Fix SimpleDateFormat concurrent issue in CspFormatter of sentinel-core (#548)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/log/CspFormatter.java

@@ -1,17 +1,14 @@
 /*
  * Copyright 1999-2018 Alibaba Group Holding Ltd.
  *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
  *
- *      http://www.apache.org/licenses/LICENSE-2.0
+ * http://www.apache.org/licenses/LICENSE-2.0
  *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
  */
 package com.alibaba.csp.sentinel.log;
 
@@ -28,10 +25,16 @@ import java.util.logging.LogRecord;
  */
 class CspFormatter extends Formatter {
 
-    private final DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
+    private final ThreadLocal<SimpleDateFormat> dateFormatThreadLocal = new ThreadLocal<SimpleDateFormat>() {
+        @Override
+        public SimpleDateFormat initialValue() {
+            return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
+        }
+    };
 
     @Override
     public String format(LogRecord record) {
+        final DateFormat df = dateFormatThreadLocal.get();
         StringBuilder builder = new StringBuilder(1000);
         builder.append(df.format(new Date(record.getMillis()))).append(" ");
         builder.append(formatMessage(record));