多线程与高并发三-Atomic类和线程同步新机制
Atomic Vs Sync Vs LongAdder
package com.mashibing.juc.c_018_00_AtomicXXX;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;
public class T02_AtomicVsSyncVsLongAdder {
static long count2 = 0L;
static AtomicLong count1 = new AtomicLong(0L);
static LongAdder count3 = new LongAdder();
public static void main(String[] args) throws Exception {
Thread[] threads = new Thread[1000];
for(int i=0; i<threads.length; i++) {
threads[i] =
new Thread(()-> {
for(int k=0; k<100000; k++) count1.incrementAndGet();
});
}
long start = System.currentTimeMillis();
for(Thread t : threads ) t.start();
for (Thread t : threads) t.join();
long end = System.currentTimeMillis();
//TimeUnit.SECONDS.sleep(10);
System.out.println("Atomic: " + count1.get() + " time " + (end-start));
//-----------------------------------------------------------
Object lock = new Object();
for(int i=0; i<threads.length; i++) {
threads[i] =
new Thread(new Runnable() {
@Override
public void run() {
for (int k = 0; k < 100000; k++)
synchronized (lock) {
count2++;
}
}
});
}
start = System.currentTimeMillis();
for(Thread t : threads ) t.start();
for (Thread t : threads) t.join();
end = System.currentTimeMillis();
System.out.println("Sync: " + count2 + " time " + (end-start));
//----------------------------------
for(int i=0; i<threads.length; i++) {
threads[i] =
new Thread(()-> {
for(int k=0; k<100000; k++) count3.increment();
});
}
start = System.currentTimeMillis();
for(Thread t : threads ) t.start();
for (Thread t : threads) t.join();
end = System.currentTimeMillis();
//TimeUnit.SECONDS.sleep(10);
System.out.println("LongAdder: " + count1.longValue() + " time " + (end-start));
}
static void microSleep(int m) {
try {
TimeUnit.MICROSECONDS.sleep(m);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
结果
Atomic: 100000000 time 1459
Sync: 100000000 time 3666
LongAdder: 100000000 time 407
Process finished with exit code 0
为什么Atomic要比Sync快?
因为不加锁,刚刚我们说了synchronized是要加锁的,有可能它要去操作系统申请重量级锁,所以synchronized效率偏低,在这种情形下效率偏低。
LongAdder为什么要比Atomic效率要高呢?
是因为LongAdder的内部做了一个分段锁,类似于分段锁的概念。在它内部的时候,会把一个值放到一个数组里,比如说数组长度是4,最开始是0,1000个线程,250个线程锁在第一个数租元素里,以此类推,每一个都往上递增算出来结果在加到一起。
ReentrantLock
可重入锁ReentranLlock
写synchronized的地方换写lock.lock(),加完锁之后需要注意的是记得lock.unlock()解锁,由于synchronized是自动解锁的,大括号执行完就结束了。lock就不行,lock必须得手动解锁,手动解锁一定要写在try...fifinally里面保证最好一定要解锁
/**
* reentrantlock用于替代synchronized
* 由于m1锁定this,只有m1执行完毕的时候,m2才能执行
* 这里是复习synchronized最原始的语义
*
* 使用reentrantlock可以完成同样的功能
* 需要注意的是,必须要必须要必须要手动释放锁(重要的事情说三遍)
* 使用syn锁定的话如果遇到异常,jvm会自动释放锁,但是lock必须手动释放锁,因此经常在finally中进行锁的释放
* @author mashibing
*/
package com.mashibing.juc.c_020;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class T02_ReentrantLock2 {
Lock lock = new ReentrantLock();
void m1() {
try {
lock.lock(); //synchronized(this)
for (int i = 0; i < 10; i++) {
TimeUnit.SECONDS.sleep(1);
System.out.println(i);
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
void m2() {
try {
lock.lock();
System.out.println("m2 ...");
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
T02_ReentrantLock2 rl = new T02_ReentrantLock2();
new Thread(rl::m1).start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
new Thread(rl::m2).start();
}
}
特性
- 用tryLock进行尝试锁定
- ReentrantLock还可以用lock.lockInterruptibly() 这个类,对interrupt()方法做出相应,可以被打断的加锁
- ReentrantLock默认是非公平锁。当我们new一个ReentrantLock你可以传一个参数为true,这个true表示公平锁,公平锁的意思是谁等在前面就先让谁执行,而不是说谁后来了之后就马上让谁执行。如果说这个锁不公平,来了一个线程上来就抢,它是有可能抢到的,如果说这个锁是个公平锁,这个线程上来会先检查队列里有没有原来等着的,如果有的话他就先进队列里等着别人先运行,这是公平锁的概念。
Reentrantlock vs synchronized
ReentrantLock可以替代synchronized这是没问题的,他也可以重入,可以锁定的。本身的底层是cas
- trylock:自己来控制,我锁不住该怎么办
- lockInterruptibly:这个类,中间你还可以被打断
- 它还可以公平和非公平的切换
CountDownLatch
