Java编程之多线程死锁与线程间通信简单实现代码
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:
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:
thread-1 acquie lock is obj thread-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:
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:
thread-1 consume num is : guyue--3938 thread-0 produce num : guyue--3939 thread-1 consume num is : guyue--3939 thread-0 produce num : guyue--3940 thread-1 consume num is : guyue--3940 thread-0 produce num : guyue--3941 thread-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:
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:
thread-0 produce num : guyue--50325 thread-2 consume num is : guyue--50325 thread-1 produce num : guyue--50326 thread-3 consume num is : guyue--50326 thread-0 produce num : guyue--50327 thread-2 consume num is : guyue--50327 thread-1 produce num : guyue--50328 thread-3 consume num is : guyue--50328
2、采用lock-condition方法实现如下:
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:
thread-0 produce num : guyue--20305 thread-3 consumer num is : guyue--20305 thread-1 produce num : guyue--20306 thread-2 consumer num is : guyue--20306 thread-0 produce num : guyue--20307 thread-3 consumer num is : guyue--20307 thread-1 produce num : guyue--20308 thread-2 consumer num is : guyue--20308
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