公平锁
是指多个线程按照申请锁的顺序来获取锁类似排队打饭,先来后到
非公平锁
是指在多线程获取锁的顺序并不是按照申请锁的顺序,有可能后申请的线程比先申请的线程优先获取到锁,在高并发的情况下,有可能造成优先级反转或者饥饿现象
两者区别
公平锁
Threads acquire a fair lock in the order in which they requested it.
公平锁,就是很公平,在并发环境中,每个线程在获取锁时会先查看此锁维护的等待队列,如果为空,或者当前线程是等待队列的第一个,就占有锁,否则就会加入到等待队列中,以后会按照 FIFO 的规则从队列中取到自己
非公平锁
a nonfair lock permits barging: threads requesting a lock can jump ahead of the queue of waiting threads if the lock happens to be available when it is requested.
非公平锁比较粗鲁,上来就直接尝试占有锁,如果尝试失败,就再采用类似公平锁那种方式。
可重入锁
可重入锁(也叫做递归锁),指的是同一线程外层函数获得锁之后,内层递归函数仍然能获取该锁的代码,在同一个线程在外层方法获取锁的时候,在进入内层方法会自动获取锁。也即是说,线程可以进入任何一个它已经拥有的锁所同步着的代码块。
可重入锁最大的作用是避免死锁
在 java 中,synchronized 和 ReenterLock 都是可重入锁。synchronized 是非公平锁,ReentrantLock 可以通过构造方法传布尔值来确定是公平锁还是非公平锁。
SyncDemo
class People{
public synchronized void look() {
System.out.println(Thread.currentThread().getName()+"\t invoked look()");
eat();
}
public synchronized void eat() {
System.out.println(Thread.currentThread().getName()+"\t invoked eat()");
}
}
/* Java 可重入锁 SynchronizedDemo 示例 */
public class SynchronizedDemo {
public static void main(String[] args) {
People people = new People();
new Thread(()->{
people.look();
},"t1").start();
new Thread(()->{
people.look();
},"t2").start();
}
}
ReentrantLockDemo
class People{
ReentrantLock lock = new ReentrantLock();
public void look() {
lock.lock();
System.out.println(Thread.currentThread().getName()+"\t invoked look()");
eat();
lock.unlock();
}
public void eat() {
lock.lock();
System.out.println(Thread.currentThread().getName()+"\t invoked eat()");
lock.unlock();
}
}
/* Java 可重入锁 ReentrantLockDemo 示例 */
public class ReentrantLockDemo {
public static void main(String[] args) {
People people = new People();
new Thread(()->{
people.look();
},"t1").start();
new Thread(()->{
people.look();
},"t2").start();
}
}
自旋锁
自旋锁(spinlock)是指尝试获取锁的线程不会立即阻塞,而是采用循环的方式去尝试获取锁,这样的好处是减少线程上下文切换的消耗,缺点是循环会消耗CPU。
手撸自旋锁
自旋锁 = while循环 + CAS
import java.util.concurrent.atomic.AtomicReference;
/* 手写一个自旋锁 */
public class SpinLockDemo {
// 原子引用 Thread
AtomicReference<Thread> atomicReference = new AtomicReference<>();
// 上锁
public void spinLock(){
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName()+"\t invoked lock");
while(!atomicReference.compareAndSet(null, thread)) {
// 自旋
}
}
// 解锁
public void spinUnlock(){
Thread thread = Thread.currentThread();
atomicReference.compareAndSet(thread, null); //释放锁
System.out.println(Thread.currentThread().getName()+"\t invoked unlock");
}
public static void main(String[] args) {
SpinLockDemo lock = new SpinLockDemo();
new Thread(()->{
lock.spinLock();
try {
Thread.sleep(2000);//休眠5s
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.spinUnlock();
},"t1").start();
new Thread(()->{
lock.spinLock();
try {
Thread.sleep(1000);//休眠5s
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.spinUnlock();
},"t2").start();
}
}