finagle-core: Java compat for TypeAgnostic.Identity

Problem

Java users don't have a clean API to access `Filter.TypeAgnostic.Identity`.

Solution

Introduce `Filter.typeAgnosticIdentity()`.

JIRA Issues: CSL-7227

Differential Revision: https://phabricator.twitter.biz/D242006

CHANGELOG.rst

@@ -7,8 +7,14 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+New Features
+~~~~~~~~~~~~
+
+* finagle-core: Add Java compatibility for `c.t.f.Filter.TypeAgnostic.Identity`
+  via `c.t.f.Filter.typeAgnosticIdentity()`. ``PHAB_ID=D242006``
+
 * finagle-thrift: Add `c.t.finagle.thrift.MethodMetadata` which provides a `LocalContext` Key
-  for setting information about the current Thrift method and an accessor for retrieving 
+  for setting information about the current Thrift method and an accessor for retrieving
   the currently set value. ``PHAB_ID=D241295``
 
 18.11.0

finagle-core/src/main/scala/com/twitter/finagle/Filter.scala

@@ -300,9 +300,16 @@ object Filter {
   }
 
   object TypeAgnostic {
+
+    /**
+     * A pass-through [[TypeAgnostic]] that in turn uses a
+     * [[Filter.Identity]] for its implementation.
+     *
+     * @see [[Filter.typeAgnosticIdentity]] for Java compatibility.
+     */
     val Identity: TypeAgnostic =
       new TypeAgnostic {
-        def toFilter[Req, Rep]: Filter[Req, Rep, Req, Rep] = identity[Req, Rep]
+        def toFilter[Req, Rep]: Filter[Req, Rep, Req, Rep] = Filter.identity[Req, Rep]
 
         override def andThen(next: TypeAgnostic): TypeAgnostic = next
 
@@ -320,9 +327,20 @@ object Filter {
       }
   }
 
+  /**
+   * Returns a [[Filter]] that does nothing beyond passing inputs through
+   * to the [[Service]].
+   */
   def identity[Req, Rep]: SimpleFilter[Req, Rep] =
     Identity.asInstanceOf[SimpleFilter[Req, Rep]]
 
+  /**
+   * A pass-through [[TypeAgnostic]] that in turn uses a
+   * [[Filter.Identity]] for its implementation.
+   */
+  def typeAgnosticIdentity: TypeAgnostic =
+    TypeAgnostic.Identity
+
   def mk[ReqIn, RepOut, ReqOut, RepIn](
     f: (ReqIn, ReqOut => Future[RepIn]) => Future[RepOut]
   ): Filter[ReqIn, RepOut, ReqOut, RepIn] = new Filter[ReqIn, RepOut, ReqOut, RepIn] {
@@ -341,7 +359,7 @@ object Filter {
   def choose[Req, Rep](
     pf: PartialFunction[Req, Filter[Req, Rep, Req, Rep]]
   ): Filter[Req, Rep, Req, Rep] = new Filter[Req, Rep, Req, Rep] {
-    private[this] val const: (Req => SimpleFilter[Req, Rep]) =
+    private[this] val const: Req => SimpleFilter[Req, Rep] =
       Function.const(Filter.identity[Req, Rep])
 
     def apply(request: Req, service: Service[Req, Rep]): Future[Rep] =

finagle-core/src/test/java/com/twitter/finagle/FilterCompilationTest.java

@@ -35,13 +35,13 @@ public Future<U> apply(T request, Service<T, U> service) {
     };
 
     passThruFilter.andThen(Filter.identity());
-    typeAgnosticPassThruFilter.andThen(Filter.TypeAgnostic$.MODULE$.Identity());
+    typeAgnosticPassThruFilter.andThen(Filter.typeAgnosticIdentity());
 
     passThruFilter.agnosticAndThen(typeAgnosticPassThruFilter);
     typeAgnosticPassThruFilter.andThen(passThruFilter);
 
     Filter.<Object, String>identity().andThen(passThruFilter);
-    Filter.TypeAgnostic$.MODULE$.Identity().andThen(typeAgnosticPassThruFilter);
+    Filter.typeAgnosticIdentity().andThen(typeAgnosticPassThruFilter);
 
     passThruFilter.andThen(constSvc);
     typeAgnosticPassThruFilter.andThen(constSvc);
@@ -96,7 +96,7 @@ public String toString() {
 
     assertEquals(
         "com.twitter.finagle.TypeAgnosticPassThruFilter",
-        typeAgnosticPassThruFilter.andThen(Filter.TypeAgnostic$.MODULE$.Identity()).toString());
+        typeAgnosticPassThruFilter.andThen(Filter.typeAgnosticIdentity()).toString());
 
     assertEquals(
         "com.twitter.finagle.PassThruFilter"
@@ -114,7 +114,7 @@ public String toString() {
 
     assertEquals(
         "com.twitter.finagle.TypeAgnosticPassThruFilter",
-        Filter.TypeAgnostic$.MODULE$.Identity().andThen(typeAgnosticPassThruFilter).toString());
+        Filter.typeAgnosticIdentity().andThen(typeAgnosticPassThruFilter).toString());
 
     assertEquals(
         "com.twitter.finagle.PassThruFilter"