diff -r 4ebc2e2fb97c -r 71c04702a3d5 src/java.base/share/classes/jdk/internal/module/ModuleInfoWriter.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/java.base/share/classes/jdk/internal/module/ModuleInfoWriter.java	Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package jdk.internal.module;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.lang.module.ModuleDescriptor;
+import java.nio.ByteBuffer;
+import java.util.stream.Stream;
+
+import jdk.internal.org.objectweb.asm.ClassWriter;
+import jdk.internal.org.objectweb.asm.Opcodes;
+
+import static jdk.internal.module.ClassFileAttributes.*;
+import static jdk.internal.module.ClassFileConstants.ACC_MODULE;
+
+/**
+ * Utility class to write a ModuleDescriptor as a module-info.class.
+ */
+
+public final class ModuleInfoWriter {
+
+    private ModuleInfoWriter() { }
+
+    /**
+     * Writes the given module descriptor to a module-info.class file,
+     * returning it in a byte array.
+     */
+    private static byte[] toModuleInfo(ModuleDescriptor md, ModuleTarget target) {
+        ClassWriter cw = new ClassWriter(0);
+        cw.visit(Opcodes.V1_9, ACC_MODULE, "module-info", null, null, null);
+        cw.visitAttribute(new ModuleAttribute(md));
+
+        // for tests: write the ModulePackages attribute when there are packages
+        // that aren't exported or open
+        Stream<String> exported = md.exports().stream()
+                .map(ModuleDescriptor.Exports::source);
+        Stream<String> open = md.opens().stream()
+                .map(ModuleDescriptor.Opens::source);
+        long exportedOrOpen = Stream.concat(exported, open).distinct().count();
+        if (md.packages().size() > exportedOrOpen)
+            cw.visitAttribute(new ModulePackagesAttribute(md.packages()));
+
+        // write ModuleMainClass if the module has a main class
+        md.mainClass().ifPresent(mc -> cw.visitAttribute(new ModuleMainClassAttribute(mc)));
+
+        // write ModuleTarget if there is a target platform
+        if (target != null) {
+            cw.visitAttribute(new ModuleTargetAttribute(target.targetPlatform()));
+        }
+
+        cw.visitEnd();
+        return cw.toByteArray();
+    }
+
+    /**
+     * Writes a module descriptor to the given output stream as a
+     * module-info.class.
+     */
+    public static void write(ModuleDescriptor descriptor,
+                             ModuleTarget target,
+                             OutputStream out)
+        throws IOException
+    {
+        byte[] bytes = toModuleInfo(descriptor, target);
+        out.write(bytes);
+    }
+
+    /**
+     * Writes a module descriptor to the given output stream as a
+     * module-info.class.
+     */
+    public static void write(ModuleDescriptor descriptor, OutputStream out)
+        throws IOException
+    {
+        write(descriptor, null, out);
+    }
+
+    /**
+     * Returns a {@code ByteBuffer} containing the given module descriptor
+     * in module-info.class format.
+     */
+    public static ByteBuffer toByteBuffer(ModuleDescriptor descriptor) {
+        byte[] bytes = toModuleInfo(descriptor, null);
+        return ByteBuffer.wrap(bytes);
+    }
+}