Java&Xml教程（十）XML作为属性文件使用

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 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");      }    }

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 2012db.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 readPropertyFileDB.properties::db.host = localhostDB.properties::db.user =   userDB.properties::db.pwd = passwordDB.properties::XYZ = nullDB.xml::db.host =   localhostDB.xml::db.user = userDB.xml::db.pwd = passwordDB.xml::XYZ = null  End of readPropertyFile  Start of readAllKeysDB.properties:: Key=db.user::value=userDB.properties::   Key=db.host::value=localhostDB.properties:: Key=db.pwd::value=passwordDB.xml:: Key=db.user::value=userDB.xml::   Key=db.host::value=localhostDB.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)

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。

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 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");      }    }

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 2012db.user=user  db.host=localhost  db.pwd=password

DB.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?><!DOCTYPE properties SYSTEM "   <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 readPropertyFileDB.properties::db.host = localhostDB.properties::db.user = userDB.properties::db.pwd = passwordDB.properties::XYZ =   nullDB.xml::db.host = localhostDB.xml::db.user = userDB.xml::db.pwd = passwordDB.xml::XYZ = null  End of readPropertyFile  Start of readAllKeysDB.properties:: Key=db.user::value=userDB.properties:: Key=db.host::value=localhostDB.properties:: Key=db.pwd::value=passwordDB.xml::   Key=db.user::value=userDB.xml:: Key=db.host::value=localhostDB.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)

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