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<h2 id="1-实现多线程有几种方式？有什么区别？"><a href="#1-实现多线程有几种方式？有什么区别？" class="headerlink" title="1. 实现多线程有几种方式？有什么区别？"></a>1. 实现多线程有几种方式？有什么区别？</h2><p>实现多线程有3种方式。</p>
<h3 id="1-1-继承-Thread-类"><a href="#1-1-继承-Thread-类" class="headerlink" title="1.1 继承 Thread 类"></a>1.1 继承 Thread 类</h3><p>继承 Thread 类，重新 run() 方法。实现代码如下：</p>
<pre><code>public class ExtendsThread extends Thread{
@Override
public void run() {
System.out.println("run ExtendsThread");
}
}
使用线程：
public class LearningThread {
public static void main(String[] args) {
func1();
}
public static void func1(){
ExtendsThread extendsThread = new ExtendsThread();
System.out.println("run func1");
extendsThread.start();
System.out.println("run func1 end");
}
}
</code></pre>
<h3 id="1-2-实现-Runnable-接口"><a href="#1-2-实现-Runnable-接口" class="headerlink" title="1.2 实现 Runnable 接口"></a>1.2 实现 Runnable 接口</h3><p>实现 Runnable 接口，实现 run() 方法，通过 Thread 类来开启线程。代码如下：</p>
<pre><code>public class ImplRunnable implements Runnable {
@Override
public void run() {
System.out.println("run ImplRunnable");
}
}
使用线程：
public class LearningThread {
public static void main(String[] args) {
func2();
}
public static void func2(){
ImplRunnable implRunnable = new ImplRunnable();
System.out.println("run func2");
Thread thread = new Thread(implRunnable);
thread.start();
System.out.println("run func2 end");
}
}
</code></pre>
<h3 id="1-3-实现-Callable-接口"><a href="#1-3-实现-Callable-接口" class="headerlink" title="1.3 实现 Callable 接口"></a>1.3 实现 Callable 接口</h3><p>实现 Callable 接口，实现 call() 方法。代码如下：</p>
<pre><code>public class ImplCallable implements Callable<Integer> {
@Override
public Integer call() throws Exception {
return 2020;
}
}
使用线程：
public class LearningThread {
public static void main(String[] args) {
func3();
}
public static void func3() throws ExecutionException, InterruptedException {
Callable<Integer> integerCallable = new ImplCallable();
System.out.println("run func3");
FutureTask<Integer> futureTask = new FutureTask<>(integerCallable);
Thread thread = new Thread(futureTask);
thread.start();
System.out.println("run func3 end");
System.out.println("futureTask.get = " + futureTask.get());
}
}
</code></pre>
<p>这三种方式都可以实现多线程，第一种由于 Java 的单继承，不建议使用。至于实现 Runnable 接口，与实现 Callable 接口，的区别是，实现 Callable 接口后通过 futureTask.get() 方法可以获取线程内的执行结果。而 Runnable 是没有返回值的。</p>
<h2 id="2-为什么要用线程池？"><a href="#2-为什么要用线程池？" class="headerlink" title="2. 为什么要用线程池？"></a>2. 为什么要用线程池？</h2><p>线程池主要是为了减少每次创建线程时的资源消耗，重复利用创建好的线程，提高资源利用率。使用线程池由于减少了线程创建的过程，在每次接到请求时可以及时响应，提高响应速度。通过线程池统一管理线程，方便线程的分配，调优和监控。</p>
<h2 id="3-创建线程池的时候有哪些参数？"><a href="#3-创建线程池的时候有哪些参数？" class="headerlink" title="3. 创建线程池的时候有哪些参数？"></a>3. 创建线程池的时候有哪些参数？</h2><p>在 Java 源码中，创建线程池的构造方法最多有 7 个参数。</p>
<pre><code>public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
</code></pre>
<ul>
<li>corePoolSize: 核心线程数，线程池的工作线程数量。</li>
<li>maximumPoolSize: 最大线程数，线程池中可以存活的最多的线程数量。当队列满了之后，会启用非核心线程，此时的线程池的大小变为最大线程数。</li>
<li>keepAliveTime: 非核心线程如果没有任务的话，可以存活的时间。</li>
<li>unit: 非核心线程存活时间的单位。</li>
<li>workQueue: 工作队列。当线程池中的线程达到 corePoolSize ,如果再来任务，就会放到工作队列里。</li>
<li>threadFactory: 线程工厂,线程池创建线程时使用的工厂。</li>
<li>handler：拒绝策略。如果线程池的线程达到了 maximumPoolSize ,如果再来任务，则执行拒绝策略。</li>
</ul>
