docs: Improve Examples (#420)

I am currently incorporating DB-Scheduler into a project I am working on
(10000 onetimetasks processed per hour in an Oracle database). Thanks
for your awesome work!!

I decided to contribute something very modest back.

## Changes

* Removed ExampleHelpers.java: method sleep was unused, method
registerShutdownHook is replaced by registerShutdownHook in
SchedulerBuilder
* Add missing main method in RecurringTaskMain.java

## Reminders
- [-] Added/ran automated tests
- [-] Update README and/or examples
- [X] Ran `mvn spotless:apply`

---
cc @kagkarlsson

examples/features/src/main/java/com/github/kagkarlsson/examples/CheckForNewBatchDirectlyMain.java

@@ -13,10 +13,7 @@
  */
 package com.github.kagkarlsson.examples;
 
-import static com.github.kagkarlsson.examples.helpers.ExampleHelpers.sleep;
-
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.helper.OneTimeTask;
 import com.github.kagkarlsson.scheduler.task.helper.Tasks;
@@ -43,10 +40,9 @@ public void run(DataSource dataSource) {
         Scheduler.create(dataSource, onetimeTask)
             .pollingInterval(Duration.ofSeconds(10))
             .threads(2)
+            .registerShutdownHook()
             .build();
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
 
     sleep(2);

examples/features/src/main/java/com/github/kagkarlsson/examples/CronMain.java

@@ -14,7 +14,6 @@
 package com.github.kagkarlsson.examples;
 
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.helper.RecurringTask;
 import com.github.kagkarlsson.scheduler.task.helper.Tasks;
@@ -45,10 +44,9 @@ public void run(DataSource dataSource) {
         Scheduler.create(dataSource)
             .startTasks(cronTask)
             .pollingInterval(Duration.ofSeconds(1))
+            .registerShutdownHook()
             .build();
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
   }
 }

examples/features/src/main/java/com/github/kagkarlsson/examples/DeletingUnresolvedTasksMain.java

@@ -14,7 +14,6 @@
 package com.github.kagkarlsson.examples;
 
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.SchedulerClient;
 import com.github.kagkarlsson.scheduler.task.helper.RecurringTask;
@@ -56,10 +55,9 @@ public void run(DataSource dataSource) {
             .pollingInterval(Duration.ofSeconds(1))
             .heartbeatInterval(Duration.ofSeconds(5))
             .deleteUnresolvedAfter(Duration.ofSeconds(20))
+            .registerShutdownHook()
             .build();
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
 
     IntStream.range(0, 5)

examples/features/src/main/java/com/github/kagkarlsson/examples/EnableImmediateExecutionMain.java

@@ -13,10 +13,7 @@
  */
 package com.github.kagkarlsson.examples;
 
-import static com.github.kagkarlsson.examples.helpers.ExampleHelpers.sleep;
-
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.helper.OneTimeTask;
 import com.github.kagkarlsson.scheduler.task.helper.Tasks;
@@ -44,10 +41,9 @@ public void run(DataSource dataSource) {
         Scheduler.create(dataSource, onetimeTask)
             .pollingInterval(Duration.ofSeconds(20))
             .enableImmediateExecution()
+            .registerShutdownHook()
             .build();
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
 
     sleep(2000);

examples/features/src/main/java/com/github/kagkarlsson/examples/ExponentialBackoffMain.java

@@ -16,7 +16,6 @@
 import static java.time.Duration.*;
 
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.FailureHandler;
 import com.github.kagkarlsson.scheduler.task.helper.OneTimeTask;
@@ -41,12 +40,13 @@ public void run(DataSource dataSource) {
                 });
 
     final Scheduler scheduler =
-        Scheduler.create(dataSource, failingTask).pollingInterval(ofSeconds(2)).build();
+        Scheduler.create(dataSource, failingTask)
+            .pollingInterval(ofSeconds(2))
+            .registerShutdownHook()
+            .build();
 
     scheduler.schedule(failingTask.instance("1"), Instant.now());
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
   }
 }

examples/features/src/main/java/com/github/kagkarlsson/examples/ExponentialBackoffWithMaxRetriesMain.java

@@ -16,7 +16,6 @@
 import static java.time.Duration.ofSeconds;
 
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.FailureHandler;
 import com.github.kagkarlsson.scheduler.task.helper.OneTimeTask;
@@ -43,12 +42,13 @@ public void run(DataSource dataSource) {
                 });
 
     final Scheduler scheduler =
-        Scheduler.create(dataSource, failingTask).pollingInterval(ofSeconds(2)).build();
+        Scheduler.create(dataSource, failingTask)
+            .pollingInterval(ofSeconds(2))
+            .registerShutdownHook()
+            .build();
 
     scheduler.schedule(failingTask.instance("1"), Instant.now());
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
   }
 }

examples/features/src/main/java/com/github/kagkarlsson/examples/MaxRetriesMain.java

@@ -14,7 +14,6 @@
 package com.github.kagkarlsson.examples;
 
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.FailureHandler;
 import com.github.kagkarlsson.scheduler.task.helper.OneTimeTask;
@@ -52,12 +51,13 @@ public void run(DataSource dataSource) {
                 });
 
     final Scheduler scheduler =
-        Scheduler.create(dataSource, failingTask).pollingInterval(Duration.ofSeconds(2)).build();
+        Scheduler.create(dataSource, failingTask)
+            .pollingInterval(Duration.ofSeconds(2))
+            .registerShutdownHook()
+            .build();
 
     scheduler.schedule(failingTask.instance("1"), Instant.now());
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
   }
 }

examples/features/src/main/java/com/github/kagkarlsson/examples/RecurringTaskMain.java

@@ -26,6 +26,10 @@
 public class RecurringTaskMain extends Example {
   private static final Logger LOG = LoggerFactory.getLogger(RecurringTaskMain.class);
 
+  public static void main(String[] args) {
+    new RecurringTaskMain().runWithDatasource();
+  }
+
   @Override
   public void run(DataSource dataSource) {
 

examples/features/src/main/java/com/github/kagkarlsson/examples/SchedulerMain.java

@@ -13,10 +13,7 @@
  */
 package com.github.kagkarlsson.examples;
 
-import static com.github.kagkarlsson.examples.helpers.ExampleHelpers.sleep;
-
 import com.github.kagkarlsson.examples.helpers.Example;
-import com.github.kagkarlsson.examples.helpers.ExampleHelpers;
 import com.github.kagkarlsson.scheduler.Scheduler;
 import com.github.kagkarlsson.scheduler.task.ExecutionContext;
 import com.github.kagkarlsson.scheduler.task.TaskInstance;
@@ -118,10 +115,9 @@ public void run(DataSource dataSource) {
     final Scheduler scheduler =
         Scheduler.create(dataSource, onetime1, onetime2)
             .startTasks(recurring1, recurring2, custom1)
+            .registerShutdownHook()
             .build();
 
-    ExampleHelpers.registerShutdownHook(scheduler);
-
     scheduler.start();
 
     sleep(3000);