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/* gnu.classpath.tools.gjdoc.TemporaryStore

   Copyright (C) 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2, or (at your option)

any later version.

GNU Classpath 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 for more details.

You should have received a copy of the GNU General Public License

along with GNU Classpath; see the file COPYING.  If not, write to the

Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA

02111-1307 USA.

Linking this library statically or dynamically with other modules is

making a combined work based on this library.  Thus, the terms and

conditions of the GNU General Public License cover the whole

combination.

As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent

modules, and to copy and distribute the resulting executable under

terms of your choice, provided that you also meet, for each linked

independent module, the terms and conditions of the license of that

module.  An independent module is a module which is not derived from

or based on this library.  If you modify this library, you may extend

this exception to your version of the library, but you are not

obligated to do so.  If you do not wish to do so, delete this

exception statement from your version. */

package gnu.classpath.tools.gjdoc;

/**

 *  Useful for passing big objects that are no longer needed by the

 *  calling method, reducing memory usage.  <p/>

 *

 *  Consider the following problem:

 *  <pre>

 *   public class A {

 *     public static void foo() {

 *       long[] hugeArray = new long[1000000]; // takes around 8 MB

 *       // ... fill hugeArray with some information ...

 *       bar(hugeArray);

 *       // ... hugeArray is no more required at this point

 *     }

 *     public static void bar(long[] arr) {

 *       // ... process contents of arr ...

 *       arr = null;

 *       System.gc();      // NOTE: will not collect arr!

 *       // ... do something memory-intensive where arr is not needed

 *     }

 *  }

 *  </pre>

 *

 *  In method <code>bar()</code>, the array cannot be garbage

 *  collected because the local variable <code>hugeArray</code> in

 *  method <code>foo()</code> still holds a reference to the array.

 *  <p/>

 *

 *  When calling <code>bar(new long[1000000]);</code> in

 *  <code>arr</code> the array <i>can</i> be collected in

 *  <code>bar()</code>, but that way it can't be initialized in

 *  <code>foo()</code>. A local variable is needed for

 *  initialization, but the variable can't be cleared before it is

 *  passed to <code>bar()</code>!  <p/>

 *

 *  <code>TemporaryStore</code> is the solution for this

 *  dilemma. The modified method <code>foo()</code> which uses a

 *  <code>TemporaryStore</code> object would look like this:

 *

 *  <pre>

 *     public static void foo() {

 *       long[] hugeArray = new long[1000000]; // takes around 7 MB

 *       // ... fill hugeArray with some very important information ...

 *       TemporaryStore tstore = new TemporaryStore(hugeArray);

 *       hugeArray = null;

 *       bar((long[])tstore.getAndClear());

 *     }

 *  </pre>

 *

 *  When control flow is transferred to <code>bar()</code>,

 *  <code>foo()</code> will hold no more references to the array

 *  and so it can be garbage collected in <code>bar()</code>.

 *

 */

public class TemporaryStore {

   private Object storedObject;

   /**

    *  Temporarily store the given object for passing it to a

    *  different method.  <p/>

    *

    *  The method constructing a new TemporaryStore object should

    *  clear all other references to the stored object, so that

    *  this TemporaryStore is the only object referencing it.

    *

    *  @param storedObject  the object to store temporarily

    *

    */

   public TemporaryStore(Object storedObject) {

      this.storedObject = storedObject;

   }

   /**

    *  Return the stored object after clearing the reference to it.

    *  <p/>

    *

    *  When the user of this class followed the recommendations in

    *  the documentation of @link{TemporaryStore(Object)}, the

    *  returned reference will be the only reference to the stored

    *  object after this method returns. If the returned reference

    *  is passed in a method call, the called method will hold the

    *  only reference to the stored object and can release it by

    *  nulling the corresponding parameter.

    *

    *  @return the object which was passed to the constructor.

    *

    */

   public Object getAndClear() {

      Object rc = this.storedObject;

      this.storedObject = null;

      return rc;

   }

}