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POjava thread wait and auto wake up
 

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    <p>I have an web application which accept some data from user to create a task, then the task should be executed. </p> <p>Since the execution of the Task is to download something from the internet which will cost some time, so I tried to create a new Thread to do the job.</p> <p>This is my idea:</p> <ol> <li><p>create a <code>LoaderThread</code> used to download data. And the <code>LoaderThread</code> hold a field of <code>ArrayList</code> used for put the <code>Task</code>.</p></li> <li><p>A <code>Servlet</code> to handle the request and response.</p></li> <li><p>When the <code>Servlet</code> startup, start the <code>LoaderThread</code></p></li> <li><p>During the servlet run, add task to the <code>LoaderThread</code>.</p></li> </ol> <p>This is the code(some of them is omitted):</p> <pre><code>public class RwdServlet extends HttpServlet { private StaticMapLoader loader; @Override public void init() throws ServletException { super.init(); loader = new StaticMapLoader(); loader.startRunning(); } @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { Task t=createTask(req); loader.addTask(t); } @Override public void destroy() { loader.stopRunning(); } } public class StaticMapLoader extends Thread { private List&lt;Task&gt; tasks = new ArrayList&lt;Task&gt;(); private boolean running = false; @Override public void run() { while (running) { if (tasks.size() &gt; 0) { Task t = tasks.get(0); log.info(t); if (t != null &amp;&amp; t.status == Status.waiting) { tasks.remove(0); t.status = Status.running; downLoad(t); } } } } private void downLoad(Task t) { //download file } public void addTask(Task t) { tasks.add(t); } public void startRunning() { running = true; this.start(); } public void stopRunning() { running = false; this.interrupt(); } } </code></pre> <p>The above code worked, but I found that even the <code>tasks</code> were empty and there are no new <code>task</code> added, the loop will keep running.</p> <p>So I though if I can make the <code>LoaderThread</code> suspend when there are no tasks, and notify it when new task come out.</p> <p>So I tried this:</p> <pre><code>@Override public void run() { while (running) { if (tasks.size() &gt; 0) { Task t = tasks.get(0); log.info(t); if (t != null &amp;&amp; t.status == Status.waiting) { tasks.remove(0); t.status = Status.running; downLoad(t); } } else { try { wait(); } catch (InterruptedException e) { e.printStackTrace(); } } } } </code></pre> <p>I tried to call the <code>wait()</code> if the <code>tasks</code> is empty.</p> <p>But I do not know how to wake up it?</p> <p>Also, is there anything I should know to improve the application?</p> <p>BWT, is it possible that more than one <code>LoaderThread</code> instance will be created? If so , how to avoid it?</p> <hr> <p>It seems that I can use other implementation, but I wonder if my case is refactor-able?</p> <p>Since I want to learn some thing I have missed. :) Thanks. </p>
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