Use unsigned byte string representation for bootstrap REST API

leshan-bsserver-demo/src/main/java/org/eclipse/leshan/server/bootstrap/demo/servlet/BootstrapServlet.java

@@ -18,6 +18,7 @@
 
 import java.io.IOException;
 import java.io.InputStreamReader;
+import java.lang.reflect.Type;
 
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
@@ -31,6 +32,14 @@
 import org.eclipse.leshan.server.bootstrap.demo.ConfigurationChecker.ConfigurationException;
 
 import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonParseException;
+import com.google.gson.JsonPrimitive;
+import com.google.gson.JsonSerializationContext;
+import com.google.gson.JsonSerializer;
 import com.google.gson.JsonSyntaxException;
 
 /**
@@ -39,14 +48,29 @@
 @SuppressWarnings("serial")
 public class BootstrapServlet extends HttpServlet {
 
+    private static class SignedByteUnsignedByteAdapter implements JsonSerializer<Byte>, JsonDeserializer<Byte> {
+
+        @Override
+        public Byte deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
+                throws JsonParseException {
+            return json.getAsByte();
+        }
+
+        @Override
+        public JsonElement serialize(Byte src, Type typeOfSrc, JsonSerializationContext context) {
+            return new JsonPrimitive((int) src & 0xff);
+        }
+    }
+
     private final BootstrapStoreImpl bsStore;
 
     private final Gson gson;
 
     public BootstrapServlet(BootstrapStoreImpl bsStore) {
         this.bsStore = bsStore;
 
-        this.gson = new Gson();
+        this.gson = new GsonBuilder().registerTypeHierarchyAdapter(Byte.class, new SignedByteUnsignedByteAdapter())
+                .create();
     }
 
     @Override