1. 同步控制
synchronized的扩展:重入锁
同步控制不仅有synchronized配合object.wait()以及object.notify(),也有增强版的reentrantLock(重入锁)
public class ReenterLock implements Runnable{
public static ReentrantLock lock=new ReentrantLock();
public static int i=0;
@Override
public void run() {
for(int j=0;j<10000000;j++){
lock.lock();
lock.lock(); //此处演示重入性
try{
i++;
}finally{
lock.unlock(); //退出临界区必须解锁
lock.unlock();
}
}
}
public static void main(String[] args) throws InterruptedException {
ReenterLock tl=new ReenterLock();
Thread t1=new Thread(tl);
Thread t2=new Thread(tl);
t1.start();t2.start();
t1.join();t2.join();
System.out.println(i); //计算结果为 20000000
}
}我们来看下reentrantlock相比synchronized锁有何优点:
中断响应
面对死锁,似乎synchronized没有任何主动解决策略,而reentrantlock则可以轻松解决
public class IntLock implements Runnable {
public static ReentrantLock lock1 = new ReentrantLock();
public static ReentrantLock lock2 = new ReentrantLock();
int lock;
/**
* 控制加锁顺序，方便构造死锁
* @param lock
*/
public IntLock(int lock) {
this.lock = lock;
}
@Override
public void run() {
try {
if (lock == 1) {
lock1.lockInterruptibly(); //可中断的加锁
try{
Thread.sleep(500);
}catch(InterruptedException e){}
lock2.lockInterruptibly();
} else {
lock2.lockInterruptibly();
try{
Thread.sleep(500);
}catch(InterruptedException e){}
lock1.lockInterruptibly();
}
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println(Thread.currentThread().getName()+":线程被中断");
} finally {
if (lock1.isHeldByCurrentThread())
lock1.unlock();
if (lock2.isHeldByCurrentThread())
lock2.unlock();
System.out.println(Thread.currentThread().getName()+":线程退出");
}
}
public static void main(String[] args) throws InterruptedException {
IntLock r1 = new IntLock(1);
IntLock r2 = new IntLock(2);
Thread t1 = new Thread(r1,"线程1");
Thread t2 = new Thread(r2,"线程2");
t1.start();t2.start();
Thread.sleep(1000);
//中断其中一个线程
t2.interrupt();
}
}
// 输出结果:
// java.lang.InterruptedException
// at //java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:898)
// at //java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
// at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
// at geym.conc.ch3.synctrl.IntLock.run(IntLock.java:31)
// at java.lang.Thread.run(Thread.java:745)
// 线程2:线程被中断
// 线程2:线程退出
// 线程1:线程退出由上可知,当t1,t2形成死锁时,可以主动利用中断来解开,但完成任务的只有t1,t2被中断. 而如果换成synchronized则将无法进行中断
锁申请等待时限
lock1.tryLock(); //尝试获取锁,获得立即返回true,未获得立即返回false
lock1.tryLock(5, TimeUnit.SECONDS); //尝试获取锁,5秒内未获得则返回false,获得返回truepublic class TryLock implements Runnable {
public static ReentrantLock lock1 = new ReentrantLock();
public static ReentrantLock lock2 = new ReentrantLock();
int lock;
public TryLock(int lock) {
this.lock = lock;
}
@Override
public void run() {
if (lock == 1) {
while (true) {
if (lock1.tryLock()) {
try {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
}
if (lock2.tryLock()) {
try {
System.out.println(Thread.currentThread()
.getId() + ":My Job done");
return;
} finally {
lock2.unlock();
}
}
} finally {
lock1.unlock();
}
}
}
} else {
while (true) {
if (lock2.tryLock()) {
try {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
}
if (lock1.tryLock()) {
try {
System.out.println(Thread.currentThread()
.getId() + ":My Job done");
return;
} finally {
lock1.unlock();
}
}
} finally {
lock2.unlock();
}
}
}
}
}
public static void main(String[] args) throws InterruptedException {
TryLock r1 = new TryLock(1);
TryLock r2 = new TryLock(2);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
