Java｜线程池学习总结
目录
常用接口及实现类
1. Executor、ExecutorService
Executor 里定义了一个 execute 方法，这个方法接受一个实现 Runnable 的对象。
public interface Executor {
void execute(Runnable command);
}
ExecutorService 是继承 Excutor 的一个接口，它定义了 Executor 框架常用的方法，如提交任务、线程池关闭、判断线程池的状态等方法。
2. Runnable、Callable
Runnable 定义了一个没有返回值的可执行方法，Callable定义了一个有返回值的方法。
实现这两个接口的类都可以做为线程 Thread 执行的对象，这里演示一个 Callable。
public void test() throws Exception {
FutureTask<String> task = new FutureTask<>(new Callable() {
@Override
public Object call() throws Exception {
String res = "hello world";
Thread.sleep(3000);
return res;
}
});
new Thread(task).start();
task.get();//call return前阻塞
}
public static void main(String[] args) throws ExecutionException, InterruptedException {
Callable<String> c = new Callable() {
@Override
public String call() throws Exception {
return "Hello Callable";
}
};
ExecutorService service = Executors.newCachedThreadPool();
Future<String> future = service.submit(c); //异步
System.out.println(future.get());//获取到结果前阻塞
service.shutdown();
}
3. Executors 线程池工具
Executors 是线程池的工具类，类似于集合框架的 Collections 工具类。
已经预定义了一些线程池，设置参数后可以直接使用。
4. 线程池的关闭
线程不是立即关闭（终结）的，关闭和终结是两个不同的状态。
线程池的关闭即遍历每个线程然后触发线程的中断（interrupt）。
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newFixedThreadPool(5); //execute submit
for (int i = 0; i < 6; i++) {
service.execute(() -> {
try {
TimeUnit.MILLISECONDS.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName());
});
}
System.out.println(service);
// 关闭线程
service.shutdown();
// isTerminated() 为false说明线程run方法还未执行完毕
System.out.println(service.isTerminated());
//此时线程已经处在关闭状态，但是还未终结（终结是线程执行完毕从run方法退出）
System.out.println(service.isShutdown());
System.out.println(service);
// 在休眠会查看
TimeUnit.SECONDS.sleep(5);
// true 所有线程已终结
System.out.println(service.isTerminated());
// true 所有线程已关闭
System.out.println(service.isShutdown());
System.out.println(service);
}
// output
java.util.concurrent.ThreadPoolExecutor@76ccd017[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
false
true
java.util.concurrent.ThreadPoolExecutor@76ccd017[Shutting down, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
pool-1-thread-1
pool-1-thread-4
pool-1-thread-3
pool-1-thread-2
pool-1-thread-5
pool-1-thread-1
true
true
java.util.concurrent.ThreadPoolExecutor@76ccd017[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 6]
5. Futrue 异步任务
Future 是异步作业，它表示提交作业未来的一个计算结果，在使用 get 方法获取结果时如果 call()未返回结果则会阻塞当前线程。
FutrueTask 是 Futrue 的实现类，可以接受 Runnable、Callable 实现类为一个执行作业。
public static void main(String[] args) throws InterruptedException, ExecutionException {
FutureTask<Integer> task = new FutureTask<>(()->{
TimeUnit.MILLISECONDS.sleep(500);
return 1000;
}); //new Callable () { Integer call();}
new Thread(task).start();
System.out.println(task.get()); //阻塞，知道call返回结果
}
5.1 CompletableFuture
CompletableFuture 是一个处理异步任务的线程工具，使用它可以对异步任务进行控制或者继续进行之后的操作（这点类似 Stream）。
下面是个使用示例。
/**
* 假设能够提供一个服务
* 这个服务查询各大电商网站同一类产品的价格并汇总展示
* 可以使用单线程一个一个查，也可以使用异步并行方式查
*/
import java.io.IOException;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
public class TestCompletableFuture {
public static void main(String[] args) throws ExecutionException, InterruptedException {
long start, end;
// 方案1，单线程计算
/*start = System.currentTimeMillis();
priceOfTM();
priceOfTB();
priceOfJD();
end = System.currentTimeMillis();
System.out.println("use serial method call! " + (end - start));*/
// ----------------------------------------------------------------------------------
// 方案2，异步并行计算
start = System.currentTimeMillis();
// 提交异步任务
CompletableFuture<Double> futureTM = CompletableFuture.supplyAsync(()->priceOfTM());
CompletableFuture<Double> futureTB = CompletableFuture.supplyAsync(()->priceOfTB());
CompletableFuture<Double> futureJD = CompletableFuture.supplyAsync(()->priceOfJD());
// 等待作业完成
CompletableFuture.allOf(futureTM, futureTB, futureJD).join();
// CompletableFuture 还可以链式进行其他操作
/*CompletableFuture.supplyAsync(()->priceOfTM())
.thenApply(String::valueOf)
.thenApply(str-> "price " + str)
.thenAccept(System.out::println);*/
end = System.currentTimeMillis();
System.out.println("use completable future! " + (end - start));
// 阻塞等待结果
try {
System.in.read();
} catch (IOException e) {
e.printStackTrace();
}
}
private static double priceOfTM() {
delay();
return 1.00;
}
private static double priceOfTB() {
delay();
return 2.00;
}
private static double priceOfJD() {
delay();
return 3.00;
}
// 延迟工具类
private static void delay() {
// int time = new Random().nextInt(500);
int time = 150;
try {
