Java多线程间同步
1、什么是线程安全
通过一个案例了解线程安全
案例:需求现在有100张火车票，有两个窗口同时抢火车票，请使用多线程模拟抢票效果。
先来看一个线程不安全的例子
class SellTicketRunnable implements Runnable {
public int count = 100;
@Override
public void run() {
while (count > 0) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
int index = 100 - count + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
count--;
}
}
}
public class JavaSyncDemo {
public static void main(String[] args) {
SellTicketRunnable runnable = new SellTicketRunnable();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
可以看到两个线程同时卖票的时候，会出现漏卖，多卖同一张票，还会出现超卖的问题，这就是线程不安全的问题。
当多个线程同时共享，同一个全局变量或静态变量，做写的操作时，可能会发生数据冲突问题，也就是线程安全问题。但是做读操作是不会发生数据冲突问题。
2、线程安全问题的解决办法
(1)使用同步代码块
class SellTicketRunnable implements Runnable {
public int count = 100;
private Object lock = new Object();
@Override
public void run() {
while (count > 0) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock) {
if (count > 0) {
int index = 100 - count + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
count--;
}
}
}
}
}
public class JavaSyncDemo {
public static void main(String[] args) {
SellTicketRunnable runnable = new SellTicketRunnable();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
从上面的案例可以看出，使用synchronized同步代码块包裹住写操作，每个线程在调用同步代码块中逻辑的时候，都需要先获取同步锁，所以避免了多线程写操作数据的冲突问题。
(2)使用同步函数
class SellTicketRunnable01 implements Runnable {
public int count = 100;
@Override
public void run() {
while (count > 0) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
this.sale();
}
}
synchronized void sale() {
if (count > 0) {
int index = 100 - count + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
count--;
}
}
}
public class JavaSyncDemo01 {
public static void main(String[] args) {
SellTicketRunnable01 runnable = new SellTicketRunnable01();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
synchronized包裹的函数，其实就是给该函数块添加了一把this锁。
注意：synchronized 修饰静态方法使用锁是当前类的字节码文件（即
类名.class），同理，如果在静态方法中添加个同步代码块，可以获取类名.class为代码块加锁
class SellTicketRunnable02 implements Runnable {
public static int count = 100;
@Override
public void run() {
while (count > 0) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
SellTicketRunnable02.sale();
}
}
static void sale() {
synchronized (SellTicketRunnable02.class) {
if (count > 0) {
int index = 100 - count + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
count--;
}
}
}
}
public class JavaSyncDemo02 {
public static void main(String[] args) {
SellTicketRunnable02 runnable = new SellTicketRunnable02();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
(3)使用lock锁
class SellTicketRunnable03 implements Runnable {
public int count = 100;
private Lock lock = new ReentrantLock();
@Override
public void run() {
while (count > 0) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.lock();
if (count > 0) {
int index = 100 - count + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
count--;
}
lock.unlock();
}
}
}
public class JavaSyncDemo03 {
public static void main(String[] args) {
SellTicketRunnable03 runnable = new SellTicketRunnable03();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
lock和synchronized的区别
①lock在使用时需要手动的获取锁和释放锁;
②lock可以尝试非阻塞的获取锁，如果这一时刻锁没有被其他线程获取到，则成功获取并持有锁；
③lock锁可以响应中断，当获取到锁的线程被中断时，中断异常会被抛出，同时锁被释放；
④lock在指定截至时间之前获取锁，如果解释时间到了依旧无法获取锁，就返回。
// lock锁的安全使用方法
class lockDemo {
Lock lock = new ReentrantLock();
void demoFun() {
lock.lock();
try {
// 可能出现线程安全的操作
} finally {
lock.unlock();
}
}
}
(4)使用Java原子类
java.util.concurrent.atomic.AtomicBoolean;java.util.concurrent.atomic.AtomicInteger;java.util.concurrent.atomic.AtomicLong;java.util.concurrent.atomic.AtomicReference;
class SellTicketRunnable04 implements Runnable {
public AtomicInteger count = new AtomicInteger(100);
@Override
public void run() {
while (true) {
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (count.get() > 0) {
int index = 100 - count.getAndDecrement() + 1;
System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
}
}
}
}
public class JavaSyncDemo04 {
public static void main(String[] args) {
SellTicketRunnable04 runnable = new SellTicketRunnable04();
Thread sellThread1 = new Thread(runnable);
Thread sellThread2 = new Thread(runnable);
sellThread1.start();
sellThread2.start();
}
}
3、死锁
先看一个死锁的示例
public class DeadLockDemo01 {
private static Object lock1 = new Object();
private static Object lock2 = new Object();
public static void main(String[] args) {
new Thread() { //线程1
public void run() {
while (true) {
synchronized (lock1) {
System.out.println(this.getName() + ":获取lock1锁");
synchronized (lock2) {
System.out.println(this.getName() + ":获取lock2锁");
}
}
}
}
}.start();
new Thread() { //线程2
public void run() {
while (true) {
synchronized (lock2) {
System.out.println(this.getName() + ":获取lock2锁");
synchronized (lock1) {
System.out.println(this.getName() + "::获取lock1锁");
}
}
}
}
}.start();
}
}
运行上面的代码，可以观察到线程卡死，就是出现了死锁
线程1先拿到lock1锁，再拿到lock2锁，执行完成后才能释放所有锁；
线程2先拿到lock2锁，再拿到lock1锁，执行完成后才能释放所有锁。
如果在线程1获取到lock1锁的时候，线程2获取到lock2还没释放，线程1无法获取lock2锁，也就无法释放lock2锁，这时系统就会出现死锁。
线程死锁的避免办法：不要在同步中嵌套同步