Module java.rmi
Package java.rmi.server

Class 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:
    1.1
    See Also:
    Serialized Form

    • Constructor Summary

      Constructors 
      Constructor Description
      UID​()
      Generates a UID that is unique over time with respect to the host that it was generated on.
      UID​(short num)
      Creates a "well-known" UID.

    • 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

    • Method Detail

      • 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(), HashMap

      • 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