一、 多线程锁synchronized
- 概念说明:synchronized是java语言的关键字,当它用来修饰一个方法或者一个代码块的时候,能保证在同一时刻最多只有一个线程执行该段代码。
- 使用方式:
a. 修饰代码块:注意在锁的时候,一定要保证是同一个对象
/**
* 不使用synchronized加锁的情况下,线程A,B交替执行
* 可能输出的结果:
* Thread B print 0
* Thread B print 1
* Thread B print 2
* Thread A print 0
* Thread B print 3
* Thread A print 1
* Thread B print 4
* Thread A print 2
* Thread A print 3
* Thread A print 4
*/
public class SyncDemo implements Runnable {
@Override
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + " print " + i);
}
}
public static void main(String[] args) {
SyncDemo syncDemo = new SyncDemo();
Thread threadA = new Thread(syncDemo, "Thread A");
Thread threadB = new Thread(syncDemo, "Thread B");
threadA.start();
threadB.start();
}
}
/**
* 使用synchronized加锁的情况下,线程A和线程B必须排队等待其中一个线程执行完成释放锁后才能继续执行
* 可能输出的结果:
* Thread A print 0
* Thread A print 1
* Thread A print 2
* Thread A print 3
* Thread A print 4
* Thread B print 0
* Thread B print 1
* Thread B print 2
* Thread B print 3
* Thread B print 4
*/
public class SyncDemo implements Runnable {
@Override
public void run() {
synchronized (this) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + " print " + i);
}
}
}
public static void main(String[] args) {
SyncDemo syncDemo = new SyncDemo();
Thread threadA = new Thread(syncDemo, "Thread A");
Thread threadB = new Thread(syncDemo, "Thread B");
threadA.start();
threadB.start();
// 稍微改动一下,此时为两个不同对象,则无法保证线程C和线程D必须等待对方执行完成才继续执行
Thread threadC = new Thread(new SyncDemo(), "Thread C");
Thread threadD = new Thread(new SyncDemo(), "Thread D");
threadC.start();
threadD.start();
}
}
b. 修饰普通方法:
/**
* 使用synchronized修饰普通方法,线程A和线程B必须排队等待其中一个线程执行完成释放锁后才能继续执行
* 可能输出的结果:
* Thread A print 0
* Thread A print 1
* Thread A print 2
* Thread A print 3
* Thread A print 4
* Thread B print 0
* Thread B print 1
* Thread B print 2
* Thread B print 3
* Thread B print 4
*/
public class SyncDemo implements Runnable {
@Override
public synchronized void run() {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + " print " + i);
}
}
public static void main(String[] args) {
SyncDemo syncDemo = new SyncDemo();
Thread threadA = new Thread(syncDemo, "Thread A");
Thread threadB = new Thread(syncDemo, "Thread B");
threadA.start();
threadB.start();
}
}
c. 修饰静态方法:上面的演示了,当创建两个不同的对象时,添加synchronized块或者是在方法上面添加synchronized,两个不同对象是彼此独立的锁,但是使用在静态方法上面使用synchronized,两个对象却保持了同步,这是因为run中调用的静态方法,而静态方法是属于同一个类的,所有两个对象相当与持有了同一把锁。
/**
* synchronized修饰静态方法,当两个不同的对象同时启动静态方法,因为静态方法是同一个方法,
* 无论你创建多少个类对象,静态方法都是同一个,则线程A和线程B在调用静态方法时,都持有同一
* 把锁
* 可能输出的结果:
* Thread A print 0
* Thread A print 1
* Thread A print 2
* Thread A print 3
* Thread A print 4
* Thread B print 0
* Thread B print 1
* Thread B print 2
* Thread B print 3
* Thread B print 4
*/
public class SyncDemo implements Runnable {
@Override
public void run() {
method();
}
public synchronized static void method() {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + " print " + i);
}
}
public static void main(String[] args) {
// 此时线程A和线程B为不同对象的线程
Thread threadA = new Thread(new SyncDemo(), "Thread A");
Thread threadB = new Thread(new SyncDemo(), "Thread B");
threadA.start();
threadB.start();
}
}
二. 多线程之volatile
- 概念:volatile关键字修饰一个共享变量(类的成员变量、类的静态成员变量),则会具备两层意义:
a. 保证了不同线程对这个变量进行操作时的可见性,即一个线程修改了某个变量的值,则新值对其他线程来说是立即可见的。
b. 禁止指令进行重排序
/**
* 使用volatile终止线程
*/
public class VolatileDemo implements Runnable {
// 如果不是用volatile关键字,可以能导致线程死锁,永远无法终止线程
private static volatile boolean flag = false;
@Override
public void run() {
while (!flag) {
System.out.println(Thread.currentThread().getName() + "线程被中断");
}
}
public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(new VolatileDemo(), "Thread A ");
thread.start();
TimeUnit.SECONDS.sleep(5);
flag = true;
}
}
- volatile关键字无法保证原子性,仅仅只能保证可见性:
/**
* volatile关键字无法保证操作的原子性,只能保证可见性
* 结果始终都会小于10000
*/
public class VolatileDemo2 {
private volatile int num = 0;
public void increase() {
num++;
}
public static void main(String[] args) throws InterruptedException {
CountDownLatch countDownLatch = new CountDownLatch(10);
final VolatileDemo2 volatileDemo = new VolatileDemo2();
for (int i = 0; i < 10; i++) {
new Thread(() -> {
try {
for (int j = 0; j < 1000; j++) {
volatileDemo.increase();
}
} finally {
countDownLatch.countDown();
}
}).start();
}
// 需要测试结果,需要让所有的线程全部执行完成
countDownLatch.await();
System.out.println(volatileDemo.num);
}
}