【1.0】 Executors类提供了几种不同特性的线程池，其主要实现类都离不开ThreadPoolExecutor，先看一下ThreadPoolExecutor的构建方法
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
【1.1】 参数说明
corePoolSize保持最小的核心线程数，不会被回收，除非allowCoreThreadTimeOut被设置
maximumPoolSize 允许被执行时最大的线程数，当核心线程池数和队列满了，会加入最大线程数中执行
keepAliveTime 当线程数大于核心线程数，超过的线程保持的时间，和unit搭配使用
unit 缓存时间单位
workQueue任务队列，可进行任务阻塞等待，保存将要执行的队列
threadFactory创建新线程的的工厂对象
handler 线程数或者队列容量达到极限，造成线程池阻塞是的处理器
【1.2】 执行方法
当调用 execute() 方法添加一个任务时，线程池会做如下判断：
- 如果正在运行的线程数量小于 corePoolSize，那么马上创建线程运行这个任务；
- 如果正在运行的线程数量大于或等于 corePoolSize，那么将这个任务放入队列。
- 如果这时候队列满了，而且正在运行的线程数量小于 maximumPoolSize，那么还是要创建线程运行这个任务；
- 如果队列满了，而且正在运行的线程数量大于或等于 maximumPoolSize，那么线程池会抛出异
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//检查执行任务数
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))//加入核心线程池执行
return;
c = ctl.get();
}
//判断线程池状态，加入任务队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//判断线程池是否关闭
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))//加入队列失败，会加入最大线程数执行
reject(command);
}
// wc >= (core ? corePoolSize : maximumPoolSize)) 线程数不超过maximumPoolSize
【1.3】 线程池会创建一样Worker来封装Thread，然后不断的执行队列的任务
private boolean addWorker(Runnable firstTask, boolean core) {
......
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//创建一个Worker
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
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) {
//启动线程
t.start();
workerStarted = true;
}
}
......
}
【1.4】 Worker继承了Runnable的接口，在初始化的时候会设置到新建的线程，当start线程就中心runWorker，将不循环断获取任务列表的任务执行，代码如下
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);
}
public void run() {
runWorker(this);
}
【1.5】 循环执行任务
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//循环获取任务，如果队列数据为空，线程会进入等待
while (task != null || (task = getTask()) != null) {
w.lock();
......
}
} finally {
processWorkerExit(w, completedAbruptly);
}
}
【2.0】 常用的任务队列
【2.1】 ArrayBlockingQueue 数组储存，有界队列，超过会从队头覆盖对象，FIFO
final Object[] items;
private void insert(E x) {
items[putIndex] = x;
//计算下个index
putIndex = inc(putIndex);
++count;
notEmpty.signal();
}
final int inc(int i) {
return (++i == items.length) ? 0 : i;
}
【2.2】 LinkedBlockingQueue 单向链表储存，有界列表，默认数是2的31次方2147483648，
static class Node {
E item;
Node next;
Node(E x) { item = x; }
}
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity)
return false;
int c = -1;
Node node = new Node(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}
【2.3】 DelayedWorkQueue 数组存储，队长可自增长，周期性任务使用
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
//对象必须的RunnableScheduledFuture的子类
RunnableScheduledFuture e = (RunnableScheduledFuture)x;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = size;
if (i >= queue.length)
grow();//自动增长
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0);
} else {
siftUp(i, e);
}
if (queue[0] == e) {
leader = null;
available.signal();
}
} finally {
lock.unlock();
}
return true;
}
【2.4】 SynchronousQueue使用队列或者栈进行储存
public SynchronousQueue(boolean fair) {
transferer = fair ? new TransferQueue() : new TransferStack();
}
【3.0】 DefaultThreadFactory 线程创建工厂类
public Thread newThread(Runnable r) {
//直接实例一个线程
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
【4.0】 Executors为我们通过一些实例不同特性的线程池方法，主要有以下几个
【4.1】 Executors.newScheduledThreadPool() 可执行周期性任务的线程池
实例是ScheduledThreadPoolExecutor，ScheduledThreadPoolExecutor继承ThreadPoolExecutor，队列使用LinkedBlockingQueue
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
new DelayedWorkQueue());
}
ScheduledFutureTask(Runnable r, V result, long ns, long period) {
super(r, result);
this.time = ns;//执行时间
this.period = period;//重复次数
this.sequenceNumber = sequencer.getAndIncrement();
}
【4.2】 Executors.newCachedThreadPool()线程数可自动扩展，闲着线程60秒将会被回收
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue());
}
【4.3】 Executors.newFixedThreadPool() 执行固定线程数的线程池，核心线程数等于最大线程数，且有限，队列无限
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
【4.4】 Executors.newSingleThreadExecutor() 单线程的线程池，FinalizableDelegatedExecutorService 只是包装了 ThreadPoolExecutor，对象在回收时会关闭线程池
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()));
}
static class FinalizableDelegatedExecutorService
extends DelegatedExecutorService {
FinalizableDelegatedExecutorService(ExecutorService executor) {
super(executor);
}
protected void finalize() {
super.shutdown();
}
}
【4.5】 Executors.newSingleThreadScheduledExecutor()
具有单线程池和周期性的特点，线程池包装了ScheduledThreadPoolExecutor，
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}