}
// 15:My Job done
// 14:My Job done使用trylock可以有效地避免产生死锁
公平锁
synchronized锁为非公平锁,而reentrantLock既可以是公平锁也可以是非公平锁
非公平锁容易产生饥饿,公平锁先进先出,但效率不敌非公平锁
public ReentrantLock(boolean fair)重入锁的搭档Condition
Condition和object.wait(),object.notify()方法类似
condition的基本方法如下:
void await() throws InterruptedException; //使当前线程等待,释放锁,能响应signal和signalAll方法,响应中断
void awaitUninterruptibly(); //类似 await,但不响应中断
long awaitNanos(long nanosTimeout)throws InterruptedException; //等待一段时间
boolean await (long time,TimeUnit unit)throws InterruptedException;
boolean awaitUntil(Date deadline)throws InterruptedException;
void signal()； //唤醒一个等待中的线程
void signalAll(); //唤醒所有等待中的线程JDK内部就有很多对于ReentrantLock的使用,如ArrayBlockingQueue
//在 ArrayBlockingQueue中的一些定义
boolean fair = true;
private final ReentrantLock lock = new ReentrantLock(fair);
private final Condition notEmpty = lock.newCondition();
private final Condition notFull = lock.newCondition();
//put(方法的实现
public void put(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
final E[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lockInterruptibly(); //put方法做同步
try {
try {
while (count == items.length) //队列已满
notFull.await(); //等待队列有足够的空间
} catch (InterruptedException ie) {
notFull.signal();
throw ie;
}
insert(e); //notFull被通知时,说明有足够的空间
} finally {
lock.unlock();
}
}
private void insert(E x) {
items[putIndex] = x;
putIndex = inc(putIndex);
++count;
notFull.signal(); //通知take方法的线程,队列已有数据
}
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly(); //对take()方法做同步
try {
try {
while (count == 0) //如果队列为空
notEmpty.await(); //则消费者队列要等待一个非空的信号
} catch (InterruptedException ie) {
notEmpty.signal();
throw ie;
}
E x = extract();
return x;
} finally {
lock.unlock();
}
}
private E extract() {
final E[] items = this.items;
E x = items[takeIndex];
items[takeIndex] = null;
takeIndex = inc(takeIndex);
--count;
notFull.signal(); //通知put线程队列已有空闲空间
return x;
}多线程同时访问:信号量(semaphore)
同步锁只能允许一个线程进行访问,信号量可以指定多个线程同时访问同一个资源.
//构造方法
public Semaphore(int permits) //传入int表示能同时访问的线程数
public Semaphore(int permits, boolean fair) //线程数,是否公平锁
//实例方法
public void acquire() throws InterruptedException //获取一个访问权限,会阻塞线程,会被打断
public void acquireUninterruptibly() //获取一个访问权限,会阻塞线程,不会被打断
public boolean tryAcquire() //获取一个访问权限,立即返回
public boolean tryAcquire(long timeout, TimeUnit unit) //获取一个访问权限,尝试一段时间
public void release() //释放一个访问权限public class SemapDemo implements Runnable {
final Semaphore semp = new Semaphore(5);
@Override
public void run() {
try {
semp.acquire();
Thread.sleep(2000);
System.out.println(Thread.currentThread().getId() + ":done!");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
semp.release(); //使用完后要释放,否则会引起信号量泄漏
}
}
public static void main(String[] args) {
ExecutorService exec = Executors.newFixedThreadPool(20);
final SemapDemo demo = new SemapDemo();
for (int i = 0; i < 20; i++) {
exec.submit(demo);
}
}
}
//输出结果
//每次输出5个结果,对应信号量的5个许可读写锁ReadWriteLock
读写锁适用于读多写少的场景,读读之间为并行,读写之间为串行,写写之间也为串行public class ReadWriteLockDemo {
private static Lock lock=new ReentrantLock();
private static ReentrantReadWriteLock readWriteLock=new ReentrantReadWriteLock(); //获取读写锁
private static Lock readLock = readWriteLock.readLock(); //读锁
private static Lock writeLock = readWriteLock.writeLock(); //写锁
private int value;
public Object handleRead(Lock lock) throws InterruptedException{
try{
lock.lock(); //模拟读操作
Thread.sleep(1000); //读操作的耗时越多，读写锁的优势就越明显
return value;
