死锁定义
死锁是指两个或者多个线程被永久阻塞的一种局面，产生的前提是要有两个或两个以上的线程，并且来操作两个或者多个以上的共同资源；我的理解是用两个线程来举例，现有线程A和B同时操作两个共同资源a和b，A操作a的时候上锁LockA,继续执行的时候，A还需要LockB进行下面的操作，这个时候b资源在被B线程操作，刚好被上了锁LockB，假如此时线程B刚好释放了LockB则没有问题，但没有释放LockB锁的时候，线程A和B形成了对LockB锁资源的争夺，从而造成阻塞，形成死锁；具体其死锁代码如下：
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public class MyDeadLockTest { public static void main(String[] args){ Object obj1 = new Object(); Thread thread1 = new Thread(new DeadRes(true,obj1)); Thread thread2 = new Thread(new DeadRes(false,obj1)); thread1.start(); thread2.start(); }}class DeadRes implements Runnable{ boolean flag; Object obj; public DeadRes(boolean flag, Object obj1) { this.flag = flag; this.obj = obj1; } @Override public void run() { if(flag){ synchronized (DeadRes.class){ System.out.println(Thread.currentThread().getName()+" acquie lock is DeadRes.class"); synchronized (obj){ System.out.println(Thread.currentThread().getName()+" acquie lock is obj"); } } }else{ synchronized (obj){ System.out.println(Thread.currentThread().getName()+" acquie lock is obj"); synchronized (DeadRes.class){ System.out.println(Thread.currentThread().getName()+" acquie lock is DeadRes.class"); } } } }} |
执行结果如下图：
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Thread-1 acquie lock is objThread-0 acquie lock is DeadRes.class |
当然每次执行的结果不一样，有可能是一种和谐状态，没有发生死锁，此时为保证每次死锁，可以让run()方法中，执行while(true)循环，这样保证了每次必定发生死锁；当然实际应用中，我们应该尽量避免死锁，当有多线程操作多个共同资源的时候，避免发生同一锁对象的同步嵌套。
线程间的通讯—-生产者与消费者模式
1、让两个线程交替进行操作，当生产了一个数字后，紧接着消费一个，首先采用Object对象中的wait-notify来实现，具体代码如下:
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public class ThreadProConsume { public static void main(String[] args){ Product product = new Product(); Thread thread1 = new Thread(new Producer(product)); Thread thread2 = new Thread(new Consumer(product)); thread1.start(); thread2.start(); }}class Product{ String name; private int count = 1; boolean flag = false; public synchronized void set(String name){ if(flag){ try { this.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } this.name = name +"--"+count++; flag = true; System.out.println(Thread.currentThread().getName()+" produce num : "+this.name); this.notify(); } public synchronized void out(){ if(!flag){ try { this.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println(Thread.currentThread().getName()+" consume num is : "+this.name); flag = false; this.notify(); }}class Producer implements Runnable{ Product res; public Producer(Product product) { this.res = product; } @Override public void run() { while(true){ res.set("guyue"); } }}class Consumer implements Runnable{ Product res; public Consumer(Product product) { this.res = product; } @Override public void run() { while(true){ res.out(); } }} |
执行结果如图：
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Thread-1 consume num is : guyue--3938Thread-0 produce num : guyue--3939Thread-1 consume num is : guyue--3939Thread-0 produce num : guyue--3940Thread-1 consume num is : guyue--3940Thread-0 produce num : guyue--3941Thread-1 consume num is : guyue--3941 |
当超过两个以上线程操作的时候，这里需要在set()与out()方法中的if判断改为while，并且notif方法，改为notifyAll()，这样多个线程操作的时候，便可以交替进行，具体代码如下：
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public class ThreadProConsume { public static void main(String[] args){ Product product = new Product(); Thread thread1 = new Thread(new Producer(product)); Thread thread3 = new Thread(new Producer(product)); Thread thread2 = new Thread(new Consumer(product)); Thread thread4 = new Thread(new Consumer(product)); thread1.start(); thread3.start(); thread2.start(); thread4.start(); }}class Product{ String name; private int count = 1; boolean flag = false; public synchronized void set(String name){ while(flag){ try { this.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } this.name = name +"--"+count++; flag = true; System.out.println(Thread.currentThread().getName()+" produce num : "+this.name); this.notifyAll(); } public synchronized void out(){ while (!flag){ try { this.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println(Thread.currentThread().getName()+" consume num is : "+this.name); flag = false; this.notifyAll(); }} |
执行结果如下：
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Thread-0 produce num : guyue--50325Thread-2 consume num is : guyue--50325Thread-1 produce num : guyue--50326Thread-3 consume num is : guyue--50326Thread-0 produce num : guyue--50327Thread-2 consume num is : guyue--50327Thread-1 produce num : guyue--50328Thread-3 consume num is : guyue--50328 |
2、采用Lock-Condition方法实现如下：
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class Product{ String name; private int count = 1; boolean flag = false; Lock lock = new ReentrantLock(); Condition conditon = lock.newCondition(); public void set(String name){ try{ lock.lock(); while(flag){ conditon.await(); } this.name = name +"--"+count++; flag = true; System.out.println(Thread.currentThread().getName()+" produce num : "+this.name); conditon.signalAll(); }catch (Exception e){ }finally { lock.unlock(); } } public void out(){ try{ lock.lock(); while(!flag){ conditon.await(); } flag = false; System.out.println(Thread.currentThread().getName()+" consumer num is : "+this.name); conditon.signalAll(); }catch (Exception e){ }finally { lock.unlock(); } }} |
执行结果如下：
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Thread-0 produce num : guyue--20305Thread-3 consumer num is : guyue--20305Thread-1 produce num : guyue--20306Thread-2 consumer num is : guyue--20306Thread-0 produce num : guyue--20307Thread-3 consumer num is : guyue--20307Thread-1 produce num : guyue--20308Thread-2 consumer num is : guyue--20308 |
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原文链接：http://blog.csdn.net/yongxihu/article/details/70277372
