Java多線程之死結與線程間通訊簡單案例

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死結定義
死結是指兩個或者多個線程被永久阻塞的一種局面，產生的前提是要有兩個或兩個以上的線程，並且來操作兩個或者多個以上的共同資源；我的理解是用兩個線程來舉例，現有線程A和B同時操作兩個共同資源a和b，A操作a的時候上鎖LockA,繼續執行的時候，A還需要LockB進行下面的操作，這個時候b資源在被B線程操作，剛好被上了鎖LockB，假如此時線程B剛好釋放了LockB則沒有問題，但沒有釋放LockB鎖的時候，線程A和B形成了對LockB鎖資源的爭奪，從而造成阻塞，形成死結；具體其死結代碼如下：

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");                    }                }            }    }}

執行結果如：

Thread-1 acquie lock is objThread-0 acquie lock is DeadRes.class

當然每次執行的結果不一樣，有可能是一種和諧狀態，沒有發生死結，此時為保證每次死結，可以讓run()方法中，執行while(true)迴圈，這樣保證了每次必定發生死結；當然實際應用中，我們應該盡量避免死結，當有多線程操作多個共同資源的時候，避免發生同一鎖對象的同步嵌套。

線程間的通訊—-生產者與消費者模式
1、讓兩個線程交替進行操作，當生產了一個數字後，緊接著消費一個，首先採用Object對象中的wait-notify來實現，具體代碼如下:

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();        }    }}

執行結果

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

當超過兩個以上線程操作的時候，這裡需要在set()與out()方法中的if判斷改為while，並且notif方法，改為notifyAll()，這樣多個線程操作的時候，便可以交替進行，具體代碼如下：

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();    }}

執行結果如下：

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、採用Lock-Condition方法實現如下：

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();        }    }}

執行結果如下：

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

Java多線程之死結與線程間通訊簡單案例