hadoop作为分布式的系统， 集群机器之间的通信是最基本，最常见的需求。
这种需求本质上是IPC, 即进程间通信。 按照传统的UINX编程模型，进程间通信无非是如下的几种方式:
管道， FIFO， 消息队列， 信号量， 共享存储， 套接字。只有套接字是可以跨机器的网络通信， 能满足hadoop的需求。
通常情况下， 网络通信的程序使用显式网络编程（即直接使用java.net包）。比如Web浏览器， Web服务器等。
但也有另一部分程序使用隐式网络编程， 比如利用hadoop RPC这种封装了底层通信细节的工具包。
目前成都创新互联公司已为上1000+的企业提供了网站建设、域名、虚拟主机、网站运营、企业网站设计、柞水网站维护等服务，公司将坚持客户导向、应用为本的策略，正道将秉承"和谐、参与、激情"的文化，与客户和合作伙伴齐心协力一起成长，共同发展。
这样做使得底层的网络通信对于程序员透明。一则减轻了程序员的负担， 二则抽象了功能模块， 使得模块之间职责更清晰， 便于维护。
首先展示一个hadoop RPC功能demo， 了解hadoop RPC的用法。
step 1: 在pom.xml文件中添加依赖项
org.apache.hadoop hadoop-common 2.6.0
step 2: 在src/main/resources中添加log4j.properties
log4j.rootLogger=DEBUG,console log4j.additivity.org.apache=true # (console) log4j.appender.console=org.apache.log4j.ConsoleAppender log4j.appender.console.Threshold=INFO log4j.appender.console.ImmediateFlush=true log4j.appender.console.Target=System.err log4j.appender.console.layout=org.apache.log4j.PatternLayout log4j.appender.console.layout.ConversionPattern=[%-5p] %d(%r) --> [%t] %l: %m %x %n
step 3: 定义RPC协议
package hadooprpc.demo;
import java.io.IOException;
interface ClientProtocol extends org.apache.hadoop.ipc.VersionedProtocol {
// 版本号,默认情况下,不同版本号的 RPC Client 和 Server 之间不能相互通信
public static final long versionID = 1L;
String echo(String value) throws IOException;
int add(int v1, int v2) throws IOException;
}step 4: 实现RPC协议
package hadooprpc.demo;
import java.io.IOException;
import org.apache.hadoop.ipc.ProtocolSignature;
public class ClientProtocolImpl implements ClientProtocol {
// 重载的方法,用于获取自定义的协议版本号,
public long getProtocolVersion(String protocol, long clientVersion) {
return ClientProtocol.versionID;
}
// 重载的方法,用于获取协议签名
public ProtocolSignature getProtocolSignature(String protocol, long clientVersion,
int hashcode) {
return new ProtocolSignature(ClientProtocol.versionID, null);
}
public String echo(String value) throws IOException {
return value;
}
public int add(int v1, int v2) throws IOException {
return v1 + v2;
}
}step 5； 构造并启动 RPC Server
package hadooprpc.demo;
import java.io.IOException;
import org.apache.hadoop.HadoopIllegalArgumentException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.ipc.RPC;
import org.apache.hadoop.ipc.Server;
public class RPCServer {
public static void main(String[] args) throws HadoopIllegalArgumentException, IOException {
Configuration conf = new Configuration();
Server server = new RPC.Builder(conf).setProtocol(ClientProtocol.class)
.setInstance(new ClientProtocolImpl()).setBindAddress("localhost").setPort(8097)
.setNumHandlers(5).build();
server.start();
}
}step 6: 构造 RPC Client 并发送 RPC 请求
package hadooprpc.demo;
import java.io.IOException;
import java.net.InetSocketAddress;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.ipc.RPC;
public class RPCClient {
public static void main(String[] args) throws IOException {
ClientProtocol client = (ClientProtocol)RPC.getProxy(ClientProtocol.class, ClientProtocol.versionID,
new InetSocketAddress(8097), new Configuration());
int result = client.add(5, 6);
System.out.println(result);
String echoResult = client.echo("result");
System.out.println(echoResult);
RPC.stopProxy(client); -- 关闭连接
}
}step 7: 启动RPC Server, 然后执行 RPC Client.
通过上面的例子可以发现， 通过这种编程方式，不用考虑网络层的细节，只需要编写接口和接口实现即可。
问渠那得清如许？ 通过hadoop RPC的源码， 或许可以管中窥豹。
hadoop RPC的实现原理很简单。
Client:
1. 通过动态代理，获取到调用接口的方法，参数类型。
2. 将调用信息编码，发送到服务器
3. 获取服务器的返回值， 并解码。
4. 返回调用方法的返回值。
Server:
1. 启动服务器， 并监听客户端。
2. 获取客户端发送过来的调用方法， 参数。
3. 执行实现类中相关的方法。
4. 将返回值发送到客户端。
理解原理后， 自己动手实现了一个非常粗糙， 只具备演示功能的RPC框架。
package srpc;
import java.io.*;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.text.MessageFormat;
import java.util.HashMap;
import java.util.Map;
/**
* Created by shgy on 17-5-7.
