Use UTF-8 for captured responses if charset is not set explicitly (#39)

In the newest versions of Spring Framework, after a change[1] to align
it with the RFC8259[2], the character encoding is no longer set to
UTF-8 for responses with `application/json` content-type.

This fix makes a `JsonResponseWrapper` use UTF-8 by default, instead of
getting charset from the wrapped `Response` instance which could have
been not set and so it returns default ISO-8895-1.

[1]: spring-projects/spring-framework#22788
[2]: https://tools.ietf.org/html/rfc8259#section-11

Author: sainaen

filters/src/main/java/ua/net/tokar/json/rainbowrest/HtmlResponseWrapper.java renamed to filters/src/main/java/ua/net/tokar/json/rainbowrest/JsonResponseWrapper.java

@@ -9,13 +9,20 @@
 import java.io.IOException;
 import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
+import java.nio.charset.StandardCharsets;
 
-class HtmlResponseWrapper extends HttpServletResponseWrapper {
+class JsonResponseWrapper extends HttpServletResponseWrapper {
+    // RFC 8259 (section 11) specifies that 'charset' parameter for media
+    // type 'application/json' has no effect and JSON text MUST be encoded
+    // using UTF-8 (section 8.1)
+    private static final String DEFAULT_CHARSET = StandardCharsets.UTF_8.name();
+
+    private String charset = DEFAULT_CHARSET;
     private final ByteArrayOutputStream capture;
     private ServletOutputStream output;
     private PrintWriter writer;
 
-    HtmlResponseWrapper(ServletResponse response) {
+    JsonResponseWrapper(ServletResponse response) {
         super((HttpServletResponse)response);
         capture = new ByteArrayOutputStream(response.getBufferSize());
     }
@@ -84,6 +91,13 @@ public void flushBuffer() throws IOException {
         }
     }
 
+    @Override
+    public void setCharacterEncoding(String charset) {
+        // for backward compatibility use non-default charset if set explicitly
+        super.setCharacterEncoding(charset);
+        this.charset = charset != null ? charset : DEFAULT_CHARSET;
+    }
+
     private byte[] getCaptureAsBytes() throws IOException {
         if (writer != null) {
             writer.close();
@@ -95,6 +109,6 @@ private byte[] getCaptureAsBytes() throws IOException {
     }
 
     String getCaptureAsString() throws IOException {
-        return new String(getCaptureAsBytes(), getCharacterEncoding());
+        return new String(getCaptureAsBytes(), charset);
     }
 }

filters/src/main/java/ua/net/tokar/json/rainbowrest/RainbowRestBatchFilter.java

@@ -123,11 +123,10 @@ protected void doFilterInternal(
                 return;
             }
             response.setContentType( "application/json" );
-            response.getWriter().write(
-                    getBatchResults(
-                            map,
-                            (HttpServletRequest) request
-                    ).toString() );
+            mapper.writeValue(
+                    response.getOutputStream(),
+                    getBatchResults( map, (HttpServletRequest) request )
+            );
         }
     }
 

filters/src/main/java/ua/net/tokar/json/rainbowrest/RainbowRestOncePerRequestFilter.java

@@ -14,6 +14,7 @@
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Enumeration;
@@ -109,7 +110,7 @@ protected String getResponseViaInternalDispatching(
             ServletRequest request,
             ServletResponse response
     ) throws ServletException, IOException {
-        HtmlResponseWrapper responseWrapper = new HtmlResponseWrapper( response );
+        JsonResponseWrapper responseWrapper = new JsonResponseWrapper( response );
         request.getRequestDispatcher( relativeUrl )
                .forward(
                        new GetHttpServletRequest( (HttpServletRequest) request ),
@@ -138,7 +139,7 @@ protected URI buildUri(
         List<NameValuePair> params = new ArrayList<>();
         params.addAll( URLEncodedUtils.parse(
                 new URI( relativeUrl ),
-                Charset.forName( "UTF-8" )
+                StandardCharsets.UTF_8
         ) );
         params.addAll( additionalRequestParams );
 

filters/src/main/java/ua/net/tokar/json/rainbowrest/RainbowRestWebFilter.java

@@ -124,7 +124,7 @@ protected void doFilterInternal(
             return;
         }
 
-        HtmlResponseWrapper capturingResponseWrapper = new HtmlResponseWrapper( response );
+        JsonResponseWrapper capturingResponseWrapper = new JsonResponseWrapper( response );
         filterChain.doFilter( request, capturingResponseWrapper );
 
         Set<String> includeFields = new HashSet<>();
@@ -159,7 +159,7 @@ protected void doFilterInternal(
             filterTree( tree, includeFields, excludeFields );
         }
 
-        response.getWriter().write( tree.toString() );
+        mapper.writeValue(response.getOutputStream(), tree);
     }
 
     private void processIncludes(