避免每次新起线程对内存的消耗，降低资源消耗，提高内存利用率，使用线程池将线程管理起来。
线程池 Executor
// 基础用法
public class ExecutorTest {
public static void main(String[] args) {
// 可伸缩的线程池 大小为10
Executor executor = Executors.newFixedThreadPool(10);
// 提交20个线程
for (int i = 0; i < 20; i++) {
executor.execute(new RunnableCase());
}
}
}
class RunnableCase implements Runnable{
@Override
public void run() {
System.out.println("Hello" + Thread.currentThread().getName());
}
}Executor 框架管理了所有线程的生命周期
Executors
Executors是一个工厂类，可以生成多种线程池。就newFixedThreadPool来看
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}- corePoolSize：线程池中的核心线程数，如果线程池中执行的线程数等于corePoolSize的时候，如果有新任务，则会放到阻塞队列里。线程池的prestartAllCoreThreads()方法可以提前创建并启动所有核心线程。
- maxmiumPoolSize：线程池中允许最大的线程数，当阻塞队列满的时候，如果线程池中的线程数目小雨maxmiumPooSize，则会创建新的线程执行。
- keepAliveTime：线程空闲时的存活时间，即当线程没有任务执行时，继续存活的时间；默认情况下，该参数只在线程数大于corePoolSize时才有用
- unit：表示的时间单位。
workQueue：用于保存超出corePoolSize的线程，具有如下特性：
1、ArrayBlockingQueue：基于数组结构的有界阻塞队列，按FIFO排序任务；
2、LinkedBlockingQuene：基于链表结构的阻塞队列，按FIFO排序任务，吞吐量通常要高于ArrayBlockingQuene；
3、SynchronousQuene：一个不存储元素的阻塞队列，每个插入操作必须等到另一个线程调用移除操作，否则插入操作一直处于阻塞状态，吞吐量通常要高于LinkedBlockingQuene；
4、priorityBlockingQuene：具有优先级的无界阻塞队列；threadFactory
DefaultThreadFactory() { SecurityManager s = System.getSecurityManager(); group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup(); namePrefix = "pool-" + poolNumber.getAndIncrement() + "-thread-"; }线程创建工厂，给线程定义线程名。
handler
线程池以及队列满了以后的如果有任务提交的处理策略。
“`
// 默认的是AbortPolicy
private static final RejectedExecutionHandler defaultHandler =new AbortPolicy();
// AbortPolicy 拒绝策略
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException(“Task ” + r.toString()
+ ” rejected from ” + e.toString());
}
}
“`
- AbortPolicy：默认策略、直接抛出异常。
- CallerRunsPolicy：用调用者所在的线程来执行任务；
- DiscardOldestPolicy：丢弃阻塞队列中靠最前的任务，并执行当前任务；
- DiscardPolicy：直接丢弃任务；
注意：如果以上策略都不满足的话，可实现RejectedExecutionHandler接口，自定义处理策略。
各种线程池说明
newFixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}corePoolSize == maximumPoolSize，Queue为LinkingBlockingQueue，当线程池没有可执行任务时，也不会释放线程。
newCachedThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}可缓存的线程池，默认缓存60s，线程池的线程数可达到Integer.MAX_VALUE，使用SynchronousQueue作为阻塞队列；
newCachedThreadPool在没有任务执行时，当线程的空闲时间超过keepAliveTime，会自动释放线程资源，当提交新任务时，如果没有空闲线程，则创建新线程执行任务，会导致一定的系统开销；
ScheduledThreadPoolExecutor
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}周期性提交任务。
newSingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}线程池中只有一个线程，如果线程异常结束，则会创建一个新的线程继续执行任务。
实现原理
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 29
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
// 11100000000000000000000000000000
private static final int RUNNING = -1 << COUNT_BITS;
// 0
private static final int SHUTDOWN = 0 << COUNT_BITS;
// 100000000000000000000000000000
private static final int STOP = 1 << COUNT_BITS;
// 1000000000000000000000000000000
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }利用高3位表示线程状态。
1、RUNNING：111，正常状态，接受新的任务，并处理任务队列中的任务；
2、SHUTDOWN：000，不接受新的任务，但是处理已经在任务队列中的任务；
3、STOP ： 001，不接受新的任务，也不处理已经在任务队列中的任务，同时会尝试停止正在执行任务的线程；
4、TIDYING ： 010，线程池和任务队列都为空，该状态下线程会执行 terminated() 方法；
5、TERMINATED：011，terminated() 方法执行完毕；
提交任务方式
两种提交方式Executor.execute() 和ExecutorService.submit()
Executor.execute()
void execute(Runnable command);只接受实现了Runnable接口的对象，无返回值，无法获取线程结果。
ExecutorService.submit()
<T> Future<T> submit(Callable<T> task);可以通过Future获取返回值。
任务执行
Executor.execute()
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}流程说明
1. 判定线程池任务是否小于核心线程数，如果小于则执行addWorker方法创建新的线程执行任务，如果大于执行步骤2
2. 如果线程池是running状态，则把任务放进阻塞队列，然后执行步骤3，如果放入失败则执行步骤4。
