Improvements to work with Java 9

Changes include:

* update to dynapath 0.2.5 (0.2.4 was broken when AOT'd under < Java 9,
  but used under Java 9)
* conditionally seal the AppClassLoader if it is available (it isn't
  under Java 9)
* seal the new ParentClassLoader from boot-bin
* use a URLClassLoader subclass that exposes .addURL so it can be
  modified (we can't call .setAccessible on URLClassLoader.addURL by
  default under Java 9) as the highest loader in the shim

boot/base/src/main/java/boot/AddableClassLoader.java

@@ -0,0 +1,14 @@
+package boot;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+// Allows us to have a modifiable ClassLoader without having to call
+// .setAccessible on URLClassLoader.addURL(), since that's not allowed
+// by default under Java 9
+public class AddableClassLoader extends URLClassLoader {
+    public AddableClassLoader(URL[] urls, ClassLoader parent) {
+        super(urls, parent); }
+
+    public void addURL(URL url) {
+	super.addURL(url); }}

boot/base/src/main/java/boot/App.java

@@ -7,7 +7,6 @@
 import java.nio.channels.FileChannel;
 import java.lang.ref.WeakReference;
 import java.net.URL;
-import java.net.URLClassLoader;
 import java.util.Map;
 import java.util.Date;
 import java.util.UUID;
@@ -306,7 +305,7 @@ public class App {
 
         for (int i=0; i<jarFiles.length; i++) urls[i] = jarFiles[i].toURI().toURL();
 
-        ClassLoader cl = new URLClassLoader(urls, App.class.getClassLoader());
+        ClassLoader cl = new AddableClassLoader(urls, App.class.getClassLoader());
         ClojureRuntimeShim rt = ClojureRuntimeShim.newRuntime(cl);
 
         rt.setName(name != null ? name : "anonymous");
@@ -319,6 +318,7 @@ public class App {
 
         rt.require("boot.pod");
         rt.invoke("boot.pod/seal-app-classloader");
+        rt.invoke("boot.pod/extend-addable-classloader");
         rt.invoke("boot.pod/set-data!", data);
         rt.invoke("boot.pod/set-pods!", pods);
         rt.invoke("boot.pod/set-this-pod!", new WeakReference<ClojureRuntimeShim>(rt));

boot/pod/project.clj

@@ -17,5 +17,5 @@
                    :url "http://www.eclipse.org/legal/epl-v10.html"}
   :dependencies   [[boot/base                               ~version :scope "provided"]
                    [org.clojure/clojure                     "1.6.0"  :scope "provided"]
-                   [org.tcrawley/dynapath                   "0.2.4"  :scope "compile"]
+                   [org.tcrawley/dynapath                   "0.2.5"  :scope "compile"]
                    [org.projectodd.shimdandy/shimdandy-impl "1.2.0"  :scope "compile"]])

boot/pod/src/boot/pod.clj

@@ -26,11 +26,32 @@
   (let [[group artifact] ((juxt namespace name) sym)]
     [(or group artifact) artifact]))
 
+(defn extend-addable-classloader
+  "Opens up the class loader used to create the shim for
+  modification. Once seal-app-classloader is called, this will be the
+  highest class loader user code can modify.
+
+  This function is called during Boot's bootstrapping phase, and shouldn't
+  be needed in client code under normal circumstances."
+  []
+  (extend boot.AddableClassLoader
+    cp/DynamicClasspath
+    (assoc dynapath.dynamic-classpath/base-readable-addable-classpath
+      :classpath-urls #(seq (.getURLs %))
+      :add-classpath-url #(.addURL %1 %2))))
+
+(def sealed-classloader-fns
+  (assoc cp/base-readable-addable-classpath
+    :classpath-urls #(seq (.getURLs %))
+    :can-add? (constantly false)))
+
 (defn seal-app-classloader
-  "Implements the DynamicClasspath protocol to the AppClassLoader class such
-  that instances of this class will refuse attempts at runtime modification
-  by libraries that do so via dynapath[1]. The system class loader is of the
-  type AppClassLoader.
+  "Implements the DynamicClasspath protocol to the AppClassLoader and
+  boot's ParentClassLoader classes such that instances of those
+  classes will refuse attempts at runtime modification by libraries
+  that do so via dynapath[1]. The system class loader is of the type
+  AppClassLoader under Java < 9, and the top-level class loader used
+  by boot is a ParentClassLoader.
 
   The purpose of this is to ensure that Clojure libraries do not pollute the
   higher-level class loaders with classes and interfaces created dynamically
@@ -42,11 +63,21 @@
   [1]: https://github.com/tobias/dynapath
   [2]: https://github.com/clojure-emacs/cider-nrepl/blob/36333cae25fd510747321f86e2f0369fcb7b4afd/README.md#with-jboss-asjboss-eapwildfly"
   []
-  (extend sun.misc.Launcher$AppClassLoader
-    cp/DynamicClasspath
-    (assoc cp/base-readable-addable-classpath
-      :classpath-urls #(seq (.getURLs %))
-      :can-add? (constantly false))))
+  (try
+    ;; this import will fail if the user doesn't have a new enough boot.sh
+    (import boot.bin.ParentClassLoader)
+    (eval '(extend boot.bin.ParentClassLoader
+             dynapath.dynamic-classpath/DynamicClasspath
+             boot.pod/sealed-classloader-fns))
+    (catch Exception _))
+
+  (try
+    ;; this import will fail if the user is using Java 9
+    (import sun.misc.Launcher$AppClassLoader)
+    (eval '(extend sun.misc.Launcher$AppClassLoader
+             dynapath.dynamic-classpath/DynamicClasspath
+             boot.pod/sealed-classloader-fns))
+    (catch Exception _)))
 
 (defn ^{:boot/from :cemerick/pomegranate} classloader-hierarchy
   "Returns a seq of classloaders, with the tip of the hierarchy first.