TimeUnit.MILLISECONDS.sleep(time);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.printf("After %s sleep!\n", time);
}
}
Executors
Executors 是线程工具类，提供了一套开箱即用的线程池，只需要简单设置参数即可使用。
1. SingleThreadExecutor
SingleThreadExecutor 单线程线程池, 只会有一个线程，少于一个线程会创建一个线程。
使用
public static void main(String[] args) {
ExecutorService service = Executors.newSingleThreadExecutor();
for(int i=0; i<5; i++) {
final int j = i;
service.execute(()->{
System.out.println(j + " " + Thread.currentThread().getName());
});
}
}
源码
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
// 核心线程数1，最大线程数1
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
2. 缓存线程池
使用
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newCachedThreadPool();
// 刚初始化，线程数0
System.out.println(service);
for (int i = 0; i < 2; i++) {
service.execute(() -> {
try {
TimeUnit.MILLISECONDS.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName());
});
}
//size=2 线程数2,active threads=2，活跃线程数2
System.out.println(service);
//60秒内无作业处理线程就会被关闭
TimeUnit.SECONDS.sleep(80);
//size=0 线程数0，active threads=0 活跃线程0
System.out.println(service);
}
源码
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
//活跃时间60秒
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
2. FixedThreadPool
固定线程池一直保持固定个线程活跃。
使用
public class T09_FixedThreadPool {
public static void main(String[] args) throws InterruptedException, ExecutionException {
// 单线程处理判断20W个数是否为质数
// 测试1，单线程
long start = System.currentTimeMillis();
getPrime(1, 200000);
long end = System.currentTimeMillis();
System.out.println("单线程耗时:"+(end - start));
// 测试2，多线程，线程池，4个线程
final int cpuCoreNum = 4;
ExecutorService service = Executors.newFixedThreadPool(cpuCoreNum);
// 20W个数分四个作业
MyTask t1 = new MyTask(1, 80000); //1-5 5-10 10-15 15-20
MyTask t2 = new MyTask(80001, 130000);
MyTask t3 = new MyTask(130001, 170000);
MyTask t4 = new MyTask(170001, 200000);
// 计时
start = System.currentTimeMillis();
// 提交作业
Future<List<Integer>> f1 = service.submit(t1);
Future<List<Integer>> f2 = service.submit(t2);
Future<List<Integer>> f3 = service.submit(t3);
Future<List<Integer>> f4 = service.submit(t4);
// 阻塞执行完成
f1.get();
f2.get();
f3.get();
f4.get();
end = System.currentTimeMillis();
System.out.println("线程池耗时:"+(end - start));
}
static class MyTask implements Callable<List<Integer>> {
int startPos, endPos;
MyTask(int s, int e) {
this.startPos = s;
this.endPos = e;
}
@Override
public List<Integer> call() throws Exception {
List<Integer> r = getPrime(startPos, endPos);
return r;
}
}
static boolean isPrime(int num) {
for(int i=2; i<=num/2; i++) {
if(num % i == 0) return false;
}
return true;
}
static List<Integer> getPrime(int start, int end) {
List<Integer> results = new ArrayList<>();
for(int i=start; i<=end; i++) {
if(isPrime(i)) results.add(i);
}
return results;
}
}
源码
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
4. ScheduledThreadPoolExecutor
定时任务线程池，指定固定个线程，延迟或重复执行作业。
使用
// 设置一个定时任务，每2秒执行一次作业处理
public static void main(String[] args) {
ScheduledExecutorService service = Executors.newScheduledThreadPool(4);
service.scheduleAtFixedRate(() -> {
int s = new Random().nextInt(1000);
try {
TimeUnit.MILLISECONDS.sleep(s);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " 耗时 " + s + " ms 处理了作业");
}, 0, 2, TimeUnit.SECONDS);
}
5. WorkStealingPool
newWorkStealingPool，这个是 JDK1.8 版本加入的一种线程池，stealing 翻译为抢断、窃取的意思。
特点：
- 抢占工作
- 作业无序执行
源码
// 使用的是WorkJoinPool，与上边的几个不同
public static ExecutorService newWorkStealingPool() {
return new ForkJoinPool
(Runtime.getRuntime().availableProcessors(),
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}
5.1 ForkJoinPool
https://zhuanlan.zhihu.com/p/90958193
ThreadPoolExecutor 源码分析
1. 常用变量的解释
// 1. `ctl`，可以看做一个int类型的数字，高3位表示线程池状态，低29位表示worker数量
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 2. `COUNT_BITS`，`Integer.SIZE`为32，所以`COUNT_BITS`为29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 3. `CAPACITY`，线程池允许的最大线程数。1左移29位，然后减1，即为 2^29 - 1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
// 4. 线程池有5种状态，按大小排序如下：RUNNING < SHUTDOWN < STOP < TIDYING < TERMINATED
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
// 5. `runStateOf()`，获取线程池状态，通过按位与操作，低29位将全部变成0
private static int runStateOf(int c) { return c & ~CAPACITY; }
// 6. `workerCountOf()`，获取线程池worker数量，通过按位与操作，高3位将全部变成0
private static int workerCountOf(int c) { return c & CAPACITY; }
// 7. `ctlOf()`，根据线程池状态和线程池worker数量，生成ctl值
private static int ctlOf(int rs, int wc) { return rs | wc; }
/*
* Bit field accessors that don't require unpacking ctl.
