概述
多线程技术主要解决处理器单元内多个线程执行的问题，它可以显著减少处理器单元的闲置时间，增加处理器单元的吞吐能力。
假设一个服务器完成一项任务所需时间为：T1 创建线程时间，T2 在线程中执行任务的时间，T3 销毁线程时间。
如果：T1 + T3 远大于 T2，则可以采用线程池，以提高服务器性能。
一个线程池包括以下四个基本组成部分：
- 线程池管理器（ThreadPool）：用于创建并管理线程池，包括 创建线程池，销毁线程池，添加新任务；
- 工作线程（PoolWorker）：线程池中线程，在没有任务时处于等待状态，可以循环的执行任务；
- 任务接口（Task）：每个任务必须实现的接口，以供工作线程调度任务的执行，它主要规定了任务的入口，任务执行完后的收尾工作，任务的执行状态等；
- 任务队列（taskQueue）：用于存放没有处理的任务。提供一种缓冲机制。
线程池技术正是关注如何缩短或调整T1,T3时间的技术，从而提高服务器程序性能的。
它把T1，T3分别安排在服务器程序的启动和结束的时间段或者一些空闲的时间段，这样在服务器程序处理客户请求时，不会有T1，T3的开销了。
线程池不仅调整T1,T3产生的时间段，而且它还显著减少了创建线程的数目，看一个例子：
假设一个服务器一天要处理50000个请求，并且每个请求需要一个单独的线程完成。在线程池中，线程数一般是固定的，所以产生线程总数不会超过线程池中线程的数目，而如果服务器不利用线程池来处理这些请求则线程总数为50000。一般线程池大小是远小于50000。所以利用线程池的服务器程序不会为了创建50000而在处理请求时浪费时间，从而提高效率。
线程的使用在java中占有极其重要的地位，在jdk1.4极其之前的jdk版本中，关于线程池的使用是极其简陋的。在jdk1.5之后这一情况有了很大的改观。Jdk1.5之后加入了java.util.concurrent包，这个包中主要介绍java中线程以及线程池的使用。为我们在开发中处理线程的问题提供了非常大的帮助。
为什么要使用线程池
- 1.创建/销毁线程伴随着系统开销，过于频繁的创建/销毁线程，会很大程度上影响处-理效率
- 2.可以根据系统的承受能力，调整线程池中工作线线程的数目，防止因为消耗过多的内存，而把服务器累趴下(每个线程需要大约1MB内存，线程开的越多，消耗的内存也就越大，最后死机)
- 3.对线程进行一些简单的管理
ThreadPoolExecutor
Java里面线程池的顶级接口是Executor，但是严格意义上讲Executor并不是一个线程池，而只是一个执行线程的工具。
真正的线程池接口是ExecutorService。即：ExecutorService接口是Executor的子接口，而ThreadPoolExecutor实现了ExecutorService，是具体的实现类:
学习Java中的线程池，就可以直接学习ThreadPoolExecutor，对线程池的配置就是对ThreadPoolExecutor构造函数的参数的配置：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
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;
}
各个参数的含义：
-
int corePoolSize：该线程池中核心线程数最大值
核心线程：线程池新建线程的时候，如果当前线程总数小于corePoolSize，则新建的是核心线程，如果超过corePoolSize，则新建的是非核心线程，核心线程默认情况下会一直存活在线程池中，即使这个核心线程啥也不干(闲置状态)。
如果指定ThreadPoolExecutor的allowCoreThreadTimeOut这个属性为true，那么核心线程如果不干活(闲置状态)的话，超过一定时间(时长下面参数决定)，就会被销毁掉。 -
int maximumPoolSize： 该线程池中线程总数最大值
线程总数 = 核心线程数 + 非核心线程数。 -
long keepAliveTime：该线程池中非核心线程闲置超时时长
一个非核心线程，如果不干活(闲置状态)的时长超过这个参数所设定的时长，就会被销毁掉，如果设置allowCoreThreadTimeOut = true，则同样会作用于核心线程。 -
TimeUnit unit：keepAliveTime的单位
TimeUnit是一个枚举类型，其包括：
