lockfile.java

纯Java的数据库

     * This method should return <tt>false</tt> if optional locking work is not
     * performed or if it fails, else <tt>true</tt>. <p>
     *
     * In general, if optional locking work fails, then any resources
     * acquired in the process should be freed before this method returns.
     * In this way, the surrounding implementation can take advantage of a
     * <tt>false</tt> return value to avoid calling {@link
     * #doOptionalReleaseActions() doOptionalReleaseActions()} as part of the
     * {@link #tryRelease() tryRelease()} method. <p>
     *
     * <b>Note:</b><p>
     *
     * The default implementation does nothing and always returns
     * <tt>false</tt>. <p>
     *
     * @return <tt>true</tt> if optional lock actions are performed and they
     *      succeed, else <tt>false</tt>
     */
    protected boolean doOptionalLockActions() {
        return false;
    }

    /**
     * Provides any optional release actions for the {@link #tryRelease()
     * tryRelease()} template method. <p>
     *
     * <b>PRE:</b> <p>
     *
     * It is important that this method does not rethrow any exceptions
     * it encounters as unchecked exceptions to the calling context. <p>
     *
     * <b>POST:</b> <p>
     *
     * In general, <tt>false</tt> should be returned if optional locking work
     * is not performed or if it fails, else <tt>true</tt>.  However, the return
     * value is currenly treated  as purely informative. <p>
     *
     * <b>Note:</b> <p>
     *
     * The default implementation does nothing and always returns false. <p>
     *
     * @return <tt>true</tt> if optional release actions are performed and they
     *      succeed, else <tt>false</tt>
     */
    protected boolean doOptionalReleaseActions() {
        return false;
    }

    /**
     * Initializes this object with a <tt>File</tt> object whose path has the
     * canonical form of the given <tt>path</tt> argument. <p>
     *
     *  <b>PRE</b>:<p>
     *
     * <ol>
     *    <li>This method is called once and <em>only</em> once per
     *        <tt>Lockfile</tt> instance.
     *
     *    <li>It is <em>always</em> the first method called after
     *        <tt>LockFile</tt> construction
     *
     *    <li>The supplied <tt>path</tt> argument is <em>never</em>
     *        <tt>null</tt>.
     * </ol>
     *
     * @param path the abstract path representing the file this object is to
     *        use as its lock file
     * @throws FileCanonicalizationException if an I/O error occurs upon
     *         canonicalization of the given path, which is possible because
     *         the given path may be illegal on the runtime file system or
     *         because construction of the canonical pathname may require
     *         native file system queries
     * @throws FileSecurityException if a required system property value cannot
     *         be accessed, or if a Java security manager exists and its
     *        <tt>{@link java.lang.SecurityManager#checkRead}</tt> method denies
     *         read access to the file; or if its <tt>{@link
     *         java.lang.SecurityManager#checkRead(java.lang.String)}</tt>
     *         method does not permit verification of the existence of
     *         all necessary parent directories; or if
     *         its <tt>{@link
     *         java.lang.SecurityManager#checkWrite(java.lang.String)}</tt>
     *         method does not permit all necessary parent directories to be
     *         created
     */
    private final void setPath(String path)
    throws LockFile.FileCanonicalizationException,
           LockFile.FileSecurityException {

        // Should at least be absolutized for reporting purposes, just in case
        // a security or canonicalization exception gets thrown.
        path = FileUtil.getDefaultInstance().canonicalOrAbsolutePath(path);
        this.file = new File(path);

        try {
            FileUtil.getDefaultInstance().makeParentDirectories(this.file);
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "setPath", ex);
        }

        try {
            this.file = FileUtil.getDefaultInstance().canonicalFile(path);
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "setPath", ex);
        } catch (IOException ex) {
            throw new FileCanonicalizationException(this, "setPath", ex);
        }

        this.cpath = this.file.getPath();
    }

    /**
     * Opens (constructs) this object's {@link #raf RandomAccessFile}. <p>
     *
     * @throws UnexpectedFileNotFoundException if a
     *         <tt>FileNotFoundException</tt> is thrown in reponse to
     *         constructing the <tt>RandomAccessFile</tt> object.
     * @throws FileSecurityException if a required system property value cannot
     *         be accessed, or if a Java security manager exists and its
     *         <tt>{@link java.lang.SecurityManager#checkRead}</tt> method
     *         denies read access to the file; or if its <tt>{@link
     *         java.lang.SecurityManager#checkWrite(java.lang.String)}</tt>
     *         method denies write access to the file
     */
    private final void openRAF()
    throws LockFile.UnexpectedFileNotFoundException,
           LockFile.FileSecurityException {

        try {
            raf = new RandomAccessFile(file, "rw");
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "openRAF", ex);
        } catch (FileNotFoundException ex) {
            throw new UnexpectedFileNotFoundException(this, "openRAF", ex);
        } catch (IOException ex) {
            throw new UnexpectedFileNotFoundException(this, "openRAF", ex);
        }
    }

