diff --git a/include/mapnik/feature_style_processor.hpp b/include/mapnik/feature_style_processor.hpp
index 6fb1c1533..89caaec25 100644
--- a/include/mapnik/feature_style_processor.hpp
+++ b/include/mapnik/feature_style_processor.hpp
@@ -93,7 +93,7 @@ public:
         
         for (;metaItr!=metaItrEnd; ++metaItr)
         {
-            metaItr->second->start();
+            metaItr->second->start(m_.metawriter_output_properties);
         }
                        
         try
diff --git a/include/mapnik/map.hpp b/include/mapnik/map.hpp
index a19a6acf2..58052e5cc 100644
--- a/include/mapnik/map.hpp
+++ b/include/mapnik/map.hpp
@@ -420,7 +420,7 @@ public:
     /*!
      * @brief Get a metawriter property.
      */
-    std::string get_metawriter_property(std::string name); /*TODO: const */
+    std::string get_metawriter_property(std::string name) const;
 
 private:
     void fixAspectRatio();
diff --git a/include/mapnik/metawriter.hpp b/include/mapnik/metawriter.hpp
index a61e0ad90..6dc2c18ee 100644
--- a/include/mapnik/metawriter.hpp
+++ b/include/mapnik/metawriter.hpp
@@ -40,6 +40,18 @@
 
 namespace mapnik {
 
+class metawriter_property_map
+{
+public:
+    metawriter_property_map() {}
+    UnicodeString const& operator[](std::string const& key) const;
+    UnicodeString& operator[](std::string const& key) {return m_[key];}
+private:
+    std::map<std::string, UnicodeString> m_;
+    UnicodeString not_found_;
+};
+
+
 /** All properties to be output by a metawriter. */
 class metawriter_properties : public std::set<std::string>
 {
@@ -67,7 +79,7 @@ class metawriter
                              proj_transform const& prj_trans,
                              CoordTransform const &t,
                              metawriter_properties const& properties = metawriter_properties())=0;
-        virtual void start() {};
+        virtual void start(metawriter_property_map const& properties) {};
         virtual void stop() {};
     protected:
         metawriter_properties dflt_properties_;
diff --git a/include/mapnik/metawriter_json.hpp b/include/mapnik/metawriter_json.hpp
index a8cb87e16..5f250e3ee 100644
--- a/include/mapnik/metawriter_json.hpp
+++ b/include/mapnik/metawriter_json.hpp
@@ -26,28 +26,46 @@
 
 // Mapnik
 #include <mapnik/metawriter.hpp>
+#include <mapnik/parse_path.hpp>
 
 // STL
-#include <ostream>
+#include <fstream>
 
 namespace mapnik {
-/** JSON writer. */
-class metawriter_json : public metawriter, private boost::noncopyable
+
+
+/** Write JSON data to a stream object. */
+class metawriter_json_stream : public metawriter, private boost::noncopyable
 {
-    public:
-        metawriter_json(metawriter_properties dflt_properties, std::string fn);
-        metawriter_json(metawriter_properties dflt_properties, std::ostream);
-        ~metawriter_json();
-        virtual void add_box(box2d<double> box, Feature const &feature,
-                             proj_transform const& prj_trans,
-                             CoordTransform const& t,
-                             metawriter_properties const& properties);
-        virtual void start();
-        virtual void stop();
-    private:
-        std::ostream *f;
-        std::string fn_;
-        int count;
+public:
+    metawriter_json_stream(metawriter_properties dflt_properties);
+    virtual void add_box(box2d<double> box, Feature const &feature,
+                         proj_transform const& prj_trans,
+                         CoordTransform const& t,
+                         metawriter_properties const& properties);
+
+    virtual void start(metawriter_property_map const& properties);
+    virtual void stop();
+    void set_stream(std::ostream *f) { f_ = f; }
+
+private:
+    std::ostream *f_;
+    int count;
+};
+
+/** JSON writer. */
+class metawriter_json : public metawriter_json_stream
+{
+public:
+    metawriter_json(metawriter_properties dflt_properties, path_expression_ptr fn);
+    ~metawriter_json();
+
+    virtual void start(metawriter_property_map const& properties);
+    virtual void stop();
+
+private:
+    path_expression_ptr fn_;
+    std::fstream f_;
 };
 
 };
diff --git a/src/load_map.cpp b/src/load_map.cpp
index 94e0ecafc..349b54882 100644
--- a/src/load_map.cpp
+++ b/src/load_map.cpp
@@ -363,7 +363,7 @@ void map_parser::parse_metawriter(Map & map, ptree const & pt)
         if (type == "json") {
             string file = get_attr<string>(pt, "file");
             optional<string> properties = get_opt_attr<string>(pt, "default-output");
-            writer = metawriter_ptr(new metawriter_json(properties, file));
+            writer = metawriter_ptr(new metawriter_json(properties, parse_path(file)));
         } else {
             throw config_error(string("Unknown type '") + type + "'");
         }
diff --git a/src/map.cpp b/src/map.cpp
index d86ccd9eb..87d734cb4 100644
--- a/src/map.cpp
+++ b/src/map.cpp
@@ -614,7 +614,7 @@ void Map::set_metawriter_property(std::string name, std::string value)
     metawriter_output_properties[name] = UnicodeString::fromUTF8(value);
 }
 
