Avoid adding inner classes of java agent jars to the classpath

The New Relic agent jar contains embedded sub jars, it gets confused
if these are loaded both by the New Relic framework and later by
EmJar. If recursion into jars that have manifests declaring
Premain-Class: is desired, add these jars to the emjar.class.path
property explicitly.

Author: argggh

emjar/src/main/java/com/comoyo/emjar/EmJarClassLoader.java

@@ -33,6 +33,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.jar.JarEntry;
 import java.util.jar.JarFile;
+import java.util.jar.Manifest;
 import java.io.File;
 import java.io.IOException;
 
@@ -56,6 +57,16 @@
  * must for obvious reasons be stored directly inside the bundle jar;
  * i.e not within an embedded jar.)
  *
+ * <p/>
+ * All jars given as part of the classpath will be recursively
+ * inspected for embedded jars, and these will in turn be added to the
+ * classpath.  Jar files on the classpath having manifests declaring
+ * <strong><code>Premain-Class:</code></strong> (i.e Java Agent jars)
+ * will not be inspected by default, but can be added to the
+ * EmJar-specific classpath contained in the
+ * <strong><code>emjar.class.path</code></strong> property if this is
+ * desired.
+
  * <p/>
  * For a less manual approach that embeds all configuration in the
  * bundled jar, see {@link Boot}.
@@ -115,16 +126,19 @@ private static URL[] getClassPath(final Properties props, final Handler handler)
         DEBUG = "true".equalsIgnoreCase(props.getProperty(EMJAR_LOG_DEBUG_PROP, ""));
 
         final ArrayList<URL> urls = new ArrayList<>();
-        addClassPathUrls(props.getProperty(JAVA_CLASS_PATH_PROP), urls, handler);
-        addClassPathUrls(props.getProperty(EMJAR_CLASS_PATH_PROP), urls, handler);
+        addClassPathUrls(props.getProperty(JAVA_CLASS_PATH_PROP), urls, handler, false);
+        addClassPathUrls(props.getProperty(EMJAR_CLASS_PATH_PROP), urls, handler, true);
         if (DEBUG) {
             System.err.println("EmJar: using classpath " + urls);
         }
         return urls.toArray(new URL[0]);
     }
 
     private static void addClassPathUrls(
-            final String classPath, final List<URL> urls, final Handler handler) {
+            final String classPath,
+            final List<URL> urls,
+            final Handler handler,
+            final boolean force) {
         if (classPath == null) {
             return;
         }
@@ -136,6 +150,15 @@ private static void addClassPathUrls(
                     continue;
                 }
                 final JarFile jar = new JarFile(file);
+                final Manifest mf = jar.getManifest();
+                if (mf != null) {
+                    if ((mf.getMainAttributes().getValue("Premain-Class") != null) && !force) {
+                        if (DEBUG) {
+                            System.err.println("EmJar: skipping java agent jar: " + elem);
+                        }
+                        continue;
+                    }
+                }
                 final Enumeration<JarEntry> embedded = jar.entries();
                 while (embedded.hasMoreElements()) {
                     final JarEntry entry = embedded.nextElement();