相信我们在学习有关线程池的时候最先接触的很大概率是这个类。这个类提供了一些简单的线程池的实现方法,所以本篇文章从Executors类开始一步步的深挖线程池的具体实现。
下面是一个简单的线程池的所以例子
public class PoolDemo {
public static void main(String[] args) {
ExecutorService threadPool = Executors.newFixedThreadPool(3);
for (int i = 0; i < 3; i++) {
threadPool.submit(() -> {
for (int j = 0; j < 3; j++) {
System.out.println(Thread.currentThread().getName() + ":" + j);
}
});
}
}
}
执行结果:
pool-1-thread-1:0
pool-1-thread-1:1
pool-1-thread-1:2
pool-1-thread-3:0
pool-1-thread-3:1
pool-1-thread-3:2
pool-1-thread-2:0
pool-1-thread-2:1
pool-1-thread-2:2
Executors可以很轻松的创建一个线程池并执行任务。在Executors中有三类线程池由三种方法实现:
在创建完线程后使用submit,execute的方法其执行任务。
在讲完Executors中的三种线程池后我们来看看其具体实现:
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
可以看到线程池最终都是由ThreadPoolExecutor类实现的,下面分析一下ThreadPoolExecutor类
Executors类生成线程池时都是通过ThreadPoolExecutor的构造方法传入不同的值来实现的。下面我们来讲一讲ThreadPoolExecutor类的构造方法。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
构造方法中有7个参数,其各自含以为:
对上述的几个重要参数进行详解:
就是一个队列,额外的支持一些操作,如获取元素的时候等待队列变为非空;在存储以上的时候将等待空间变为可用。其具体实现使用了大量的CAS,AQS等之前写过的Lock的基础知识。这里我只是对BlockingQueue做简单的介绍,其详细的介绍后续内容中给出。
其具体实现我简单写几个:
这是一个有界队基于链表线程的阻塞队列,按照FIFO的方式对元素进行排序,故队头元素就是存放在队列中时间最长的元素;队尾元素是存放在队列时间最短的元素,因为新的元素会被插入到队列1尾端,队列的获取或者检索操作会从队头开始。基于链表的阻塞队列,比基于数组的队列有更高的吞吐量。
其内部维护了一个Node为节点的链表
private transient Node<E> last;
static class Node<E> {
E item;
/**
* One of:
* - the real successor Node
* - this Node, meaning the successor is head.next
* - null, meaning there is no successor (this is the last node)
*/
Node<E> next;
Node(E x) { item = x; }
}
其内部还维护了两个ReentrantLock和两个Condition
/** Lock held by take, poll, etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();
/** Lock held by put, offer, etc */
private final ReentrantLock putLock = new ReentrantLock();
/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();
是一个通过数组维护的阻塞队列,其他介绍同LinkedBlockingQueue。只是ArrayBlockingQueue吞吐量比LinkedBlockingQueue低。
其内部维护了一个item数组来存放队列的元素
/** The queued items */
final Object[] items;
这是一个阻塞队列,每一个插入操作必须等待其他线程的移出操作完成才能执行,反之亦然。从这可用直到SynchronousQueue队列只维护了一个元素
ThreadFactory:根据需要创建新的线程,使用线程工厂就减少了直接使用new Thread,使创建线程更加简便,可用应用自定义其子类实现具体线程的创建。
其最简单的实现就是:
class SimpleThreadFactory implements ThreadFactory {
public Thread newThread(Runnable r) {
return new Thread(r);
}
}
线程工厂使用的最多的子类为Executors.DefaultThreadFactory
其实现为:
static class DefaultThreadFactory implements ThreadFactory {
//poolNumber定义为static是要确保不同的线程池使用同一个工厂时,按照同一个基准来对
//线程池号自增
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
//threadNumber线程计数为非静态,确保不同线程池使用同一个工厂生成的线程名称都是从1
//开始
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
//线程名称的前缀,
namePrefix = "pool-" +
poolNumber.getAndIncrement() +
"-thread-";
}
public Thread newThread(Runnable r) {
//通过传入的任务创建线程对象,四个参数分别为
//线程组、要运行的对象、线程名称、
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
//确保线程池中的线程都为用户线程
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
//确保创建的线程优先级都是正常优先级(5)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
当任务数量超出阻塞队列时会执行,拒绝策略。还有一种情况,就是线程池执行了shutdown方法时,因为线程池已经关闭了,所以对后续任务也要执行拒绝策略。
RejectedExecutionHandler有四个实现了,代表了四种不同的拒绝策略。
这个处理器对被拒绝的任务会抛出一个异常
其实现为:
/**
* A handler for rejected tasks that throws a
* {@code RejectedExecutionException}.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an {@code AbortPolicy}.