* 创建一个简单的PRC框架, 参数和返回值只支持基础类型
*/
// 接口
interface MyProtocol{
String echo(String msg);
int add(int a,int b);
}
// 实现
class MyProtocolImp implements MyProtocol{
@Override
public String echo(String msg) {
return msg;
}
@Override
public int add(int a,int b){
return a+b;
}
}
public class ImitateRPC {
private static final Map> PRIMITIVE_NAMES = new HashMap>();
static {
PRIMITIVE_NAMES.put("boolean", Boolean.TYPE);
PRIMITIVE_NAMES.put("byte", Byte.TYPE);
PRIMITIVE_NAMES.put("char", Character.TYPE);
PRIMITIVE_NAMES.put("short", Short.TYPE);
PRIMITIVE_NAMES.put("int", Integer.TYPE);
PRIMITIVE_NAMES.put("long", Long.TYPE);
PRIMITIVE_NAMES.put("float", Float.TYPE);
PRIMITIVE_NAMES.put("double", Double.TYPE);
PRIMITIVE_NAMES.put("void", Void.TYPE);
}
public static class Server{
private Class protocolClass;
private Object protocolImpl;
private ServerSocket server;
public Server(Class protocolClass, Object protocolImpl) throws IOException {
if(protocolImpl == null){
throw new IllegalArgumentException("protocolImpl is not set");
}
if (protocolClass == null) {
throw new IllegalArgumentException("protocolClass is not set");
} else {
if (!protocolClass.isAssignableFrom(protocolImpl.getClass())) {
throw new IOException("protocolClass "+ protocolClass +
" is not implemented by protocolImpl which is of class " +
protocolImpl.getClass());
}
}
this.protocolClass = protocolClass;
this.protocolImpl = protocolImpl;
}
public void start(){
System.out.println("start server");
try{
this.server = new ServerSocket(8189);
listen();
}catch(Exception e){
e.printStackTrace();
}
}
public void close(){
System.out.println("close server");
try {
this.server.close();
} catch (IOException e) {}
}
private void listen(){
new Thread(){
@Override
public void run(){
while (!server.isClosed()){
Socket incoming = null;
try {
incoming = server.accept();
DataInputStream inStream = new DataInputStream(incoming.getInputStream());
// 从客户端读取出 调用方法的信息
int dataLen = inStream.readInt();
byte[] data = new byte[dataLen];
inStream.read(data,0, dataLen);
DataInputStream contentStream = new DataInputStream(new ByteArrayInputStream(data));
String methodName = contentStream.readUTF();
int paramCount = contentStream.readInt();
Class[] paramTypes = new Class[paramCount];
Object[] args = new Object[paramCount];
for(int i=0;i declaredClass = PRIMITIVE_NAMES.get(className);
if(declaredClass == null){
declaredClass = String.class;
}
paramTypes[i] = declaredClass;
if(declaredClass == String.class){
args[i] = contentStream.readUTF();
}else if (declaredClass.isPrimitive()) { // primitive types
if (declaredClass == Boolean.TYPE) { // boolean
args[i] = Boolean.valueOf(contentStream.readBoolean());
} else if (declaredClass == Character.TYPE) { // char
args[i] = Character.valueOf(contentStream.readChar());
} else if (declaredClass == Byte.TYPE) { // byte
args[i] = Byte.valueOf(contentStream.readByte());
} else if (declaredClass == Short.TYPE) { // short
args[i] = Short.valueOf(contentStream.readShort());
} else if (declaredClass == Integer.TYPE) { // int
args[i] = Integer.valueOf(contentStream.readInt());
} else if (declaredClass == Long.TYPE) { // long
args[i] = Long.valueOf(contentStream.readLong());
} else if (declaredClass == Float.TYPE) { // float
args[i] = Float.valueOf(contentStream.readFloat());
} else if (declaredClass == Double.TYPE) { // double
args[i] = Double.valueOf(contentStream.readDouble());
} else if (declaredClass == Void.TYPE) { // void
args[i] = null;
} else {
throw new IllegalArgumentException("Not a primitive: "+declaredClass);
}
}
}
Method method = protocolClass.getMethod(methodName, paramTypes);
Object obj = method.invoke(protocolImpl, args);
Class retType = method.getReturnType();
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
DataOutput out = new DataOutputStream(buffer);
out.writeUTF(retType.getCanonicalName());
if(retType == String.class){
out.writeUTF((String)obj);
}else if (retType.isPrimitive()) { // primitive types
if (retType == Boolean.TYPE) { // boolean
out.writeBoolean((Boolean)obj);
} else if (retType == Character.TYPE) { // char
out.writeChar((Character) obj);
} else if (retType == Byte.TYPE) { // byte
out.writeBoolean((Boolean)obj);
} else if (retType == Short.TYPE) { // short
out.writeShort((Short) obj);
} else if (retType == Integer.TYPE) { // int
out.writeInt((Integer) obj);
} else if (retType == Long.TYPE) { // long
out.writeLong((Long) obj);
} else if (retType == Float.TYPE) { // float
out.writeFloat((Float) obj);
} else if (retType == Double.TYPE) { // double
out.writeDouble((Double) obj);