}finally{
lock.unlock();
}
}
public void handleWrite(Lock lock,int index) throws InterruptedException{
try{
lock.lock(); //模拟写操作
Thread.sleep(1000);
value=index;
}finally{
lock.unlock();
}
}
public static void main(String[] args) {
final ReadWriteLockDemo demo=new ReadWriteLockDemo();
Runnable readRunnale=new Runnable() {
@Override
public void run() {
try {
demo.handleRead(readLock);
// demo.handleRead(lock);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
Runnable writeRunnale=new Runnable() {
@Override
public void run() {
try {
demo.handleWrite(writeLock,new Random().nextInt());
// demo.handleWrite(lock,new Random().nextInt());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
for(int i=0;i<18;i++){
new Thread(readRunnale).start();
}
for(int i=18;i<20;i++){
new Thread(writeRunnale).start();
}
}
}
//结果:
//读写锁明显要比单纯的锁要更快结束,说明读写锁确实提升不少效率倒计数器CountDownLatch
让一个线程等待,知道倒计时结束public class CountDownLatchDemo implements Runnable {
static final CountDownLatch end = new CountDownLatch(10); //构造倒计时器,倒计数为10
static final CountDownLatchDemo demo=new CountDownLatchDemo();
@Override
public void run() {
try {
//模拟检查任务
Thread.sleep(new Random().nextInt(10)*1000);
System.out.println("check complete");
end.countDown(); //倒计时器减1
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws InterruptedException {
ExecutorService exec = Executors.newFixedThreadPool(10);
for(int i=0;i<10;i++){
exec.submit(demo);
}
//等待检查
end.await(); //主线程阻塞,待其他线程全部完成后再唤醒主线程
//发射火箭
System.out.println("Fire!");
exec.shutdown();
}
}循环栅栏CyclicBarrier
循环栅栏类似于倒计时器,但是计数器可以反复使用,cyclicBarrier比CountDownLatch稍微强大些,可以传入一个barrierAction,barrierAction指每次完成计数便出发一次public CyclicBarrier(int parties,Runnable barrierAction) //构造方法public class CyclicBarrierDemo {
public static class Soldier implements Runnable {
private String soldier;
private final CyclicBarrier cyclic;
Soldier(CyclicBarrier cyclic, String soldierName) {
this.cyclic = cyclic;
this.soldier = soldierName;
}
public void run() {
try {
//等待所有士兵到齐
cyclic.await(); //触发一次循环栅栏,达到计数器后才会进行下一步工作
doWork();
//等待所有士兵完成工作
cyclic.await(); //再次触发循环栅栏,达到计数器后才会进行下一步工作
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
void doWork() {
try {
Thread.sleep(Math.abs(new Random().nextInt()%10000)); //模拟工作
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(soldier + ":任务完成");
}
}
public static class BarrierRun implements Runnable { //用于传入CyclicBarrier的构造方法,作为达到计数器数值后的触发任务, 可以被多次调用
boolean flag;
int N;
public BarrierRun(boolean flag, int N) {
this.flag = flag;
this.N = N;
}
public void run() {
if (flag) {
System.out.println("司令:[士兵" + N + "个，任务完成！]");
} else {
System.out.println("司令:[士兵" + N + "个，集合完毕！]");
flag = true;
}
}
}
public static void main(String args[]) throws InterruptedException {
final int N = 10;
Thread[] allSoldier=new Thread[N];
boolean flag = false;
CyclicBarrier cyclic = new CyclicBarrier(N, new BarrierRun(flag, N));
//设置屏障点，主要是为了执行这个方法
System.out.println("集合队伍！");
for (int i = 0; i < N; ++i) {
System.out.println("士兵 "+i+" 报道！");
allSoldier[i]=new Thread(new Soldier(cyclic, "士兵 " + i));
allSoldier[i].start();
}
}
}注意:
一旦其中一个被interrupt后,很可能会抛出一个interruptExpection和9个BrokenBarrierException,表示该循环栅栏已破损,防止其他线程进行无所谓的长久等待
线程阻塞工具LockSupport
LockSupport是一个非常实用的线程阻塞工具,不需要获取某个对象的锁(如wait),也不会抛出interruptedException异常public static void park() //挂起当前线程,
public static void park(Object blocker) //挂起当前线程,显示阻塞对象,parking to wait for <地址值>public class LockSupportDemo {
