单例模式-键值生成器

    最近看<JAVA与模式>，单例章节的例子，我作了修改之后的练习，用properties文件代替数据库，略有不同。

    需求:某个系统需要的多键值生成器,键值唯一，键值缓存(避免多次访问,节约资源)

    实现:

 

    一个标准单例：

package singleton.keyGenerator;

import java.util.HashMap;

public class KeyGenerator {

	private static KeyGenerator keyGenerator = new KeyGenerator();
	private HashMap<String, KeyBuffer> KeyPool = new HashMap<String, KeyBuffer>();//缓存

	private KeyGenerator() {

	}

	public static KeyGenerator getInstance() {
		return keyGenerator;
	}

        //第一次使用，放入缓存，否则直接使用
	public int getNextKey(String key) {
		if (!KeyPool.containsKey(key)) {
			KeyPool.put(key, new KeyBuffer(key));
		}
		return KeyPool.get(key).getNextVal(key);
	}

}

 

    单键的缓存:

package singleton.keyGenerator;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Properties;

//单键值的缓存类
public class KeyBuffer {
	private int nextKey;
	private int maxKey;
	// 键缓存大小
	private int PoolSize = 3;
	
	public KeyBuffer(String key) {
		getFromFile(key);
	}

	public synchronized int getNextVal(String key) {
		if (nextKey > maxKey) {
			getFromFile(key);
		}
		return nextKey++;
	}

	private synchronized void getFromFile(String key) {
		Properties prop = new Properties();
		System.out.println("ReadFromFile:"+key);
		try {
			InputStream in = new FileInputStream("c:\\key.properties");
			prop.load(in);
			if (!prop.containsKey(key)) {
                                //文件中没有此键，异常
				throw new RuntimeException("key not defined");
			}
			nextKey = Integer.valueOf((String) prop.get(key)) + 1;
			maxKey = nextKey + PoolSize - 1;
			in.close();
			//
			OutputStream out = new FileOutputStream("c:\\key.properties");
			prop.setProperty(key, Integer.toString(maxKey));
			prop.store(out, "From KeyGenerator");
			out.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}

     测试：

package singleton.keyGenerator;

public class Client {

	public static void main(String[] args) {
		KeyGenerator keyGenerator = KeyGenerator.getInstance();
		System.out.println(keyGenerator.getNextKey("KEY"));
		System.out.println(keyGenerator.getNextKey("NEWKEY"));
		System.out.println(keyGenerator.getNextKey("KEY"));
		System.out.println(keyGenerator.getNextKey("NEWKEY"));
		System.out.println(keyGenerator.getNextKey("KEY"));
		System.out.println(keyGenerator.getNextKey("NEWKEY"));
		System.out.println(keyGenerator.getNextKey("KEY"));
		System.out.println(keyGenerator.getNextKey("NEWKEY"));
		System.out.println(keyGenerator.getNextKey("KEY"));
		System.out.println(keyGenerator.getNextKey("NEWKEY"));
	}
}

 

测试结果：

ReadFromFile:KEY

71

ReadFromFile:NEWKEY

1016

72

1017

73

1018

ReadFromFile:KEY

74

ReadFromFile:NEWKEY

1019

75

1020

 

文件 key.properties

#From KeyGenerator

#Fri Jan 24 20:33:20 CST 2014

KEY=76

NEWKEY=1021

 

总结：单例是个很有意思的模式