/*
* @(#)FileLock.java 1.8 03/12/19
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.nio.channels;
import java.io.IOException;
/**
* A token representing a lock on a region of a file.
*
* <p> A file-lock object is created each time a lock is acquired on a file via
* one of the {@link FileChannel#lock(long,long,boolean) lock} or {@link
* FileChannel#tryLock(long,long,boolean) tryLock} methods of the {@link
* FileChannel} class.
*
* <p> A file-lock object is initially valid. It remains valid until the lock
* is released by invoking the {@link #release release} method, by closing the
* channel that was used to acquire it, or by the termination of the Java
* virtual machine, whichever comes first. The validity of a lock may be
* tested by invoking its {@link #isValid isValid} method.
*
* <p> A file lock is either <i>exclusive</i> or <i>shared</i>. A shared lock
* prevents other concurrently-running programs from acquiring an overlapping
* exclusive lock, but does allow them to acquire overlapping shared locks. An
* exclusive lock prevents other programs from acquiring an overlapping lock of
* either type. Once it is released, a lock has no further effect on the locks
* that may be acquired by other programs.
*
* <p> Whether a lock is exclusive or shared may be determined by invoking its
* {@link #isShared isShared} method. Some platforms do not support shared
* locks, in which case a request for a shared lock is automatically converted
* into a request for an exclusive lock.
*
* <p> The locks held on a particular file by a single Java virtual machine do
* not overlap. The {@link #overlaps overlaps} method may be used to test
* whether a candidate lock range overlaps an existing lock.
*
* <p> A file-lock object records the file channel upon whose file the lock is
* held, the type and validity of the lock, and the position and size of the
* locked region. Only the validity of a lock is subject to change over time;
* all other aspects of a lock's state are immutable.
*
* <p> File locks are held on behalf of the entire Java virtual machine.
* They are not suitable for controlling access to a file by multiple
* threads within the same virtual machine.
*
* <p> File-lock objects are safe for use by multiple concurrent threads.
*
*
* <a name="pdep">
* <h4> Platform dependencies </h4>
*
* <p> This file-locking API is intended to map directly to the native locking
* facility of the underlying operating system. Thus the locks held on a file
* should be visible to all programs that have access to the file, regardless
* of the language in which those programs are written.
*
* <p> Whether or not a lock actually prevents another program from accessing
* the content of the locked region is system-dependent and therefore
* unspecified. The native file-locking facilities of some systems are merely
* <i>advisory</i>, meaning that programs must cooperatively observe a known
* locking protocol in order to guarantee data integrity. On other systems
* native file locks are <i>mandatory</i>, meaning that if one program locks a
* region of a file then other programs are actually prevented from accessing
* that region in a way that would violate the lock. On yet other systems,
* whether native file locks are advisory or mandatory is configurable on a
* per-file basis. To ensure consistent and correct behavior across platforms,
* it is strongly recommended that the locks provided by this API be used as if
* they were advisory locks.
*
* <p> On some systems, acquiring a mandatory lock on a region of a file
* prevents that region from being {@link java.nio.channels.FileChannel#map
* </code>mapped into memory<code>}, and vice versa. Programs that combine
* locking and mapping should be prepared for this combination to fail.
*
* <p> On some systems, closing a channel releases all locks held by the Java
* virtual machine on the underlying file regardless of whether the locks were
* acquired via that channel or via another channel open on the same file. It
* is strongly recommended that, within a program, a unique channel be used to
* acquire all locks on any given file.
*
* <p> Some network filesystems permit file locking to be used with
* memory-mapped files only when the locked regions are page-aligned and a
* whole multiple of the underlying hardware's page size. Some network
* filesystems do not implement file locks on regions that extend past a
* certain position, often 2<sup>30</sup> or 2<sup>31</sup>. In general, great
* care should be taken when locking files that reside on network filesystems.
*
*
* @author Mark Reinhold
* @author JSR-51 Expert Group
* @version 1.8, 03/12/19
* @since 1.4
*/
public abstract class FileLock {
private final FileChannel channel;
private final long position;
private final long size;
private final boolean shared;
/**
* Initializes a new instance of this class. </p>
*
* @param channel
* The file channel upon whose file this lock is held
*
* @param position
* The position within the file at which the locked region starts;
* must be non-negative
*
* @param size
* The size of the locked region; must be non-negative, and the sum
* <tt>position</tt> + <tt>size</tt> must be non-negative
*
* @param shared
* <tt>true</tt> if this lock is shared,
* <tt>false</tt> if it is exclusive
*
* @throws IllegalArgumentException
* If the preconditions on the parameters do not hold
*/
protected FileLock(FileChannel channel,
long position, long size, boolean shared)
{
if (position < 0)
throw new IllegalArgumentException("Negative position");
if (size < 0)
throw new IllegalArgumentException("Negative size");
if (position + size < 0)
throw new IllegalArgumentException("Negative position + size");
this.channel = channel;
this.position = position;
this.size = size;
this.shared = shared;
}
/**
* Returns the file channel upon whose file this lock is held. </p>
*
* @return The file channel
*/
public final FileChannel channel() {
return channel;
}
/**
* Returns the position within the file of the first byte of the locked
* region.
*
* <p> A locked region need not be contained within, or even overlap, the
* actual underlying file, so the value returned by this method may exceed
* the file's current size. </p>
*
* @return The position
*/
public final long position() {
return position;
}
/**
* Returns the size of the locked region in bytes.
*
* <p> A locked region need not be contained within, or even overlap, the
* actual underlying file, so the value returned by this method may exceed
* the file's current size. </p>
*
* @return The size of the locked region
*/
public final long size() {
return size;
}
/**
* Tells whether this lock is shared. </p>
*
* @return <tt>true</tt> if lock is shared,
* <tt>false</tt> if it is exclusive
*/
public final boolean isShared() {
return shared;
}
/**
* Tells whether or not this lock overlaps the given lock range. </p>
*
* @return <tt>true</tt> if, and only if, this lock and the given lock
* range overlap by at least one byte
*/
public final boolean overlaps(long position, long size) {
if (position + size <= this.position)
return false; // That is below this
if (this.position + this.size <= position)
return false; // This is below that
return true;
}
/**
* Tells whether or not this lock is valid.
*
* <p> A lock object remains valid until it is released or the associated
* file channel is closed, whichever comes first. </p>
*
* @return <tt>true</tt> if, and only if, this lock is valid
*/
public abstract boolean isValid();
/**
* Releases this lock.
*
* <p> If this lock object is valid then invoking this method releases the
* lock and renders the object invalid. If this lock object is invalid
* then invoking this method has no effect. </p>
*
* @throws ClosedChannelException
* If the channel that was used to acquire this lock
* is no longer open
*
* @throws IOException
* If an I/O error occurs
*/
public abstract void release() throws IOException;
/**
* Returns a string describing the range, type, and validity of this lock.
*
* @return A descriptive string
*/
public final String toString() {
return (this.getClass().getName()
+ "[" + position
+ ":" + size
+ " " + (shared ? "shared" : "exclusive")
+ " " + (isValid() ? "valid" : "invalid")
+ "]");
}
}
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