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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