BAEL-4795: Static garbage collected fields (#12693)

* BAEL-4795: Static garbage collected fields

* BAEL-4795: Fix readAllBytes()

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/CustomClassloader.java

@@ -0,0 +1,36 @@
+package com.baeldung.staticgc;
+
+import java.io.File;
+import java.io.IOException;
+import org.apache.commons.io.IOUtils;
+
+public class CustomClassloader extends ClassLoader {
+
+    public static final String PREFIX = "com.baeldung.classloader";
+
+    public CustomClassloader(ClassLoader parent) {
+        super(parent);
+    }
+
+    @Override
+    public Class<?> loadClass(String name) throws ClassNotFoundException {
+        if (name.startsWith(PREFIX)) {
+            return getClass(name);
+        } else {
+            return super.loadClass(name);
+        }
+    }
+
+    private Class<?> getClass(String name) {
+        String fileName = name.replace('.', File.separatorChar) + ".class";
+        try {
+            byte[] byteArr = IOUtils.toByteArray(getClass().getClassLoader().getResourceAsStream(fileName));
+            Class<?> c = defineClass(name, byteArr, 0, byteArr.length);
+            resolveClass(c);
+            return c;
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/GarbageCollectionExplicitExample.java

@@ -0,0 +1,28 @@
+package com.baeldung.staticgc;
+
+public class GarbageCollectionExplicitExample {
+
+    public static final String METHOD_NAME = "printValue";
+
+    public static void main(String[] args) throws InterruptedException {
+        loadClass();
+        System.gc();
+        Thread.sleep(1000);
+    }
+
+
+    /**
+     * The method loads a class and creates its instance. After the invocation of this method all the local variables go outside the scope.
+     */
+    private static void loadClass() {
+        try {
+            final String className = "com.baeldung.classloader.GarbageCollectedStaticFieldHolder";
+            CustomClassloader loader = new CustomClassloader(GarbageCollectionExplicitExample.class.getClassLoader());
+            Class<?> clazz = loader.loadClass(className);
+            Object instance = clazz.getConstructor().newInstance();
+            clazz.getMethod(METHOD_NAME).invoke(instance);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/GarbageCollectionImplicitExample.java

@@ -0,0 +1,28 @@
+package com.baeldung.staticgc;
+
+public class GarbageCollectionImplicitExample {
+
+    public static final String METHOD_NAME = "printValue";
+
+public static void main(String[] args) {
+    while (true) {
+        loadClass();
+    }
+}
+
+
+    /**
+     * The method loads a class and creates its instance. After the invocation of this method all the local variables go outside the scope.
+     */
+    private static void loadClass() {
+        try {
+            final String className = "com.baeldung.classloader.GarbageCollectedStaticFieldHolder";
+            CustomClassloader loader = new CustomClassloader(GarbageCollectionImplicitExample.class.getClassLoader());
+            Class<?> clazz = loader.loadClass(className);
+            Object instance = clazz.getConstructor().newInstance();
+            clazz.getMethod(METHOD_NAME).invoke(instance);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/GarbageCollectionNullExample.java

@@ -0,0 +1,28 @@
+package com.baeldung.staticgc;
+
+public class GarbageCollectionNullExample {
+
+    public static final String METHOD_NAME = "printValue";
+
+public static void main(String[] args) throws InterruptedException {
+    CustomClassloader loader;
+    Class<?> clazz;
+    Object instance;
+    try {
+        final String className = "com.baeldung.classloader.GarbageCollectedStaticFieldHolder";
+        loader = new CustomClassloader(GarbageCollectionNullExample.class.getClassLoader());
+        clazz = loader.loadClass(className);
+        instance = clazz.getConstructor().newInstance();
+        clazz.getMethod(METHOD_NAME).invoke(instance);
+    } catch (Exception e) {
+        throw new RuntimeException(e);
+    }
+    loader = null;
+    clazz = null;
+    instance = null;
+    System.gc();
+    Thread.sleep(1000);
+}
+
+
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/StaticField.java

@@ -0,0 +1,19 @@
+package com.baeldung.staticgc;
+
+public class StaticField {
+
+    private final String value;
+
+    public StaticField(final String value) {
+        this.value = value;
+    }
+
+    public String getValue() {
+        return value;
+    }
+
+    @Override
+    protected void finalize() {
+        System.out.println("The object is garbage now");
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/staticgc/StaticFieldHolder.java

@@ -0,0 +1,7 @@
+package com.baeldung.staticgc;
+
+public class StaticFieldHolder {
+
+    private static StaticField staticField = new StaticField("Hello from static field");
+
+}

core-java-modules/core-java-jvm-2/src/main/resources/com/baeldung/classloader/GarbageCollectedStaticFieldHolder.java

@@ -0,0 +1,31 @@
+package com.baeldung.classloader;
+
+import java.util.Scanner;
+
+public class GarbageCollectedStaticFieldHolder {
+
+    private static GarbageCollectedInnerObject garbageCollectedInnerObject =
+        new GarbageCollectedInnerObject("Hello from a garbage collected static field");
+
+    public void printValue(){
+        System.out.println(garbageCollectedInnerObject.getMessage() );
+    }
+}
+
+class GarbageCollectedInnerObject {
+
+    private final String message;
+
+    public GarbageCollectedInnerObject(final String message) {
+        this.message = message;
+    }
+
+    public String getMessage() {
+        return message;
+    }
+
+    @Override
+    protected void finalize() {
+        System.out.println("The object is garbage now");
+    }
+}