content/api/user-guide/6.27-oid.mdtext

Title: 6.27 - Oid
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# 6.27 - Oid

## Introduction

An *OID* stands for *Object* *ID*entifier. It's part of the *ASN/1*  standard. Basically, it's a hierarchy, named a 'registration tree'. It's a list of 'arcs', which values start at '0', and each arc is separated with a dot ( '.' ).

There are three defined root arcs :

    * itu-t (0)
    * iso (1)
    * joint-iso-itu-t (2) 

There are some places on the internet where one can look at the existing OIDs. Here is one: [OID repository](http://www.oid-info.com/cgi-bin/display?tree=&see=all).

## Syntax

The OID syntax is pretty simple.  Numbers are separated with periods between them, e.g. 0.1.245

There are a few extra rules:

    * The first number must be 0, 1 or 2
    * If it's 0 or 1, then the second number must be between 0 and 39
    * A period must be preceded and followed by a number
    * A 0 at first position is never followed by another digit
    * There is no limit for the number of digits.  For example, 2.25.81407072025111374527560065493494091452, is a legitimate OID value.

## API

The *Apache LDAP API* offers a few functions that can be used to manipulate an OID. In any case, there is not a constructor for an OID.

The class to use is *org.apache.directory.api.asn1.util.Oid*. It creates immutable instances.

### static Oid fromBytes( byte[] )

Returns an *Oid* instance constructed using a *byte[]* representing an encoded OID.

### static Oid fromString( String )

Returns an *Oid* instance constructed from a *String* representing an OID.

### static boolean isOid( String )

Tells if a *String* is a valid OID.

### String toString()

Return the *String* representation of an *Oid* instance.

### byte[] toBytes()

Return the *byte[]* representation of an *Oid* instance.

## OID parsing

We use a *FSA* (Finite State Automaton) to parse a String to get an internal representation of an OID. Here is a picture showing the *FSA* we are using:

![OID FSA](images/OID-state-automaton.png)