} else if (retType == Void.TYPE) { // void
} else {
throw new IllegalArgumentException("Not a primitive: "+retType);
}
}
byte[] array = buffer.toByteArray();
//将返回结果写回到客户端
DataOutputStream outStream = new DataOutputStream(incoming.getOutputStream());
outStream.writeInt(array.length);
outStream.write(array);
outStream.flush();
}catch (SocketException e){
}catch (Exception e){
e.printStackTrace();
}finally {
try {
if(incoming!=null){ incoming.close(); }
}catch (Exception e){}
}
}
}
}.start();
}
}
public static T getProxy(Class protocol){
T proxy = (T) Proxy.newProxyInstance(protocol.getClassLoader(), new Class[]{protocol},
new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Class[] paramTypes = method.getParameterTypes();
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
DataOutput out = new DataOutputStream(buffer);
out.writeUTF(method.getName()); // 写入方法名
out.writeInt(paramTypes.length); // 写入参数个数
// 写入参数
for (int i = 0; i < paramTypes.length; i++) {
Class declaredClass = paramTypes[i];
Object instance = args[i];
String paramTypeName = declaredClass.getCanonicalName();
out.writeUTF(paramTypeName);
if (declaredClass == String.class) {
out.writeUTF((String)instance);
}else if (declaredClass.isPrimitive()) { // primitive type
if (declaredClass == Boolean.TYPE) { // boolean
out.writeBoolean(((Boolean) instance).booleanValue());
} else if (declaredClass == Character.TYPE) { // char
out.writeChar(((Character) instance).charValue());
} else if (declaredClass == Byte.TYPE) { // byte
out.writeByte(((Byte) instance).byteValue());
} else if (declaredClass == Short.TYPE) { // short
out.writeShort(((Short) instance).shortValue());
} else if (declaredClass == Integer.TYPE) { // int
out.writeInt(((Integer) instance).intValue());
} else if (declaredClass == Long.TYPE) { // long
out.writeLong(((Long) instance).longValue());
} else if (declaredClass == Float.TYPE) { // float
out.writeFloat(((Float) instance).floatValue());
} else if (declaredClass == Double.TYPE) { // double
out.writeDouble(((Double) instance).doubleValue());
} else if (declaredClass == Void.TYPE) { // void
} else {
throw new IllegalArgumentException("Not a primitive: " + declaredClass);
}
}else{
throw new IOException("Can't write: "+instance+" as "+declaredClass);
}
}
// 发送到服务器端
byte[] array = buffer.toByteArray();
Socket client = new Socket("127.0.0.1", 8189);
try{
DataOutputStream outStream = new DataOutputStream(client.getOutputStream());
outStream.writeInt(array.length);
outStream.write(array);
outStream.flush();
// 读取服务器的应答
DataInputStream inStream = new DataInputStream(client.getInputStream());
int bufSize = inStream.readInt();
byte[] retArray = new byte[bufSize];
inStream.read(retArray,0, bufSize);
DataInputStream contentStream = new DataInputStream(new ByteArrayInputStream(retArray));
String retType = contentStream.readUTF();
Class retTypeClass = PRIMITIVE_NAMES.get(retType);
if(retTypeClass == null){
retTypeClass = String.class;
}
if(retTypeClass == String.class){
return contentStream.readUTF();
}else if (retTypeClass.isPrimitive()) { // primitive types
if (retTypeClass == Boolean.TYPE) { // boolean
return Boolean.valueOf(contentStream.readBoolean());
} else if (retTypeClass == Character.TYPE) { // char
return Character.valueOf(contentStream.readChar());
} else if (retTypeClass == Byte.TYPE) { // byte
return Byte.valueOf(contentStream.readByte());
} else if (retTypeClass == Short.TYPE) { // short
return Short.valueOf(contentStream.readShort());
} else if (retTypeClass == Integer.TYPE) { // int
return Integer.valueOf(contentStream.readInt());
} else if (retTypeClass == Long.TYPE) { // long
return Long.valueOf(contentStream.readLong());
} else if (retTypeClass == Float.TYPE) { // float
return Float.valueOf(contentStream.readFloat());
} else if (retTypeClass == Double.TYPE) { // double
return Double.valueOf(contentStream.readDouble());
} else if (retTypeClass == Void.TYPE) { // void
return null;
} else {
throw new IllegalArgumentException("Not a primitive: "+retTypeClass);
}
}
}finally {
try {
client.close();
}catch (Exception e){}
}
return null;
}
});
return proxy;
}
public static void main(String[] args) throws IOException {
// start server
Server server = new Server(MyProtocol.class, new MyProtocolImp());
server.start();
// call rpc
MyProtocol client = ImitateRPC.getProxy(MyProtocol.class);
System.out.println(client.echo("hello world"));
System.out.println(client.add(1, 4));
server.close();
}
} 上面的代码只有300多行， 而且在编码/解码部分有重叠的代码， 可以优化缩减， 功能也不完善。 但是仿照Hadoop RPC, 演示了RPC的原理。 对照着Hadoop RPC的源码， 自己编码实现， 对于源码中各个类的功能， 会有更清晰的理解。
