公平锁和非公平锁
1.公平锁，是指多个线程按照申请的顺序来获取锁，类似排队打饭，先来后到。
2.非公平锁，是指多个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锁，在高并发情况下，有可能会造成优先级反转或者饥饿现象。
Java ReentrantLock而言，通过构造函数指定该锁是否是公平锁，默认是非公平锁，非公平锁的优点在于吞吐量比公平锁大。
对于Synchronized而言，也是一种非公平锁。
可重入锁（也叫做递归锁）
指的是同一个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。
ReentrantLock/Synchronized就是一个典型的可重入锁，可重入锁的最大作用是避免死锁。
例子：
class Photo implements Runnable{
public synchronized void sendSMS() throws Exception {
System.out.println(Thread.currentThread().getName()+" invoked sendSMS()");
sendEmail();
}
public synchronized void sendEmail() throws Exception {
System.out.println(Thread.currentThread().getName()+" invoked sendEmail()");
}
@Override
public void run() {
get();
}
Lock lock = new ReentrantLock();
public void get() {
lock.lock();
try {
System.out.println(Thread.currentThread().getName()+" invoked sendSMS()");
set();
} finally {
lock.unlock();
}
}
public void set() {
lock.lock();
try {
System.out.println(Thread.currentThread().getName()+" invoked sendEmail()");
} finally {
lock.unlock();
}
}
}
public class ReentrantLockDemo {
public static void main(String[] args) {
Photo photo = new Photo();
new Thread(() ->{
try {
photo.sendSMS();
} catch (Exception e) {
e.printStackTrace();
}
},"t1").start();
new Thread(() ->{
try {
photo.sendSMS();
} catch (Exception e) {
e.printStackTrace();
}
},"t2").start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("------------------------");
Thread t3 = new Thread(photo,"t3");
Thread t4 = new Thread(photo,"t4");
t3.start();
t4.start();
}
}
运行结果见下图
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
//unsafe.getAndAddInt(Object var1, long var2,int var4) {
int var 5;
do {
var5 = this.getIntVolatile(var1,var2);
} while(!this.compareAndSwapInt(var1,var2,var5,var5 + var4))
return var5;
}
例子：
public class SpinLockDemo {
//原子引用线程
AtomicReference<Thread> atomicReference = new AtomicReference<>();
public void myLock() {
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName()+" come in");
while (!atomicReference.compareAndSet(null,thread)){
}
}
public void myUnlock() {
Thread thread = Thread.currentThread();
atomicReference.compareAndSet(thread,null);
System.out.println(Thread.currentThread().getName()+" come out");
}
public static void main(String[] args) {
SpinLockDemo spinLockDemo = new SpinLockDemo();
new Thread(() ->{
spinLockDemo.myLock();
try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
spinLockDemo.myUnlock();
},"t1").start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
new Thread(() ->{
spinLockDemo.myLock();
spinLockDemo.myUnlock();
},"t2").start();
}
}
结果如下：
独占锁：指该锁一次只能被一个线程所持有，对ReentrantLock和Synchronized而言都是独占锁
共享锁：指该锁可被多个线程所持有，对于ReentrantReadWriteLock其读锁是共享锁，其写锁是独占锁。
读锁的共享锁可保证并发读是非常高效的，读写，写读，写写的过程是互斥的
读写锁分离例子：
class MyReadWrite {
private volatile Map<String, Object> map = new HashMap<>();
//private Lock lock = new ReentrantLock();
private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
public void put(String key, Object value) {
readWriteLock.writeLock().lock();
try {
System.out.println(Thread.currentThread().getName() + " 正在写入：" + key);
try {
TimeUnit.MILLISECONDS.sleep(300);
} catch (InterruptedException e) {
e.printStackTrace();
}
map.put(key, value);
System.out.println(Thread.currentThread().getName() + " 写入完成：" + key);
} catch (Exception e) {
e.printStackTrace();
}finally {
readWriteLock.writeLock().unlock();
}
}
public void get(String key) {
readWriteLock.readLock().lock();
try {
System.out.println(Thread.currentThread().getName() + " 正在读取：");
try {
TimeUnit.MILLISECONDS.sleep(300);
} catch (InterruptedException e) {
e.printStackTrace();
}
Object result = map.get(key);
System.out.println(Thread.currentThread().getName() + " 读取完成：" + result);
} catch (Exception e) {
e.printStackTrace();
}finally {
readWriteLock.readLock().unlock();
}
}
}
public class ReadWriteLockDemo {
public static void main(String[] args) {
MyReadWrite myReadWrite = new MyReadWrite();
//模拟5个线程读写
for(int i = 1; i <= 5; i++) {
final int tempInt = i;
new Thread(() ->{
myReadWrite.put(tempInt+"",tempInt+"");
},String.valueOf(i)).start();
}
for(int i = 1; i <= 5; i++) {
final int tempInt = i;
new Thread(() ->{
myReadWrite.get(tempInt+"");
},String.valueOf(i)).start();
}
}
}
结果如下：
