JAVA-2111: Split or move core-java-jvm module (#9688)

* JAVA-2111: Moved boolean Memory Layout article

* JAVA-2111: Moved Shutdown Hooks article

* JAVA-2111: Updated README files

* JAVA-2111: Fixed prev link

core-java-modules/core-java-jvm-2/README.md

@@ -2,4 +2,10 @@
 
 This module contains articles about working with the Java Virtual Machine (JVM).
 
-### Relevant Articles: 
+### Relevant Articles: 
+
+- [Memory Layout of Objects in Java](https://www.baeldung.com/java-memory-layout)
+- [Measuring Object Sizes in the JVM](https://www.baeldung.com/jvm-measuring-object-sizes)
+- [Adding Shutdown Hooks for JVM Applications](https://www.baeldung.com/jvm-shutdown-hooks)
+- [boolean and boolean[] Memory Layout in the JVM](https://www.baeldung.com/jvm-boolean-memory-layout)
+- More articles: [[<-- prev]](/core-java-modules/core-java-jvm)

core-java-modules/core-java-jvm-2/src/test/java/com/baeldung/boolsize/BooleanSizeUnitTest.java

@@ -0,0 +1,25 @@
+package com.baeldung.boolsize;
+
+import org.junit.Test;
+import org.openjdk.jol.info.ClassLayout;
+import org.openjdk.jol.vm.VM;
+
+public class BooleanSizeUnitTest {
+
+    @Test
+    public void printingTheVMDetails() {
+        System.out.println(VM.current().details());
+    }
+
+    @Test
+    public void printingTheBoolWrapper() {
+        System.out.println(ClassLayout.parseClass(BooleanWrapper.class).toPrintable());
+    }
+
+    @Test
+    public void printingTheBoolArray() {
+        boolean[] value = new boolean[3];
+
+        System.out.println(ClassLayout.parseInstance(value).toPrintable());
+    }
+}

core-java-modules/core-java-jvm-2/src/test/java/com/baeldung/boolsize/BooleanWrapper.java

@@ -0,0 +1,5 @@
+package com.baeldung.boolsize;
+
+class BooleanWrapper {
+    private boolean value;
+}

core-java-modules/core-java-jvm-2/src/test/java/com/baeldung/shutdownhook/ShutdownHookUnitTest.java

@@ -0,0 +1,47 @@
+package com.baeldung.shutdownhook;
+
+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();
+    }
+}