Reloadable MessageSources

As we saw in the previous post, the ResourceBundleMessageSource uses underlying JDK's ResourceBundle implementation. The java.util.ResourceBundle loads locale specific files or bundles only once. It caches loaded bundles forever: Reloading a bundle during program execution is not possible. As this MessageSource relies on ResourceBundle, it faces the same limitation.So we are stuck with the same set of values. Spring provides us with an alternative implementation - ReloadableResourceBundleMessageSource.
Consider the bean implementation for the same.

<bean id="messageSource"
        class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
         <property name="basenames">
      <list>
        <value>classpath:standard</value>
        <value>com/resources/failure</value>        
      </list>
    </property>
     <property name="cacheSeconds" value="1"/> 
</bean>

The above bean implementation includes a "cacheSeconds" property that tells the MessageSource when to reload the bundles. A value of -1 implies that it will be cached forever.
The basenames property is also slightly more advanced than the   ResourceBundleMessageSource. Using it we can refer to any Spring resource location and not just classpath resources. In case we need to use classpath resources, a "classpath:" prefix can be used. So standard.properties is loaded from the classpath.
I wrote the below code to test the working of the MessageSource:

static String FILE_NAME = "<eclipse workspace path>"
        + "Spring - I18N\\bin\\com\\resources" + "\\failure.properties";

public static void main(String[] args) throws InterruptedException {
    ApplicationContext applicationContext = new ClassPathXmlApplicationContext(
        "reloadable-messages.xml");
    System.out.println(applicationContext.getMessage("argument.required",
        new Object[] { "Application Context" }, "Required", null));
    System.out.println("Updating the properties file");
    FileOutputStream fos = null;
    try {
        fos = new FileOutputStream(FILE_NAME);
        fos.write("argument.required=The \''{0}\'' argument is A NEW VALUE."
            .getBytes("UTF-8"));
        fos.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
    System.out.println("going to sleep for 10 secs while the bundles are reloaded");
    Thread.sleep(10 * 1000);
    System.out.println(applicationContext.getMessage("argument.required",
        new Object[] { "Application Context" }, "Required", null));
}

I read the property and then rewrote the message value. I then let the main thread sleep for ten seconds. After this on executing the code:

The 'Application Context' argument is required.
Updating the properties file
going to sleep for 10 secs while the bundles are reloaded
The 'Application Context' argument is A NEW VALUE.

As can be seen, the message loaded was different at both times and this was achieved without restarting the program. I was able to reproduce the same with the static message.

static String FILE_NAME = "D:\\work\\Spring\\Workspace\\Spring ws\\"
    + "Spring - I18N\\bin" + "\\standard.properties";

public static void main(String[] args) throws InterruptedException {
    ApplicationContext applicationContext = new ClassPathXmlApplicationContext(
            "reloadable-messages.xml");
    System.out.println(applicationContext.getMessage("rockMessage",
            null, "Required", null));
    System.out.println("Updating the properties file");
    FileOutputStream fos = null;
    try {
        fos = new FileOutputStream(FILE_NAME);
        fos.write("rockMessage=Alligators never rock ever"
            .getBytes("UTF-8"));
        fos.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
    System.out.println("going to sleep for 10 secs while the bundles are reloaded");
        Thread.sleep(10 * 1000);
    System.out.println(applicationContext.getMessage("rockMessage",
        null, "Required", null));
}

The output indicates that the change was loaded:

Alligators rock!
Updating the properties file
going to sleep for 10 secs while the bundles are reloaded
Alligators never rock ever

The other way to reload the bundles is to programmatically clear the cached bundles.

public static void clearByProg(final ApplicationContext applicationContext) {
     ReloadableResourceBundleMessageSource res = (ReloadableResourceBundleMessageSource) 
                  applicationContext.getBean("messageSource");
     res.clearCache();    
}