类型信息，字符串类型的方法名，４字节的序列号seqId

2. writeFieldBegin，写了１个字节的字段数据类型，和2个字节字段的顺序号

3. writeI32，写了４个字节的字节数组

4. writeString,先写４字节消息头表示字符串长度，再写字符串字节

5. writeBinary,先写４字节消息头表示字节数组长度，再写字节数组内容

6.readMessageBegin时，先读４字节版本和类型信息，再读字符串，再读４字节序列号

7.readFieldBegin，先读1个字节的字段数据类型，再读2个字节的字段顺序号

8. readString时，先读４字节字符串长度，再读字符串内容。字符串统一采用UTF-8编码

 public void writeMessageBegin(TMessage message) throws TException {
    if (strictWrite_) {
      int version = VERSION_1 | message.type;
      writeI32(version);
      writeString(message.name);
      writeI32(message.seqid);
    } else {
      writeString(message.name);
      writeByte(message.type);
      writeI32(message.seqid);
    }
  }

public void writeFieldBegin(TField field) throws TException {
    writeByte(field.type);
    writeI16(field.id);
  }

private byte[] i32out = new byte[4];
  public void writeI32(int i32) throws TException {
    i32out[0] = (byte)(0xff & (i32 >> 24));
    i32out[1] = (byte)(0xff & (i32 >> 16));
    i32out[2] = (byte)(0xff & (i32 >> 8));
    i32out[3] = (byte)(0xff & (i32));
    trans_.write(i32out, 0, 4);
  }

public void writeString(String str) throws TException {
    try {
      byte[] dat = str.getBytes("UTF-8");
      writeI32(dat.length);
      trans_.write(dat, 0, dat.length);
    } catch (UnsupportedEncodingException uex) {
      throw new TException("JVM DOES NOT SUPPORT UTF-8");
    }
  }

public void writeBinary(ByteBuffer bin) throws TException {
    int length = bin.limit() - bin.position();
    writeI32(length);
    trans_.write(bin.array(), bin.position() + bin.arrayOffset(), length);
  }

public TMessage readMessageBegin() throws TException {
    int size = readI32();
    if (size < 0) {
      int version = size & VERSION_MASK;
      if (version != VERSION_1) {
        throw new TProtocolException(TProtocolException.BAD_VERSION, "Bad version in readMessageBegin");
      }
      return new TMessage(readString(), (byte)(size & 0x000000ff), readI32());
    } else {
      if (strictRead_) {
        throw new TProtocolException(TProtocolException.BAD_VERSION, "Missing version in readMessageBegin, old client?");
      }
      return new TMessage(readStringBody(size), readByte(), readI32());
    }
  }

public TField readFieldBegin() throws TException {
    byte type = readByte();
    short id = type == TType.STOP ? 0 : readI16();
    return new TField("", type, id);
  }

public String readString() throws TException {
    int size = readI32();

    if (trans_.getBytesRemainingInBuffer() >= size) {
      try {
        String s = new String(trans_.getBuffer(), trans_.getBufferPosition(), size, "UTF-8");
        trans_.consumeBuffer(size);
        return s;
      } catch (UnsupportedEncodingException e) {
        throw new TException("JVM DOES NOT SUPPORT UTF-8");
      }
    }

    return readStringBody(size);
  }

TProtocol定义了基本的协议信息，包括传输什么数据，如何解析传输的数据的基本方法。

还存在一个问题，就是服务器端如何知道客户端发送过来的数据是怎么组合的，比如第一个字段是字符串类型，第二个字段是int。这个信息是在IDL生成客户端时生成的代码时提供了。Thrift生成的客户端代码提供了读写参数的方法，这两个方式是一一对应的，包括字段的序号，类型等等。客户端使用写参数的方法，服务器端使用读参数的方法。

关于IDL生成的客户端代码会在后面的文章具体描述。下面简单看一下自动生成的代码

1. 方法的调用从writeMessageBegin开始，发送了消息头信息

2. 写方法的参数，也就是写消息体。方法参数由一个统一的接口TBase描述，提供了read和write的统一接口。自动生成的代码提供了read, write方法参数的具体实现

