byte-buddy appendToBootstrapClassLoaderSearch, and jdk instrumentation

AppendToBootstrapClassLoaderSearch.java

  /**
    * All codes from stagemonitor apm, which gives a example about how to use instrumentation::appendToBootstrapClassLoaderSearch
    * This will be very useful for instrument some class in rt.jar
    */
  private static boolean initInstrumentation() {
		try {
			/**
			 * this try-catch block shows two ways about instrumentation.
			 * ref issue: https://github.com/raphw/byte-buddy/issues/237
			 */
			try {
				instrumentation = ByteBuddyAgent.getInstrumentation();
			} catch (IllegalStateException e) {
				instrumentation = ByteBuddyAgent.install(
						new ByteBuddyAgent.AttachmentProvider.Compound(
								new EhCacheAttachmentProvider(),
								ByteBuddyAgent.AttachmentProvider.DEFAULT));
			}
			ensureDispatcherIsAppendedToBootstrapClasspath(instrumentation);
			if (!Dispatcher.getValues().containsKey(IGNORED_CLASSLOADERS_KEY)) {
				Dispatcher.put(IGNORED_CLASSLOADERS_KEY, Collections.newSetFromMap(new ConcurrentHashMap<Integer, Boolean>()));
			}
			hashCodesOfClassLoadersToIgnore = Dispatcher.get(IGNORED_CLASSLOADERS_KEY);
			return true;
		} catch (Exception e) {
			logger.warn("Failed to perform runtime attachment of the stagemonitor agent. Make sure that you run your " +
					"application with a JDK (not a JRE)." +
					"To make stagemonitor work with a JRE, you have to add the following command line argument to the " +
					"start of the JVM: -javaagent:/path/to/byte-buddy-agent-<version>.jar", e);
			return false;
		}
	}

  private static void ensureDispatcherIsAppendedToBootstrapClasspath(Instrumentation instrumentation)
			throws ClassNotFoundException, IOException {
		final ClassLoader bootstrapClassloader = ClassLoader.getSystemClassLoader().getParent();
		try {
			bootstrapClassloader.loadClass(DISPATCHER_CLASS_NAME);
			// already injected
		} catch (ClassNotFoundException e) {
			final JarFile jarfile = new JarFile(createTempDispatcherJar());
			instrumentation.appendToBootstrapClassLoaderSearch(jarfile);
			bootstrapClassloader.loadClass(DISPATCHER_CLASS_NAME);
		}
	}

	private static File createTempDispatcherJar() throws IOException {
		final InputStream input = AgentAttacher.class.getClassLoader()
				.getResourceAsStream("stagemonitor-dispatcher.jar.gradlePleaseDontExtract");
		if (input == null) {
			throw new IllegalStateException("If you see this exception inside you IDE, you have to execute gradle " +
					"processResources before running the tests.");
		}
		final File tempDispatcherJar = File.createTempFile("stagemonitor-dispatcher", ".jar");
		tempDispatcherJar.deleteOnExit();
		IOUtils.copy(input, new FileOutputStream(tempDispatcherJar));
		return tempDispatcherJar;
	}