Add pre-loading setting to load OpenAPI early

Add configuration setting to `SpringDocConfigProperties` to enable pre-loading of OpenAPI. This setting is false by default. When instantiating `AbstractOpenApiResource` the OpenAPI is generated in a separate thread. Fixes #850

Author: Dodecahedra

springdoc-openapi-common/src/main/java/org/springdoc/api/AbstractOpenApiResource.java

@@ -37,6 +37,7 @@
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
+import java.util.concurrent.Executors;
 import java.util.stream.Collectors;
 
 import com.fasterxml.jackson.annotation.JsonView;
@@ -200,6 +201,10 @@ protected AbstractOpenApiResource(String groupName, ObjectFactory<OpenAPIBuilder
 			operationCustomizers.get().removeIf(Objects::isNull);
 		this.operationCustomizers = operationCustomizers;
 		this.actuatorProvider = actuatorProvider;
+		if (springDocConfigProperties.isPreLoadingEnabled()) {
+			Executors.newSingleThreadExecutor()
+					.execute(this::getOpenApi);
+		}
 	}
 
 	/**

springdoc-openapi-common/src/main/java/org/springdoc/core/SpringDocConfigProperties.java

@@ -130,6 +130,11 @@ public class SpringDocConfigProperties {
 	 */
 	private boolean showLoginEndpoint;
 
+	/**
+	 * Allow for pre-loading OpenAPI
+	 */
+	private boolean preLoadingEnabled = false;
+
 	/**
 	 * Is use fqn boolean.
 	 *
@@ -472,6 +477,14 @@ public void setWriterWithDefaultPrettyPrinter(boolean writerWithDefaultPrettyPri
 		this.writerWithDefaultPrettyPrinter = writerWithDefaultPrettyPrinter;
 	}
 
+	public void setPreLoadingEnabled(boolean preLoadingEnabled) {
+		this.preLoadingEnabled = preLoadingEnabled;
+	}
+
+	public boolean isPreLoadingEnabled() {
+		return preLoadingEnabled;
+	}
+
 	/**
 	 * The type Model converters.
 	 */