小谈java多线程
关键字：Synchronized、wait/notify、ReentrantLock、生产-消费者模型
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这篇博文仅仅对java多线程的部分知识做个小结，欢迎大家补充。
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Synchronized
?? ? 说到多线程，大家（对于初学者来说）可能就会想到Synchronized这个关键字。Synchronized可以用在方法级别上，用来同步一个方法的访问，也可以同步一个代码块。形式如下：
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// 同步方法public synchronized void method(){ // 方法体}// 同步代码快synchronized(object){ // 代码块}Synchronized 需要注意的是：Synchronized作用于方法时，Synchronized首先会对方法所属的对象加锁，对象的其他被Synchronized修饰的方法都不可以被其他线程访问，非Synchronized修饰发方法可以被多个线程同时访问。
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wait/notify
?? ?wait/notify必须在Synchronized的作用范围内使用，也就是说wait/notify必须在同步方法体内或同步代码快内才有效，其他地方就会报IllegalMonitorStateException。
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基于Synchronized、wait/notify机理实现生产-消费者模型
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public class Basket {private List<String> basket = new ArrayList<String>(10);private boolean running = false ;public void run(){this.running = true;}public void stop(){this.running = false;}public boolean isRunning(){return running;}public void add(String e) {synchronized (basket) {while (basket.size() == 10) {try {basket.wait(); // 访问的线程会等待在此处，当再次被唤醒，从此处继续执行} catch (InterruptedException ex) {ex.printStackTrace();}}basket.add(e);String currentThreadName = Thread.currentThread().getName();System.out.println("线程["+currentThreadName+"] "+e + " 生产一个产品");basket.notifyAll();}}public void remove(String msg) {synchronized (basket) {String currentThreadName = Thread.currentThread().getName();while (basket.size() == 0) {try {System.out.println("线程["+currentThreadName+"] "+msg + ": please wait a little");basket.wait();} catch (InterruptedException e) {e.printStackTrace();}}String result = basket.get(0);basket.remove(0);System.out.println("线程["+currentThreadName+"] "+msg + " 消费：" + result);basket.notifyAll();}}public void getResult() {System.out.println(basket.toString());}}生产者
class UProducer implements Runnable {private String name = "";private Basket basket;public UProducer(String name, Basket basket) {this.name = name;this.basket = basket;}@Overridepublic void run() {int i = 0;while(basket.isRunning()){basket.add(name + i++);try {Thread.sleep(500);} catch (InterruptedException e) {e.printStackTrace();}}}public String getName() {return name;}}?消费者
class Customer implements Runnable {private String name = "";private Basket basket;public Customer(String name, Basket basket) {super();this.name = name;this.basket = basket;}@Overridepublic void run() {int i = 0;while(basket.isRunning()){basket.remove(name + i++);try {Thread.sleep(500);} catch (InterruptedException e) {e.printStackTrace();}}}public String getName() {return name;}}?测试代码
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public static void main(String[] args) {Basket basket = new Basket();basket.run();UProducer mrZhang = new UProducer("张三", basket);UProducer mrLi = new UProducer("李四", basket);Customer missWang = new Customer("王五", basket);Customer missZhao = new Customer("赵六", basket);new Thread(missWang, missWang.getName()).start();new Thread(missZhao, missZhao.getName()).start();try {Thread.sleep(5000);} catch (InterruptedException e) {e.printStackTrace();}new Thread(mrZhang, mrZhang.getName()).start();new Thread(mrLi, mrLi.getName()).start();try {Thread.sleep(10000);} catch (InterruptedException e) {e.printStackTrace();}basket.stop();basket.getResult();}?Synchronized关键字可以很方便的实现简单的多线程问题，jvm默认的为我们提供了对象的加锁、释放锁功能。Synchronized关键字同时存在一些问题。如一个线程得不到对象的锁会永远的等待。
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ReentrantLock
??ReentrantLock是对Synchronized的加强，提供了线程竞争锁的等待时间、多条件下的线程等待策略等等强大的功能。
具体的方法请参考API
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以下是对生产-消费者模型的改写
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class Basket {private static final int CAPACITY = 10;private List<Bread> container = new ArrayList<Bread>(CAPACITY);private ReentrantLock lock = new ReentrantLock();private Condition notFull = lock.newCondition();private Condition notEmpty = lock.newCondition();public void add(Bread bread) {try {lock.lockInterruptibly();while (container.size() == CAPACITY) {System.out.println("货源[" + Thread.currentThread().getName() + "]：no more ,i will call you if i need");notFull.await();}container.add(bread);System.out.println("货源[" + Thread.currentThread().getName() + "]：进货：" + bread);notEmpty.signalAll();} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}public void remove() {try {lock.lockInterruptibly();while (container.size() == 0) {System.out.println("销售[" + Thread.currentThread().getName() + "]: please wait a little");notEmpty.await();}Bread bread = container.get(0);container.remove(0);System.out.println("销售[" + Thread.currentThread().getName() + "]：销售：" + bread);notFull.signalAll();} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}}class Bread {private String name;public Bread(String name) {this.name = name;}@Overridepublic String toString() {return name;}}??