[[java-rest-high-exists-alias]]
=== Exists Alias API

[[java-rest-high-exists-alias-request]]
==== Exists Alias Request

The Exists Alias API uses `GetAliasesRequest` as its request object.
One or more aliases can be optionally provided either at construction
time or later on through the relevant setter method.

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-request]
--------------------------------------------------

==== Optional arguments
The following arguments can optionally be provided:

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-request-alias]
--------------------------------------------------
<1> One or more aliases to look for

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-request-indices]
--------------------------------------------------
<1> The index or indices that the alias is associated with

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-request-indicesOptions]
--------------------------------------------------
<1> Setting `IndicesOptions` controls how unavailable indices are resolved and
how wildcard expressions are expanded

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-request-local]
--------------------------------------------------
<1> The `local` flag (defaults to `false`) controls whether the aliases need
to be looked up in the local cluster state or in the cluster state held by
the elected master node.

[[java-rest-high-exists-alias-sync]]
==== Synchronous Execution

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-execute]
--------------------------------------------------

[[java-rest-high-exists-alias-async]]
==== Asynchronous Execution

The asynchronous execution of a exists alias request requires both a `GetAliasesRequest`
instance and an `ActionListener` instance to be passed to the asynchronous
method:

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-execute-async]
--------------------------------------------------
<1> The `GetAliasesRequest` to execute and the `ActionListener` to use when
the execution completes

The asynchronous method does not block and returns immediately. Once it is
completed the `ActionListener` is called back using the `onResponse` method
if the execution successfully completed or using the `onFailure` method if
it failed.

A typical listener for the `Boolean` response looks like:

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/IndicesClientDocumentationIT.java[exists-alias-listener]
--------------------------------------------------
<1> Called when the execution is successfully completed. The response is
provided as an argument
<2> Called in case of failure. The raised exception is provided as an argument

[[java-rest-high-exists-alias-response]]
==== Exists Alias Response

The Exists Alias API returns a `boolean` that indicates whether the provided
alias (or aliases) was found or not.