java.rmi.server
Class UID

java.lang.Object
  java.rmi.server.UID

All Implemented Interfaces:

Serializable

public final class UID

extends Object

implements Serializable

A UID represents an identifier that is unique over time with respect to the host it is generated on, or one of 2¹⁶ "well-known" identifiers.

The UID() constructor can be used to generate an identifier that is unique over time with respect to the host it is generated on. The UID(short) constructor can be used to create one of 2¹⁶ well-known identifiers.

A UID instance contains three primitive values:

• unique, an int that uniquely identifies the VM that this UID was generated in, with respect to its host and at the time represented by the time value (an example implementation of the unique value would be a process identifier), or zero for a well-known UID
• time, a long equal to a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive, or zero for a well-known UID
• count, a short to distinguish UIDs generated in the same VM with the same time value

An independently generated UID instance is unique over time with respect to the host it is generated on as long as the host requires more than one millisecond to reboot and its system clock is never set backward. A globally unique identifier can be constructed by pairing a UID instance with a unique host identifier, such as an IP address.

Since:

JDK1.1

Author:

Ann Wollrath, Peter Jones

See Also:

Serialized Form

Field Summary

private short	count 16-bit number to distinguish UID instances created in the same VM with the same time value
private static int	hostUnique
private static short	lastCount
private static long	lastTime
private static Object	lock
private static long	ONE_SECOND
private static long	serialVersionUID indicate compatibility with JDK 1.1.x version of class
private long	time a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive
private int	unique number that uniquely identifies the VM that this UID was generated in with respect to its host and at the given time

Constructor Summary

	UID() Generates a UID that is unique over time with respect to the host that it was generated on.
private	UID(int unique, long time, short count) Constructs a UID given data read from a stream.
	UID(short num) Creates a "well-known" UID.

Method Summary

boolean	equals(Object obj) Compares the specified object with this UID for equality.
private static int	getHostUniqueNum() In the absence of an actual pid, just do something somewhat random.
int	hashCode() Returns the hash code value for this UID.
static UID	read(DataInput in) Constructs and returns a new UID instance by unmarshalling a binary representation from an DataInput instance.
String	toString() Returns a string representation of this UID.
void	write(DataOutput out) Marshals a binary representation of this UID to a DataOutput instance.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

ONE_SECOND

private static long ONE_SECOND

hostUnique

private static final int hostUnique

lock

private static final Object lock

lastTime

private static long lastTime

lastCount

private static short lastCount

serialVersionUID

private static final long serialVersionUID

indicate compatibility with JDK 1.1.x version of class

See Also:

Constant Field Values

unique

private final int unique

number that uniquely identifies the VM that this UID was generated in with respect to its host and at the given time

time

private final long time

a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive

count

private final short count

16-bit number to distinguish UID instances created in the same VM with the same time value

Constructor Detail

UID

public UID()

Generates a UID that is unique over time with respect to the host that it was generated on.

UID

public UID(short num)

Creates a "well-known" UID. There are 2¹⁶ possible such well-known ids.

A UID created via this constructor will not clash with any UIDs generated via the no-arg constructor.

Parameters:

num - number for well-known UID

UID

private UID(int unique,
            long time,
            short count)

Constructs a UID given data read from a stream.

Method Detail

getHostUniqueNum

private static int getHostUniqueNum()

In the absence of an actual pid, just do something somewhat random.

hashCode

public int hashCode()

Returns the hash code value for this UID.

Overrides:

hashCode in class Object

Returns:

the hash code value for this UID

See Also:

Object.equals(java.lang.Object), Hashtable

equals

public boolean equals(Object obj)

Compares the specified object with this UID for equality. This method returns true if and only if the specified object is a UID instance with the same unique, time, and count values as this one.

Overrides:

equals in class Object

Parameters:

obj - the object to compare this UID to

Returns:

true if the given object is equivalent to this one, and false otherwise

See Also:

Object.hashCode(), Hashtable

toString

public String toString()

Returns a string representation of this UID.

Overrides:

toString in class Object

Returns:

a string representation of this UID

write

public void write(DataOutput out)
           throws IOException

Marshals a binary representation of this UID to a DataOutput instance.

Specifically, this method first invokes the given stream's DataOutput.writeInt(int) method with this UID's unique value, then it invokes the stream's DataOutput.writeLong(long) method with this UID's time value, and then it invokes the stream's DataOutput.writeShort(int) method with this UID's count value.

Parameters:

out - the DataOutput instance to write this UID to

Throws:

IOException - if an I/O error occurs while performing this operation

read

public static UID read(DataInput in)
                throws IOException

Constructs and returns a new UID instance by unmarshalling a binary representation from an DataInput instance.

Specifically, this method first invokes the given stream's DataInput.readInt() method to read a unique value, then it invoke's the stream's DataInput.readLong() method to read a time value, then it invoke's the stream's DataInput.readShort() method to read a count value, and then it creates and returns a new UID instance that contains the unique, time, and count values that were read from the stream.

Parameters:

in - the DataInput instance to read UID from

Returns:

unmarshalled UID instance

Throws:

IOException - if an I/O error occurs while performing this operation