[Cronet] Implement methods to get response headers in HttpURLConnection.

components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java

Index: components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java
diff --git a/components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java b/components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java
index 9c308ae1f5fba209df0e7c83a9db181bbcb233e6..0d965000c72e1b844a83fc29c1e80cc6aac7d03b 100644
--- a/components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java
+++ b/components/cronet/android/java/src/org/chromium/net/urlconnection/CronetHttpURLConnection.java
@@ -114,6 +114,77 @@ public class CronetHttpURLConnection extends HttpURLConnection {
     }
 
     /**
+     * Returns an unmodifiable map of the response-header fields and values.
+     */
+    @Override
+    public Map<String, List<String>> getHeaderFields() {
+        try {
+            connect();
+        } catch (IOException e) {
+            return Collections.emptyMap();
+        }
+        return mResponseInfo.getAllHeaders();
+    }
+
+    /**
+     * Returns the value of the named header field. If called on a connection
+     * that sets the same header multiple times with possibly different values,
+     * only the last value is returned.
+     */
+    @Override
+    public final String getHeaderField(String fieldName) {
+        try {
+            connect();
+        } catch (IOException e) {
+            return null;
+        }
+        Map<String, List<String>> map = mResponseInfo.getAllHeaders();

[mef, 2015/01/23 17:08:02]

Getting all headers on every call to getHeaderField() seems really inefficient
as it involves a lengthy map buildup. 
Would it make sense to either cache headers map in CronetHttpURLConnection or in
ResponseInfo?

Alternatively, would it make sense to getAllHeadersAsList() and just loop here
looking for first fieldName? This would at least avoid all the copies /
allocations and shorten the loop in comparison with getAllHeaders.

[miloslav, 2015/01/23 19:17:33]

+1 for caching getAllHeaders in ResponseInfo. Also the unmodifiable collection
returned by NativeResponseInfo.getAllHeadersAsList should be cached too. 

Are ResponseInfo instances supposed to be modified after construction?

[xunjieli, 2015/01/23 20:06:41]

I don't think ResponseInfo instances are modified after construction. 
I cached the headers in NativeResponseInfo, as ResponseInfo is an interface. 
Should we cache them in ResponseInfo? that will involve making ResponseInfo as
an abstract class. WDYT? PTAL.

[miloslav, 2015/01/23 20:21:55]

Sorry, by ResponseInfo instances I was referring to its implementations, which
currently is only NativeResponseInfo, so caching there is fine.

For a moment I got confused that NativeResponseInfo.getAllHeaders does not cache
the result, so the code here should be fine. 

I asked whether NativeInfo is to be modified after construction, because if it
doesn't then making all fields final and computing the map at construction could
be easier (and as a bonus thread safe). However, since it is fine now, it is not
worth it changing it.

+        if (!map.containsKey(fieldName)) {
+            return null;
+        }
+        List<String> values = map.get(fieldName);
+        return values.get(values.size() - 1);
+    }
+
+    /**
+     * Returns the name of the header field at the given position pos, or null
+     * if there are fewer than pos fields.
+     */
+    @Override
+    public final String getHeaderFieldKey(int pos) {
+        try {
+            connect();
+        } catch (IOException e) {
+            return null;
+        }
+        List<Pair<String, String>> headers =
+                mResponseInfo.getAllHeadersAsList();
+        if (pos >= headers.size()) {
+            return null;
+        }
+        return headers.get(pos).first;
+    }
+
+    /**
+     * Returns the header value at the field position pos or null if the header
+     * has fewer than pos fields.
+     */
+    @Override
+    public final String getHeaderField(int pos) {
+        try {
+            connect();
+        } catch (IOException e) {
+            return null;
+        }
+        List<Pair<String, String>> headers =
+                mResponseInfo.getAllHeadersAsList();
+        if (pos >= headers.size()) {
+            return null;
+        }
+        return headers.get(pos).second;
+    }
+
+    /**
      * Returns an InputStream for reading data from the resource pointed by this
      * URLConnection.
      * @throws FileNotFoundException if http response code is equal or greater