【设计模式：单例模式】使用单例模式加载properties文件

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先准备测试程序：

package org.jediael.util;import static org.junit.Assert.*;import org.junit.Test;public class BasicConfigurationTest {@Testpublic 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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