CountDownLatch是一个同步工具类,用来协调多个线程之间的同步,或者说起到线程之间的通信(而不是用作互斥的作用)。在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。
package com.mashibing.juc.c_020;
import java.util.concurrent.CountDownLatch;
public class T06_TestCountDownLatch {
public static void main(String[] args) {
usingJoin();
usingCountDownLatch();
}
private static void usingCountDownLatch() {
Thread[] threads = new Thread[100];
CountDownLatch latch = new CountDownLatch(threads.length);
for(int i=0; i<threads.length; i++) {
threads[i] = new Thread(()->{
int result = 0;
for(int j=0; j<10000; j++) result += j;
latch.countDown();
});
}
for (int i = 0; i < threads.length; i++) {
threads[i].start();
}
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("end latch");
}
private static void usingJoin() {
Thread[] threads = new Thread[100];
for(int i=0; i<threads.length; i++) {
threads[i] = new Thread(()->{
int result = 0;
for(int j=0; j<10000; j++) result += j;
});
}
for (int i = 0; i < threads.length; i++) {
threads[i].start();
}
for (int i = 0; i < threads.length; i++) {
try {
threads[i].join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("end join");
}
}
CyclicBarrier
现实生活中我们经常会遇到这样的情景,在进行某个活动前需要等待人全部都齐了才开始。例如吃饭时要等全家人都上座了才动筷子,旅游时要等全部人都到齐了才出发,比赛时要等运动员都上场后才开始。利用CyclicBarrier类可以实现一组线程相互等待,当所有线程都到达某个屏障点后再进行后续的操作。
可以用于多线程计算数据,最后合并计算结果的场景。
package com.mashibing.juc.c_020;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class T07_TestCyclicBarrier {
public static void main(String[] args) {
//CyclicBarrier barrier = new CyclicBarrier(20);
CyclicBarrier barrier = new CyclicBarrier(20, () -> System.out.println("满人"));
/*CyclicBarrier barrier = new CyclicBarrier(20, new Runnable() {
@Override
public void run() {
System.out.println("满人,发车");
}
});*/
for(int i=0; i<100; i++) {
new Thread(()->{
try {
barrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}).start();
}
}
}
Phaser
Phaser一种可重用的同步屏障,功能上类似于CyclicBarrier和CountDownLatch,但使用上更为灵活。非常适用于在多线程环境下同步协调分阶段计算任务(Fork/Join框架中的子任务之间需同步时,优先使用Phaser)
package com.mashibing.juc.c_020;
import java.util.Random;
import java.util.concurrent.Phaser;
import java.util.concurrent.TimeUnit;
public class T09_TestPhaser2 {
static Random r = new Random();
static MarriagePhaser phaser = new MarriagePhaser();
static void milliSleep(int milli) {
try {
TimeUnit.MILLISECONDS.sleep(milli);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
phaser.bulkRegister(7);
for(int i=0; i<5; i++) {
new Thread(new Person("p" + i)).start();
}
new Thread(new Person("新郎")).start();
new Thread(new Person("新娘")).start();
}
static class MarriagePhaser extends Phaser {
@Override
protected boolean onAdvance(int phase, int registeredParties) {
switch (phase) {
case 0:
System.out.println("所有人到齐了!" + registeredParties);
System.out.println();
return false;
case 1:
System.out.println("所有人吃完了!" + registeredParties);
System.out.println();
return false;
case 2:
System.out.println("所有人离开了!" + registeredParties);
System.out.println();
return false;
case 3:
System.out.println("婚礼结束!新郎新娘抱抱!" + registeredParties);
return true;
default:
return true;
}
}
}
static class Person implements Runnable {
String name;
public Person(String name) {
this.name = name;
}
public void arrive() {
milliSleep(r.nextInt(1000));
System.out.printf("%s 到达现场!\n", name);
phaser.arriveAndAwaitAdvance();
}
public void eat() {
milliSleep(r.nextInt(1000));
System.out.printf("%s 吃完!\n", name);
phaser.arriveAndAwaitAdvance();
}
public void leave() {
milliSleep(r.nextInt(1000));
System.out.printf("%s 离开!\n", name);
phaser.arriveAndAwaitAdvance();
}
private void hug() {
if(name.equals("新郎") || name.equals("新娘")) {
milliSleep(r.nextInt(1000));
System.out.printf("%s 洞房!\n", name);
phaser.arriveAndAwaitAdvance();
} else {
phaser.arriveAndDeregister();
//phaser.register()
}
}
@Override