* These depend on the bit layout and on workerCount being never negative.
*/
// 8. `runStateLessThan()`，线程池状态小于xx
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
// 9. `runStateAtLeast()`，线程池状态大于等于xx
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
2. 构造方法
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
// 基本类型参数校验
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
// 空指针校验
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
// 根据传入参数`unit`和`keepAliveTime`，将存活时间转换为纳秒存到变量`keepAliveTime `中
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
3. 提交执行 task 的过程
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();
// worker数量比核心线程数小，直接创建worker执行任务
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// worker数量超过核心线程数，任务直接进入队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
// 线程池状态不是RUNNING状态，说明执行过shutdown命令，需要对新加入的任务执行reject()操作。
// 这儿为什么需要recheck，是因为任务入队列前后，线程池的状态可能会发生变化。
if (! isRunning(recheck) && remove(command))
reject(command);
// 这儿为什么需要判断0值，主要是在线程池构造方法中，核心线程数允许为0
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
// 如果线程池不是运行状态，或者任务进入队列失败，则尝试创建worker执行任务。
// 这儿有3点需要注意：
// 1. 线程池不是运行状态时，addWorker内部会判断线程池状态
// 2. addWorker第2个参数表示是否创建核心线程
// 3. addWorker返回false，则说明任务执行失败，需要执行reject操作
else if (!addWorker(command, false))
reject(command);
}
4. addworker 源码解析
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
// 外层自旋
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 这个条件写得比较难懂，我对其进行了调整，和下面的条件等价
// (rs > SHUTDOWN) ||
// (rs == SHUTDOWN && firstTask != null) ||
// (rs == SHUTDOWN && workQueue.isEmpty())
// 1. 线程池状态大于SHUTDOWN时，直接返回false
// 2. 线程池状态等于SHUTDOWN，且firstTask不为null，直接返回false
// 3. 线程池状态等于SHUTDOWN，且队列为空，直接返回false
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
// 内层自旋
for (;;) {
int wc = workerCountOf(c);
// worker数量超过容量，直接返回false
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 使用CAS的方式增加worker数量。
// 若增加成功，则直接跳出外层循环进入到第二部分
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
// 线程池状态发生变化，对外层循环进行自旋
if (runStateOf(c) != rs)
continue retry;
// 其他情况，直接内层循环进行自旋即可
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// worker的添加必须是串行的，因此需要加锁
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
// 这儿需要重新检查线程池状态
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
// worker已经调用过了start()方法，则不再创建worker
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
// worker创建并添加到workers成功
workers.add(w);
// 更新`largestPoolSize`变量
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
// 启动worker线程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
// worker线程启动失败，说明线程池状态发生了变化（关闭操作被执行），需要进行shutdown相关操作
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
5. 线程池 worker 任务单元
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
// 这儿是Worker的关键所在，使用了线程工厂创建了一个线程。传入的参数为当前worker
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
// 省略代码...
}
6. 核心线程执行逻辑-runworker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
// 调用unlock()是为了让外部可以中断
w.unlock(); // allow interrupts
// 这个变量用于判断是否进入过自旋（while循环）
boolean completedAbruptly = true;
try {
// 这儿是自旋
// 1. 如果firstTask不为null，则执行firstTask；
// 2. 如果firstTask为null，则调用getTask()从队列获取任务。
// 3. 阻塞队列的特性就是：当队列为空时，当前线程会被阻塞等待
while (task != null || (task = getTask()) != null) {
// 这儿对worker进行加锁，是为了达到下面的目的
// 1. 降低锁范围，提升性能
// 2. 保证每个worker执行的任务是串行的
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
// 如果线程池正在停止，则对当前线程进行中断操作
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
// 执行任务，且在执行前后通过`beforeExecute()`和`afterExecute()`来扩展其功能。
// 这两个方法在当前类里面为空实现。
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
// 帮助gc
task = null;
// 已完成任务数加一
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// 自旋操作被退出，说明线程池正在结束
processWorkerExit(w, completedAbruptly);
}
}