One could use the queryParam method, passing it parameter name and an array of values:
public WebTarget queryParam(String name, Object... values);
Example (jersey-client 2.23.2):
WebTarget target = ClientBuilder.newClient().target(URI.create("http://localhost"));
target.path("path")
.queryParam("param_name", Arrays.asList("paramVal1", "paramVal2").toArray())
.request().get();
This will issue request to following URL:
http://localhost/path?param_name=paramVal1¶m_name=paramVal2