lucene/solr/solr-ref-guide/src/velocity-response-writer.adoc

= Velocity Response Writer
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The VelocityResponseWriter is an optional plugin available in the `contrib/velocity` directory. It powers the /browse user interfaces when using configurations such as "_default", "techproducts", and "example/files".

Its JAR and dependencies must be added (via `<lib>` or solr/home lib inclusion), and must be registered in `solrconfig.xml` like this:

[source,xml]
----
<queryResponseWriter name="velocity" class="solr.VelocityResponseWriter">
  <str name="template.base.dir">${velocity.template.base.dir:}</str>

<!--
  <str name="init.properties.file">velocity-init.properties</str>
  <bool name="params.resource.loader.enabled">true</bool>
  <bool name="solr.resource.loader.enabled">false</bool>
  <lst name="tools">
    <str name="mytool">com.example.MyCustomTool</str>
  </lst>
-->
</queryResponseWriter>
----

The above example shows the optional initialization and custom tool parameters used by VelocityResponseWriter; these are detailed in the following table. These initialization parameters are only specified in the writer registration in `solrconfig.xml`, not as request-time parameters. See further below for request-time parameters.

== Configuration & Usage

=== VelocityResponseWriter Initialization Parameters

`template.base.dir`::
If specified and exists as a file system directory, a file resource loader will be added for this directory. Templates in this directory will override "solr" resource loader templates.

`init.properties.file`:: Specifies a properties file name which must exist in the Solr `conf/` directory (*not* under a `velocity/` subdirectory) or root of a JAR file in a <lib>.

`params.resource.loader.enabled`::
The "params" resource loader allows templates to be specified in Solr request parameters. For example:
+
[source,bash]
http://localhost:8983/solr/gettingstarted/select?q=\*:*&wt=velocity&v.template=custom&v.template.custom=CUSTOM%3A%20%23core_name
+
where `v.template=custom` says to render a template called "custom" and the value of `v.template.custom` is the custom template. This is `false` by default; it'd be a niche, unusual, use case to need this enabled.

`solr.resource.loader.enabled`::
The "solr" resource loader is the only template loader registered by default. Templates are served from resources visible to the SolrResourceLoader under a `velocity/` subdirectory. The VelocityResponseWriter itself has some built-in templates (in its JAR file, under `velocity/`) that are available automatically through this loader. These built-in templates can be overridden when the same template name is in conf/velocity/ or by using the `template.base.dir` option.

`tools`::
External "tools" can be specified as list of string name/value (tool name / class name) pairs. Tools, in the Velocity context, are simply Java objects. Tool classes are constructed using a no-arg constructor (or a single-SolrCore-arg constructor if it exists) and added to the Velocity context with the specified name.
+
A custom registered tool can override the built-in context objects with the same name, except for `$request`, `$response`, `$page`, and `$debug` (these tools are designed to not be overridden).

=== VelocityResponseWriter Request Parameters

`v.template`::
Specifies the name of the template to render.

`v.layout`::
Specifies a template name to use as the layout around the main, `v.template`, specified template.
+
The main template is rendered into a string value included into the layout rendering as `$content`.

`v.layout.enabled`::
Determines if the main template should have a layout wrapped around it. The default is `true`, but requires `v.layout` to specified as well.

`v.contentType`::
Specifies the content type used in the HTTP response. If not specified, the default will depend on whether `v.json` is specified or not.
+
The default without `v.json=wrf`: `text/html;charset=UTF-8`.
+
The default with `v.json=wrf`: `application/json;charset=UTF-8`.

`v.json`::
Specifies a function name to wrap around the response rendered as JSON. If specified, the content type used in the response will be "application/json;charset=UTF-8", unless overridden by `v.contentType`.
+
Output will be in this format (with `v.json=wrf`):
+
`wrf("result":"<Velocity generated response string, with quotes and backslashes escaped>")`

`v.locale`::
Locale to use with the `$resource` tool and other LocaleConfig implementing tools. The default locale is `Locale.ROOT`. Localized resources are loaded from standard Java resource bundles named `resources[_locale-code].properties`.
+
Resource bundles can be added by providing a JAR file visible by the SolrResourceLoader with resource bundles under a velocity sub-directory. Resource bundles are not loadable under `conf/`, as only the class loader aspect of SolrResourceLoader can be used here.

`v.template._template_name_`:: When the "params" resource loader is enabled, templates can be specified as part of the Solr request.


=== VelocityResponseWriter Context Objects

// TODO: Change column width to %autowidth.spread when https://github.com/asciidoctor/asciidoctor-pdf/issues/599 is fixed

[cols="30,70",options="header"]
|===
|Context Reference |Description
|`request` |{solr-javadocs}solr-core/org/apache/solr/request/SolrQueryRequest.html[SolrQueryRequest] javadocs
|`response` |{solr-javadocs}solr-core/org/apache/solr/response/SolrQueryResponse.html[QueryResponse] most of the time, but in some cases where QueryResponse doesn't like the request handler's output (https://wiki.apache.org/solr/AnalysisRequestHandler[AnalysisRequestHandler], for example, causes a ClassCastException parsing "response"), the response will be a SolrResponseBase object.
|`esc` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#EscapeTool[EscapeTool] instance
|`date` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#ComparisonDateTool[ComparisonDateTool] instance
|`list` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/apidocs/org/apache/velocity/tools/generic/ListTool.html[ListTool] instance
|`math` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#MathTool[MathTool] instance
|`number` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#NumberTool[NumberTool] instance
|`sort` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#SortTool[SortTool] instance
|`display` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#DisplayTool[DisplayTool] instance
|`resource` |A Velocity http://velocity.apache.org/tools/{ivy-velocity-tools-version}/tools-summary.html#ResourceTool[ResourceTool] instance
|`engine` |The current VelocityEngine instance
|`page` |An instance of Solr's PageTool (only included if the response is a QueryResponse where paging makes sense)
|`debug` |A shortcut to the debug part of the response, or null if debug is not on. This is handy for having debug-only sections in a template using `#if($debug)...#end`
|`content` |The rendered output of the main template, when rendering the layout (`v.layout.enabled=true` and `v.layout=<template>`).
|[custom tool(s)] |Tools provided by the optional "tools" list of the VelocityResponseWriter registration are available by their specified name.
|===