Initial implementation of Cronet Async API.

components/cronet/android/java/src/org/chromium/net/UrlRequestContext.java

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.net;
+
+import android.content.Context;
+import android.util.Log;
+
+import java.lang.reflect.Constructor;
+import java.util.concurrent.Executor;
+
+/**
+ * A context for {@link UrlRequest}'s, which uses the best HTTP stack
+ * available on the current platform.
+ */
+public abstract class UrlRequestContext {
+    private static final String TAG = "UrlRequestFactory";
+    private static final String CRONET_URL_REQUEST_CONTEXT =
+            "org.chromium.net.CronetUrlRequestContext";
+
+    /**
+     * Creates an UrlRequest object. All UrlRequest functions must
+     * be called on the Executor's thread, and all callbacks will be called
+     * on the Executor's thread as well. Executor must not run tasks on the
+     * current thread to prevent network jank and exception during shutdown.
+     *
+     * createRequest itself may be called on any thread.
+     * @param url URL for the request.
+     * @param listener Callback interface that gets called on different events.
+     * @param executor Executor on which all callbacks will be called.
+     * @return new request.
+     */
+    public abstract UrlRequest createRequest(String url,
+            UrlRequestListener listener, Executor executor);
+
+    /**
+     * @return true if the context is enabled.
+     */
+    public abstract boolean isEnabled();
+
+    /**
+     * @return a human-readable version string of the context.
+     */
+    public abstract String getVersionString();
+
+    /**
+     * Shutdown the UrlRequestContext if there are no active requests, otherwise
+     * throw an exception. Cannot be called on network thread.

[mmenke, 2014/11/06 17:31:29]

nit:  Maybe "Cannot be called on network thread - the thread Cronet calls into
Executor on (Which is different from the thread the Executor invokes callbacks
on).  May block until all the Context's resources have been cleaned up."

[mef, 2014/11/06 22:51:47]

Done.

+     */
+    public abstract void shutdown();
+
+    /**
+     * Create context with given config. If config.legacyMode is true, or
+     * native library is not available, then creates HttpUrlConnection-based
+     * context.
+     * @param context application context.
+     * @param config context configuration.
+     */
+    public static UrlRequestContext createContext(Context context,
+            UrlRequestContextConfig config) {
+        UrlRequestContext urlRequestContext = null;
+        if (config.userAgent().isEmpty()) {
+            config.setUserAgent(UserAgent.from(context));
+        }
+        if (!config.legacyMode()) {
+            urlRequestContext = createCronetContext(context, config);
+        }
+        if (urlRequestContext == null) {
+            // TODO(mef): Fallback to stub implementation. Once stub
+            // implementation is available merge with createCronetFactory.
+            urlRequestContext = createCronetContext(context, config);
+        }
+        Log.i(TAG, "Using network stack: "
+                + urlRequestContext.getVersionString());
+        return urlRequestContext;
+    }
+
+    private static UrlRequestContext createCronetContext(Context context,
+            UrlRequestContextConfig config) {
+        UrlRequestContext urlRequestContext = null;
+        try {
+            Class<? extends UrlRequestContext> contextClass =
+                    UrlRequestContext.class.getClassLoader()
+                            .loadClass(CRONET_URL_REQUEST_CONTEXT)
+                            .asSubclass(UrlRequestContext.class);
+            Constructor<? extends UrlRequestContext> constructor =
+                    contextClass.getConstructor(
+                            Context.class, UrlRequestContextConfig.class);
+            UrlRequestContext cronetContext =
+                    constructor.newInstance(context, config);
+            if (cronetContext.isEnabled()) {
+                urlRequestContext = cronetContext;
+            }
+        } catch (ClassNotFoundException e) {
+            // Leave as null.
+        } catch (Exception e) {
+            throw new IllegalStateException(
+                    "Cannot instantiate: " + CRONET_URL_REQUEST_CONTEXT,
+                    e);
+        }
+        return urlRequestContext;
+    }
+}