com.fasterxml.uuid.ext

Class FileBasedTimestampSynchronizer

java.lang.Object

com.fasterxml.uuid.TimestampSynchronizer

com.fasterxml.uuid.ext.FileBasedTimestampSynchronizer

public final class FileBasedTimestampSynchronizer
extends TimestampSynchronizer

Implementation of TimestampSynchronizer, which uses file system as the storage and locking mechanism.

Synchronization is achieved by obtaining an exclusive file locks on two specified lock files, and by using the files to store first "safe" timestamp value that the generator can use; alternating between one to use to ensure one of them always contains a valid timestamp. Latter is needed to guard against system clock moving backwards after UUID generator restart.

Note: this class will only work on JDK 1.4 and above, since it requires NIO package to do proper file locking (as well as new opening mode for RandomAccessFile).

Also note that it is assumed that the caller has taken care to synchronize access to method to be single-threaded. As such, none of the methods is explicitly synchronized here.

Field Summary

Fields

Modifier and Type	Field and Description
protected static String	DEFAULT_LOCK_FILE_NAME1
protected static String	DEFAULT_LOCK_FILE_NAME2
protected long	mInterval
protected com.fasterxml.uuid.ext.LockedFile	mLocked1
protected com.fasterxml.uuid.ext.LockedFile	mLocked2

Constructor Summary

Constructors

Constructor and Description
FileBasedTimestampSynchronizer() Constructor that uses default values for names of files to use (files will get created in the current working directory), as well as for the update frequency value (10 seconds).
FileBasedTimestampSynchronizer(File lockFile1, File lockFile2)
FileBasedTimestampSynchronizer(File lockFile1, File lockFile2, long interval)

Method Summary

Methods

Modifier and Type	Method and Description
void	deactivate() Method UUIDTimer will call if this synchronizer object is being replaced by another synchronizer (or removed, that is, no more synchronization is to be done).
protected static void	doDeactivate(com.fasterxml.uuid.ext.LockedFile lf1, com.fasterxml.uuid.ext.LockedFile lf2)
protected long	initialize() This method is to be called only once by UUIDTimer.
void	setUpdateInterval(long interval)
long	update(long now) Method called by UUIDTimer to indicate that it has generated a timestamp value that is beyond last legal timestamp value.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_LOCK_FILE_NAME1

protected static final String DEFAULT_LOCK_FILE_NAME1

See Also:

Constant Field Values

DEFAULT_LOCK_FILE_NAME2

protected static final String DEFAULT_LOCK_FILE_NAME2

See Also:

Constant Field Values

mInterval

protected long mInterval

mLocked1

protected final com.fasterxml.uuid.ext.LockedFile mLocked1

mLocked2

protected final com.fasterxml.uuid.ext.LockedFile mLocked2

Constructor Detail

FileBasedTimestampSynchronizer

public FileBasedTimestampSynchronizer()
                               throws IOException

Constructor that uses default values for names of files to use (files will get created in the current working directory), as well as for the update frequency value (10 seconds).

Throws:

IOException

FileBasedTimestampSynchronizer

public FileBasedTimestampSynchronizer(File lockFile1,
                              File lockFile2)
                               throws IOException

Throws:

IOException

FileBasedTimestampSynchronizer

public FileBasedTimestampSynchronizer(File lockFile1,
                              File lockFile2,
                              long interval)
                               throws IOException

Throws:

IOException

Method Detail

setUpdateInterval

public void setUpdateInterval(long interval)

initialize

protected long initialize()
                   throws IOException

This method is to be called only once by UUIDTimer. It should fetch the persisted timestamp value, which indicates first timestamp value that is guaranteed NOT to have used by a previous incarnation. If it can not determine such value, it is to return 0L as a marker.

Specified by:

initialize in class TimestampSynchronizer

Returns:

First timestamp value that was NOT locked by lock files; 0L to indicate that no information was read.

Throws:

IOException

deactivate

public void deactivate()
                throws IOException

Description copied from class: TimestampSynchronizer

Method UUIDTimer will call if this synchronizer object is being replaced by another synchronizer (or removed, that is, no more synchronization is to be done). It will not be called if JVM terminates.

Specified by:

deactivate in class TimestampSynchronizer

Throws:

IOException

update

public long update(long now)
            throws IOException

Description copied from class: TimestampSynchronizer

Method called by UUIDTimer to indicate that it has generated a timestamp value that is beyond last legal timestamp value. The method should only return once it has "locked" specified timestamp value (and possible additional ones).

Specified by:

update in class TimestampSynchronizer

Parameters:

now - Timestamp value caller wants to use, and that the synchronizer is asked to protect.

Returns:

Timestamp value that the caller can NOT use. That is, all timestamp values prior to (less than) this value can be used ok, but this value and ones after can only be used by first calling update.

Throws:

IOException

doDeactivate

protected static void doDeactivate(com.fasterxml.uuid.ext.LockedFile lf1,
                com.fasterxml.uuid.ext.LockedFile lf2)