public static Object u = new Object();
static ChangeObjectThread t1 = new ChangeObjectThread("t1");
static ChangeObjectThread t2 = new ChangeObjectThread("t2");
public static class ChangeObjectThread extends Thread {
public ChangeObjectThread(String name){
super.setName(name);
}
@Override
public void run() {
synchronized (u) {
System.out.println("in "+getName());
LockSupport.park(this);
}
}
}
public static void main(String[] args) throws InterruptedException {
t1.start();
Thread.sleep(100);
t2.start();
LockSupport.unpark(t1);
LockSupport.unpark(t2); //即使unpark发生在park前,也可以使程序正常结束
t1.join();
t2.join();
}
}LockSupport使用了类似信号量的机制,它为每个线程准备一个许可,如果许可可用,park立即返回,并且消费这个许可(转为不可用),如果许可不可用,就会阻塞,而unpark方法就是使一个许可变为可用
locksupport.park()可以相应中断,但是不会抛出interruptedException,我们可以用Thread.interrupted等方法中获取中断标记.public class LockSupportIntDemo {
public static Object u = new Object();
static ChangeObjectThread t1 = new ChangeObjectThread("t1");
static ChangeObjectThread t2 = new ChangeObjectThread("t2");
public static class ChangeObjectThread extends Thread {
public ChangeObjectThread(String name){
super.setName(name);
}
@Override
public void run() {
synchronized (u) {
System.out.println("in "+getName());
LockSupport.park();
if(Thread.interrupted()){ //检测到中断位,并清除中断状态
System.out.println(getName()+" 被中断了");
}
if (Thread.currentThread().isInterrupted()){ //中断状态已被清除,无法检测到
System.out.println(1);
}
}
System.out.println(getName()+"执行结束");
}
}
public static void main(String[] args) throws InterruptedException {
t1.start();
Thread.sleep(100);
t2.start();
t1.interrupt();
LockSupport.unpark(t2);
}
}
//输出:
//in t1
//t1 被中断了
//t1执行结束
//in t2
//t2执行结束Guava和Limiter限流
限流算法一般有两种:漏桶算法和令牌桶算法
漏桶算法: 利用缓存区,所有请求进入系统,都在缓存区中保存,然后以固定的流速流出缓存区进行处理.
令牌桶算法: 桶中存放令牌,每个请求拿到令牌后才能进行处理,如果没有令牌,请求要么等待,要么丢弃.RateLimiter就是采用这种算法public class RateLimiterDemo {
static RateLimiter limiter = RateLimiter.create(2); //每秒处理2个请求
public static class Task implements Runnable {
@Override
public void run() {
System.out.println(System.currentTimeMillis());
}
}
public static void main(String args[]) throws InterruptedException {
for (int i = 0; i < 50; i++) {
limiter.acquire(); //过剩流量会等待
new Thread(new Task()).start();
}
}
}
// 某些场景倾向于丢弃过剩流量,tryAcquire则是立即返回,不会阻塞
// for (int i = 0; i < 50; i++) {
// if(!limiter.tryAcquire()) {
// continue;
// }
// new Thread(new Task()).start();
// }2. 线程池
Executors框架
Executor框架提供了各种类型的线程池,主要有以下工厂方法:
//固定线程数量,当有新任务提交时,若池中有空闲线程则立即执行,若没有空闲线程,任务会被暂存在一个任务队列中,直到有空闲线程
public static ExecutorService newFixedThreadPool(int nThreads)
//返回只有一个线程的线程池,多余任务被保存到一个任务队列中,线程空闲时,按先入先出的顺序执行队列中的任务
public static ExecutorService newSingleThreadPoolExecutor()
//线程数量不固定,优先使用空闲线程,多余任务会创建新线程
public static ExecutorService newCachedThreadPool()
//线程数量为1,给定时间执行某任务,或周期性执行任务
public static ScheduledExecutorService newSingleThreadScheduledExecutor()
//线程数量可以指定,定时或周期性执行任务
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize)计划任务:newScheduledThreadPool主要方法
//给定时间,对任务进行一次调度
public ScheduledFuture schedule(Runnable command,long delay, TimeUnit unit);
//周期调度,以任务完成后间隔固定时间调度下一个任务,(两者相加)
public ScheduledFuture scheduleWithFixedDelay(Runnable command,long initialDelay,
long delay,
TimeUnit unit);
//周期调度,两个任务开始的时间差为固定间隔,如果任务时间大于间隔时间则以任务时间为准(两者取其大者)
public ScheduledFuture scheduleAtFixedRate(Runnable command,long initialDelay,
long period,
TimeUnit unit);注意:
任务异常时后续所有任务都将停止调度,因此必须保证所有任务异常均被正常处理.
