Set charset in web hook and replace magic number (#2986)

oap-server/server-alarm-plugin/src/main/java/org/apache/skywalking/oap/server/core/alarm/provider/WebhookCallback.java

@@ -21,7 +21,12 @@ package org.apache.skywalking.oap.server.core.alarm.provider;
 import com.google.gson.Gson;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
+import java.nio.charset.StandardCharsets;
 import java.util.List;
+
+import io.netty.handler.codec.http.HttpHeaderValues;
+import org.apache.http.HttpHeaders;
+import org.apache.http.HttpStatus;
 import org.apache.http.StatusLine;
 import org.apache.http.client.ClientProtocolException;
 import org.apache.http.client.config.RequestConfig;
@@ -58,7 +63,8 @@ public class WebhookCallback implements AlarmCallback {
             .setSocketTimeout(HTTP_SOCKET_TIMEOUT).build();
     }
 
-    @Override public void doAlarm(List<AlarmMessage> alarmMessage) {
+    @Override
+    public void doAlarm(List<AlarmMessage> alarmMessage) {
         if (remoteEndpoints.size() == 0) {
             return;
         }
@@ -68,16 +74,16 @@ public class WebhookCallback implements AlarmCallback {
             remoteEndpoints.forEach(url -> {
                 HttpPost post = new HttpPost(url);
                 post.setConfig(requestConfig);
-                post.setHeader("Accept", "application/json");
-                post.setHeader("Content-type", "application/json");
+                post.setHeader(HttpHeaders.ACCEPT, HttpHeaderValues.APPLICATION_JSON.toString());
+                post.setHeader(HttpHeaders.CONTENT_TYPE, HttpHeaderValues.APPLICATION_JSON.toString());
 
-                StringEntity entity = null;
+                StringEntity entity;
                 try {
-                    entity = new StringEntity(gson.toJson(alarmMessage));
+                    entity = new StringEntity(gson.toJson(alarmMessage), StandardCharsets.UTF_8);
                     post.setEntity(entity);
                     CloseableHttpResponse httpResponse = httpClient.execute(post);
                     StatusLine statusLine = httpResponse.getStatusLine();
-                    if (statusLine != null && statusLine.getStatusCode() != 200) {
+                    if (statusLine != null && statusLine.getStatusCode() != HttpStatus.SC_OK) {
                         logger.error("send alarm to " + url + " failure. Response code: " + statusLine.getStatusCode());
                     }
                 } catch (UnsupportedEncodingException e) {