Smarter classpath detection on Java9

On Java9 cannot rely on URLClassLoader being found from which
to determine classpath so use the environment variable. This may
have issues if loaders are being constructed that specifically
deviate from the java.class.path.

weaver/src/org/aspectj/weaver/tools/WeavingAdaptor.java

@@ -20,7 +20,17 @@ import java.io.PrintWriter;
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.security.ProtectionDomain;
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Properties;
+import java.util.Set;
+import java.util.StringTokenizer;
 
 import org.aspectj.bridge.AbortException;
 import org.aspectj.bridge.IMessage;
@@ -134,11 +144,20 @@ public class WeavingAdaptor implements IMessageContext {
 				warn("cannot determine classpath");
 			}
 		}
-		list.addAll(0, makeClasspath(System.getProperty("sun.boot.class.path")));
-		// On Java9 the sun.boot.class.path won't be set. System classes accessible through JRT filesystem 
+		// On Java9 it is possible to fail to find a URLClassLoader from which to derive a suitable classpath
+		// For now we can determine it from the java.class.path:
         if (LangUtil.is19VMOrGreater()) {
-        		list.add(0, LangUtil.getJrtFsFilePath());
+	    		list.add(0, LangUtil.getJrtFsFilePath());
+			List<String> javaClassPathEntries = makeClasspath(System.getProperty("java.class.path"));
+			for (int i=javaClassPathEntries.size()-1;i>=0;i--) {
+				String javaClassPathEntry = javaClassPathEntries.get(i);
+				if (!list.contains(javaClassPathEntry)) {
+					list.add(0,javaClassPathEntry);
+				}
+			}
         }
+		// On Java9 the sun.boot.class.path won't be set. System classes accessible through JRT filesystem 
+		list.addAll(0, makeClasspath(System.getProperty("sun.boot.class.path")));
 		return list;
 	}