如何使变量/常量初始化后始终可用？

Question

我有一个 swing 应用程序，必须连接到数据库才能获取某些资源，为此我使用 .properties 文件来存储数据库属性，并且可以在运行时读取该属性。
为此，我使用以下代码

    public void readPropertiesFile(){
       try{
         InputStream is = ReadValues.class.getResourceAsStream(PROP_FILE);
         Properties prop = new Properties();
         prop.load(is);
         String URL = prop.getProperty("DB_URL");
         String user = prop.getProperty("DB_USER");
         String  pwd = prop.getProperty("DB_PWD");
         is.close();
      /* code to use values read from the file*/
       }catch(Exception e){
         System.out.println("Failed to read from " + PROP_FILE + " file.");
       }
   }

，但每当我想连接到数据库（对于 Connection 对象）时，我都必须调用此方法。 我知道现在处理速度足够快，可以在微秒内运行这些行，但据我自己的知识，建议我在应用程序启动或用户第一次尝试连接到数据库时存储这些数据库值的方法对于此类对象或变量或常量中的任何操作，这些操作在应用程序重新启动之前都可用，并且可以直接调用而无需读取文件。

PS：我知道数据库值不会经常改变，如果发生这种情况，我会很乐意重新启动我的应用程序:)

Answer 1

通过将这些静态字段放在单独的类中，直到您第一次访问 URL、USER 或 PASSWORD 时才会加载它们。

public class DbProps {
  public static final String URL;
  public static final String USER;
  public static final String PASSWORD;

  static {
       try{
         InputStream is = ReadValues.class.getResourceAsStream(PROP_FILE);
         try {
           Properties prop = new Properties();
           prop.load(is);
           URL = prop.getProperty("DB_URL");
           USER = prop.getProperty("DB_USER");
           PASSWORD = prop.getProperty("DB_PWD");
         } finally {
           is.close();
         }
       }catch(Exception e){
         throw new RuntimeException("Failed to read from " + PROP_FILE + " file.", e);
       }
  }
}

Answer 2

您可以取消检查条件，该条件将检查是否是第一次，然后设置该值，否则使用现有值

public static boolean isFirstTime = true;
public static String URL = true;
public static String user = true;
public static String pwd = true;
public void readPropertiesFile(){
if(isFirstTime){
       try{

         InputStream is = ReadValues.class.getResourceAsStream(PROP_FILE);
         Properties prop = new Properties();
         prop.load(is);
         URL = prop.getProperty("DB_URL");
         user = prop.getProperty("DB_USER");
         pwd = prop.getProperty("DB_PWD");
isFirstTime = false;
         is.close();
      /* code to use values read from the file*/
       }catch(Exception e){
         System.out.println("Failed to read from " + PROP_FILE + " file.");
       }
}
   }
//use this URL user and pwd in your application

Answer 3

这是适合您的通用环境类。您可以获取数据库道具，例如 Environment.getEnvironment().getProperty("DB_URL") 等。

public class Environment {
   private static final String PROP_FILE = "somefilename";

   private static final Environment singleton = new Environment();

   public static Environment getEnvironment() {
      return singleton;
   }

   private Properties properties = new Properties();

   protected Environment() {
      super();
      loadProperties();
   }

   public Properties getProperties() {
      return properties;
   }

   public String getProperty(String propertyName) {
      return getProperty(propertyName, System.getProperty(propertyName));
   }

   public String getProperty(String propertyName, String defaultValue) {
      return getProperties().getProperty(propertyName, defaultValue);
   }

   public void loadProperties() {
      URL resourceURL = null;

      try {
         resourceURL = Thread.currentThread().getContextClassLoader()
               .getResource(PROP_FILE);
         getProperties().load(resourceURL.openStream());
         System.out.println("Loaded properties from "
               + resourceURL.toExternalForm());
      } catch (IOException ioe) {
         System.err.println("Failed to load properties from "
               + resourceURL.toExternalForm());
      }
   }
}