<h2 id="4-线程池是如何工作的？"><a href="#4-线程池是如何工作的？" class="headerlink" title="4. 线程池是如何工作的？"></a>4. 线程池是如何工作的？</h2><p>当有任务提交到线程池时，首先启动核心线程。随着任务的增加，当核心线程用完之后，再次提交的线程将会进入工作队列。当工作队列满了之后，如果再次提交到线程池任务，将会判断，核心线程数是否小于最大线程数，如果小于，将会启用非核心线程，当工作的线程达到最大线程数后，如果还继续提交任务到线程池，则会执行拒绝策略来拒绝任务。</p>
<h2 id="5-常见的拒绝策略有哪些？"><a href="#5-常见的拒绝策略有哪些？" class="headerlink" title="5. 常见的拒绝策略有哪些？"></a>5. 常见的拒绝策略有哪些？</h2><p>JDK 自带了 4 种拒绝策略。</p>
<h3 id="5-1-AbortPolicy"><a href="#5-1-AbortPolicy" class="headerlink" title="5.1 AbortPolicy"></a>5.1 AbortPolicy</h3><p>直接丢弃任务，抛出 RejectedExecutionException</p>
<pre><code>public class ThreadPoolRunnable implements Runnable {
private int number = 0;
public ThreadPoolRunnable(int number) {
this.number = number;
}
@Override
public void run() {
System.out.println("run "+Thread.currentThread().getName()+", number = " + number);
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
使用线程池：
public static void func4(){
ThreadPoolExecutor executor = new ThreadPoolExecutor(2,3,10,
TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.AbortPolicy());
for (int i = 0; i < 7; i++) {
Runnable runnable = new ThreadPoolRunnable(i);
executor.execute(runnable);
}
executor.shutdown();
}
执行结果：
run pool-1-thread-1, number = 0
run pool-1-thread-3, number = 3
run pool-1-thread-2, number = 1
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task com.learning.thread.ThreadPoolRunnable@1d44bcfa rejected from java.util.concurrent.ThreadPoolExecutor@266474c2[Running, pool size = 3, active threads = 3, queued tasks = 1, completed tasks = 0]
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 com.learning.thread.LearningThread.func4(LearningThread.java:45)
at com.learning.thread.LearningThread.main(LearningThread.java:11)
run pool-1-thread-1, number = 2
</code></pre>
<p>由于最大线程数是 3 ,队列大小是 1，所以线程池最多可以同时存在 4 个线程，当提交第 5 任务时，主线程抛出异常。</p>
<h3 id="5-2-CallerRunsPolicy"><a href="#5-2-CallerRunsPolicy" class="headerlink" title="5.2 CallerRunsPolicy"></a>5.2 CallerRunsPolicy</h3><p>调用启用线程池的线程进行处理，不过会阻塞主线程。</p>
<pre><code> public static void func4(){
ThreadPoolExecutor executor = new ThreadPoolExecutor(2,3,10,
TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.CallerRunsPolicy());
for (int i = 0; i < 7; i++) {
Runnable runnable = new ThreadPoolRunnable(i);
executor.execute(runnable);
}
executor.shutdown();
}
执行结果：
run pool-1-thread-1, number = 0
run main, number = 4
run pool-1-thread-3, number = 3
run pool-1-thread-2, number = 1
run main, number = 5
run pool-1-thread-3, number = 2
run pool-1-thread-3, number = 6
</code></pre>
<p>可以看到 第 5 个任务，即 number = 4，是在主线程中执行的，由于主线程被阻塞，导致第 6 个任务不能立即提交，当主线程执行结束后再提交时，线程池里已经有可用的线程了，所以第 6 个任务是线程池执行的。</p>
<h3 id="5-3-DiscardPolicy"><a href="#5-3-DiscardPolicy" class="headerlink" title="5.3 DiscardPolicy"></a>5.3 DiscardPolicy</h3><p>直接拒绝任务，不抛出任何异常</p>
<pre><code> public static void func4(){
ThreadPoolExecutor executor = new ThreadPoolExecutor(2,3,10,
TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.DiscardPolicy());
for (int i = 0; i < 7; i++) {
Runnable runnable = new ThreadPoolRunnable(i);
executor.execute(runnable);
}
executor.shutdown();
}
执行结果：
run pool-1-thread-2, number = 1
run pool-1-thread-3, number = 3
run pool-1-thread-1, number = 0
run pool-1-thread-2, number = 2
</code></pre>
