请教帖（phonegap中的cordova中的插件管理器（PluginManager）的同步和异步如何实现的）

   /**
     * Receives a request for execution and fulfills it by finding the appropriate
     * Java class and calling it's execute method.
     *
     * PluginManager.exec can be used either synchronously or async. In either case, a JSON encoded
     * string is returned that will indicate if any errors have occurred when trying to find
     * or execute the class denoted by the clazz argument.
     *
     * @param service       String containing the service to run
     * @param action        String containing the action that the class is supposed to perform. This is
     *                      passed to the plugin execute method and it is up to the plugin developer
     *                      how to deal with it.
     * @param callbackId    String containing the id of the callback that is execute in JavaScript if
     *                      this is an async plugin call.
     * @param rawArgs       An Array literal string containing any arguments needed in the
     *                      plugin execute method.
     */
    public void exec(final String service, final String action, final String callbackId, final String rawArgs) {
        if (numPendingUiExecs.get() > 0) {
            numPendingUiExecs.getAndIncrement();//获取并自增
            this.ctx.getActivity().runOnUiThread(new Runnable() {
                public void run() {
                    execHelper(service, action, callbackId, rawArgs);
                    numPendingUiExecs.getAndDecrement();//获取并自减
                }
            });
        } else {
            execHelper(service, action, callbackId, rawArgs);
        }
    }

 代码如上，小弟最近在看cordova，对于 PluginManager.exec can be used either synchronously or async，这句很不理解，希望大神们帮助解答一下，还有cordova是怎么实现同步和异步的？最好详细一些，小弟对此研究不深.