change logger style to slf4j for cleaner code

webmagic-core/src/main/java/us/codecraft/webmagic/downloader/HttpClientDownloader.java

@@ -74,7 +74,7 @@ public class HttpClientDownloader extends AbstractDownloader {
         } else {
             acceptStatCode = Sets.newHashSet(200);
         }
-        logger.info("downloading page " + request.getUrl());
+        logger.info("downloading page {}" , request.getUrl());
         RequestBuilder requestBuilder = RequestBuilder.get().setUri(request.getUrl());
         if (headers != null) {
             for (Map.Entry<String, String> headerEntry : headers.entrySet()) {

webmagic-extension/src/main/java/us/codecraft/webmagic/model/PageModelExtractor.java

@@ -340,9 +340,7 @@ class PageModelExtractor {
     private Object convert(String value, ObjectFormatter objectFormatter) {
         try {
             Object format = objectFormatter.format(value);
-            if (logger.isDebugEnabled()) {
-                logger.debug("String " + value + " is converted to " + format);
-            }
+            logger.debug("String {} is converted to {}", value, format);
             return format;
         } catch (Exception e) {
             logger.error("convert " + value + " to " + objectFormatter.clazz() + " error!", e);

webmagic-extension/src/main/java/us/codecraft/webmagic/scheduler/FileCacheQueueScheduler.java

@@ -149,9 +149,7 @@ public class FileCacheQueueScheduler implements Scheduler {
         if (!inited.get()) {
             init(task);
         }
-        if (logger.isDebugEnabled()) {
-            logger.debug("push to queue " + request.getUrl());
-        }
+        logger.debug("push to queue {}" , request.getUrl());
         if (urls.add(request.getUrl())) {
             queue.add(request);
             fileUrlWriter.println(request.getUrl());