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;
}
}
~~~
所谓协议就是客户端和服务器端约定传输什么数据，如何解析传输的数据。对于一个RPC调用的协议来说，要传输的数据主要有:
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;
｝
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
~~~
public void read(org.apache.thrift.protocol.TProtocol iprot, handle_args struct) throws org.apache.thrift.TException {
org.apache.thrift.protocol.TField schemeField;
iprot.readStructBegin();
while (true)
{
schemeField = iprot.readFieldBegin();
if (schemeField.type == org.apache.thrift.protocol.TType.STOP) {
break;
}
switch (schemeField.id) {
case 1: // IDENTITY
if (schemeField.type == org.apache.thrift.protocol.TType.STRING) {
struct.identity = iprot.readString();
struct.setIdentityIsSet(true);
} else {
org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
}
break;
case 2: // UID
if (schemeField.type == org.apache.thrift.protocol.TType.I64) {
struct.uid = iprot.readI64();
struct.setUidIsSet(true);
} else {
org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
}
break;
case 3: // SID
if (schemeField.type == org.apache.thrift.protocol.TType.STRING) {
struct.sid = iprot.readString();
struct.setSidIsSet(true);
} else {
org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
}
break;
case 4: // TYPE
if (schemeField.type == org.apache.thrift.protocol.TType.I32) {
struct.type = iprot.readI32();
struct.setTypeIsSet(true);
} else {
org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
}
break;
｝
~~~