NANOSECONDS ： 1微毫秒 = 1微秒 / 1000
MICROSECONDS ： 1微秒 = 1毫秒 / 1000
MILLISECONDS ： 1毫秒 = 1秒 /1000
SECONDS ： 秒
MINUTES ： 分
HOURS ： 小时
DAYS ： 天
-
BlockingQueue workQueue：该线程池中的任务队列：维护着等待执行的Runnable对象
当所有的核心线程都在干活时，新添加的任务会被添加到这个队列中等待处理，如果队列满了，则新建非核心线程执行任务。
常用的workQueue类型:
1.SynchronousQueue：
这个队列接收到任务的时候，会直接提交给线程处理，而不保留它。
如果所有线程都在工作怎么办？
那就新建一个线程来处理这个任务！所以为了保证不出现<线程数达到了maximumPoolSize而不能新建线程>的错误，使用这个类型队列的时候，maximumPoolSize一般指定成Integer.MAX_VALUE，即无限大。
2.LinkedBlockingQueue：
这个队列接收到任务的时候，如果当前线程数小于核心线程数，则新建线程(核心线程)处理任务；如果当前线程数等于核心线程数，则进入队列等待。由于这个队列默认大小为Integer.MAX_VALUE，即所有超过核心线程数的任务都将被添加到队列中，所以如果未通过构造方法设置队列长度就会导致maximumPoolSize的设定失效，因为总线程数永远不会超过corePoolSize。
3.ArrayBlockingQueue：
可以限定队列的长度，接收到任务的时候，如果没有达到corePoolSize的值，则新建线程(核心线程)执行任务，如果达到了，则入队等候，如果队列已满，则新建线程(非核心线程)执行任务，又如果总线程数到了maximumPoolSize，并且队列也满了，则发生错误。
4.DelayQueue：
队列内元素必须实现Delayed接口，这就意味着你传进去的任务必须先实现Delayed接口。这个队列接收到任务时，首先先入队，只有达到了指定的延时时间，才会执行任务。
如何向ThreadPoolExecutor添加任务
通过ThreadPoolExecutor.execute(Runnable command)方法即可向线程池内添加一个任务。
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) {
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);
}
RejectedExecutionHandler拒绝策略
RejectedExecutionHandler是一个接口，当要创建的线程数量大于线程池的最大线程数的时候，新的任务就会被拒绝，就会调用这个接口里的rejectedExecution方法：
package java.util.concurrent;
/**
* A handler for tasks that cannot be executed by a {@link ThreadPoolExecutor}.
*
* @since 1.5
* @author Doug Lea
*/
public interface RejectedExecutionHandler {
/**
* Method that may be invoked by a {@link ThreadPoolExecutor} when
* {@link ThreadPoolExecutor#execute execute} cannot accept a
* task. This may occur when no more threads or queue slots are
* available because their bounds would be exceeded, or upon
* shutdown of the Executor.
*
* <p>In the absence of other alternatives, the method may throw
* an unchecked {@link RejectedExecutionException}, which will be
* propagated to the caller of {@code execute}.
