1 /*
   2  * Copyright (c) 2007, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.nio.file;
  27 
  28 import java.util.Iterator;
  29 import java.io.Closeable;
  30 import java.io.IOException;
  31 
  32 /**
  33  * An object to iterate over the entries in a directory. A directory stream
  34  * allows for the convenient use of the for-each construct to iterate over a
  35  * directory.
  36  *
  37  * <p> {@code DirectoryStream} extends {@code IterableOnce}. It is not a
  38  * general-purpose {@code Iterable} as it supports only a single {@code
  39  * Iterator}; invoking the {@link #iterator iterator} method to obtain a second
  40  * or subsequent iterator throws {@code IllegalStateException}.
  41  *
  42  * <p> An important property of the directory stream's {@code Iterator} is that
  43  * its {@link Iterator#hasNext() hasNext} method is guaranteed to read-ahead by
  44  * at least one element. If {@code hasNext} method returns {@code true}, and is
  45  * followed by a call to the {@code next} method, it is guaranteed that the
  46  * {@code next} method will not throw an exception due to an I/O error, or
  47  * because the stream has been {@link #close closed}. The {@code Iterator} does
  48  * not support the {@link Iterator#remove remove} operation.
  49  *
  50  * <p> A {@code DirectoryStream} is opened upon creation and is closed by
  51  * invoking the {@code close} method. Closing a directory stream releases any
  52  * resources associated with the stream. Failure to close the stream may result
  53  * in a resource leak. The try-with-resources statement provides a useful
  54  * construct to ensure that the stream is closed:
  55  * <pre>
  56  *   Path dir = ...
  57  *   try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir)) {
  58  *       for (Path entry: stream) {
  59  *           ...
  60  *       }
  61  *   }
  62  * </pre>
  63  *
  64  * <p> Once a directory stream is closed, then further access to the directory,
  65  * using the {@code Iterator}, behaves as if the end of stream has been reached.
  66  * Due to read-ahead, the {@code Iterator} may return one or more elements
  67  * after the directory stream has been closed. Once these buffered elements
  68  * have been read, then subsequent calls to the {@code hasNext} method returns
  69  * {@code false}, and subsequent calls to the {@code next} method will throw
  70  * {@code NoSuchElementException}.
  71  *
  72  * <p> A directory stream is not required to be <i>asynchronously closeable</i>.
  73  * If a thread is blocked on the directory stream's iterator reading from the
  74  * directory, and another thread invokes the {@code close} method, then the
  75  * second thread may block until the read operation is complete.
  76  *
  77  * <p> If an I/O error is encountered when accessing the directory then it
  78  * causes the {@code Iterator}'s {@code hasNext} or {@code next} methods to
  79  * throw {@link DirectoryIteratorException} with the {@link IOException} as the
  80  * cause. As stated above, the {@code hasNext} method is guaranteed to
  81  * read-ahead by at least one element. This means that if {@code hasNext} method
  82  * returns {@code true}, and is followed by a call to the {@code next} method,
  83  * then it is guaranteed that the {@code next} method will not fail with a
  84  * {@code DirectoryIteratorException}.
  85  *
  86  * <p> The elements returned by the iterator are in no specific order. Some file
  87  * systems maintain special links to the directory itself and the directory's
  88  * parent directory. Entries representing these links are not returned by the
  89  * iterator.
  90  *
  91  * <p> The iterator is <i>weakly consistent</i>. It is thread safe but does not
  92  * freeze the directory while iterating, so it may (or may not) reflect updates
  93  * to the directory that occur after the {@code DirectoryStream} is created.
  94  *
  95  * <p> <b>Usage Examples:</b>
  96  * Suppose we want a list of the source files in a directory. This example uses
  97  * both the for-each and try-with-resources constructs.
  98  * <pre>
  99  *   List&lt;Path&gt; listSourceFiles(Path dir) throws IOException {
 100  *       List&lt;Path&gt; result = new ArrayList&lt;&gt;();
 101  *       try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir, "*.{c,h,cpp,hpp,java}")) {
 102  *           for (Path entry: stream) {
 103  *               result.add(entry);
 104  *           }
 105  *       } catch (DirectoryIteratorException ex) {
 106  *           // I/O error encounted during the iteration, the cause is an IOException
 107  *           throw ex.getCause();
 108  *       }
 109  *       return result;
 110  *   }
 111  * </pre>
 112  * @param   <T>     The type of element returned by the iterator
 113  *
 114  * @since 1.7
 115  *
 116  * @see Files#newDirectoryStream(Path)
 117  */
 118 
 119 public interface DirectoryStream<T>
 120     extends Closeable, IterableOnce<T> {
 121     /**
 122      * An interface that is implemented by objects that decide if a directory
 123      * entry should be accepted or filtered. A {@code Filter} is passed as the
 124      * parameter to the {@link Files#newDirectoryStream(Path,DirectoryStream.Filter)}
 125      * method when opening a directory to iterate over the entries in the
 126      * directory.
 127      *
 128      * @param   <T>     the type of the directory entry
 129      *
 130      * @since 1.7
 131      */
 132     @FunctionalInterface
 133     public static interface Filter<T> {
 134         /**
 135          * Decides if the given directory entry should be accepted or filtered.
 136          *
 137          * @param   entry
 138          *          the directory entry to be tested
 139          *
 140          * @return  {@code true} if the directory entry should be accepted
 141          *
 142          * @throws  IOException
 143          *          If an I/O error occurs
 144          */
 145         boolean accept(T entry) throws IOException;
 146     }
 147 
 148     /**
 149      * Returns the iterator associated with this {@code DirectoryStream}.
 150      *
 151      * @return  the iterator associated with this {@code DirectoryStream}
 152      *
 153      * @throws  IllegalStateException
 154      *          if this directory stream is closed or the iterator has already
 155      *          been returned
 156      */
 157     @Override
 158     Iterator<T> iterator();
 159 }