學習java tutorial中的rmi章節，讀書筆記

JAVA-RMI 所采用的機制：

1. 某Object需要remote call ，則它所有的方法都放在一個新聲明的接口中，該接口必須extends          Remote接口

   Compute接口中存放所有需要進行遠程調用的方法，讓實現該接口的類來實現這些被遠程調用的方法

package compute;

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Compute extends Remote {
    Object executeTask(Task t) throws RemoteException;
}

2.采用遠程調用的方法同樣有一些參數和返回值，對于這些參數和返回值，在不同JAVA虛擬機間傳輸，RM  I采取的是值傳遞，所以所有的參數和返回值必須extends Serializable

package compute;

import java.io.Serializable;

public interface Task extends Serializable {
    Object execute();
}

/*******************************************************************************************
上面為第一部分，在這當中，我們設計了一個接口，其中包含需要遠程調用的方法，然后讓該接口的實現者（即遠程對象的owner）來實現這些方法
*******************************************************************************************/

3. 現在我們為了得到遠程對象，就來實現一下上面定義的接口：
   main函數中，新建一個遠程對象engine,并將它與名字name邦定：Naming.rebind(name, engine);

注：The superclass UnicastRemoteObject supplies implementations for a number of     java.lang.Object methods (equals, hashCode, toString) so that they are defined     appropriately for remote objects. UnicastRemoteObjectalso includes constructors and     static methods used to export a remote object, that is, make the remote object available     to receive incoming calls from clients.

package engine;

import java.rmi.*;
import java.rmi.server.*;
import compute.*;

public class ComputeEngine extends UnicastRemoteObject
                           implements Compute
{
    public ComputeEngine() throws RemoteException {
        super();
    }

    public Object executeTask(Task t) {
        return t.execute();
    }

    public static void main(String[] args) {
        if (System.getSecurityManager() == null) {
            System.setSecurityManager(new RMISecurityManager());
        }
        String name = "http://host/Compute";
        try {
            Compute engine = new ComputeEngine();
            Naming.rebind(name, engine);
            System.out.println("ComputeEngine bound");
        } catch (Exception e) {
            System.err.println("ComputeEngine exception: " +
          e.getMessage());
            e.printStackTrace();
        }
    }
}

/*******************************************************************************************
上面為第二部分，在這當中，我們得到了一個實現了Compute接口的遠程對象engine，注意：engine中只有那些Compute接口中的方法能夠被遠程調用，其他方法只能本地調用。現在，將遠程對象engine邦定到RMI Registry 。 client可以從registry那里得到engine的引用，實際上是一個stub，相當于一個engine的代理，遠程方法被client調用以后，可以改變engine的狀態
*******************************************************************************************/
  
4. 下面我們實現那些遠程方法調用時所要用到的參數：Task

package client;

import compute.*;
import java.math.*;

public class Pi implements Task {

    /** constants used in pi computation */
    private static final BigDecimal ZERO =
        BigDecimal.valueOf(0);
    private static final BigDecimal  ONE =
        BigDecimal.valueOf(1);
    private static final BigDecimal FOUR =
        BigDecimal.valueOf(4);

    /** rounding mode to use during pi computation */
    private static final int roundingMode =
        BigDecimal.ROUND_HALF_EVEN;

    /** digits of precision after the decimal point */
    private int digits;
   
    /**
     * Construct a task to calculate pi to the specified
     * precision.
     */
    public Pi(int digits) {
        this.digits = digits;
    }

    /**
     * Calculate pi.
     */
    public Object execute() {
        return computePi(digits);
    }

    /**
     * Compute the value of pi to the specified number of
     * digits after the decimal point.  The value is
     * computed using Machin's formula:
     *
     *          pi/4 = 4*arctan(1/5) - arctan(1/239)
     *
     * and a power series expansion of arctan(x) to
     * sufficient precision.
     */
    public static BigDecimal computePi(int digits) {
        int scale = digits + 5;
        BigDecimal arctan1_5 = arctan(5, scale);
        BigDecimal arctan1_239 = arctan(239, scale);
        BigDecimal pi = arctan1_5.multiply(FOUR).subtract(
                                  arctan1_239).multiply(FOUR);
        return pi.setScale(digits,
                           BigDecimal.ROUND_HALF_UP);
    }
    /**
     * Compute the value, in radians, of the arctangent of
     * the inverse of the supplied integer to the speficied
     * number of digits after the decimal point.  The value
     * is computed using the power series expansion for the
     * arc tangent:
     *
     * arctan(x) = x - (x^3)/3 + (x^5)/5 - (x^7)/7 +
     *     (x^9)/9 ...
     */  
    public static BigDecimal arctan(int inverseX,
                                  int scale)
    {
        BigDecimal result, numer, term;
        BigDecimal invX = BigDecimal.valueOf(inverseX);
        BigDecimal invX2 =
            BigDecimal.valueOf(inverseX * inverseX);

        numer = ONE.divide(invX, scale, roundingMode);

        result = numer;
        int i = 1;
        do {
            numer =
                numer.divide(invX2, scale, roundingMode);
            int denom = 2 * i + 1;
            term =
                numer.divide(BigDecimal.valueOf(denom),
                             scale, roundingMode);
            if ((i % 2) != 0) {
                result = result.subtract(term);
            } else {
                result = result.add(term);
            }
            i++;
        } while (term.compareTo(ZERO) != 0);
        return result;
    }
}

5. 最后我們實現client對象
   遠程對象comp是通過RMI的注冊表來查找到的

package client;

import java.rmi.*;
import java.math.*;
import compute.*;

public class ComputePi {
    public static void main(String args[]) {
        if (System.getSecurityManager() == null) {
            System.setSecurityManager(new RMISecurityManager());
        }
        try {
            String name = "http://" + args[0] + "/Compute";
            Compute comp = (Compute) Naming.lookup(name);
            Pi task = new Pi(Integer.parseInt(args[1]));
            BigDecimal pi = (BigDecimal) (comp.executeTask(task));
            System.out.println(pi);
        } catch (Exception e) {
            System.err.println("ComputePi exception: " +
                               e.getMessage());
            e.printStackTrace();
        }
    }
}