学习java tutorial中的rmi章节,读书笔记
JAVA-RMI 所采用的机制:
1. 某Object需要remote call ,则它所有的方法都放在一个新声明的接口中,该接口必须extends Remote接口
Compute接口中存放所有需要进行远程调用的方法,让实现该接口的类来实现这些被远程调用的方法
package compute;
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface Compute extends Remote {
Object executeTask(Task t) throws RemoteException;
}
2.采用远程调用的方法同样有一些参数和返回值,对于这些参数和返回值,在不同JAVA虚拟机间传输,RM I采取的是值传递,所以所有的参数和返回值必须extends Serializable
package compute;
import java.io.Serializable;
public interface Task extends Serializable {
Object execute();
}
/*******************************************************************************************
上面为第一部分,在这当中,我们设计了一个接口,其中包含需要远程调用的方法,然后让该接口的实现者(即远程对象的owner)来实现这些方法
*******************************************************************************************/
3. 现在我们为了得到远程对象,就来实现一下上面定义的接口:
main函数中,新建一个远程对象engine,并将它与名字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 = "//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();
}
}
}
/*******************************************************************************************
上面为第二部分,在这当中,我们得到了一个实现了Compute接口的远程对象engine,注意:engine中只有那些Compute接口中的方法能够被远程调用,其他方法只能本地调用。现在,将远程对象engine邦定到RMI Registry 。 client可以从registry那里得到engine的引用,实际上是一个stub,相当于一个engine的代理,远程方法被client调用以后,可以改变engine的状态
*******************************************************************************************/
4. 下面我们实现那些远程方法调用时所要用到的参数: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. 最后我们实现client对象
远程对象comp是通过RMI的注册表来查找到的
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 = "//" + 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();
}
}
}
posted on 2008-04-03 13:25
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