java并发编程基础---Sky

1.线程及启动和终止

1.1 线程 -进程/优先级

操作系统调度的最小单元是线程，线程是轻量级进程。

线程优先级由setPriority(int)方法来设置，默认优先级是5，等级1~10.等级越高分的时间片越多。

1.2 线程的状态

new 初始化 》》Runable 运行》》Blocked阻塞 》》Wating等待》》 time_Wating超时等待》》 temerinated终止状态。

1.3 Daemon辅助线程

主线程终止后，辅助线程也就结束。thread.setDaemon(true)设置在线程开始之前。

1.4 过期suspend() , resume(), stop()为啥不建议使用？

suspend()调用后太霸道，不释放占有资源，而是抱着占有资源去睡觉，这样容易死锁。站着茅坑不拉屎。

resume()、stop()不保证线程资源正常释放，那要你们何用？

安全的终止线程：有中断操作，和自定义cancel（）方法。

 package Thread;

 import java.util.concurrent.TimeUnit;

 /**
  * Created by Sky on 2016/9/22.
  * @author xiaoyongyong
  */
 public class Shutdown {
     public static void main(String[] args) throws InterruptedException {
         Runner one = new Runner();
         Thread countThread = new Thread(one,"CountThread");
         countThread.start();
         TimeUnit.SECONDS.sleep(1);
         countThread.interrupt();

         Runner two = new Runner();
         countThread = new Thread(two,"CountThread");
         countThread.start();
         TimeUnit.SECONDS.sleep(1);
         two.cancle();
     }

     private static class Runner  implements Runnable{
         private  long i;
         private volatile boolean on =true;
         @Override
         public void run() {
             while (on && !Thread.currentThread().isInterrupted()){
                 i++;
             }
             System.out.println("Count i ="+ i);
         }
         public void cancle(){
             on = false;
         }
     }
 }

2. 线程之间通信

2.1 volatile 和 synchronized 关键字。

volatile：它能保证所有线程对变量访问的可见性，即某线程对定义变量的修改，其他线程可见。谨慎使用，过多使用会降低程序运行效率。

syncchronized：保证只有一个线程能对其定义的变量修改。保证其变量访问的可见性和排他性。

2.2 通知/等待机制

可以确保及时性和降低开销。

相关方法：notify()、nitifyAll()、wait()、wait(long)、wait(long,int)。

注意点：1）使用notify()、nitifyAll()、wait()时需要先对调用对象加锁。2）notify()、nitifyAll()方法调用后，等待线程依旧不会从wait()方法返回，

需要调用notify()、nitifyAll()方法的线程，释放锁之后，等待线程才有机会从wait()返回。等待队列-》同步队列。3）wait()方法返回的前提是获取对象锁。

这种机制可以运用在生产者-消费者模式中。

 package Thread;

 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.concurrent.TimeUnit;

 /**
  * Created by Sky on 2016/9/22.
  *
  * @author xiaoyongyong
  */
 public class WaitNotify {
     static boolean flag = true;
     static final Object lock = new Object();

     public static void main(String[] args) throws Exception {
         Thread waitTread = new Thread(new Wait(), "WaitTread");
         waitTread.start();
         TimeUnit.SECONDS.sleep(1);

         Thread notifyTread = new Thread(new NotifyTread(), "NotifyTread");
         notifyTread.start();
     }

     private static class Wait implements Runnable {
         @Override
         public void run() {
             synchronized (lock) {
                 while (flag) {
                     try {
                         System.out.println(Thread.currentThread() + " flag is true.wait. " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                         lock.wait();
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     }
                 }
                 System.out.println(Thread.currentThread() + " flag is true.running. " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
             }
         }
     }

     private static class NotifyTread  implements Runnable {
         @Override
         public void run() {
             synchronized (lock){
                 System.out.println(Thread.currentThread() + " hold lock.notify. " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                 lock.notifyAll();
                 flag = false;
                 try {
                     TimeUnit.SECONDS.sleep(2);
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             }
             synchronized (lock) {
                 System.out.println(Thread.currentThread() + " hold lock.again.sleep " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                 try {
                     TimeUnit.SECONDS.sleep(2);
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             }
         }
     }
 }

返回结果：

Thread[WaitTread,5,main] flag is true.wait. 22:03:45
Thread[NotifyTread,5,main] hold lock.notify. 22:03:46
Thread[NotifyTread,5,main] hold lock.again.sleep 22:03:48
Thread[WaitTread,5,main] flag is true.running. 22:03:50

2.3 管道输入/输出流

主要用于线程之间的数据传输，传输媒介是内存，有PipedOutputStream/PipedInputStream（面向字节）  和PipedReader/PipedWriter（面向字符）。

 package Thread;

 import java.io.IOException;
 import java.io.PipedReader;
 import java.io.PipedWriter;

