Fixed 'PackageInternalsFinder.fuse' throws an exception when jar in jar is empty (#2941)

memorycompiler/src/main/java/com/taobao/arthas/compiler/PackageInternalsFinder.java

@@ -98,18 +98,22 @@ public class PackageInternalsFinder {
 
             JarURLConnection jarConn = (JarURLConnection) packageFolderURL.openConnection();
             String rootEntryName = jarConn.getEntryName();
-            int rootEnd = rootEntryName.length() + 1;
 
-            Enumeration<JarEntry> entryEnum = jarConn.getJarFile().entries();
+            if (rootEntryName != null) {
-            while (entryEnum.hasMoreElements()) {
+                //可能为 null（自己没有类文件时）
-                JarEntry jarEntry = entryEnum.nextElement();
+                int rootEnd = rootEntryName.length() + 1;
-                String name = jarEntry.getName();
-                if (name.startsWith(rootEntryName) && name.indexOf('/', rootEnd) == -1 && name.endsWith(CLASS_FILE_EXTENSION)) {
-                    URI uri = URI.create(jarUri + "!/" + name);
-                    String binaryName = name.replaceAll("/", ".");
-                    binaryName = binaryName.replaceAll(CLASS_FILE_EXTENSION + "$", "");
 
-                    result.add(new CustomJavaFileObject(binaryName, uri));
+                Enumeration<JarEntry> entryEnum = jarConn.getJarFile().entries();
+                while (entryEnum.hasMoreElements()) {
+                    JarEntry jarEntry = entryEnum.nextElement();
+                    String name = jarEntry.getName();
+                    if (name.startsWith(rootEntryName) && name.indexOf('/', rootEnd) == -1 && name.endsWith(CLASS_FILE_EXTENSION)) {
+                        URI uri = URI.create(jarUri + "!/" + name);
+                        String binaryName = name.replaceAll("/", ".");
+                        binaryName = binaryName.replaceAll(CLASS_FILE_EXTENSION + "$", "");
+
+                        result.add(new CustomJavaFileObject(binaryName, uri));
+                    }
                 }
             }
         } catch (Exception e) {