MAPREDUCE-7251: Present friendlier error for non-static main

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/RunJar.java

@@ -24,6 +24,7 @@
 import java.io.OutputStream;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLClassLoader;
@@ -315,6 +316,11 @@ public void run() {
     Thread.currentThread().setContextClassLoader(loader);
     Class<?> mainClass = Class.forName(mainClassName, true, loader);
     Method main = mainClass.getMethod("main", String[].class);
+
+    if (!Modifier.isStatic(main.getModifiers())) {
+      throw new IOException("Method main must be static: " + mainClassName);
+    }
+
     List<String> newArgsSubList = Arrays.asList(args)
         .subList(firstArg, args.length);
     String[] newArgs = newArgsSubList

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/util/NonStaticMain.java

@@ -0,0 +1,27 @@
+/**
+ * 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.hadoop.util;
+
+/**
+ * NonStaticMain is used to test classes which provide a main which
+ * isn't static.
+ */
+public class NonStaticMain {
+  public void main(String[] args) {
+  }
+}

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/util/TestRunJar.java

@@ -57,7 +57,7 @@ public class TestRunJar {
   private static final int BUFF_SIZE = 2048;
   private File TEST_ROOT_DIR;
 
-  private static final String TEST_JAR_NAME="test-runjar.jar";
+  private static final String TEST_JAR_NAME = "test-runjar.jar";
   private static final String TEST_JAR_2_NAME = "test-runjar2.jar";
   private static final long MOCKED_NOW = 1_460_389_972_000L;
   private static final long MOCKED_NOW_PLUS_TWO_SEC = MOCKED_NOW + 2_000;
@@ -297,4 +297,35 @@ public void testUnJar2() throws IOException {
           "would create file outside of", e);
     }
   }
-}
+
+  /**
+   * Tests that RunJar errors appropriately for classes with non-static main
+   * methods.
+   */
+  @Test
+  public void testRunNonStaticMain() throws Throwable {
+    RunJar runJar = spy(new RunJar());
+    // enable the client classloader
+    when(runJar.useClientClassLoader()).thenReturn(true);
+    // set the system classes and blacklist the test main class so it
+    // can be loaded by the application classloader
+    String mainCls = NonStaticMain.class.getName();
+    String systemClasses = "-" + mainCls + ","
+        + ApplicationClassLoader.SYSTEM_CLASSES_DEFAULT;
+    when(runJar.getSystemClasses()).thenReturn(systemClasses);
+
+    // create the test jar
+    File testJar = JarFinder.makeClassLoaderTestJar(this.getClass(),
+        TEST_ROOT_DIR, TEST_JAR_2_NAME, BUFF_SIZE, mainCls);
+    // form the args
+    String[] args = new String[] {testJar.getAbsolutePath(), mainCls};
+
+    // run RunJar
+    try {
+      runJar.run(args);
+      fail("run should throw IOException.");
+    } catch (IOException e) {
+      GenericTestUtils.assertExceptionContains("Method main must be static", e);
+    }
+  }
+}