*
* @param r the runnable task requested to be executed
* @param executor the executor attempting to execute this task
* @throws RejectedExecutionException if there is no remedy
*/
void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
}
Java中为我们提供了4种拒绝策略：
-
AbortPolicy
ThreadPoolExecutor中默认的拒绝策略就是AbortPolicy，直接抛出异常。
源码如下：
/**
* 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) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
简单粗暴，直接抛出个RejectedExecutionException异常，也不执行这个任务了。
-
CallerRunsPolicy
CallerRunsPolicy在任务被拒绝添加后，会调用当前线程池的所在的线程去执行被拒绝的任务。
源码如下：
/**
* 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) {
if (!e.isShutdown()) {
r.run();
}
}
}
-
DiscardPolicy
采用这个拒绝策略，会让被线程池拒绝的任务直接抛弃，不会抛异常也不会执行。
源码如下：
/**
* 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) {
}
}
-
DiscardOldestPolicy
DiscardOldestPolicy策略的作用是：当任务被拒绝添加时，会抛弃任务队列中最旧的任务也就是最先加入队列的，再把这个新任务添加进去。
源码如下：
/**
* 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) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}
当然我们也可以自定义拒绝策略，只需要实现RejectedExecutionHandler接口并重写rejectedExecution方法即可，例如让被拒绝的任务在一个新的线程中执行：
static class MyRejectedExecutionHandler implements RejectedExecutionHandler {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
new Thread(r,"新线程"+new Random().nextInt(10)).start();
}
}
Java自带的四种线程池
如果你不想自己写一个线程池，Java通过Executors这个类提供了四种线程池，这四种线程池都是直接或间接配置ThreadPoolExecutor的参数实现的。
- 1.CachedThreadPool：可缓存线程池
/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available. These pools will typically improve the performance
* of programs that execute many short-lived asynchronous tasks.
* Calls to {@code execute} will reuse previously constructed
* threads if available. If no existing thread is available, a new
* thread will be created and added to the pool. Threads that have
* not been used for sixty seconds are terminated and removed from
* the cache. Thus, a pool that remains idle for long enough will
* not consume any resources. Note that pools with similar
* properties but different details (for example, timeout parameters)
* may be created using {@link ThreadPoolExecutor} constructors.
*
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available, and uses the provided
* ThreadFactory to create new threads when needed.
* @param threadFactory the factory to use when creating new threads
* @return the newly created thread pool
* @throws NullPointerException if threadFactory is null
*/
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(),
threadFactory);
}
根据源码可以看出：
1.这种线程池内部没有核心线程，线程的数量是有没限制的。
2.在创建任务时，若有空闲的线程时则复用空闲的线程，若没有则新建线程。
3.没有工作的线程（闲置状态）在超过了60S还不做事，就会销毁。
- 2.FixedThreadPool：定长线程池
/**
* Creates a thread pool that reuses a fixed number of threads
* operating off a shared unbounded queue. At any point, at most
* {@code nThreads} threads will be active processing tasks.
* If additional tasks are submitted when all threads are active,
* they will wait in the queue until a thread is available.
* If any thread terminates due to a failure during execution
* prior to shutdown, a new one will take its place if needed to
* execute subsequent tasks. The threads in the pool will exist
* until it is explicitly {@link ExecutorService#shutdown shutdown}.
*
* @param nThreads the number of threads in the pool
* @return the newly created thread pool
* @throws IllegalArgumentException if {@code nThreads <= 0}
*/
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
/**
* Creates a thread pool that reuses a fixed number of threads
* operating off a shared unbounded queue, using the provided
* ThreadFactory to create new threads when needed. At any point,
* at most {@code nThreads} threads will be active processing
* tasks. If additional tasks are submitted when all threads are
* active, they will wait in the queue until a thread is
* available. If any thread terminates due to a failure during
* execution prior to shutdown, a new one will take its place if
* needed to execute subsequent tasks. The threads in the pool will
* exist until it is explicitly {@link ExecutorService#shutdown
* shutdown}.
*
* @param nThreads the number of threads in the pool
* @param threadFactory the factory to use when creating new threads
* @return the newly created thread pool
* @throws NullPointerException if threadFactory is null
* @throws IllegalArgumentException if {@code nThreads <= 0}
*/
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
根据源码可以看出：
1.该线程池的最大线程数等于核心线程数，所以在默认情况下，该线程池的线程不会因为闲置状态超时而被销毁。
2.如果当前线程数小于核心线程数，并且也有闲置线程的时候提交了任务，这时也不会去复用之前的闲置线程，会创建新的线程去执行任务。如果当前执行任务数大于了核心线程数，大于的部分就会进入队列等待。等着有闲置的线程来执行这个任务。
- SingleThreadPool单线程线程池
/**
* Creates an Executor that uses a single worker thread operating
* off an unbounded queue. (Note however that if this single
* thread terminates due to a failure during execution prior to
* shutdown, a new one will take its place if needed to execute
* subsequent tasks.) Tasks are guaranteed to execute
* sequentially, and no more than one task will be active at any
* given time. Unlike the otherwise equivalent
* {@code newFixedThreadPool(1)} the returned executor is
* guaranteed not to be reconfigurable to use additional threads.
