Optimize DefaultHeaders#toString and implement HttpHeaders#toString

Motivation:

For debugging/logging purpose, it would be convenient to have
HttpHeaders#toString implemented.

DefaultHeaders does implement toString be the implementation is suboptimal and allocates a Set for the names and Lists for values.

Modification:

* Introduce HeadersUtil#toString that provides a convenient optimized helper to implement toString for various headers implementations
* Have DefaultHeaders#toString and HttpHeaders#toString delegate their toString implementation to HeadersUtil

Result:

Convenient HttpHeaders#toString. Optimized DefaultHeaders#toString.

codec-http/src/main/java/io/netty/handler/codec/http/HttpHeaders.java

@@ -19,6 +19,7 @@ import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.handler.codec.DateFormatter;
 import io.netty.handler.codec.Headers;
+import io.netty.handler.codec.HeadersUtils;
 import io.netty.util.AsciiString;
 import io.netty.util.CharsetUtil;
 
@@ -1687,4 +1688,9 @@ public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>>
     public boolean contains(CharSequence name, CharSequence value, boolean ignoreCase) {
         return contains(name.toString(), value.toString(), ignoreCase);
     }
+
+    @Override
+    public String toString() {
+        return HeadersUtils.toString(getClass(), iteratorCharSequence(), size());
+    }
 }

codec-http/src/test/java/io/netty/handler/codec/http/DefaultHttpHeadersTest.java

@@ -209,6 +209,26 @@ public class DefaultHttpHeadersTest {
         assertDefaultValues(headers, HeaderValue.THREE);
     }
 
+    @Test
+    public void toStringOnEmptyHeaders() {
+        assertEquals("DefaultHttpHeaders[]", newDefaultDefaultHttpHeaders().toString());
+    }
+
+    @Test
+    public void toStringOnSingleHeader() {
+        assertEquals("DefaultHttpHeaders[foo: bar]", newDefaultDefaultHttpHeaders()
+                .add("foo", "bar")
+                .toString());
+    }
+
+    @Test
+    public void toStringOnMultipleHeaders() {
+        assertEquals("DefaultHttpHeaders[foo: bar, baz: qix]", newDefaultDefaultHttpHeaders()
+                .add("foo", "bar")
+                .add("baz", "qix")
+                .toString());
+    }
+
     private static void assertDefaultValues(final DefaultHttpHeaders headers, final HeaderValue headerValue) {
         assertTrue(contentEquals(headerValue.asList().get(0), headers.get(HEADER_NAME)));
         List<CharSequence> expected = headerValue.asList();

codec/src/main/java/io/netty/handler/codec/DefaultHeaders.java

@@ -889,17 +889,7 @@ public class DefaultHeaders<K, V, T extends Headers<K, V, T>> implements Headers
 
     @Override
     public String toString() {
-        StringBuilder builder = new StringBuilder(getClass().getSimpleName()).append('[');
-        String separator = "";
-        for (K name : names()) {
-            List<V> values = getAll(name);
-            for (int i = 0; i < values.size(); ++i) {
-                builder.append(separator);
-                builder.append(name).append(": ").append(values.get(i));
-                separator = ", ";
-            }
-        }
-        return builder.append(']').toString();
+        return HeadersUtils.toString(getClass(), iterator(), size());
     }
 
     protected HeaderEntry<K, V> newHeaderEntry(int h, K name, V value, HeaderEntry<K, V> next) {

codec/src/main/java/io/netty/handler/codec/HeadersUtils.java

@@ -72,6 +72,31 @@ public final class HeadersUtils {
         return new StringEntryIterator(headers.iterator());
     }
 
+    /**
+     * Helper for implementing toString for {@link DefaultHeaders} and wrappers such as DefaultHttpHeaders.
+     * @param headersClass the class of headers
+     * @param headersIt the iterator on the actual headers
+     * @param size the size of the iterator
+     * @return a String representation of the headers
+     */
+    public static <K, V> String toString(Class<?> headersClass, Iterator<Entry<K, V>> headersIt, int size) {
+        String simpleName = headersClass.getSimpleName();
+        if (size == 0) {
+            return simpleName + "[]";
+        } else {
+            // original capacity assumes 20 chars per headers
+            StringBuilder sb = new StringBuilder(simpleName.length() + 2 + size * 20)
+                    .append(simpleName)
+                    .append('[');
+            while (headersIt.hasNext()) {
+                Entry<?, ?> header = headersIt.next();
+                sb.append(header.getKey()).append(": ").append(header.getValue()).append(", ");
+            }
+            sb.setLength(sb.length() - 2);
+            return sb.append(']').toString();
+        }
+    }
+
     /**
      * {@link Headers#names()} and convert each element of {@link Set} to a {@link String}.
      * @param headers the headers to get the names from