3. 写完结束 

 public void write_args(org.apache.thrift.protocol.TProtocol prot) throws org.apache.thrift.TException {
        prot.writeMessageBegin(new org.apache.thrift.protocol.TMessage("handle", org.apache.thrift.protocol.TMessageType.CALL, 0));
        handle_args args = new handle_args();
        args.setIdentity(identity);
        args.setUid(uid);
        args.setSid(sid);
        args.setType(type);
        args.setMessage(message);
        args.setParams(params);
        args.write(prot);
        prot.writeMessageEnd();
      }

public interface TBase<T extends TBase<?,?>, F extends TFieldIdEnum> extends Comparable<T>,  Serializable {

  public void read(TProtocol iprot) throws TException;

  public void write(TProtocol oprot) throws TException;
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public static class handle_args <strong>implements org.apache.thrift.TBase</strong><handle_args, handle_args._Fields>, java.io.Serializable, Cloneable   {
    private static final org.apache.thrift.protocol.TStruct STRUCT_DESC = new org.apache.thrift.protocol.TStruct("handle_args");

    private static final org.apache.thrift.protocol.TField IDENTITY_FIELD_DESC = new org.apache.thrift.protocol.TField("identity", org.apache.thrift.protocol.TType.STRING, (short)1);
    private static final org.apache.thrift.protocol.TField UID_FIELD_DESC = new org.apache.thrift.protocol.TField("uid", org.apache.thrift.protocol.TType.I64, (short)2);
    private static final org.apache.thrift.protocol.TField SID_FIELD_DESC = new org.apache.thrift.protocol.TField("sid", org.apache.thrift.protocol.TType.STRING, (short)3);
    private static final org.apache.thrift.protocol.TField TYPE_FIELD_DESC = new org.apache.thrift.protocol.TField("type", org.apache.thrift.protocol.TType.I32, (short)4);
    private static final org.apache.thrift.protocol.TField MESSAGE_FIELD_DESC = new org.apache.thrift.protocol.TField("message", org.apache.thrift.protocol.TType.STRING, (short)5);
    private static final org.apache.thrift.protocol.TField PARAMS_FIELD_DESC = new org.apache.thrift.protocol.TField("params", org.apache.thrift.protocol.TType.MAP, (short)6);

    private static final Map<Class<? extends IScheme>, SchemeFactory> schemes = new HashMap<Class<? extends IScheme>, SchemeFactory>();
    static {
      schemes.put(StandardScheme.class, new handle_argsStandardSchemeFactory());
      schemes.put(TupleScheme.class, new handle_argsTupleSchemeFactory());
    }

    public String identity; // required
    public long uid; // required
    public String sid; // required
    public int type; // required
    public String message; // required
    public Map<String,String> params; // required

    /** The set of fields this struct contains, along with convenience methods for finding and manipulating them. */
    public enum _Fields implements org.apache.thrift.TFieldIdEnum {
      IDENTITY((short)1, "identity"),
      UID((short)2, "uid"),
      SID((short)3, "sid"),
      TYPE((short)4, "type"),
      MESSAGE((short)5, "message"),
      PARAMS((short)6, "params");
｝

//　　自动生成的写方法参数的方法，按照字段顺序写，给客户端代码使用
      public void write(org.apache.thrift.protocol.TProtocol oprot, handle_args struct) throws org.apache.thrift.TException {
        struct.validate();

        oprot.writeStructBegin(STRUCT_DESC);
        if (struct.identity != null) {
          oprot.writeFieldBegin(IDENTITY_FIELD_DESC);
          oprot.writeString(struct.identity);
          oprot.writeFieldEnd();
        }
        oprot.writeFieldBegin(UID_FIELD_DESC);
        oprot.writeI64(struct.uid);
        oprot.writeFieldEnd();
        if (struct.sid != null) {
          oprot.writeFieldBegin(SID_FIELD_DESC);
          oprot.writeString(struct.sid);
          oprot.writeFieldEnd();
        }
        oprot.writeFieldBegin(TYPE_FIELD_DESC);
        oprot.writeI32(struct.type);
        oprot.writeFieldEnd();
        if (struct.message != null) {
          oprot.writeFieldBegin(MESSAGE_FIELD_DESC);
          oprot.writeString(struct.message);
          oprot.writeFieldEnd();
        }
｝

<pre name="code" class="java">//　　自动生成的读方法参数的方法，按照字段顺序读，给服务器端代码使用

Thrift源码分析（二）-- 协议和编解码