    /**
     * Checks whether the given <tt>DataInputStream</tt> contains the
     * {@link #MAGIC} value.
     *
     * @param  dis the stream to check
     * @throws FileSecurityException if a required system property value cannot
     *         be accessed, or if a Java security manager exists and its
     *         <tt>{@link java.lang.SecurityManager#checkRead}</tt> method
     *         denies read access to the file
     * @throws UnexpectedEndOfFileException if an <tt>EOFException</tt> is
     *         thrown while reading the <tt>DataInputStream</tt>
     * @throws UnexpectedFileIOException if an <tt>IOException</tt> other than
     *         <tt>EOFException</tt> is thrown while reading the
     *         <tt>DataInputStream</tt>
     * @throws WrongMagicException if a value other than <tt>MAGIC</tt> is read
     *         from the <tt>DataInputStream</tt>
     */
    private final void checkMagic(final DataInputStream dis)
    throws LockFile.FileSecurityException,
           LockFile.UnexpectedEndOfFileException,
           LockFile.UnexpectedFileIOException, LockFile.WrongMagicException {

        boolean      success = true;
        final byte[] magic   = new byte[MAGIC.length];

        try {
            for (int i = 0; i < MAGIC.length; i++) {
                magic[i] = dis.readByte();

                if (MAGIC[i] != magic[i]) {
                    success = false;
                }
            }
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "checkMagic", ex);
        } catch (EOFException ex) {
            throw new UnexpectedEndOfFileException(this, "checkMagic", ex);
        } catch (IOException ex) {
            throw new UnexpectedFileIOException(this, "checkMagic", ex);
        }

        if (!success) {
            throw new WrongMagicException(this, "checkMagic", magic);
        }
    }

    /**
     * Retrieves the last written hearbeat timestamp from this object's lock
     * file.  If this object's lock file does not exist, then <tt>Long.MIN_VALUE
     * </tt> (the earliest time representable as a <tt>long</tt> in Java) is
     * returned immediately. <p>
     *
     * @return the hearbeat timestamp read from this object's lock file,
     *         as a <tt>long</tt> value or, if this object's lock
     *         file does not exist, <tt>Long.MIN_VALUE</tt>, the earliest time
     *         representable as a <tt>long</tt> in Java.
     * @throws FileSecurityException if a required system property value cannot
     *         be accessed, or if a Java security manager exists and its
     *         <tt>{@link java.lang.SecurityManager#checkRead}</tt> method
     *         denies read access to the file
     * @throws UnexpectedEndOfFileException if an <tt>EOFException</tt> is
     *         thrown while attempting to read the target file's <tt>MAGIC</tt>
     *         or heartbeat timestamp value
     * @throws UnexpectedFileNotFoundException if, after successfully testing
     *         for existence, the target file is not found a moment later while
     *         attempting to read its <tt>MAGIC</tt> and heartbeat timestamp
     *         values
     * @throws UnexpectedFileIOException if any other input stream error occurs
     * @throws WrongMagicException if the lock file does not start with the
     *         the {@link #MAGIC} value
     */
    private final long readHeartbeat()
    throws LockFile.FileSecurityException,
           LockFile.UnexpectedFileNotFoundException,
           LockFile.UnexpectedEndOfFileException,
           LockFile.UnexpectedFileIOException, LockFile.WrongMagicException {

        FileInputStream fis = null;
        DataInputStream dis = null;

        try {
            if (!file.exists()) {
                return Long.MIN_VALUE;
            }

            fis = new FileInputStream(file);
            dis = new DataInputStream(fis);

            checkMagic(dis);

            return dis.readLong();
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "readHeartbeat", ex);
        } catch (FileNotFoundException ex) {
            throw new UnexpectedFileNotFoundException(this, "readHeartbeat",
                    ex);
        } catch (EOFException ex) {
            throw new UnexpectedEndOfFileException(this, "readHeartbeat", ex);
        } catch (IOException ex) {
            throw new UnexpectedFileIOException(this, "readHeartbeat", ex);
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException ioe) {

                    // ioe.printStackTrace();
                }
            }
        }
    }

    /**
     * Schedules the lock heartbeat task.
     */
    private final void startHeartbeat() {

        if (timerTask == null || HsqlTimer.isCancelled(timerTask)) {
            Runnable runner = new HeartbeatRunner();

            timerTask = timer.schedulePeriodicallyAfter(0, HEARTBEAT_INTERVAL,
                    runner, true);
        }
    }

    /**
     * Cancels the lock heartbeat task.
     */
    private final void stopHeartbeat() {

        if (timerTask != null && !HsqlTimer.isCancelled(timerTask)) {
            HsqlTimer.cancel(timerTask);

            timerTask = null;
        }
    }

    /**
     * Writes the {@link #MAGIC} value to this object's lock file that
     * distiguishes it as an HSQLDB lock file. <p>
     *
     * @throws FileSecurityException possibly never (seek and write are native
     *      methods whose JavaDoc entries do not actually specify throwing
     *      <tt>SecurityException</tt>).  However, it is conceivable that these
     *      native methods may, in turn, access Java methods that do
     *      throw <tt>SecurityException</tt>. In this case, a
     *      <tt>SecurityException</tt> might be thrown if a required system
     *      property value cannot be accessed, or if a security manager exists
     *      and its <tt>{@link
     *      java.lang.SecurityManager#checkWrite(java.io.FileDescriptor)}</tt>
     *      method denies write access to the file
     * @throws UnexpectedEndOfFileException if an end of file exception is
     *      thrown while attempting to write the <tt>MAGIC</tt> value to the
     *      target file (typically, this cannot happen, but the case is
     *      included to distiguish it from the general <tt>IOException</tt>
     *      case).
     * @throws UnexpectedFileIOException if any other I/O error occurs while
     *      attepting to write the <tt>MAGIC</tt> value to the target file.
     */
    private final void writeMagic()
    throws LockFile.FileSecurityException,
           LockFile.UnexpectedEndOfFileException,
           LockFile.UnexpectedFileIOException {

        try {
            raf.seek(0);
            raf.write(MAGIC);
        } catch (SecurityException ex) {
            throw new FileSecurityException(this, "writeMagic", ex);
        } catch (EOFException ex) {
            throw new UnexpectedEndOfFileException(this, "writeMagic", ex);
        } catch (IOException ex) {
            throw new UnexpectedFileIOException(this, "writeMagic", ex);
        }