[Improvement][common] HttpClient singleton mode #3332 (#3350)

* Optimize the httpclient

* Optimize the httpclient singleton

* Merge branch 'github-local/dev' into feature/httpClientsingleton

# Conflicts:
#	dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/utils/HttpUtils.java
#	dolphinscheduler-common/src/test/java/org/apache/dolphinscheduler/common/utils/HttpUtilsTest.java

* Merge branch 'github-local/dev' into feature/httpClientsingleton

# Conflicts:
#	dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/utils/HttpUtils.java
#	dolphinscheduler-common/src/test/java/org/apache/dolphinscheduler/common/utils/HttpUtilsTest.java

* Merge branch 'github-local/dev' into feature/httpClientsingleton

# Conflicts:
#	dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/utils/HttpUtils.java
#	dolphinscheduler-common/src/test/java/org/apache/dolphinscheduler/common/utils/HttpUtilsTest.java

dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/utils/HttpUtils.java

@@ -53,13 +53,19 @@ public class HttpUtils {
 
 	public static final Logger logger = LoggerFactory.getLogger(HttpUtils.class);
 
+	private HttpUtils() {
 
-	private static CloseableHttpClient httpClient;
+	}
 
-	private HttpUtils() {
+	public static CloseableHttpClient getInstance(){
+		return HttpClientInstance.httpClient;
+	}
 
+	private static class HttpClientInstance{
+		private static final CloseableHttpClient httpClient = HttpClients.custom().setConnectionManager(cm).setDefaultRequestConfig(requestConfig).build();
 	}
 
+
 	private static PoolingHttpClientConnectionManager cm;
 
 	private static SSLContext ctx = null;
@@ -112,20 +118,14 @@ public class HttpUtils {
 	}
 
 
-	public static synchronized CloseableHttpClient getHttpClient() {
-		if (null == httpClient) {
-			httpClient = HttpClients.custom().setConnectionManager(cm).setDefaultRequestConfig(requestConfig).build();
-			;
-		}
-		return httpClient;
-	}
+
 	/**
 	 * get http request content
 	 * @param url url
 	 * @return http get request response content
 	 */
 	public static String get(String url){
-		CloseableHttpClient httpclient = HttpUtils.getHttpClient();
+		CloseableHttpClient httpclient = HttpUtils.getInstance();
 
 		HttpGet httpget = new HttpGet(url);
 		String responseContent = null;

dolphinscheduler-common/src/test/java/org/apache/dolphinscheduler/common/utils/HttpUtilsTest.java

@@ -46,8 +46,8 @@ public class HttpUtilsTest {
 
 	@Test
 	public void testGetHttpClient() {
-		CloseableHttpClient httpClient1 = HttpUtils.getHttpClient();
-		CloseableHttpClient httpClient2 = HttpUtils.getHttpClient();
+		CloseableHttpClient httpClient1 = HttpUtils.getInstance();
+		CloseableHttpClient httpClient2 = HttpUtils.getInstance();
 		Assert.assertEquals(httpClient1, httpClient2);
 	}
 }