在Java5.0中对于RMI的使用发生了一些改变, 使得RMI的使用更加容易方便.
一. 定义RMI接口, 这一步最简单, 与以前相比, 也没有变化.
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface Payment extends Remote {
public double calculatePayment(double principal, double annualRate, int terms)
throws RemoteException;
}
注意点:
1. 必须继承java.rmi.Remote
2. 接口中的方法必须抛出java.rmi.RemoteException异常
二. 定义远程类实现接口
与以前相比, 也没有发生变化
import java.rmi.RemoteException;
public class PaymentImpl implements Payment {
public double calculatePayment(double principal, double annRate, int years)
throws RemoteException {
double monthlyInt = annRate / 12;
double monthlyPayment = (principal * monthlyInt)
/ (1 - Math.pow(1/ (1 + monthlyInt), years * 12));
return format(monthlyPayment, 2);
}
public double format(double amount, int places) {
double temp = amount;
temp = temp * Math.pow(10, places);
temp = Math.round(temp);
temp = temp/Math.pow(10, places);
return temp;
}
}
注意点:
1. 远程对象必须实现第一步定义的接口, 在这里是Payment接口.
2. 远程对象中可以定义接口中不包括的方法, 比如: format, 但是这些方法不能远程使用.
3. 实现远程方法时, 可以不抛出java.rmi.RemoteException异常
三. 定义远程服务
此处与以前相比, 发生了一些变化, 需要注意.
import java.rmi.registry.Registry;
import java.rmi.registry.LocateRegistry;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class Server {
public Server() {}
public static void main(String args[]) {
try {
PaymentImpl robj = new PaymentImpl();
Payment stub = (Payment) UnicastRemoteObject.exportObject(robj, 0);
Registry registry = LocateRegistry.getRegistry();
registry.bind("Mortgage", stub);
System.out.println("Mortgage Server is ready to listen");
} catch (Exception e) {
System.err.println("Server exception thrown: " + e.toString());
e.printStackTrace();
}
}
}
而传统的做法是这样的:
public class Server extends java.rmi.server.UnicastRemoteObject implements aRemoteInterface{
public Server(int port) {
super(port);
}
....
Naming.bind(uniqueName, this);
....
}
请自行进行比较.
四. 定义客户端类
相应地, 这部分也有了变化.
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
public class Client {
private static Payment stub = null;
private Client() {}
public static void main(String[] args) {
double payment, principal = 80000;
double annualInterest = .065;
int years = 15;
try {
Registry reg = LocateRegistry.getRegistry("localhost");
stub = (Payment) reg.lookup("Mortgage");
} catch (Exception e) {
System.err.println("Client exception thrown: " + e.toString());
e.printStackTrace();
}
if (args.length == 3) {
try {
principal = Double.parseDouble(args[0]);
annualInterest = Double.parseDouble(args[1]);
years = Integer.parseInt(args[2]);
}
catch (Exception e) {
System.out.println("Wrong input " + e.getMessage() );
System.exit(0);
}
print(principal, annualInterest, years);
} else {
System.out.println("Usage: java Client principal annualInterest years ");
System.out.println("\nFor example: java Client 80000 .065 15 ");
System.out.println("\nYou will get the output like the following: \n");
print(principal, annualInterest, years);
System.exit(0);
}
}
public static void print(double pr, double annRate, int years){
double mpayment = 0;
try {
mpayment = stub.calculatePayment(pr, annRate, years);
}catch(Exception e) {
System.out.println("Remote method exception thrown: " + e.getMessage());
}
System.out.println("The principal is $" + (int)pr);
System.out.println("The annual interest rate is " + annRate*100 +"%");
System.out.println("The term is " + years + " years");
System.out.println("Your monthly payment is $" + mpayment);
}
}
注意点: lookup服务的方法与以前是不同的
五. 测试使用
现在我们有了4个类
Payment.java-- a remote interface
PaymentImpl.java-- a remote object class
Server.java-- an RMI server
Client.java-- an RMI client
1. 我们把它们都放到一个目录下, 比如: 以 c:\myrmi
2. 使用javac编译这4个类. javac -d . Payment.java PaymentImpl.java Server.java Client.java
3. 注意: Java5.0的新特性, 现在在不需要在启动服务之前生成Stub了, stub现在自动生成.
而在Java5.0以前是需要用rmic手工生成的.
4. start rmiregistry, 出现一个窗口, 不要关闭.
5. start java Server, 出现一个窗口, 显示 Mortgage server is ready to listen...
6. 启动客户端java Client, 如果出现
The principal is $150000
The annual interest rate is 6.0%
The term is 15 years
Your monthly payment is $1265.79
那就大功告成了.
注意点: 安全策略机制进行了弱化, 不再需要指定策略文件 授这个权,那个权的了.