Java多线程之死锁与线程间通信简单案例

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 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:

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--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：

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--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：

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--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