Reload and Hot Reload

Owner does support programmatic reload, as well as the automatic “hot reload” for configuration files.

There are two ways on how the automatic HotReload can be implemented with OWNER: synchronous or asynchronous.

Programmatic reload

You can manually ask a configuration object to reload. This is done via the Reloadable interface.

Example:

@Sources{...}
interface MyConfig extends Reloadable {
    String someProperties();
}

MyConfig cfg = ConfigFactory.create(MyConfig.class);
cfg.reload();

The cfg.reload() will perform the reload of all properties in the same way as when the object was initially created. If the configuration files have been altered, after the reload invocation, those changes will be reflected in the config object.

The Reloadable interface extends from Config:

Automatic “hot reload”

You can instruct OWNER to automatically reload the properties files if they are modified on the filesystem.

For instance:

@HotReload
@Sources("file:foo/bar/baz.properties")
interface MyConfig extends Config {
    @DefaultValue("localhost")
    String serverName();
}

You see in the above example we have specified the annotation @HotReload on the interface level.

The hot reload works only on filesystem URLs. This means that you can make it work with those two types of URLs:

• file:path/to/your.properties a filesystem backed URL.
• jar:file:path/to/some.jar!/path/to/your.properties a jar file in your local filesystem that contains a properties files.
• classpath:path/to/your.properties a resource loaded from the classpath, if the classpath resource is stored on filesystem (from inside a jar or from inside a classpath folder). If the ClassLoader is loading the resource from a remote url (for instance from a jar accessed via http protocol), then it won’t work. Almost always, the application loads classes and resources from a filesystem backed classpath. So this should work almost always.

If you don’t specify the @Sources annotation, then OWNER will try to load the properties file from the classpath from a resource matching the same package and class name of the mapping interface.

The hot reload annotation instructs OWNER to monitor those resources for changes and reload them when they change.

Why only ‘file:’, ‘jar:file’ and ‘classpath:’ URLs?

Monitoring remote URLs, such as “http” or “ftp”, will involve network communication to download those files from remote servers frequently just for checking if they are changed, and it is not convenient to implement the hot reload doing frequent heavy operations like these. You can still perform the reload programmatically, using the Reloadable interface, for these cases.

While instead, monitoring the filesystem is not a big deal, also because filesystems implement ‘last modification date’ that can be checked to detect modifications without actually check the content of the file for changes.

This is the reason why OWNER implements “hot reload” only on filesystem based URLs.

The @HotReload annotation

The @HotReload annotation accepts 3 optional parameters.

And is defined as:

@interface HotReload {
    long value() default 5;
    TimeUnit unit() default SECONDS;
    HotReloadType type() default SYNC;
}

enum HotReloadType {
    SYNC, ASYNC
}

You can check the latest javadocs for further details.

So you can specify also the interval for the hot reload, expressed by value and unit, and you can also specify the type of hot reload that you need.

Some examples:

// Using the default values:
// will check for MyConfig.properties file changes in classpath
// with interval of 5 seconds.
// It will use SYNC hot reload.
@HotReload
interface MyConfig extends Config { ... }

// Will check for file changes every 2 seconds.
// It will use SYNC hot reload.
@HotReload(2)
@Sources("file:foo/bar/baz.properties")
interface MyConfig extends Config { ... }

// Will check for file changes every 500 millis.
// It will use SYNC hot reload.
@HotReload(value=500, unit = TimeUnit.MILLISECONDS)
@Sources("file:foo/bar/baz.properties")
interface MyConfig extends Config { ... }

// Will use ASYNC reload type: will span a
// separate thread that will check for file
// changes every 5 seconds (default)
@HotReload(type=HotReloadType.ASYNC)
@Sources("file:foo/bar/baz.properties")
interface MyConfig extends Config { ... }

// Will use ASYNC reload type and will check every 2 seconds.
@HotReload(value=2, type=HotReloadType.ASYNC)
@Sources("file:foo/bar/baz.properties")
interface MyConfig extends Config { ... }

The difference between SYNC and ASYNC hot reload will be explained below.

As explained before the last modified date of the file will be used to detect changes on the files.

The synchronous hot reload

The synchronous hot reload works this way: every time you call a method on the config object created by ConfigFactory.create() the configuration files will be checked for modifications, then eventually reload the files.

This means that, if you don’t use the config object for long periods there will be no checks on the filesystem, and consequently no reload will be performed.

So, we can define this behavior a lazy hot reload, since it does that only when needed, at the very last time.

This is the default for the @HotReload annotation, but you can also specify this type of hot reload explicitly with type=SYNC:

@HotReload(type=HotReloadType.SYNC)

The asynchronous hot reload

The asynchronous hot reload works this way: it schedule a periodic task to be executed on a separate thread on the specified interval, to check the files for modification and eventually reload them.

This means that, if you don’t use the config object for long periods, the check on the filesystem will be done in background anyway and eventually the reload will be performed.

To enable this behavior you need to specify type=ASYNC to the hot reload annotation:

@HotReload(type=HotReloadType.ASYNC)

Intercepting reload events

Since reload can happen programmatically, and automatically synchronously and asynchronously, it may be helpful for to have some notification mechanism to intercept reload events.

For this, please look at the Reloadable interface, that allows the user to attach ReloadListeners to the config object.

Hot reload example

In the project’s sources it is included a working example:

public class AutoReloadExample {
    private static final String spec =
      "file:target/test-resources/AutoReloadExample.properties";

    private static File target;

    @Sources(spec)
    @HotReload(1)
    interface AutoReloadConfig extends Config, Reloadable {
        @DefaultValue("5")
        Integer someValue();
    }

    static {
        try {
            target = new File(new URL(spec).getFile());
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args)
        throws IOException, InterruptedException {

        save(target, new Properties() { {
            setProperty("someValue", "10");
        }});

        AutoReloadConfig cfg =
            ConfigFactory.create(AutoReloadConfig.class);

        cfg.addReloadListener(new ReloadListener() {
            public void reloadPerformed(ReloadEvent event) {
                System.out.print(
                    "\rReload intercepted at "
                    + new Date() + " \n");
            }
        });

        System.out.println("You can change the file "
            + target.getAbsolutePath() +
            " and see the changes reflected below");

        int someValue = 0;
        while (someValue >= 0) {
            someValue = cfg.someValue();
            System.out.print(
               "\rsomeValue is: " + someValue + "\t\t\t\t");
            Thread.sleep(500);
        }

    }
}

To run this example, you need to follow these steps:

# after downloading the sources in the directory 'owner'
$ cd owner
$ mvn clean compile test-compile
$ java -classpath \
       target/classes/:target/test-classes/ \
       io.github.qubitpi.owner.examples.AutoReloadExample

Then you can change the file indicated by the program to see the changes being reflected and the reload event being intercepted.