线程池
常用线程池
几种常用的的生成线程池的方法：
newCachedThreadPoolnewFixedThreadPoolnewScheduledThreadPoolnewSingleThreadExecutornewSingleThreadScheduledExecutor
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
ExecutorService在Executor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();和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
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.wait和Object.notify,实现线程间同步。
javaDoc文档例子，可阻塞队列
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
