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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