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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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 */
package example.cleaner;

import java.lang.ref.Cleaner;
import java.util.Arrays;
import java.util.Optional;

public class SensitiveData implements AutoCloseable {
    // A cleaner (preferably one shared within a library,
    // but for the sake of example, a new one is created here)
    private static final Cleaner cleaner = Cleaner.create();

    // The sensitive data
    private char[] sensitiveData;

    // The result of registering with the cleaner
    private final Cleaner.Cleanable cleanable;

    /**
     * Construct an object to hold sensitive data.
     * @param sensitiveData a char array, non-null
     */
    public SensitiveData(char[] sensitiveData) {
        final char[] chars = sensitiveData.clone();
        final Runnable F   // Pick one of the following cleaner functions
                = () -> Arrays.fill(chars, (char) 0); // A lambda
                // = new SensitiveCleanable(chars);   // a nested record class for cleanup
                // = clearCharsRunnable(chars);       // lambda from a static context
        this.sensitiveData = chars;
        this.cleanable = cleaner.register(this, F);
    }

    /**
     * Return an Optional of a copy of the char array.
     */
    public Optional<char[]> sensitiveData() {
        return Optional.ofNullable(sensitiveData == null ? null : sensitiveData.clone());
    }

    /**
     * Close and cleanup the sensitive data storage.
     */
    public void close() {
        sensitiveData = null;       // Data not available after close
        cleanable.clean();          // The cleanable already has the char array reference
    }

    // Return a lambda to do the clearing, ensure it does not reference 'this'.
    private static Runnable clearCharsRunnable(char[] chars) {
        return () -> Arrays.fill(chars, (char)0);
    }

    /*
     * Record class to perform the cleanup of a char array.
     */
    private record SensitiveCleanable(char[] sensitiveData) implements Runnable {
        public void run() {
            // cleanup action accessing SensitiveCleanable, executed at most once
            Arrays.fill(sensitiveData, (char)0);
        }
    }

    // Encapsulate a string for printing and clear the temporary buffer.
    public static void main(String[] args) {
        for (String s : args) {
            char[] chars = s.toCharArray();
            try (SensitiveData sd = new SensitiveData(chars)) {
                Arrays.fill(chars, (char) 0);
                print(sd);
            }
        }
    }

    // Print the sensitive data and clear the temporary buffer.
    private static void print(SensitiveData sd) {
        char[] chars = sd.sensitiveData().get();
        System.out.println(chars);
        Arrays.fill(chars, (char)0);
    }
}