【设计模式：单例模式】使用单例模式加载properties文件

先准备测试程序：

package org.jediael.util;
import static org.junit.Assert.*;
import org.junit.Test;

public class BasicConfigurationTest {
	@Test
	public void testGetValue(){
		BasicConfiguration configuration = BasicConfiguration.getInstance();
		assertTrue(configuration.getValue("key").equals("value"));
	}
}

其中properties文件中有一行如下：

key=value

优先选择方案三

方式一：懒汉方式

到第一次使用实例时，才加载实例

package org.jediael.util;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class BasicConfiguration {

	private static BasicConfiguration configuration = null;
	private Properties pros = null;
	
	public static synchronized BasicConfiguration getInstance(){
		if(configuration == null){
			configuration = new BasicConfiguration();
		}
		return configuration;
	}
	
	public String getValue(String key){
		return pros.getProperty(key);
	}
	
	private BasicConfiguration(){
		readConfig();
	}

	private void readConfig() {
		pros = new Properties();
		InputStream in = null;
		try {
			in = new FileInputStream(Thread.currentThread().getContextClassLoader().getResource("")
					.getPath() + "search.properties");
			pros.load(in);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}catch (IOException e) {
			e.printStackTrace();
		}finally{
			try {
				in.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

上述程序中，产生了BasicConfiguration的一个单例。

好处是只有到第一次调用getInstance才生成对象，节省了空间。不足之处在于同步锁导致有可能执行过慢。

2、饿汉方式

package org.jediael.util;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class BasicConfiguration {

	private static BasicConfiguration configuration = new BasicConfiguration();
	private Properties pros = null;
	
	public static BasicConfiguration getInstance(){
		return configuration;
	}
	
	public String getValue(String key){
		return pros.getProperty(key);
	}
	
	private BasicConfiguration(){
		readConfig();
	}

	private void readConfig() {
		pros = new Properties();
		InputStream in = null;
		try {
			in = new FileInputStream(Thread.currentThread().getContextClassLoader().getResource("")
					.getPath() + "search.properties");
			pros.load(in);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}catch (IOException e) {
			e.printStackTrace();
		}finally{
			try {
				in.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

由于BasicConfiguration的实例是static，因此，当类被加载时就会初始化，但这样即使并不需要使用此实例，也会被初始化，导致内存空间的浪费。

方式三：

package org.jediael.util;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class BasicConfiguration {

	
	private Properties pros = null;
	
	private static class ConfigurationHolder{
		private static BasicConfiguration configuration = new BasicConfiguration();
	}
	
	public static BasicConfiguration getInstance(){
		return ConfigurationHolder.configuration;
	}
	
	public String getValue(String key){
		return pros.getProperty(key);
	}
	
	private BasicConfiguration(){
		readConfig();
	}
	


	private void readConfig() {
		pros = new Properties();
		InputStream in = null;
		try {
			in = new FileInputStream(Thread.currentThread().getContextClassLoader().getResource("")
					.getPath() + "search.properties");
			pros.load(in);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}catch (IOException e) {
			e.printStackTrace();
		}finally{
			try {
				in.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

由于初始化放在内部类中，只有当此内部类被使用时，才会进行初始化。从而既节省了空间，也无需同步代码。

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