*
* @return the newly created single-threaded Executor
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
/**
* Creates an Executor that uses a single worker thread operating
* off an unbounded queue, and uses the provided ThreadFactory to
* create a new thread when needed. Unlike the otherwise
* equivalent {@code newFixedThreadPool(1, threadFactory)} the
* returned executor is guaranteed not to be reconfigurable to use
* additional threads.
*
* @param threadFactory the factory to use when creating new
* threads
*
* @return the newly created single-threaded Executor
* @throws NullPointerException if threadFactory is null
*/
public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory));
}
根据源码可以看出：
1.有且仅有一个工作线程执行任务
2.所有任务按照指定顺序执行，即遵循队列的入队出队规则
- ScheduledThreadPool延期周期执行线程池
/**
* Creates a thread pool that can schedule commands to run after a
* given delay, or to execute periodically.
* @param corePoolSize the number of threads to keep in the pool,
* even if they are idle
* @return a newly created scheduled thread pool
* @throws IllegalArgumentException if {@code corePoolSize < 0}
*/
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
/**
* Creates a thread pool that can schedule commands to run after a
* given delay, or to execute periodically.
* @param corePoolSize the number of threads to keep in the pool,
* even if they are idle
* @param threadFactory the factory to use when the executor
* creates a new thread
* @return a newly created scheduled thread pool
* @throws IllegalArgumentException if {@code corePoolSize < 0}
* @throws NullPointerException if threadFactory is null
*/
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
根据源码可以看出：
DEFAULT_KEEPALIVE_MILLIS就是默认10L，这里就是10秒。这个线程池有点像是吧CachedThreadPool和FixedThreadPool 结合了一下。
不仅设置了核心线程数，最大线程数也是Integer.MAX_VALUE。
这个线程池是上述4个中为唯一个有延迟执行和周期执行任务的线程池。
线程池的执行流程
- 新任务进来先看是否有可用核心线程，如果有，则使用核心线程处理此任务
- 如果核心线程数已满，则添加到任务队列等待处理
- 如果任务队列已满，且核心线程数已满，则使用非核心线程处理任务
- 如果队列已满，核心线程和非核心线程都已满，则执行拒绝策略
线程池的关闭
ThreadPoolExecutor提供了两个方法，用于线程池的关闭，分别是shutdown()和shutdownNow()，其中：
- shutdown()：不会立即终止线程池，而是要等所有任务缓存队列中的任务都执行完后才终止，但再也不会接受新的任务
- shutdownNow()：立即终止线程池，并尝试打断正在执行的任务，并且清空任务缓存队列，返回尚未执行的任务
如何选择线程池数量
线程池的大小决定着系统的性能，过大或者过小的线程池数量都无法发挥最优的系统性能。
当然线程池的大小也不需要做的太过于精确，只需要避免过大和过小的情况。一般来说，确定线程池的大小需要考虑CPU的数量，内存大小，任务是计算密集型还是IO密集型等因素。
NCPU = CPU的数量
UCPU = 期望对CPU的使用率 0 ≤ UCPU ≤ 1
W/C = 等待时间与计算时间的比率
如果希望处理器达到理想的使用率，那么线程池的最优大小为：
线程池大小=NCPU * UCPU(1+W/C)
在Java中使用int ncpus = Runtime.getRuntime().availableProcessors();可以获取CPU的数量。
线程池工厂
Executors的线程池如果不指定线程工厂会使用Executors中的DefaultThreadFactory
默认线程池工厂创建的线程都是非守护线程。
使用自定义的线程工厂可以做很多事情，比如可以跟踪线程池在何时创建了多少线程，也可以自定义线程名称和优先级。如果将
新建的线程都设置成守护线程，当主线程退出后，将会强制销毁线程池。
