高性能RPC over MINA&google protobuf 代码&实例 （二）

在本系列上篇http://maoyidao.iteye.com/blog/1636923 实现了基于google
protobuf的序列化反序列化，现在看看怎么把他们组装到MINA的nio中。本篇主要描述怎么处理断包。

使用MINA的CumulativeProtocolDecoder是个好主意，先从MINA自己的sample开始。在这个例子中如果接受到的IoBuffer只包含一部分消息， decoders应该继续接收消息知道接收到完整的消息之后再进行解码。

List1

Java代码


public class CrLfTerminatedCommandLineDecoder

extends CumulativeProtocolDecoder {

private Command parseCommand(IoBuffer in) {

// Convert the bytes in the specified buffer to a

// Command object.

...

}

protected boolean doDecode(

IoSession session, IoBuffer in, ProtocolDecoderOutput out)

throws Exception {

// Remember the initial position.

int start = in.position();

// Now find the first CRLF in the buffer.

byte previous = 0;

while (in.hasRemaining()) {

byte current = in.get();

if (previous == '\r' && current == '\n') {

// Remember the current position and limit.

int position = in.position();

int limit = in.limit();

try {

in.position(start);

in.limit(position);

// The bytes between in.position() and in.limit()

// now contain a full CRLF terminated line.

out.write(parseCommand(in.slice()));

} finally {

// Set the position to point right after the

// detected line and set the limit to the old

// one.

in.position(position);

in.limit(limit);

}

// Decoded one line; CumulativeProtocolDecoder will

// call me again until I return false. So just

// return true until there are no more lines in the

// buffer.

return true;

}

previous = current;

}

// Could not find CRLF in the buffer. Reset the initial

// position to the one we recorded above.

in.position(start);

return false;

}

}

IoBuffer继承java.nio.ByteBuffer，MINA定义这个类有两个原因：

ByteBuffer没有提供get/putString这样方便的方法。

ByteBuffer只能分配固定大小的空间，IoBuffer通过setAutoExpand，expand等方法支持写入变长数据，例如:

Java代码


String greeting = messageBundle.getMessage("hello");

IoBuffer buf = IoBuffer.allocate(16);

// Turn on autoExpand (it is off by default)

buf.setAutoExpand(true);

CharsetEncoder utf8encoder = Charset.forName("UTF-8").newEncoder();

buf.putString(greeting, utf8encoder);

IoBuffer重新分配了底层的ByteBuffer，因为在这个例子里数据超过了16 byte。扩展的时候，ByteBuffer容量（capacity）增倍，limit则指向写入内容的最后一个位置。

List1代码展示的有关断包的逻辑是找到了下\r\n，即认为上一个包已经全部接收到。这是一中常见的断包逻辑，但maoyidao更喜欢使用指定包长度的方法，逻辑更清晰，代码更简单。结合http://maoyidao.iteye.com/blog/1636923 中序列化时加入的验证码，decode代码应该是这样的：

Java代码


public class MaoyidaoDecoder

extends CumulativeProtocolDecoder {

protected boolean doDecode(

IoSession session, IoBuffer in, ProtocolDecoderOutput out)

throws Exception

{

CodedInputStream cis = CodedInputStream.newInstance(getBytes(in, 10));

int pos1 = in.position();

int pos2 = in.limit();

int flag1 = cis.readRawVarint32();

int flag2 = cis.readRawVarint32();

if(flag1 != 3 || flag2 != 7){

// 校验码不对，丢弃这个包

in.position(pos2);

return true;

}else{

// 校验码无问题，重设postion，读下面的内容

in.position(pos1 + 2);

}

int contentLength = CodedInputStream.newInstance(getBytes(in, 5)).readRawVarint32();

int contentLength0 = contentLength + CodedOutputStream.computeRawVarint32Size(contentLength);

// 是否有足够的数据

if(in.remaining() >= contentLength0){

try {

Maoyidao.MaoyidaoPacket.Builder builder = Maoyidao.MaoyidaoPacket.newBuilder();

CodedInputStream.newInstance(getBytes(in, protocolLength)).readMessage(builder,

ExtensionRegistryLite.getEmptyRegistry());

out.write(builder.build());

in.position(in.position() + protocolLength);

return true;

} catch (Exception e) {

}

}

// 没有足够的数据，返回false，IoBuffer退回到最初的位置

else {

in.position(pos1);

return false;

}

}

}

通过这个decodefilter，向上层的outputstream流中写入了rpc对象protobuf序列化后的字符流，这样后续处理只需根据protobuf对象，即可。

本系列（一）（二）2篇文章介绍了基于protobuf/MINA构建JAVA RPC的主要代码逻辑。

本文系maoyidao原创，转载请引用原链接：

http://maoyidao.iteye.com/blog/1637193