public void run() {
arrive();
eat();
leave();
hug();
}
}
}
ReadWriteLock
ReadWriteLock 是读写锁。读写锁的概念其实就是共享锁和排他锁,读锁就是共享锁,写锁就是排他锁。那这个是什么意思,我们先要来理解这件事儿,读写有很多种情况,比如说你数据库里的某条儿数据你放在内存里读的时候特别多,而改的时候并不多。
而读写锁有以下三个重要的特性:
- 公平选择性:支持非公平(默认)和公平的锁获取方式,吞吐量还是非公平优于公平。
- 重进入:读锁和写锁都支持线程重进入。
- 锁降级:遵循获取写锁、获取读锁再释放写锁的次序,写锁能够降级成为读锁。
package com.mashibing.juc.c_020;
import java.util.Random;
import java.util.concurrent.atomic.LongAdder;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class T10_TestReadWriteLock {
static Lock lock = new ReentrantLock();
private static int value;
static ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
static Lock readLock = readWriteLock.readLock();
static Lock writeLock = readWriteLock.writeLock();
public static void read(Lock lock) {
try {
lock.lock();
Thread.sleep(1000);
System.out.println("read over!");
//模拟读取操作
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void write(Lock lock, int v) {
try {
lock.lock();
Thread.sleep(1000);
value = v;
System.out.println("write over!");
//模拟写操作
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
//Runnable readR = ()-> read(lock);
Runnable readR = ()-> read(readLock);
//Runnable writeR = ()->write(lock, new Random().nextInt());
Runnable writeR = ()->write(writeLock, new Random().nextInt());
for(int i=0; i<18; i++) new Thread(readR).start();
for(int i=0; i<2; i++) new Thread(writeR).start();
}
}
Semaphore
Semaphore也是一个线程同步的辅助类,可以维护当前访问自身的线程个数,并提供了同步机制。使用Semaphore可以控制同时访问资源的线程个数,例如,实现一个文件允许的并发访问数。
Semaphore的含义就是限流,比如说你在买票,Semaphore写5就是只能有5个人可以同时买票。acquire的意思叫获得这把锁,线程如果想继续往下执行,必须得从Semaphore里面获得一个许可,他一共有5个许可用到0了你就得给我等着。
package com.mashibing.juc.c_020;
import java.util.concurrent.Semaphore;
public class T11_TestSemaphore {
public static void main(String[] args) {
//Semaphore s = new Semaphore(2);
Semaphore s = new Semaphore(2, true);
//允许一个线程同时执行
//Semaphore s = new Semaphore(1);
new Thread(()->{
try {
s.acquire();
System.out.println("T1 running...");
Thread.sleep(200);
System.out.println("T1 running...");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
s.release();
}
}).start();
new Thread(()->{
try {
s.acquire();
System.out.println("T2 running...");
Thread.sleep(200);
System.out.println("T2 running...");
s.release();
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
}
}
Exchanger
提供的一个用于两个工作线程之间交换数据的封装工具类,简单说就是一个线程在完成一定的事务后想与另一个线程交换数据,则第一个先拿出数据的线程会一直等待第二个线程,直到第二个线程拿着数据到来时才能彼此交换对应数据。
package com.mashibing.juc.c_020;
import java.util.concurrent.Exchanger;
public class T12_TestExchanger {
static Exchanger<String> exchanger = new Exchanger<>();
public static void main(String[] args) {
new Thread(()->{
String s = "T1";
try {
s = exchanger.exchange(s);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " " + s);
}, "t1").start();
new Thread(()->{
String s = "T2";
try {
s = exchanger.exchange(s);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " " + s);
}, "t2").start();
}
}
LockSupport
LockSupport是一个比较底层的工具类,用来创建锁和其他同步工具类的基本线程阻塞原语。java锁和同步器框架的核心 AQS:AbstractQueuedSynchronizer,就是通过调用 LockSupport .park()和 LockSupport .unpark()的方法,来实现线程的阻塞和唤醒的。
核心方法其实就两个:
park()
和unpark()
,其中park()
方法用来阻塞当前调用线程,unpark()
方法用于唤醒指定线程。
package com.mashibing.juc.c_020;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;
public class T13_TestLockSupport {
public static void main(String[] args) {
Thread t = new Thread(()->{
for (int i = 0; i < 10; i++) {
System.out.println(i);
if(i == 5) {
LockSupport.park();
}
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
t.start();
LockSupport.unpark(t);
/*try {
TimeUnit.SECONDS.sleep(8);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("after 8 senconds!");
LockSupport.unpark(t);*/
}
}
本文来自博客园,作者:gary2048,转载请注明原文链接:https://www.cnblogs.com/zhoum/p/15142390.html