java基础巩固笔记(5)-多线程之线程并发库

时间:2023-02-14 20:51:04

java基础巩固笔记(5)-多线程之线程并发库

标签: java



本文主要概述java.util.concurrent包下的相关类和使用方法

Package java.util.concurrent

原子性操作类

java.util.concurrent.atomic包下的类:

Package java.util.concurrent.atomic

线程池

java.util.concurrent:Class Executors

常用线程池

几种常用的的生成线程池的方法:

  • newCachedThreadPool
  • newFixedThreadPool
  • newScheduledThreadPool
  • newSingleThreadExecutor
  • newSingleThreadScheduledExecutor

例子:newFixedThreadPool

ExecutorService threadPool = Executors.newFixedThreadPool(3);
for(int i=0;i<10;i++){
threadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread().getName());
}
});
}

单线程newSingleThreadExecutor可用于重启

用线程池启动定时器

例子:类似Timer的定时执行

Executors.newScheduledThreadPool(3).scheduleAtFixedRate(
new Runnable() {
@Override
public void run() {
System.out.println("ScheduledThreadPool "+Thread.currentThread().getName());
}
},3,1, TimeUnit.SECONDS
);

Callable&Future

ExecutorServiceExecutor的基础上增加了一些方法,其中有两个核心的方法:

  • Future<?> submit(Runnable task)
  • <T> Future<T> submit(Callable<T> task)

这两个方法都是向线程池中提交任务,它们的区别在于Runnable在执行完毕后没有结果,Callable执行完毕后有一个结果。这在多个线程中传递状态和结果是非常有用的。另外他们的相同点在于都返回一个Future对象。Future对象可以阻塞线程直到运行完毕(获取结果,如果有的话),也可以取消任务执行,当然也能够检测任务是否被取消或者是否执行完毕。

Lock&Condition

Lock

Lock功能类似传统多线程技术里的synchronized,实现线程互斥,但更加面向对象。将需要互斥的代码片段放到lock.lock();lock.unlock();之间。

例子

class A{
private Lock lock = new ReentrantLock();

public void function(){
lock.lock();
try{
//功能代码
}finally{
lock.unlock();
}
}
}
  • 读写锁

java.util.concurrent.locks:Class ReentrantReadWriteLock

javaDoc文档读写锁例子,缓存:

class CachedData {
Object data;
volatile boolean cacheValid;
final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

void processCachedData() {
rwl.readLock().lock();
if (!cacheValid) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
try {
// Recheck state because another thread might have
// acquired write lock and changed state before we did.
if (!cacheValid) {
data = ...
cacheValid = true;
}
// Downgrade by acquiring read lock before releasing write lock
rwl.readLock().lock();
} finally {
rwl.writeLock().unlock(); // Unlock write, still hold read
}
}

try {
use(data);
} finally {
rwl.readLock().unlock();
}
}
}

重点注意在释放写锁前加读锁那部分代码,注释为// Downgrade by acquiring read lock before releasing write lock。自己挂了写锁,再挂读锁是可以的,这面涉及的技巧以后再研究。

Condition

Condition类似于传统多线程技术中的Object.waitObject.notify,实现线程间同步。

javaDoc文档例子,可阻塞队列

class BoundedBuffer例子

class BoundedBuffer {
final Lock lock = new ReentrantLock();
final Condition notFull = lock.newCondition();
final Condition notEmpty = lock.newCondition();

final Object[] items = new Object[100];
int putptr, takeptr, count;

public void put(Object x) throws InterruptedException {
lock.lock();
try {
while (count == items.length)
notFull.await();
items[putptr] = x;
if (++putptr == items.length) putptr = 0;
++count;
notEmpty.signal();
} finally {
lock.unlock();
}
}

public Object take() throws InterruptedException {
lock.lock();
try {
while (count == 0)
notEmpty.await();
Object x = items[takeptr];
if (++takeptr == items.length) takeptr = 0;
--count;
notFull.signal();
return x;
} finally {
lock.unlock();
}
}
}

使用了两个condition

同步工具

  • Semaphore

类似占坑

  • CyclicBarrier

阶段性使进度一致

  • CountDownLatch

一人通知多人/多人通知一人

  • Exchanger

线程间数据交换,都到达则自然交换

参考资料


作者@brianway更多文章:个人网站 | CSDN | oschina