-std::string Map::get_metawriter_property(std::string name)
+std::string Map::get_metawriter_property(std::string name) const
 {
     std::string result;
     to_utf8(metawriter_output_properties[name], result);
diff --git a/src/metawriter.cpp b/src/metawriter.cpp
index 562489886..40cf3aae3 100644
--- a/src/metawriter.cpp
+++ b/src/metawriter.cpp
@@ -32,48 +32,34 @@
 // STL
 #include <iomanip>
 #include <cstdio>
-#include <fstream>
 
 namespace mapnik {
 
-metawriter_json::metawriter_json(metawriter_properties dflt_properties, std::string fn)
-    : metawriter(dflt_properties), fn_(fn), count(0)
+UnicodeString const& metawriter_property_map::operator[](std::string const& key) const
 {
-
+    std::map<std::string, UnicodeString>::const_iterator it;
+    it = m_.find(key);
+    if (it != m_.end()) return not_found_;
+    return (*it).second;
 }
 
-metawriter_json::~metawriter_json()
+void metawriter_json_stream::start(metawriter_property_map const& properties)
 {
-    stop();
+    assert(f_);
+    *f_ << "{ \"type\": \"FeatureCollection\", \"features\": [\n";
 }
 
-void metawriter_json::start()
+void metawriter_json_stream::stop()
 {
-    if (!fn_.empty())
-    {
-        if (f)
-        {
-            std::cerr << "ERROR: GeoJSON metawriter is already active!\n";
-            return;
-        }
-        f = new std::fstream(fn_.c_str(), std::fstream::out | std::fstream::trunc);
-        if (f->fail()) perror((std::string("Failed to open file ") + fn_).c_str());
-    }
-    assert(f);
-    *f << "{ \"type\": \"FeatureCollection\", \"features\": [\n";
-}
-
-void metawriter_json::stop()
-{
-    if (f) *f << " ] }\n";
-    if (f && !fn_.empty())
-    {
-        dynamic_cast<std::fstream *>(f)->close();
-        f = 0;
+    if (f_) {
+        *f_ << " ] }\n";
     }
 }
 
-void metawriter_json::add_box(box2d<double> box, Feature const &feature,
+metawriter_json_stream::metawriter_json_stream(metawriter_properties dflt_properties)
+    : metawriter(dflt_properties), count(0) {}
+
+void metawriter_json_stream::add_box(box2d<double> box, Feature const &feature,
                               proj_transform const& prj_trans, CoordTransform const &t,
                               const metawriter_properties& properties)
 {
@@ -85,7 +71,7 @@ void metawriter_json::add_box(box2d<double> box, Feature const &feature,
 
     if (props.empty())
     {
-        std::cerr << "WARNING: No expression available for GeoJSON metawriter.\n";
+        std::cerr << "WARNING: No properties available for GeoJSON metawriter.\n";
     }
 
     /* Coordinate transform in renderer:
@@ -112,8 +98,8 @@ void metawriter_json::add_box(box2d<double> box, Feature const &feature,
     double maxx = box.maxx();
     double maxy = box.maxy();
 
-    if (count++) *f << ",\n";
-    *f << std::fixed << std::setprecision(8) << "{ \"type\": \"Feature\",\n  \"geometry\": { \"type\": \"Polygon\",\n    \"coordinates\": [ [ [" <<
+    if (count++) *f_ << ",\n";
+    *f_ << std::fixed << std::setprecision(8) << "{ \"type\": \"Feature\",\n  \"geometry\": { \"type\": \"Polygon\",\n    \"coordinates\": [ [ [" <<
             minx << ", " << miny << "], [" <<
             maxx << ", " << miny << "], [" <<
             maxx << ", " << maxy << "], [" <<
@@ -130,13 +116,46 @@ void metawriter_json::add_box(box2d<double> box, Feature const &feature,
             //Property found
             text = boost::replace_all_copy(boost::replace_all_copy(itr->second.to_string(), "\\", "\\\\"), "\"", "\\\"");
         }
-        if (i++) *f << ",";
-        *f << "\n    \"" << p << "\":\"" << text << "\"";
+        if (i++) *f_ << ",";
+        *f_ << "\n    \"" << p << "\":\"" << text << "\"";
     }
 
-    *f << "\n} }";
+    *f_ << "\n} }";
 }
 
+
+/********************************************************************************************/
+
+metawriter_json::metawriter_json(metawriter_properties dflt_properties, path_expression_ptr fn)
+    : metawriter_json_stream(dflt_properties), fn_(fn) {}
+
+
+metawriter_json::~metawriter_json()
+{
+    stop();
+}
+
+void metawriter_json::start(metawriter_property_map const& properties)
+{
+    std::string filename =
+        path_processor<metawriter_property_map>::evaluate(*fn_, properties);
+#ifdef MAPNIK_DEBUG
+    std::clog << "Metawriter JSON: filename=" << filename << "\n";
+#endif
+    f_.open(filename.c_str(), std::fstream::out | std::fstream::trunc);
+    if (f_.fail()) perror((std::string("Metawriter JSON: Failed to open file ") + filename).c_str());
+    set_stream(&f_);
+    metawriter_json_stream::start(properties);
+}
+
+
+void metawriter_json::stop()
+{
+    metawriter_json_stream::stop();
+    if (f_.is_open()) f_.close();
+}
+
+
 metawriter_properties::metawriter_properties(boost::optional<std::string> str)
 {
     if (str) {