使用wait()與notify()實(shí)現(xiàn)線程間協(xié)作
1. wait()與notify()/notifyAll()
調(diào)用sleep()和yield()的時(shí)候鎖并沒(méi)有被釋放，而調(diào)用wait()將釋放鎖。這樣另一個(gè)任務(wù)（線程）可以獲得當(dāng)前對(duì)象的鎖，從而進(jìn)入它的synchronized方法中。可以通過(guò)notify()/notifyAll()，或者時(shí)間到期，從wait()中恢復(fù)執(zhí)行。
只能在同步控制方法或同步塊中調(diào)用wait()、notify()和notifyAll()。如果在非同步的方法里調(diào)用這些方法，在運(yùn)行時(shí)會(huì)拋出IllegalMonitorStateException異常。
2.模擬單個(gè)線程對(duì)多個(gè)線程的喚醒
模擬線程之間的協(xié)作。Game類有2個(gè)同步方法prepare()和go()。標(biāo)志位start用于判斷當(dāng)前線程是否需要wait()。Game類的實(shí)例首先啟動(dòng)所有的Athele類實(shí)例，使其進(jìn)入wait()狀態(tài)，在一段時(shí)間后，改變標(biāo)志位并notifyAll()所有處于wait狀態(tài)的Athele線程。
Game.java
package concurrency;import java.util.Collection;import java.util.Collections;import java.util.HashSet;import java.util.Iterator;import java.util.Set;class Athlete implements Runnable { private final int id; private Game game; public Athlete(int id, Game game) { this.id = id; this.game = game; } public boolean equals(Object o) { if (!(o instanceof Athlete)) return false; Athlete athlete = (Athlete) o; return id == athlete.id; } public String toString() { return "Athlete<" + id + ">"; } public int hashCode() { return new Integer(id).hashCode(); } public void run() { try { game.prepare(this); } catch (InterruptedException e) { System.out.println(this + " quit the game"); } } }public class Game implements Runnable { private Set<Athlete> players = new HashSet<Athlete>(); private boolean start = false; public void addPlayer(Athlete one) { players.add(one); } public void removePlayer(Athlete one) { players.remove(one); } public Collection<Athlete> getPlayers() { return Collections.unmodifiableSet(players); } public void prepare(Athlete athlete) throws InterruptedException { System.out.println(athlete + " ready!"); synchronized (this) { while (!start) wait(); if (start) System.out.println(athlete + " go!"); } } public synchronized void go() { notifyAll(); } public void ready() { Iterator<Athlete> iter = getPlayers().iterator(); while (iter.hasNext()) new Thread(iter.next()).start(); } public void run() { start = false; System.out.println("Ready......"); System.out.println("Ready......"); System.out.println("Ready......"); ready(); start = true; System.out.println("Go!"); go(); } public static void main(String[] args) { Game game = new Game(); for (int i = 0; i < 10; i++) game.addPlayer(new Athlete(i, game)); new Thread(game).start(); }}結(jié)果：
Ready......Ready......Ready......Athlete<0> ready!Athlete<1> ready!Athlete<2> ready!Athlete<3> ready!Athlete<4> ready!Athlete<5> ready!Athlete<6> ready!Athlete<7> ready!Athlete<8> ready!Athlete<9> ready!Go!Athlete<9> go!Athlete<8> go!Athlete<7> go!Athlete<6> go!Athlete<5> go!Athlete<4> go!Athlete<3> go!Athlete<2> go!Athlete<1> go!Athlete<0> go!
3.模擬忙等待過(guò)程
MyObject類的實(shí)例是被觀察者，當(dāng)觀察事件發(fā)生時(shí)，它會(huì)通知一個(gè)Monitor類的實(shí)例（通知的方式是改變一個(gè)標(biāo)志位）。而此Monitor類的實(shí)例是通過(guò)忙等待來(lái)不斷的檢查標(biāo)志位是否變化。
BusyWaiting.java
import java.util.concurrent.TimeUnit;class MyObject implements Runnable { private Monitor monitor; public MyObject(Monitor monitor) { this.monitor = monitor; } public void run() { try { TimeUnit.SECONDS.sleep(3); System.out.println("i'm going."); monitor.gotMessage(); } catch (InterruptedException e) { e.printStackTrace(); } }}class Monitor implements Runnable { private volatile boolean go = false; public void gotMessage() throws InterruptedException { go = true; } public void watching() { while (go == false) ; System.out.println("He has gone."); } public void run() { watching(); }}public class BusyWaiting { public static void main(String[] args) { Monitor monitor = new Monitor(); MyObject o = new MyObject(monitor); new Thread(o).start(); new Thread(monitor).start(); }}結(jié)果：
i'm going.He has gone.
