Extension (Java SE 9 & JDK 9 )
Keywords: java.security.cert.Extension interface

Module java.base
Package java.security.cert

## Interface Extension

-     public interface Extension

  This interface represents an X.509 extension.

  Extensions provide a means of associating additional attributes with users or public keys and for managing a certification hierarchy. The extension format also allows communities to define private extensions to carry information unique to those communities.

  Each extension contains an object identifier, a criticality setting indicating whether it is a critical or a non-critical extension, and and an ASN.1 DER-encoded value. Its ASN.1 definition is:

           Extension ::= SEQUENCE {
               extnId        OBJECT IDENTIFIER,
               critical      BOOLEAN DEFAULT FALSE,
               extnValue     OCTET STRING
                       -- contains a DER encoding of a value
                       -- of the type registered for use with
                       -- the extnId object identifier value
           }

  This interface is designed to provide access to a single extension, unlike `X509Extension` which is more suitable for accessing a set of extensions.

  Since:
    1.7

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `encode​(OutputStream out)` | Generates the extension's DER encoding and writes it to the output stream.
    `String` | `getId​()` | Gets the extensions's object identifier.
    `byte[]` | `getValue​()` | Gets the extensions's DER-encoded value.
    `boolean` | `isCritical​()` | Gets the extension's criticality setting.

- - ### Method Detail

    - #### getId

          String getId​()

      Gets the extensions's object identifier.
      Returns:
        the object identifier as a String

    - #### isCritical

          boolean isCritical​()

      Gets the extension's criticality setting.
      Returns:
        true if this is a critical extension.

    - #### getValue

          byte[] getValue​()

      Gets the extensions's DER-encoded value. Note, this is the bytes that are encoded as an OCTET STRING. It does not include the OCTET STRING tag and length.
      Returns:
        a copy of the extension's value, or `null` if no extension value is present.

    - #### encode

          void encode​(OutputStream out)
               throws IOException

      Generates the extension's DER encoding and writes it to the output stream.
      Parameters:
        `out` - the output stream
      Throws:
        `IOException` - on encoding or output error.
        `NullPointerException` - if `out` is `null`.