- 核心线程池 ThreadPoolExecutor
ThreadPoolExecutor构造函数:
public ThreadPoolExecutor(int corePoolSize, //核心线程池大小
int maximumPoolSize, //最大线程池大小
long keepAliveTime, //线程池中超过corePoolSize数目的空闲线程最大存活时间；可以allowCoreThreadTimeOut(true)使得核心线程有效时间
TimeUnit unit, //keepAliveTime时间单位
BlockingQueue workQueue, //阻塞任务队列
ThreadFactory threadFactory, //新建线程工厂
RejectedExecutionHandler handler ) //当提交任务数超过maxmumPoolSize+workQueue之和时，任务会交给RejectedExecutionHandler来处理 workQueue指被提交但是未执行的任务队列,是BlockingQueue接口的对象
1.直接提交队列:SynchronousQueue,该队列没有容量,每个插入操作对应一个删除操作,即提交的任务总是会交给线程执行,如果没有空闲进程,则创建新线程,数量达最大则执行拒绝策略,一般需要设置很大的maximumPoolSize
2.有界任务队列:ArrayBlockingQueue,有新任务时,若线程池的实际线程数小于corePoolSize,优先创建新线程,若大于corePoolSize,加入到等待队列,若队列已满,不大于maximumPoolSize前提下,创建新线程执行;当且仅当等待队列满时才会创建新线程,否则数量一直维持在corePoolSize
3.无界任务队列:LinkedBlockingQueue,小于corePoolSize时创建线程,达到corePoolSize则加入队列直到资源消耗殆尽
4.优先任务队列:PriorityBlockingQueue,特殊无界队列,总是保证高优先级的任务先执行.
Executors分析
newFixedThreadPool: corePoolSize=maximumPoolSize,线程不会超过corePoolSize,使用LinkedBlockingQueue
newSingleThreadPoolExecutor: newFixedThreadPool的弱化版,corePoolSize只有1
newCachedThreadPool: corePoolSize=0,maximumPoolSize为无穷大,空闲线程60秒回收,使用SynchronousQueue队列ThreadPoolExecutor的execute()方法执行逻辑
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) { //检查是否小于corePoolSize
if (addWorker(command, true)) //添加线程,执行任务
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) { //添加进队列
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command)) //双重校验
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false)) //提交线程池失败
reject(command); //拒绝执行
}
拒绝策略
**AbortPolicy**:该策略会直接抛出异常,阻止系统正常工作。
**CallerRunsPolicy**:只要线程池未关闭,该策略直接在调用者线程中,运行当前被丢弃的任务。显然这样做不会真的丢弃任务,但是,任务提交线程的性能极有可 能会急剧下降。 任务,并尝试再次提交当前任务。
**DiscardOldestPolicy**:该策略将丢弃最老的一个请求,也就是即将被执行的一个
**DiscardPolicy**:该策略默默地丢弃无法处理的任务,不予任何处理。如果允许任务丢失,我觉得这可能是最好的一种方案了吧!自定义ThreadFactory
public static void main(String[] args) throws InterruptedException {
MyTask task = new MyTask();
ExecutorService es = new ThreadPoolExecutor(5, 5,
0L, TimeUnit.MILLISECONDS,
new SynchronousQueue(),
new ThreadFactory(){
@Override
public Thread newThread(Runnable r) { //自定义创建线程的方法
Thread t= new Thread(r);
t.setDaemon(true);
System.out.println("create "+t);
return t;
}
}
);
for (int i = 0; i < 5; i++) {
es.submit(task);
}
Thread.sleep(2000);
} 扩展线程池
public class ExtThreadPool {
public static class MyTask implements Runnable {
public String name;
public MyTask(String name) {
this.name = name;
}
@Override
public void run() {
System.out.println("正在执行" + ":Thread ID:" + Thread.currentThread().getId()
+ ",Task Name=" + name);
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) throws InterruptedException {
ExecutorService es = new ThreadPoolExecutor(5, 5, 0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()) {
@Override
protected void beforeExecute(Thread t, Runnable r) {
