API cleanup for Async Volley stack ahead of 1.2.0 release. (#398)

- Add warning to new APIs noting that they are experimental and subject to change
- Mark NetworkUtility as package-private since it should not be used externally
  (all methods were already package-private)

This unblocks a 1.2.0 release while permitting us to refine the API further
before we lock it down in a similar way to other Volley APIs.

See #181

src/main/java/com/android/volley/AsyncCache.java

@@ -18,7 +18,12 @@
 
 import androidx.annotation.Nullable;
 
-/** Asynchronous equivalent to the {@link Cache} interface. */
+/**
+ * Asynchronous equivalent to the {@link Cache} interface.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public abstract class AsyncCache {
 
     public interface OnGetCompleteCallback {

src/main/java/com/android/volley/AsyncNetwork.java

@@ -22,7 +22,12 @@
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.atomic.AtomicReference;
 
-/** An asynchronous implementation of {@link Network} to perform requests. */
+/**
+ * An asynchronous implementation of {@link Network} to perform requests.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public abstract class AsyncNetwork implements Network {
     private ExecutorService mBlockingExecutor;
     private ExecutorService mNonBlockingExecutor;

src/main/java/com/android/volley/AsyncRequestQueue.java

@@ -42,7 +42,10 @@
  * An asynchronous request dispatch queue.
  *
  * <p>Add requests to the queue with {@link #add(Request)}. Once completed, responses will be
- * delivered on the main thread (unless a custom {@link ResponseDelivery} has been provided)
+ * delivered on the main thread (unless a custom {@link ResponseDelivery} has been provided).
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
  */
 public class AsyncRequestQueue extends RequestQueue {
     /** Default number of blocking threads to start. */

src/main/java/com/android/volley/RequestTask.java

@@ -1,6 +1,11 @@
 package com.android.volley;
 
-/** Abstract runnable that's a task to be completed by the RequestQueue. */
+/**
+ * Abstract runnable that's a task to be completed by the RequestQueue.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public abstract class RequestTask<T> implements Runnable {
     final Request<T> mRequest;
 

src/main/java/com/android/volley/cronet/CronetHttpStack.java

@@ -53,6 +53,9 @@
 
 /**
  * A {@link AsyncHttpStack} that's based on Cronet's fully asynchronous API for network requests.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
  */
 public class CronetHttpStack extends AsyncHttpStack {
 

src/main/java/com/android/volley/toolbox/AsyncHttpStack.java

@@ -28,7 +28,12 @@
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.atomic.AtomicReference;
 
-/** Asynchronous extension of the {@link BaseHttpStack} class. */
+/**
+ * Asynchronous extension of the {@link BaseHttpStack} class.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public abstract class AsyncHttpStack extends BaseHttpStack {
     private ExecutorService mBlockingExecutor;
     private ExecutorService mNonBlockingExecutor;

src/main/java/com/android/volley/toolbox/BasicAsyncNetwork.java

@@ -37,7 +37,12 @@
 import java.util.Map;
 import java.util.concurrent.ExecutorService;
 
-/** A network performing Volley requests over an {@link HttpStack}. */
+/**
+ * A network performing Volley requests over an {@link HttpStack}.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public class BasicAsyncNetwork extends AsyncNetwork {
 
     private final AsyncHttpStack mAsyncStack;

src/main/java/com/android/volley/toolbox/NetworkUtility.java

@@ -42,7 +42,7 @@
  * Utility class for methods that are shared between {@link BasicNetwork} and {@link
  * BasicAsyncNetwork}
  */
-public final class NetworkUtility {
+final class NetworkUtility {
     private static final int SLOW_REQUEST_THRESHOLD_MS = 3000;
 
     private NetworkUtility() {}

src/main/java/com/android/volley/toolbox/NoAsyncCache.java

@@ -3,7 +3,12 @@
 import com.android.volley.AsyncCache;
 import com.android.volley.Cache;
 
-/** An AsyncCache that doesn't cache anything. */
+/**
+ * An AsyncCache that doesn't cache anything.
+ *
+ * <p><b>WARNING</b>: This API is experimental and subject to breaking changes. Please see
+ * https://github.com/google/volley/wiki/Asynchronous-Volley for more details.
+ */
 public class NoAsyncCache extends AsyncCache {
     @Override
     public void get(String key, OnGetCompleteCallback callback) {