*/
public AbortPolicy() { }
/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
//对被拒绝的任务抛出异常,并且把异常传播给execute方法
//RejectedExecutionException:是一个运行期异常
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
对AbortPolicy的使用:
public class PoolDemo2 {
public static void main(String[] args) {
AtomicInteger integer = new AtomicInteger(0);
ExecutorService pool = new ThreadPoolExecutor(3,5,0l, TimeUnit.MICROSECONDS,
new LinkedBlockingDeque<>(3),new ThreadPoolExecutor.AbortPolicy());
for (int i = 0; i < 9; i++) {
pool.execute(() -> {
System.out.println(Thread.currentThread().getName() + ": " + integer.incrementAndGet());
});
}
}
}
输出结果:
pool-1-thread-1: 1
pool-1-thread-1: 3
pool-1-thread-1: 4
pool-1-thread-1: 6
pool-1-thread-2: 2
pool-1-thread-3: 7
pool-1-thread-4: 5
pool-1-thread-5: 8
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task wf.threadpool.PoolDemo2$$Lambda$1/1831932724@3b9a45b3 rejected from java.util.concurrent.ThreadPoolExecutor@7699a589[Running, pool size = 5, active threads = 4, queued tasks = 0, completed tasks = 4]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
at wf.threadpool.PoolDemo2.main(PoolDemo2.java:22)
因为申请的线程池最大线程数为5,阻塞队列能存储的任务为3个,线程池同时能处理8个任务。所以我们给9个任务时最后一个被拒绝了,且抛出了异常。
会丢弃被拒绝的任务,不做任何处理
实现类:
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardPolicy}.
*/
public DiscardPolicy() { }
/**
* Does nothing, which has the effect of discarding task r.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}
使用代码:
public class PoolDemo2 {
public static void main(String[] args) {
AtomicInteger integer = new AtomicInteger(0);
ExecutorService pool = new ThreadPoolExecutor(3,5,0l, TimeUnit.MICROSECONDS,
new LinkedBlockingDeque<>(3),new ThreadPoolExecutor.DiscardPolicy());
for (int i = 0; i < 9; i++) {
pool.execute(() -> {
System.out.println(Thread.currentThread().getName() + ": " + integer.incrementAndGet());
});
}
}
}
输出结果:
pool-1-thread-1: 1
pool-1-thread-2: 2
pool-1-thread-3: 4
pool-1-thread-1: 3
pool-1-thread-3: 6
pool-1-thread-2: 5
pool-1-thread-4: 7
pool-1-thread-5: 8
最终只执行了8个任务,最后一个任务被丢弃了。
这个处理器会丢弃最老的未被处理的请求,即阻塞队列的队头元素,并且把被拒绝的任务,执行execute,尝试放入阻塞队列。除非线程池被关闭
/**
* A handler for rejected tasks that discards the oldest unhandled
* request and then retries {@code execute}, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/
public DiscardOldestPolicy() { }
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
//确定线程池不为null
if (!e.isShutdown()) {
//移出阻塞队列的队头元素
e.getQueue().poll();
//对任务执行execute方法
e.execute(r);
}
}
}
测试代码;
```cpp
public class PoolDemo2 {
public static void main(String[] args) {
AtomicInteger integer = new AtomicInteger(0);
ExecutorService pool = new ThreadPoolExecutor(3,5,0l, TimeUnit.MICROSECONDS,
