[solo 开发][插件] 插件加载不上

[WARN ]-[2019-03-20 02:22:45]-[org.b3log.latke.plugin.PluginManager:148]: Load plugin under directory[/plugins/kanbanniang/] failed
java.lang.NullPointerException
	at java.base/java.util.ArrayDeque.addLast(ArrayDeque.java:304)
	at java.base/java.util.ArrayDeque.add(ArrayDeque.java:495)
	at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:154)
	at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:171)
	at java.base/java.net.URLClassLoader.<init>(URLClassLoader.java:120)
	at org.b3log.latke.plugin.PluginManager.load(PluginManager.java:185)
	at org.b3log.latke.plugin.PluginManager.load(PluginManager.java:142)
	at org.b3log.solo.SoloServletListener.contextInitialized(SoloServletListener.java:124)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4668)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5136)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1377)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1367)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:140)
	at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:902)
	at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:831)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1377)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1367)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:140)
	at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:902)
	at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:262)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
	at org.apache.catalina.core.StandardService.startInternal(StandardService.java:423)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
	at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:928)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
	at org.apache.catalina.startup.Catalina.start(Catalina.java:638)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:350)
	at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:492)

@88250

org.b3log.latke.plugin.PluginManager

    /**
     * Loads a plugin by the specified plugin directory and put it into the specified holder.
     *
     * @param pluginDirPath the specified plugin directory
     * @param holder        the specified holder
     * @return loaded plugin
     * @throws Exception exception
     */
    private AbstractPlugin load(final String pluginDirPath, final Map<String, HashSet<AbstractPlugin>> holder) throws Exception {
        final Properties props = new Properties();
        final ServletContext servletContext = AbstractServletListener.getServletContext();

        String plugin = StringUtils.substringAfter(pluginDirPath, "/plugins");
        plugin = plugin.replace("/", "");

        final File file = Latkes.getWebFile("/plugins/" + plugin + "/plugin.properties");
        props.load(new FileInputStream(file));

        final URL defaultClassesFileDirURL = servletContext.getResource("/plugins/" + plugin + "classes");// 没有初始化
        URL classesFileDirURL = null; // 没有初始化
        try {
            classesFileDirURL = servletContext.getResource(props.getProperty("classesDirPath"));
        } catch (final MalformedURLException e) {
            LOGGER.log(Level.ERROR, "Reads [" + props.getProperty("classesDirPath") + "] failed", e);
        }

        final URLClassLoader classLoader = new URLClassLoader(new URL[]{
                defaultClassesFileDirURL, classesFileDirURL}, PluginManager.class.getClassLoader());

        classLoaders.add(classLoader);

        String pluginClassName = props.getProperty(Plugin.PLUGIN_CLASS);
        if (StringUtils.isBlank(pluginClassName)) {
            pluginClassName = NotInteractivePlugin.class.getName();
        }

        final String rendererId = props.getProperty(Plugin.PLUGIN_RENDERER_ID);

        if (StringUtils.isBlank(rendererId)) {
            LOGGER.log(Level.WARN, "no renderer defined by this plugin[" + plugin + "]，this plugin will be ignore!");
            return null;
        }

        final Class<?> pluginClass = classLoader.loadClass(pluginClassName);

        LOGGER.log(Level.TRACE, "Loading plugin class[name={0}]", pluginClassName);
        final AbstractPlugin ret = (AbstractPlugin) pluginClass.newInstance();

        ret.setRendererId(rendererId);

        setPluginProps(plugin, ret, props);
        registerEventListeners(props, classLoader, ret);
        register(ret, holder);

        ret.changeStatus();

        return ret;
    }

new URLClassLoader() 的时候有用到 ArrayDeque

        ArrayList<URL> path = new ArrayList<>(urls.length);
        ArrayDeque<URL> unopenedUrls = new ArrayDeque<>(urls.length);
        for (URL url : urls) {
            path.add(url);
            unopenedUrls.add(url);
        }

ArrayDeque 是 Deque 接口的一种具体实现，是依赖于可变数组来实现的。ArrayDeque 没有容量限制，可根据需求自动进行扩容。ArrayDeque 不支持值为 null 的元素。

Queue 也是 Java 集合框架中定义的一种接口，直接继承自 Collection 接口。除了基本的 Collection 接口规定测操作外，Queue 接口还定义一组针对队列的特殊操作。通常来说，Queue 是按照先进先出(FIFO)的方式来管理其中的元素的，但是优先队列是一个例外。

Deque 接口继承自 Queue 接口，但 Deque 支持同时从两端添加或移除元素，因此又被成为双端队列。鉴于此，Deque 接口的实现可以被当作 FIFO 队列使用，也可以当作 LIFO 队列（栈）来使用。官方也是推荐使用 Deque 的实现来替代 Stack。

	final  URL  defaultClassesFileDirURL  =  servletContext.getResource("/plugins/"  + plugin +  "classes") !=  null
		?  servletContext.getResource("/plugins/"  + plugin +  "classes")
		:  new  URL("file:");
	URL  classesFileDirURL  =  new  URL("file:");