Java&Xml教程（十）XML作为属性文件使用

港控/mmm° 2022-08-20 04:27 24阅读 0赞

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 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.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。