 /**
  * Created by Sky on 2016/9/22.
  * @author xiaoyongyong
  */
 public class Piped {
     public static void main(String[] args) throws IOException {
         PipedWriter out = new PipedWriter();
         PipedReader in = new PipedReader();
         out.connect(in);
         Thread printThread = new Thread(new Print(in), "PringThread");
         printThread.start();

         int receive;
         try {
             while ((receive = System.in.read()) != -1) {
                 out.write(receive);
             }
         } finally {
             out.close();
         }
     }

     private static class Print implements Runnable {
         private PipedReader in;
         public Print(PipedReader in) {
             this.in = in;
         }
         @Override
         public void run() {
             int receive;
             try {
                 while ((receive = in.read()) != -1) {
                     System.out.print((char) receive);
                 }
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
     }
 }

输入一串字符串，按enter后，将原样输出。管道输入输出流必须先连接才可复制，即connect()。

2.4 Thread.join()的使用

当线程A等待thread线程终止之后，才从thread.join()返回。

join方法顾名思义 就是往线程中添加东西的；join方法可以用于临时加入线程，一个线程在运算过程中，如果满足于条件，我们可以临时加入一个线程，让这个线程运算完，另外一个线程再继续运行。

Thread,yeld()和Thread.sleep()的区别？

hread.yield()方法暂停当前正在执行的线程对象，并执行其他线程。也就是交出CPU一段时间(其他同样的优先级或者更高优先级的线程可以获取到运行的机会)；不会释放锁资源。

而Thread.sleep()方法使当前线程进入停滞状态，所以执行sleep()的线程在指定的时间内肯定不会执行, 同时sleep函数不会释放锁资源；sleep可使优先级低的线程得到执行的机会，当然也可以让同优先级和高优先级的线程有执行的机会。

这两个的区别在于yield只能是同级，或者高级优先执行，而sleep可以低级，同级都可以有优先执行机会。

2.5 ThreadLocal的使用

线程变量，是一个以TreadLocal对象为键,任意对象为值得存储结构。

2.6 线程池技术及示例。

以下是线程池接口的定义和实现：

package Thread;

/**
 * Created by asus on 2016/9/27.
 */
public interface ThreadPool<Job extends Runnable> {
    void execute(Job job);
    void shutdown();
    void addWorkers(int num);
    void removeWoker(int num);
    int getJobSize();
}

package Thread;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.atomic.AtomicLong;

/**
 * Created by asus on 2016/9/27.
 *
 * @author xiaoyongyong
 */
public class DefaultThreadPool<Job extends Runnable> implements ThreadPool<Job> {
    private static final int MAX_WORKER_NUMBERS = 10;
    private static final int DEFAULT_WORKER_NUMBERS = 5;
    private static final int MIN_WORKER_NUBERS = 1;
    private final LinkedList<Job> jobs = new LinkedList<Job>();
    private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());
    private int workerNum = DEFAULT_WORKER_NUMBERS;
    private AtomicLong threadNum = new AtomicLong();

    public DefaultThreadPool() {
        initializeWorker(DEFAULT_WORKER_NUMBERS);
    }

    private void DefaultThreadPool(int num) {
        workerNum = num > MAX_WORKER_NUMBERS ? MAX_WORKER_NUMBERS : num < MIN_WORKER_NUBERS ? MAX_WORKER_NUMBERS : num;
        initializeWorker(workerNum);
    }

    @Override
    public void execute(Job job) {
        if (job != null) {
            synchronized (jobs) {
                jobs.add(job);
                jobs.notify();
            }
        }
    }

    @Override
    public void shutdown() {
        for (Worker worker : workers) {
            worker.shutdown();
        }
    }

    @Override
    public void addWorkers(int num) {
        synchronized (jobs) {
            if (num + this.workerNum > MAX_WORKER_NUMBERS) {
                initializeWorker(num);
                this.workerNum += num;
            }
        }
    }

    @Override
    public void removeWoker(int num) {
        synchronized (jobs) {
            if (num >= this.workerNum) {
                throw new IllegalArgumentException("beyond workerNum");
            }
            int count = 0;
            while (count < num) {
                Worker worker = workers.get(count);
                if (workers.remove(worker)) {
                    worker.shutdown();
                    count++;
                }
            }
            this.workerNum -= count;
        }
    }

    @Override
    public int getJobSize() {
        return jobs.size();
    }

    private void initializeWorker(int num) {
        for (int i = 0; i < num; i++) {
            Worker worker = new Worker();
            workers.add(worker);
            Thread threadWorker = new Thread(worker, "ThreadPool-Worker-" + threadNum.incrementAndGet());
            threadWorker.start();
        }
    }

    class Worker implements Runnable {
        private volatile boolean running = true;

        @Override
        public void run() {
            while (running){
                Job job = null;
                synchronized (jobs){
                    while (jobs.isEmpty()){
                        try {
                            jobs.wait();
                        } catch (InterruptedException e) {
                            Thread.currentThread().interrupt();
                            e.printStackTrace();
                            return;
                        }
                    }
                    job = jobs.removeFirst();
                }
                if(job != null){
                    job.run();
                }
            }
        }

        public void shutdown() {
            running = false;
        }
    }
}

2.7 基于县此次技术的简单Web服务器

这个服务器使用main线程不断接受客户端Socket的连接，将连接以及请求交给线程池处理，这样使得Web服务器能够同同时处理多个客户端请求。