3. 再次判断线程池状态是否为running，如果不是，则从队列里删除该命令，执行reject方法来处理。如果线程池中的任务为0，则addWorker添加空任务。
4. 执行addWorker方法创建新的线程执行任务，如果失败，则执行reject方法。
addWorker实现
addWorker在线程池中主要负责创建线程执行任务
private boolean addWorker(Runnable firstTask, boolean core) {
/*--------------------------------第1段----------------------*/
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
/*--------------------------------第2段----------------------*/
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;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}第1段
1. 如果线程池状态大于等于SHUTDOWN，则返回，除非状态为SHUTDOWN&&提交的任务为空&&队列为空。
2. 判断是否为核心线程，如果是，则大于coreSize就返回false，如果不是，则大于maxmumPoolSize就返回。
3. 跳出循环开始第2段，创建线程
第2段
加锁的情况下，新建Worker类，将worker插入到workers里，并启动worker中的线程。
Worker代码
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
/*---------------------------*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}- 继承了AbstractQueuedSynchronizer类，可控制线程的中止；
- 实现了Runnable接口，自身就是一个任务；
- 传入Runnable参数；
- 创建了线程的同时传入了自身，线程执行方法调用的是runWorker方法。
runWorker方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
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
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
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 {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}- 先通过unlock方法释放锁，completedAbruptly设为true。
- 上锁，执行beforeExecute方法；然后执行run方法，最后执行afterExecute方法
- 执行完以后，会调用getTask来从阻塞队列获取等待任务，如果没有，则挂起
getTask
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}for循环的情况下
1.workQueue.take，从阻塞线程中获取线程，如队列无线程则阻塞，如有线程，则获取并返回。
2.workQueue.poll，在keepAlive时间内还未返回，则返回null
注意：从以上程序可以看出，执行完线程之后，会尝试从队列获取线程，保证了队列中的线程可以被执行。
Future和Callable
如果需要线程返回结果，则需要用到Future和Callable，还需要使用ExecutorService.submit()方法提交。
public class ExecutorTest {
public static void main(String[] args) {
// 可伸缩的线程池
ExecutorService service = Executors.newFixedThreadPool(10);
Future<String> future = service.submit(new CallableCase());
String result = null;
try {
result = future.get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println(result);
}
}
class CallableCase implements Callable<String> {
@Override
public String call() throws Exception {
Thread.sleep(20000);
return "sleep thread";
}
}Callable负责返回值，Future可获取Callable返回的结果。
1. Future可以获取返回值以及异常值
2. Future.get方法会一直阻塞到Callable有返回值。
ExecutorService.submit方法
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}Callable任务会被封装成FutureTask对象。
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;有多种状态
public class FutureTask<V> implements RunnableFuture<V>
public interface RunnableFuture<V> extends Runnable, Future<V>可以看出来FutureTask实现了Runnable状态，所以可以使用ExecutorService来提交。最终执行的是FutureTask.run方法
FutureTask.get
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}通过awaitDone来等待结果返回
awaitDone
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}- 如果主线程被中断，则抛出中断异常；
- 判断FutureTask当前的state，如果大于COMPLETING，说明任务已经执行完成，则直接返回；
- 如果当前state等于COMPLETING，说明任务已经执行完，这时主线程只需通过yield方法让出cpu资源，等待state变成NORMAL；
- 通过WaitNode类封装当前线程，并通过UNSAFE添加到waiters链表；
- 最终通过LockSupport的park或parkNanos挂起线程；
FutureTask.run
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}- 执行Callable.call方法。
- 如果执行成功有结果，通过set保存对象。
- 如果有异常，则保存异常。
set方法
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}setException方法
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}set和setException方法中，都会通过UnSAFE修改FutureTask的状态，并执行finishCompletion方法通知主线程任务已经执行完成；
finishCompletion
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}1、执行FutureTask类的get方法时，会把主线程封装成WaitNode节点并保存在waiters链表中；
2、FutureTask任务执行完成后，通过UNSAFE设置waiters的值，并通过LockSupport类unpark方法唤醒主线程；
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