我們通常會將Java應用的配置引數儲存在屬性檔案中，Java應用的屬性檔案可以是一個正常的基於key-value對,以properties為副檔名的檔案，也可以是XML檔案.
在本案例中，將會向大家介紹如何通過Java程式輸出這兩種格式的屬性檔案，並介紹如何從classpath中載入和使用這兩種屬性檔案。
下面是案例程式程式碼：
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 properties
        prop.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 resources
        is.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 properties
        prop.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 resources
        is.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");
    }

}

當執行這段程式碼時，writePropertyFile 方法會在生成上述兩種格式的屬性檔案，並將檔案儲存在工程的根目錄下。
writePropertyFile 方法生成的兩種屬性檔案內容：
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，我們在使用prop.storeToXML(new FileOutputStream(xmlFileName), "DB Config XML file");這段程式碼時第二個引數傳入的是註釋內容，如果傳入null，生成的xml屬性檔案將沒有comment元素。
控制檯輸出內容如下：

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.NullPointerException
    at 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)

這裡報了空指標異常，原因是生成的檔案儲存在工程的根目錄下面，而讀取時是從classpath下讀取，將上面生成的兩個屬性檔案拷貝到src下再次執行程式即可。