4.使用wait()與notify()改寫上面的例子
下面的例子通過(guò)wait()來(lái)取代忙等待機(jī)制，當(dāng)收到通知消息時(shí)，notify當(dāng)前Monitor類線程。
Wait.java
package concurrency.wait;import java.util.concurrent.TimeUnit;class MyObject implements Runnable { private Monitor monitor; public MyObject(Monitor monitor) { this.monitor = monitor; }定時(shí)啟動(dòng)線程
這里提供兩種在指定時(shí)間后啟動(dòng)線程的方法。一是通過(guò)java.util.concurrent.DelayQueue實(shí)現(xiàn)；二是通過(guò)java.util.concurrent.ScheduledThreadPoolExecutor實(shí)現(xiàn)。
1. java.util.concurrent.DelayQueue
類DelayQueue是一個(gè)無(wú)界阻塞隊(duì)列，只有在延遲期滿時(shí)才能從中提取元素。它接受實(shí)現(xiàn)Delayed接口的實(shí)例作為元素。
<<interface>>Delayed.java
package java.util.concurrent;import java.util.*;public interface Delayed extends Comparable<Delayed> { long getDelay(TimeUnit unit);}getDelay()返回與此對(duì)象相關(guān)的剩余延遲時(shí)間，以給定的時(shí)間單位表示。此接口的實(shí)現(xiàn)必須定義一個(gè) compareTo 方法，該方法提供與此接口的 getDelay 方法一致的排序。
DelayQueue隊(duì)列的頭部是延遲期滿后保存時(shí)間最長(zhǎng)的 Delayed 元素。當(dāng)一個(gè)元素的getDelay(TimeUnit.NANOSECONDS) 方法返回一個(gè)小于等于 0 的值時(shí)，將發(fā)生到期。
2.設(shè)計(jì)帶有時(shí)間延遲特性的隊(duì)列
類DelayedTasker維護(hù)一個(gè)DelayQueue<DelayedTask> queue，其中DelayedTask實(shí)現(xiàn)了Delayed接口，并由一個(gè)內(nèi)部類定義。外部類和內(nèi)部類都實(shí)現(xiàn)Runnable接口，對(duì)于外部類來(lái)說(shuō)，它的run方法是按定義的時(shí)間先后取出隊(duì)列中的任務(wù)，而這些任務(wù)即內(nèi)部類的實(shí)例，內(nèi)部類的run方法定義每個(gè)線程具體邏輯。
這個(gè)設(shè)計(jì)的實(shí)質(zhì)是定義了一個(gè)具有時(shí)間特性的線程任務(wù)列表，而且該列表可以是任意長(zhǎng)度的。每次添加任務(wù)時(shí)指定啟動(dòng)時(shí)間即可。
DelayedTasker.java
package com.zj.timedtask;import static java.util.concurrent.TimeUnit.SECONDS;import static java.util.concurrent.TimeUnit.NANOSECONDS;import java.util.Collection;import java.util.Collections;import java.util.Random;import java.util.concurrent.DelayQueue;import java.util.concurrent.Delayed;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;public class DelayedTasker implements Runnable { DelayQueue<DelayedTask> queue = new DelayQueue<DelayedTask>(); public void addTask(DelayedTask e) { queue.put(e); } public void removeTask() { queue.poll(); } public Collection<DelayedTask> getAllTasks() { return Collections.unmodifiableCollection(queue); } public int getTaskQuantity() { return queue.size(); } public void run() { while (!queue.isEmpty()) try { queue.take().run(); } catch (InterruptedException e) { System.out.println("Interrupted"); } System.out.println("Finished DelayedTask"); } public static class DelayedTask implements Delayed, Runnable { private static int counter = 0; private final int id = counter++; private final int delta; private final long trigger; public DelayedTask(int delayInSeconds) { delta = delayInSeconds; trigger = System.nanoTime() + NANOSECONDS.convert(delta, SECONDS); } public long getDelay(TimeUnit unit) { return unit.convert(trigger - System.nanoTime(), NANOSECONDS); } public int compareTo(Delayed arg) { DelayedTask that = (DelayedTask) arg; if (trigger < that.trigger) return -1; if (trigger > that.trigger) return 1; return 0; } public void run() { //run all that you want to do System.out.println(this); } public String toString() { return "[" + delta + "s]" + "Task" + id; } } public static void main(String[] args) { Random rand = new Random(); ExecutorService exec = Executors.newCachedThreadPool(); DelayedTasker tasker = new DelayedTasker(); for (int i = 0; i < 10; i++) tasker.addTask(new DelayedTask(rand.nextInt(5))); exec.execute(tasker); exec.shutdown(); }}結(jié)果：
[0s]Task 1[0s]Task 2[0s]Task 3[1s]Task 6[2s]Task 5[3s]Task 8[4s]Task 0[4s]Task 4[4s]Task 7[4s]Task 9Finished DelayedTask
