java.util.concurrent

- Java's low-level concurrency primitives

synchronized/volatile/wait/notify/notifyall 

常见问题：deadlock 死锁，thread starvation 线程饥饿, race conditiond 竞争条件等。

synchronized：performance issue.

- java.util.concurrent.atomic subpackage contains low-level utility classes that support lock-free thread-safe programming on single variables.

- java.util.concurrent.locks subpackage contains low-level utility types for locking and waiting for conditions.

compare-and-swap (CAS) - cas更轻量 (java.util.concurrent.locks.reentrantLock) ，reentrantLock提供了更多锁的选择。

Executor Framework

executor的优点，在不改变代码的前提下，改变程序对task的调度。

executor的缺点，不能取消正在运行中的task，不能关闭executor或决定一个异步程序是否结束。 

常用方法：

ExecutorService newCachedThreadPool()

创建线程池时创建所需的thread，在之前构建的thread可用时会重用，60秒内没有使用的线程会被终止并从缓存中移除。可以改善short-lived异步程序的性能。

ExecutorService newSingleThreadExecutor()

使用single worker thread和 unbounded queue创建executor —— tasks加入queue中依次执行 (依次执行的task不超过一个)。如果thread如果在关闭前因为执行失败终止，新的thread会被创建并替代它的位置。

ExecutorService newFixedThreadPool(int nThreads) 

在共享的unbounded queue中重用固定数量的线程，最多可以用nThreads共享task。如果额外的task提交时，所有的线程都被占用，当前task会在queue内等待，知道有可使用的线程。如果任一thread在关闭前执行失败，如果有task需要被执行，新的线程会被创建并取代失败线程的位置。线程池在executor随着shut down而结束。

public class Server {

    static Executor pool = Executors.newFixedThreadPool(5);
    public static void main(String[] args) throws IOException {
        ServerSocket socket = new ServerSocket(9000);
        while(true){
            final Socket s = socket.accept();
            Runnable r = new Runnable(){
                @Override
                public void run(){
                    doWork(s);
                }
            };
            pool.execute(r);
        }
    }

    public static void doWork(Socket s){

    }

}

当前callable无法在5秒内完成时，get()将返回TimeoutException。

public class ReadWebPage {

    public static void main(String[] args) {
        if(args.length != 1){
            System.out.println("usage: java ReadWebPage url");
            return;
        }
        ExecutorService executor = Executors.newSingleThreadExecutor();
        Callable<List<String>> callable;
        callable = new Callable<List<String>>(){
            @Override
            public List<String> call() throws Exception {
                List<String> lines = new ArrayList<>();
                URL url = new URL(args[0]);
                HttpURLConnection con;
                con = (HttpURLConnection) url.openConnection();
                InputStreamReader isr;
                isr = new InputStreamReader(con.getInputStream());
                BufferedReader br;
                br = new BufferedReader(isr);
                String line;
                while((line = br.readLine()) != null){
                    lines.add(line);
                }
                return lines;
            }
        };

        Future<List<String>> future = executor.submit(callable);

        try{
            List<String> lines = future.get(5, TimeUnit.SECONDS);
            for(String line: lines){
                System.out.println(line);
            }
        }catch (Exception ee){
            System.err.println("Callable through exception: " + ee.getMessage());
        }
        executor.shutdown();
    }
}