FileDescriptor


1   /*
2    * @(#)FileDescriptor.java  1.3 03/12/19
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.io;
9   
10  /**
11   * Instances of the file descriptor class serve as an opaque handle
12   * to the underlying machine-specific structure representing an open
13   * file, an open socket, or another source or sink of bytes. The
14   * main practical use for a file descriptor is to create a
15   * <code>FileInputStream</code> or <code>FileOutputStream</code> to
16   * contain it.
17   * <p>
18   * Applications should not create their own file descriptors.
19   *
20   * @author  Pavani Diwanji
21   * @version 1.3, 12/19/03
22   * @see     java.io.FileInputStream
23   * @see     java.io.FileOutputStream
24   * @since   JDK1.0
25   */
26  public final class FileDescriptor {
27   
28      private int fd;
29  
30      private long handle;
31  
32      /**
33       * Constructs an (invalid) FileDescriptor
34       * object.
35       */
36      public /**/ FileDescriptor() {
37      fd = -1;
38          handle = -1;
39      }
40  
41      private /* */ FileDescriptor(int fd) {
42      this.fd = fd;
43          handle = -1;
44      }
45  
46      static {
47          initIDs();
48      }
49  
50      /**
51       * A handle to the standard input stream. Usually, this file
52       * descriptor is not used directly, but rather via the input stream
53       * known as <code>System.in</code>.
54       *
55       * @see     java.lang.System#in
56       */
57      public static final FileDescriptor   in = standardStream(0);
58  
59      /**
60       * A handle to the standard output stream. Usually, this file
61       * descriptor is not used directly, but rather via the output stream
62       * known as <code>System.out</code>.
63       * @see     java.lang.System#out
64       */
65      public static final FileDescriptor   out = standardStream(1);
66  
67      /**
68       * A handle to the standard error stream. Usually, this file
69       * descriptor is not used directly, but rather via the output stream
70       * known as <code>System.err</code>.
71       *
72       * @see     java.lang.System#err
73       */
74      public static final FileDescriptor   err = standardStream(2);
75  
76      /**
77       * Tests if this file descriptor object is valid.
78       *
79       * @return  <code>true</code> if the file descriptor object represents a
80       *          valid, open file, socket, or other active I/O connection;
81       *          <code>false</code> otherwise.
82       */
83      public boolean valid() {
84      return ((handle != -1) || (fd != -1));
85      }
86  
87      /**
88       * Force all system buffers to synchronize with the underlying
89       * device.  This method returns after all modified data and
90       * attributes of this FileDescriptor have been written to the
91       * relevant device(s).  In particular, if this FileDescriptor
92       * refers to a physical storage medium, such as a file in a file
93       * system, sync will not return until all in-memory modified copies
94       * of buffers associated with this FileDesecriptor have been
95       * written to the physical medium.
96       *
97       * sync is meant to be used by code that requires physical
98       * storage (such as a file) to be in a known state  For
99       * example, a class that provided a simple transaction facility
100      * might use sync to ensure that all changes to a file caused
101      * by a given transaction were recorded on a storage medium.
102      *
103      * sync only affects buffers downstream of this FileDescriptor.  If
104      * any in-memory buffering is being done by the application (for
105      * example, by a BufferedOutputStream object), those buffers must
106      * be flushed into the FileDescriptor (for example, by invoking
107      * OutputStream.flush) before that data will be affected by sync.
108      *
109      * @exception SyncFailedException
110      *        Thrown when the buffers cannot be flushed,
111      *        or because the system cannot guarantee that all the
112      *        buffers have been synchronized with physical media.
113      * @since     JDK1.1
114      */
115     public native void sync() throws SyncFailedException  ;
116 
117     /* This routine initializes JNI field offsets for the class */
118     private static native void initIDs();
119 
120     private static native long set(int d);
121 
122     private static FileDescriptor   standardStream(int fd) {
123         FileDescriptor   desc = new FileDescriptor  ();
124         desc.handle = set(fd);
125         return desc;
126     }
127 
128 }
129
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