Added the sample codes

core-java-modules/core-java-jvm/src/test/java/com/baeldung/hook/ShutdownHookUnitTest.java

@@ -0,0 +1,47 @@
+package com.baeldung.hook;
+
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class ShutdownHookUnitTest {
+
+    @Test
+    public void givenAHook_WhenShutsDown_ThenHookShouldBeExecuted() {
+        Thread printingHook = new Thread(() -> System.out.println("In the middle of a shutdown"));
+        Runtime.getRuntime().addShutdownHook(printingHook);
+    }
+
+    @Test
+    public void addingAHook_WhenThreadAlreadyStarted_ThenThrowsAnException() {
+        Thread longRunningHook = new Thread(() -> {
+            try {
+                Thread.sleep(300);
+            } catch (InterruptedException ignored) {}
+        });
+        longRunningHook.start();
+
+        assertThatThrownBy(() -> Runtime.getRuntime().addShutdownHook(longRunningHook))
+          .isInstanceOf(IllegalArgumentException.class)
+          .hasMessage("Hook already running");
+    }
+
+    @Test
+    public void addingAHook_WhenAlreadyExists_ThenAnExceptionWouldBeThrown() {
+        Thread unfortunateHook = new Thread(() -> {});
+        Runtime.getRuntime().addShutdownHook(unfortunateHook);
+
+        assertThatThrownBy(() -> Runtime.getRuntime().addShutdownHook(unfortunateHook))
+          .isInstanceOf(IllegalArgumentException.class)
+          .hasMessage("Hook previously registered");
+    }
+
+    @Test
+    public void removeAHook_WhenItIsAlreadyRegistered_ThenWouldDeRegisterTheHook() {
+        Thread willNotRun = new Thread(() -> System.out.println("Won't run!"));
+        Runtime.getRuntime().addShutdownHook(willNotRun);
+
+        assertThat(Runtime.getRuntime().removeShutdownHook(willNotRun)).isTrue();
+    }
+}