我们通常会将Java应用的配置参数保存在属性文件中，Java应用的属性文件可以是一个正常的基于key-value对,以properties为扩展名的文件，也可以是XML文件.
在本案例中，將会向大家介绍如何通过Java程序输出这两种格式的属性文件，并介绍如何从classpath中加载和使用这两种属性文件。
下面是案例程序代码：
PropertyFilesUtil.java

package com.journaldev.util;import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;
import java.util.Set;public class PropertyFilesUtil {public static void main(String[] args) throws IOException {String propertyFileName = "DB.properties";String xmlFileName = "DB.xml";writePropertyFile(propertyFileName, xmlFileName);readPropertyFile(propertyFileName, xmlFileName);readAllKeys(propertyFileName, xmlFileName);readPropertyFileFromClasspath(propertyFileName);}/*** read property file from classpath* @param propertyFileName* @throws IOException*/private static void readPropertyFileFromClasspath(String propertyFileName) throws IOException {Properties prop = new Properties();prop.load(PropertyFilesUtil.class.getClassLoader().getResourceAsStream(propertyFileName));System.out.println(propertyFileName +" loaded from Classpath::db.host = "+prop.getProperty("db.host"));System.out.println(propertyFileName +" loaded from Classpath::db.user = "+prop.getProperty("db.user"));System.out.println(propertyFileName +" loaded from Classpath::db.pwd = "+prop.getProperty("db.pwd"));System.out.println(propertyFileName +" loaded from Classpath::XYZ = "+prop.getProperty("XYZ"));}/*** read all the keys from the given property files* @param propertyFileName* @param xmlFileName* @throws IOException */private static void readAllKeys(String propertyFileName, String xmlFileName) throws IOException {System.out.println("Start of readAllKeys");Properties prop = new Properties();FileReader reader = new FileReader(propertyFileName);prop.load(reader);Set<Object> keys= prop.keySet();for(Object obj : keys){System.out.println(propertyFileName + ":: Key="+obj.toString()+"::value="+prop.getProperty(obj.toString()));}//loading xml file now, first clear existing propertiesprop.clear();InputStream is = new FileInputStream(xmlFileName);prop.loadFromXML(is);keys= prop.keySet();for(Object obj : keys){System.out.println(xmlFileName + ":: Key="+obj.toString()+"::value="+prop.getProperty(obj.toString()));}//Now free all the resourcesis.close();reader.close();System.out.println("End of readAllKeys");}/*** This method reads property files from file system* @param propertyFileName* @param xmlFileName* @throws IOException * @throws FileNotFoundException */private static void readPropertyFile(String propertyFileName, String xmlFileName) throws FileNotFoundException, IOException {System.out.println("Start of readPropertyFile");Properties prop = new Properties();FileReader reader = new FileReader(propertyFileName);prop.load(reader);System.out.println(propertyFileName +"::db.host = "+prop.getProperty("db.host"));System.out.println(propertyFileName +"::db.user = "+prop.getProperty("db.user"));System.out.println(propertyFileName +"::db.pwd = "+prop.getProperty("db.pwd"));System.out.println(propertyFileName +"::XYZ = "+prop.getProperty("XYZ"));//loading xml file now, first clear existing propertiesprop.clear();InputStream is = new FileInputStream(xmlFileName);prop.loadFromXML(is);System.out.println(xmlFileName +"::db.host = "+prop.getProperty("db.host"));System.out.println(xmlFileName +"::db.user = "+prop.getProperty("db.user"));System.out.println(xmlFileName +"::db.pwd = "+prop.getProperty("db.pwd"));System.out.println(xmlFileName +"::XYZ = "+prop.getProperty("XYZ"));//Now free all the resourcesis.close();reader.close();System.out.println("End of readPropertyFile");}/*** This method writes Property files into file system in property file* and xml format* @param fileName* @throws IOException*/private static void writePropertyFile(String propertyFileName, String xmlFileName) throws IOException {System.out.println("Start of writePropertyFile");Properties prop = new Properties();prop.setProperty("db.host", "localhost");prop.setProperty("db.user", "user");prop.setProperty("db.pwd", "password");prop.store(new FileWriter(propertyFileName), "DB Config file");System.out.println(propertyFileName + " written successfully");prop.storeToXML(new FileOutputStream(xmlFileName), "DB Config XML file");System.out.println(xmlFileName + " written successfully");System.out.println("End of writePropertyFile");}}

当运行这段代码时，writePropertyFile 方法会在生成上述两种格式的属性文件，并將文件存储在工程的根目录下。
writePropertyFile 方法生成的两种属性文件内容：
DB.properties

#DB Config file
#Fri Nov 16 11:16:37 PST 2012
db.user=user
db.host=localhost
db.pwd=password

DB.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
<comment>DB Config XML file</comment>
<entry key="db.user">user</entry>
<entry key="db.host">localhost</entry>
<entry key="db.pwd">password</entry>
</properties>

需要注意的是comment元素，我们在使用prop.storeToXML(new FileOutputStream(xmlFileName), "DB Config XML file");这段代码时第二个参数传入的是注释内容，如果传入null，生成的xml属性文件將没有comment元素。
控制台输出内容如下：

Start of writePropertyFile
DB.properties written successfully
DB.xml written successfully
End of writePropertyFile
Start of readPropertyFile
DB.properties::db.host = localhost
DB.properties::db.user = user
DB.properties::db.pwd = password
DB.properties::XYZ = null
DB.xml::db.host = localhost
DB.xml::db.user = user
DB.xml::db.pwd = password
DB.xml::XYZ = null
End of readPropertyFile
Start of readAllKeys
DB.properties:: Key=db.user::value=user
DB.properties:: Key=db.host::value=localhost
DB.properties:: Key=db.pwd::value=password
DB.xml:: Key=db.user::value=user
DB.xml:: Key=db.host::value=localhost
DB.xml:: Key=db.pwd::value=password
End of readAllKeys
Exception in thread "main" java.lang.NullPointerExceptionat java.util.Properties$LineReader.readLine(Properties.java:434)at java.util.Properties.load0(Properties.java:353)at java.util.Properties.load(Properties.java:341)at com.journaldev.util.PropertyFilesUtil.readPropertyFileFromClasspath(PropertyFilesUtil.java:31)at com.journaldev.util.PropertyFilesUtil.main(PropertyFilesUtil.java:21)

这里报了空指针异常，原因是生成的文件保存在工程的根目录下面，而读取时是从classpath下读取，將上面生成的两个属性文件拷贝到src下再次运行程序即可。