System.out.println("准备执行：" + ((MyTask) r).name);
}
@Override
protected void afterExecute(Runnable r, Throwable t) {
System.out.println("执行完成：" + ((MyTask) r).name);
}
@Override
protected void terminated() {
System.out.println("线程池退出");
}
};
for (int i = 0; i < 5; i++) {
MyTask task = new MyTask("TASK-GEYM-" + i);
es.execute(task);
Thread.sleep(10);
}
es.shutdown(); //等待所有任务执行完毕后再关闭
}
} 异常堆栈消息
线程池中的异常堆栈可能不会抛出,需要我们自己去包装
public class TraceThreadPoolExecutor extends ThreadPoolExecutor {
public TraceThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit, BlockingQueue workQueue) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
@Override
public void execute(Runnable task) {
super.execute(wrap(task, clientTrace(), Thread.currentThread()
.getName()));
}
@Override
public Future submit(Runnable task) {
return super.submit(wrap(task, clientTrace(), Thread.currentThread()
.getName()));
}
private Exception clientTrace() {
return new Exception("Client stack trace");
}
private Runnable wrap(final Runnable task, final Exception clientStack,
String clientThreadName) {
return new Runnable() {
@Override
public void run() {
try {
task.run(); //外层包裹trycatch,即可打印出异常
} catch (Exception e) {
clientStack.printStackTrace();
throw e;
}
}
};
}
} Fork/Join框架
类似于mapreduce,用于大数据量,fork()创造子线程,join表示等待,
public class CountTask extends RecursiveTask{
private static final int THRESHOLD = 10000; //任务分解规模
private long start;
private long end;
public CountTask(long start,long end){
this.start=start;
this.end=end;
}
@Override
public Long compute(){
long sum=0;
boolean canCompute = (end-start) subTasks=new ArrayList();
long pos=start;
for(int i=0;i<100;i++){
long lastOne=pos+step;
if(lastOne>end)lastOne=end; //最后一个任务可能小于step,故需要此步
CountTask subTask=new CountTask(pos,lastOne); //子任务
pos+=step+1; //调整下一个任务
subTasks.add(subTask);
subTask.fork(); //fork子任务
}
for(CountTask t:subTasks){
sum+=t.join(); //聚合任务
}
}
return sum;
}
public static void main(String[]args){
ForkJoinPool forkJoinPool = new ForkJoinPool();
CountTask task = new CountTask(0,200000000000L);
ForkJoinTask result = forkJoinPool.submit(task);
try{
long res = result.get();
System.out.println("sum="+res);
}catch(InterruptedException e){
e.printStackTrace();
}catch(ExecutionException e){
e.printStackTrace();
}
}
} 注意:
如果任务的划分层次很多,一直得不到返回,可能有两种原因:
1.系统内线程数量越积越多,导致性能严重下降
2.函数调用层次变多,导致栈溢出
Guava对线程池的拓展
1.特殊的DirectExecutor线程池
Executor executor=MoreExecutors.directExecutor(); // 仅在当前线程运行,用于抽象2.Daemon线程池
提供将普通线程转换为Daemon线程.很多情况下,我们不希望后台线程池阻止程序的退出
public class MoreExecutorsDemo2 {
public static void main(String[] args) {
ThreadPoolExecutor exceutor = (ThreadPoolExecutor)Executors.newFixedThreadPool(2);
MoreExecutors.getExitingExecutorService(exceutor);
exceutor.execute(() -> System.out.println("I am running in " + Thread.currentThread().getName()));
}
}3.future模式扩展
待续....