5.3.8. RMI¶
5.3.8.1. 简介¶
RMI(Remote Method Invocation，远程方法调用)能够让在客户端Java虚拟机上的对象像调用本地对象一样调用服务端Java虚拟机中的对象上的方法。其中RMI标准实现是Java RMI，之外还有Weblogic RMI、Spring RMI等不同的实现。
RMI中比较重要的两个概念是Stub和Skeleton，Stub和Skeleton对同一套接口进行实现，其中Stub由Client端调用，并不进行真正的实现，而是和Server端通信。Skeleton是Server端，监听来自Stub的连接，根据Stub发送的数据进行真正的操作。
5.3.8.2. 调用步骤¶
- 客户调用客户端辅助对象Stub上的方法
- 客户端辅助对象Stub打包调用信息(变量，方法名)，通过网络发送给服务端辅助对象Skeleton
- 服务端辅助对象Skeleton将客户端辅助对象发送来的信息解包，找出真正被调用的方法以及该方法所在对象
- 调用真正服务对象上的真正方法，并将结果返回给服务端辅助对象Skeleton
- 服务端辅助对象将结果打包，发送给客户端辅助对象Stub
- 客户端辅助对象将返回值解包，返回给调用者
- 客户获得返回值
5.3.8.3. 样例¶
一份代码样例如下(来自《Enterprise JavaBeans》)：
5.3.8.3.1. Person接口定义¶
public interface Person {
public int getAge() throws Throwable;
public String getName() throws Throwable;
}
5.3.8.3.2. 使用PersonServer实现Person¶
public class PersonServer implements Person {
private int age;
private String name;
public PersonServer(String name, int age) {
this.age = age;
this.name = name;
}
public int getAge() {
return age;
}
public String getName() {
return name;
}
}
5.3.8.3.3. 使用Person_Stub实现Person¶
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.net.Socket;
public class Person_Stub implements Person {
private Socket socket;
public Person_Stub() throws Throwable {
// connect to skeleton
socket = new Socket("computer_name", 9000);
}
public int getAge() throws Throwable {
// pass method name to skeleton
ObjectOutputStream outStream =
new ObjectOutputStream(socket.getOutputStream());
outStream.writeObject("age");
outStream.flush();
ObjectInputStream inStream =
new ObjectInputStream(socket.getInputStream());
return inStream.readInt();
}
public String getName() throws Throwable {
// pass method name to skeleton
ObjectOutputStream outStream =
new ObjectOutputStream(socket.getOutputStream());
outStream.writeObject("name");
outStream.flush();
ObjectInputStream inStream =
new ObjectInputStream(socket.getInputStream());
return (String)inStream.readObject();
}
}
5.3.8.3.4. Skeleton的实现¶
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.net.Socket;
import java.net.ServerSocket;
public class Person_Skeleton extends Thread {
private PersonServer myServer;
public Person_Skeleton(PersonServer server) {
// get reference of object server
this.myServer = server;
}
public void run() {
try {
// new socket at port 9000
ServerSocket serverSocket = new ServerSocket(9000);
// accept stub's request
Socket socket = serverSocket.accept();
while (socket != null) {
// get stub's request
ObjectInputStream inStream =
new ObjectInputStream(socket.getInputStream());
String method = (String)inStream.readObject();
// check method name
if (method.equals("age")) {
// execute object server's business method
int age = myServer.getAge();
ObjectOutputStream outStream =
new ObjectOutputStream(socket.getOutputStream());
// return result to stub
outStream.writeInt(age);
outStream.flush();
}
if(method.equals("name")) {
// execute object server's business method
String name = myServer.getName();
ObjectOutputStream outStream =
new ObjectOutputStream(socket.getOutputStream());
// return result to stub
outStream.writeObject(name);
outStream.flush();
}
}
} catch(Throwable t) {
t.printStackTrace();
System.exit(0);
}
}
public static void main(String args []) {
// new object server
PersonServer person = new PersonServer("Richard", 34);
Person_Skeleton skel = new Person_Skeleton(person);
skel.start();
}
}
5.3.8.3.5. Client实现¶
public class PersonClient {
public static void main(String [] args) {
try {
Person person = new Person_Stub();
int age = person.getAge();
String name = person.getName();
System.out.println(name + " is " + age + " years old");
} catch(Throwable t) {
t.printStackTrace();
}
}
}
5.3.8.4. T3协议¶
T3协议是用于在WebLogic服务器和其他类型的Java程序之间传输信息的协议，是Weblogic对RMI规范的实现。简单来说，可以把T3视为暴露JDNI给用户调用的接口。
5.3.8.5. JRMP¶
Java远程方法协议(Java Remote Method Protocol，JRMP)是特定于Java技术的、用于查找和引用远程对象的协议。这是运行在Java远程方法调用(RMI)之下、TCP/IP之上的线路层协议。
JRMP是一个Java特有的、适用于Java之间远程调用的基于流的协议，要求客户端和服务器上都使用Java对象。