[SPARK-21840][core] Add trait that allows conf to be directly set in application.

core/src/main/scala/org/apache/spark/deploy/SparkApplication.scala

@@ -0,0 +1,55 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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
+ *
+ * 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 org.apache.spark.deploy
+
+import java.lang.reflect.Modifier
+
+import org.apache.spark.SparkConf
+
+/**
+ * Entry point for a Spark application. Implementations must provide a no-argument constructor.
+ */
+private[spark] trait SparkApplication {
+
+  def start(args: Array[String], conf: SparkConf): Unit
+
+}
+
+/**
+ * Implementation of SparkApplication that wraps a standard Java class with a "main" method.
+ *
+ * Configuration is propagated to the application via system properties, so running multiple
+ * of these in the same JVM may lead to undefined behavior due to configuration leaks.
+ */
+private[deploy] class JavaMainApplication(klass: Class[_]) extends SparkApplication {
+
+  override def start(args: Array[String], conf: SparkConf): Unit = {
+    val mainMethod = klass.getMethod("main", new Array[String](0).getClass)
+    if (!Modifier.isStatic(mainMethod.getModifiers)) {
+      throw new IllegalStateException("The main method in the given main class must be static")
+    }
+
+    val sysProps = conf.getAll.toMap
+    sysProps.foreach { case (k, v) =>
+      sys.props(k) = v
+    }
+
+    mainMethod.invoke(null, args)
+  }
+
+}