3. java.util.concurrent.ScheduledThreadPoolExecutor
該類可以另行安排在給定的延遲后運(yùn)行任務(wù)（線程），或者定期（重復(fù)）執(zhí)行任務(wù)。在構(gòu)造子中需要知道線程池的大小。最主要的方法是：
[1] schedule
public ScheduledFuture<?> schedule(Runnable command, long delay,TimeUnit unit)
創(chuàng)建并執(zhí)行在給定延遲后啟用的一次性操作。
指定者：
-接口 ScheduledExecutorService 中的 schedule;
參數(shù)：
-command - 要執(zhí)行的任務(wù) ;
-delay - 從現(xiàn)在開(kāi)始延遲執(zhí)行的時(shí)間 ;
-unit - 延遲參數(shù)的時(shí)間單位 ;
返回：
-表示掛起任務(wù)完成的 ScheduledFuture，并且其 get() 方法在完成后將返回 null。
[2] scheduleAtFixedRate
public ScheduledFuture<?> scheduleAtFixedRate(
Runnable command,long initialDelay,long period,TimeUnit unit)
創(chuàng)建并執(zhí)行一個(gè)在給定初始延遲后首次啟用的定期操作，后續(xù)操作具有給定的周期；也就是將在 initialDelay 后開(kāi)始執(zhí)行，然后在 initialDelay+period 后執(zhí)行，接著在 initialDelay + 2 * period 后執(zhí)行，依此類推。如果任務(wù)的任何一個(gè)執(zhí)行遇到異常，則后續(xù)執(zhí)行都會(huì)被取消。否則，只能通過(guò)執(zhí)行程序的取消或終止方法來(lái)終止該任務(wù)。如果此任務(wù)的任何一個(gè)執(zhí)行要花費(fèi)比其周期更長(zhǎng)的時(shí)間，則將推遲后續(xù)執(zhí)行，但不會(huì)同時(shí)執(zhí)行。
指定者：
-接口 ScheduledExecutorService 中的 scheduleAtFixedRate;
參數(shù)：
-command - 要執(zhí)行的任務(wù) ;
-initialDelay - 首次執(zhí)行的延遲時(shí)間 ;
-period - 連續(xù)執(zhí)行之間的周期 ;
-unit - initialDelay 和 period 參數(shù)的時(shí)間單位 ;
返回：
-表示掛起任務(wù)完成的 ScheduledFuture，并且其 get() 方法在取消后將拋出異常。
4.設(shè)計(jì)帶有時(shí)間延遲特性的線程執(zhí)行者
類ScheduleTasked關(guān)聯(lián)一個(gè)ScheduledThreadPoolExcutor，可以指定線程池的大小。通過(guò)schedule方法知道線程及延遲的時(shí)間，通過(guò)shutdown方法關(guān)閉線程池。對(duì)于具體任務(wù)（線程）的邏輯具有一定的靈活性（相比前一中設(shè)計(jì)，前一種設(shè)計(jì)必須事先定義線程的邏輯，但可以通過(guò)繼承或裝飾修改線程具體邏輯設(shè)計(jì)）。
ScheduleTasker.java
package com.zj.timedtask;import java.util.concurrent.ScheduledThreadPoolExecutor;import java.util.concurrent.TimeUnit;public class ScheduleTasker { private int corePoolSize = 10; ScheduledThreadPoolExecutor scheduler; public ScheduleTasker() { scheduler = new ScheduledThreadPoolExecutor(corePoolSize); } public ScheduleTasker(int quantity) { corePoolSize = quantity; scheduler = new ScheduledThreadPoolExecutor(corePoolSize); } public void schedule(Runnable event, long delay) { scheduler.schedule(event, delay, TimeUnit.SECONDS); } public void shutdown() { scheduler.shutdown(); } public static void main(String[] args) { ScheduleTasker tasker = new ScheduleTasker(); tasker.schedule(new Runnable() { public void run() { System.out.println("[1s]Task 1"); } }, 1); tasker.schedule(new Runnable() { public void run() { System.out.println("[2s]Task 2"); } }, 2); tasker.schedule(new Runnable() { public void run() { System.out.println("[4s]Task 3"); } }, 4); tasker.schedule(new Runnable() { public void run() { System.out.println("[10s]Task 4"); } }, 10); tasker.shutdown(); }}結(jié)果：
[1s]Task 1[2s]Task 2[4s]Task 3[10s]Task 4 public void run() { try { TimeUnit.SECONDS.sleep(3); System.out.println("i'm going."); monitor.gotMessage(); } catch (InterruptedException e) { e.printStackTrace(); } }}class Monitor implements Runnable { private volatile boolean go = false; public synchronized void gotMessage() throws InterruptedException { go = true; notify(); } public synchronized void watching() throws InterruptedException { while (go == false) wait(); System.out.println("He has gone."); } public void run() { try { watching(); } catch (InterruptedException e) { e.printStackTrace(); } }}public class Wait { public static void main(String[] args) { Monitor monitor = new Monitor(); MyObject o = new MyObject(monitor); new Thread(o).start(); new Thread(monitor).start(); }}結(jié)果：
i'm going.He has gone.
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