Xhtml to xhtml to managed bean method call with parameter passing using in jsf

Question

I am using Jsf (2.2.20) and Primefaces (8.0) in my application. In a datatable column I have <h:link> which is passing a column value to another xhtml page. Code below:

<p:column>
        <h:link value="Link1" outcome="testView1.xhtml" style="font-size:8pt;" >
                <f:param name="Unique_Key" value="${row.value1}" />
        </h:link>
</p:column>

Where row.value1 is the column value. In testView1.xhtml page I have the below code :

<f:metadata>
        <f:viewParam id="unit_id" name="unit_id" value="${Unique_Key}"  />
        <f:viewAction action="#{testManagedBean.document}" />
    </f:metadata>

In testManagedBean class I have a method document() which is having code like :

FacesContext facesContext = FacesContext.getCurrentInstance();
ExternalContext externalContext = facesContext.getExternalContext();
Map<String, String> map = externalContext.getRequestParameterMap();
String param = map.get("unit_id");

But the param is coming always null. Am I doing any wrong in the code?