Double-checked locking when loading Jwk definitions

Fixes gh-1405

Author: leomillon

spring-security-oauth2/src/main/java/org/springframework/security/oauth2/provider/token/store/jwk/JwkDefinitionSource.java

@@ -85,6 +85,10 @@ JwkDefinitionHolder getDefinitionLoadIfNecessary(String keyId) {
 			return result;
 		}
 		synchronized (this.jwkDefinitions) {
+			result = this.getDefinition(keyId);
+			if (result != null) {
+				return result;
+			}
 			this.jwkDefinitions.clear();
 			for (URL jwkSetUrl : jwkSetUrls) {
 				this.jwkDefinitions.putAll(loadJwkDefinitions(jwkSetUrl));