EJB methods may be marked as annotation @javax.ejb.Asynchronous to inform the EJB container that they invoke tasks separate to the container invocation Java thread. An asynchronous method returns a void element or it is Future<V>, where V is the value type that is being returned.
Here is an example of a session bean using asynchronous invocation:
@Singleton
public class AsyncWebpageDownloader {
@Asynchronous
@Lock(LockType.READ)
public Future<String> fetchWebPage(String url)
throws IOException {
String data = createReport(url);
return new AsyncResult<String>(data);
}
private String createReport(String url) throws IOException {
URLConnection http = new URL("url")
.openConnection();
InputStream in = http.getInputStream();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
baos.write( (url+":
").getBytes());
try {
byte[] buf = new byte[8192];
int len;
while ((len = in.read(buf)) > 0) {
baos.write(buf, 0, len);
nap();
}
baos.close();
} finally {
in.close();
}
return baos.toString();
}
private void nap() {
try {
Thread.sleep(200);
} catch (InterruptedException ignored) {
}
}
}The EJB AsyncWebpageDownloader has a single method called fetchWebPage() that is annotated with @Asynchronous. We also add a concurrency hint to the container with an annotation @javax.ejb.Lock, to configure this call point with a reader's lock. The method accepts a URL and downloads a web page from the internet into memory. In order to simulate a long running operation, we add some time delays to the file download inside the I/O loop. At the end of the process, the downloaded content is returned to the caller as Future<String> instance. The result is stored in a special type javax.ejb.AsyncResult.