Add test utility for generating Jar files with compiled classes.

This was requested by a few different people and may be generally
useful, so I'd like to contribute this and not block on a different
PR for it to get in.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #326 from pwendell/class-loader-test-utils and squashes the following commits:

ff3e88e [Patrick Wendell] Add test utility for generating Jar files with compiled classes.

core/src/test/scala/org/apache/spark/TestUtils.scala

+/*
+ * 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
+
+import java.io.{File, FileInputStream, FileOutputStream}
+import java.net.{URI, URL}
+import java.util.jar.{JarEntry, JarOutputStream}
+
+import scala.collection.JavaConversions._
+
+import javax.tools.{JavaFileObject, SimpleJavaFileObject, ToolProvider}
+import com.google.common.io.Files
+
+object TestUtils {
+
+  /**
+   * Create a jar that defines classes with the given names.
+   *
+   * Note: if this is used during class loader tests, class names should be unique
+   * in order to avoid interference between tests.
+   */
+  def createJarWithClasses(classNames: Seq[String]): URL = {
+    val tempDir = Files.createTempDir()
+    val files = for (name <- classNames) yield createCompiledClass(name, tempDir)
+    val jarFile = new File(tempDir, "testJar-%s.jar".format(System.currentTimeMillis()))
+    createJar(files, jarFile)
+  }
+
+  /**
+   * Create a jar file that contains this set of files. All files will be located at the root
+   * of the jar.
+   */
+  def createJar(files: Seq[File], jarFile: File): URL = {
+    val jarFileStream = new FileOutputStream(jarFile)
+    val jarStream = new JarOutputStream(jarFileStream, new java.util.jar.Manifest())
+
+    for (file <- files) {
+      val jarEntry = new JarEntry(file.getName)
+      jarStream.putNextEntry(jarEntry)
+
+      val in = new FileInputStream(file)
+      val buffer = new Array[Byte](10240)
+      var nRead = 0
+      while (nRead <= 0) {
+        nRead = in.read(buffer, 0, buffer.length)
+        jarStream.write(buffer, 0, nRead)
+      }
+      in.close()
+    }
+    jarStream.close()
+    jarFileStream.close()
+
+    jarFile.toURI.toURL
+  }
+
+  // Adapted from the JavaCompiler.java doc examples
+  private val SOURCE = JavaFileObject.Kind.SOURCE
+  private def createURI(name: String) = {
+    URI.create(s"string:///${name.replace(".", "/")}${SOURCE.extension}")
+  }
+
+  private class JavaSourceFromString(val name: String, val code: String)
+    extends SimpleJavaFileObject(createURI(name), SOURCE) {
+    override def getCharContent(ignoreEncodingErrors: Boolean) = code
+  }
+
+  /** Creates a compiled class with the given name. Class file will be placed in destDir. */
+  def createCompiledClass(className: String, destDir: File): File = {
+    val compiler = ToolProvider.getSystemJavaCompiler
+    val sourceFile = new JavaSourceFromString(className, s"public class $className {}")
+
+    // Calling this outputs a class file in pwd. It's easier to just rename the file than
+    // build a custom FileManager that controls the output location.
+    compiler.getTask(null, null, null, null, null, Seq(sourceFile)).call()
+
+    val fileName = className + ".class"
+    val result = new File(fileName)
+    if (!result.exists()) throw new Exception("Compiled file not found: " + fileName)
+    val out = new File(destDir, fileName)
+    result.renameTo(out)
+    out
+  }
+}