OnJava8-Examples/exceptions/CleanupIdiom.java

// exceptions/CleanupIdiom.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// Disposable objects must be followed by a try-finally

class NeedsCleanup { // Construction can't fail
  private static long counter = 1;
  private final long id = counter++;
  public void dispose() {
    System.out.println(
      "NeedsCleanup " + id + " disposed");
  }
}

class ConstructionException extends Exception {}

class NeedsCleanup2 extends NeedsCleanup {
  // Construction can fail:
  public NeedsCleanup2() throws ConstructionException {}
}

public class CleanupIdiom {
  public static void main(String[] args) {
    // (1):
    NeedsCleanup nc1 = new NeedsCleanup();
    try {
      // ...
    } finally {
      nc1.dispose();
    }

    // (2):
    // If construction cannot fail you can group objects:
    NeedsCleanup nc2 = new NeedsCleanup();
    NeedsCleanup nc3 = new NeedsCleanup();
    try {
      // ...
    } finally {
      nc3.dispose(); // Reverse order of construction
      nc2.dispose();
    }

    // (3):
    // If construction can fail you must guard each one:
    try {
      NeedsCleanup2 nc4 = new NeedsCleanup2();
      try {
        NeedsCleanup2 nc5 = new NeedsCleanup2();
        try {
          // ...
        } finally {
          nc5.dispose();
        }
      } catch(ConstructionException e) { // nc5 const.
        System.out.println(e);
      } finally {
        nc4.dispose();
      }
    } catch(ConstructionException e) { // nc4 const.
      System.out.println(e);
    }
  }
}
/* Output:
NeedsCleanup 1 disposed
NeedsCleanup 3 disposed
NeedsCleanup 2 disposed
NeedsCleanup 5 disposed
NeedsCleanup 4 disposed
*/