new LinkedBlockingDeque<>(3),new ThreadPoolExecutor.DiscardOldestPolicy());
for (int i = 0; i < 9; i++) {
pool.execute(() -> {
System.out.println(Thread.currentThread().getName() + ": " + integer.incrementAndGet());
});
}
}
}
结果:
pool-1-thread-1: 1
pool-1-thread-2: 3
pool-1-thread-1: 4
pool-1-thread-1: 6
pool-1-thread-5: 7
pool-1-thread-3: 2
pool-1-thread-2: 5
pool-1-thread-4: 8
最后老任务被丢弃,最终执行任务有8个,不过因为没有提示所以看不出来。
会在调用execute方法的线程中直接运行任务的run方法。除非线程池关闭,任务会被直接丢弃。
/**
* A handler for rejected tasks that runs the rejected task
* directly in the calling thread of the {@code execute} method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code CallerRunsPolicy}.
*/
public CallerRunsPolicy() { }
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
//判断线程池是否执行shutdown方法
if (!e.isShutdown()) {
//直接调用run方法
r.run();
}
}
}
这种方法如果任务执行时间过长会阻塞调用者的线程,导致性能问题。
测代码:
public class PoolDemo2 {
public static void main(String[] args) {
AtomicInteger integer = new AtomicInteger(0);
ExecutorService pool = new ThreadPoolExecutor(3,5,0l, TimeUnit.MICROSECONDS,
new LinkedBlockingDeque<>(3),new ThreadPoolExecutor.CallerRunsPolicy());
for (int i = 0; i < 9; i++) {
pool.execute(() -> {
System.out.println(Thread.currentThread().getName() + ": " + integer.incrementAndGet());
});
}
}
}
结果:
pool-1-thread-1: 1
pool-1-thread-4: 5
main: 4
pool-1-thread-3: 2
pool-1-thread-2: 3
pool-1-thread-3: 9
pool-1-thread-4: 8
pool-1-thread-5: 7
pool-1-thread-1: 6
可以看到任务都被执行了,而被拒绝的任务有主线程执行了。
通过上面对线程池的创建的理解我们重新审视一下Executors中的三个线程池
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
//线程池创建
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
//拒绝策略
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
//阻塞队列
public LinkedBlockingQueue() {
//参数固定创建的是一个无解阻塞队列
this(Integer.MAX_VALUE);
}
/**
* Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if {@code capacity} is not greater
* than zero
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
因为其corePoolSize和maximumPoolSize都是nThreads,故线程池只能创建nThreads个线程。故keepAliveTime和unit两个属性没有直接给置为0。其采用了一个LinkedBlockingQueue无参构造创建一个无界阻塞队列。其拒绝策略为AbortPolicy,给拒绝的任务抛出异常。
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
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);
}
newSingleThreadExecutor线程池处理线程数只有1外,其他与newFixedThreadPool相同
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
其corePoolSize设置为0,maximumPoolSize设置为Integer.MAX_VALUE声明,线程池中不维护常驻线程,只有当需要时才会创建线程。而其阻塞队列使用了SynchronousQueue,这个阻塞队列只能维护一个元素,让存入到线程不会阻塞在队列上。
Executors类创建的线程池只能依靠默认的方法来对任务进行处理,导致其局限较大,在特定的环境中,不能较好的完成任务,故在开发对时候如果要使用线程池,最好根据项目需要通过ThreadPoolExecutor自己配置需要的线程池。
而且其使用了无界队列,很有可能在大的任务量情况下导致OOM异常
线程池使用时最好把偏向锁关闭,使用偏向锁会影响性能。