<h3 id="5-4-DiscardOldestPolicy"><a href="#5-4-DiscardOldestPolicy" class="headerlink" title="5.4 DiscardOldestPolicy"></a>5.4 DiscardOldestPolicy</h3><p>抛弃队列中最先加入的任务，然后将当前任务提交到线程池。</p>
<pre><code> public static void func4(){
ThreadPoolExecutor executor = new ThreadPoolExecutor(2,3,10,
TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.DiscardOldestPolicy());
for (int i = 0; i < 7; i++) {
Runnable runnable = new ThreadPoolRunnable(i);
executor.execute(runnable);
}
executor.shutdown();
}
执行结果：
run pool-1-thread-1, number = 0
run pool-1-thread-3, number = 3
run pool-1-thread-2, number = 1
run pool-1-thread-1, number = 6
</code></pre>
<p>number 0、1，使用核心线程执行。</p>
<p>number 2，放入队列。</p>
<p>number 3，启用非核心线程执行。</p>
<p>number 4，抛弃 number 2，将 number 4 放入队列</p>
<p>number 5，抛弃 number 4，将 number 5 放入队列</p>
<p>number 6，抛弃 number 5，将 number 6 放入队列</p>
<p>执行 number 6</p>
<h3 id="5-5-自定义拒绝策略"><a href="#5-5-自定义拒绝策略" class="headerlink" title="5.5 自定义拒绝策略"></a>5.5 自定义拒绝策略</h3><p>当然，如果上面的拒绝策略都不满足的话,我们也可以定义拒绝策略。</p>
<pre><code>public class MyRejectedExecutionHandler implements RejectedExecutionHandler {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
System.out.println(executor.toString());
}
}
执行结果：
run pool-1-thread-2, number = 1
run pool-1-thread-3, number = 3
run pool-1-thread-1, number = 0
java.util.concurrent.ThreadPoolExecutor@1d44bcfa[Running, pool size = 3, active threads = 3, queued tasks = 1, completed tasks = 0]
java.util.concurrent.ThreadPoolExecutor@1d44bcfa[Running, pool size = 3, active threads = 3, queued tasks = 1, completed tasks = 0]
java.util.concurrent.ThreadPoolExecutor@1d44bcfa[Running, pool size = 3, active threads = 3, queued tasks = 1, completed tasks = 0]
run pool-1-thread-2, number = 2
</code></pre>
<h2 id="6-常见的线程池有哪些？各自有什么特点？"><a href="#6-常见的线程池有哪些？各自有什么特点？" class="headerlink" title="6. 常见的线程池有哪些？各自有什么特点？"></a>6. 常见的线程池有哪些？各自有什么特点？</h2><p>JDK自带了几个常见的线程池。</p>
<h3 id="6-1-FixedThreadPool"><a href="#6-1-FixedThreadPool" class="headerlink" title="6.1 FixedThreadPool"></a>6.1 FixedThreadPool</h3><p>FixedThreadPool 被称为可重用固定线程数的线程池。创建的源码如下：</p>
<pre><code> public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
</code></pre>
<p>可以看到 FixedThreadPool 的的核心线程数和最大线程数都是传入的参数。使用的队列是 LinkedBlockingQueue 。<br>由于 LinkedBlockingQueue 是一个无界队列（队列的容量为 Intger.MAX_VALUE），所以运行中的 FixedThreadPool 不会拒绝任务，所以当任务过多的时候可能会造成 OOM 。</p>
<h3 id="6-2-SingleThreadExecutor"><a href="#6-2-SingleThreadExecutor" class="headerlink" title="6.2 SingleThreadExecutor"></a>6.2 SingleThreadExecutor</h3><p>SingleThreadExecutor 是只有一个线程的线程池。创建源码：</p>
<pre><code> public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
</code></pre>
<p>SingleThreadExecutor 的核心线程数和最大线程数都为 1，所以这个线程池只有一个线程。使用的队列也是 LinkedBlockingQueue ，所以当任务过多时也会存在 OOM 的问题。</p>
<h3 id="6-3-CachedThreadPool"><a href="#6-3-CachedThreadPool" class="headerlink" title="6.3 CachedThreadPool"></a>6.3 CachedThreadPool</h3><p>CachedThreadPool 无固定大小的线程池，随着任务的不断提交，创建新的线程来执行。创建源码：</p>
<pre><code> public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
</code></pre>
<p>CachedThreadPool 的核心线程数为 0 ，最大线程数是 Integer.MAX_VALUE，可见所有的线程都是非核心线程。如果线程池的线程 60 秒,没有执行任务则会被销毁。由于使用了 SynchronousQueue，所以当主线程通过 SynchronousQueue.offer(Runnable task) 提交任务到队列后会阻塞，如果线程池中有可用的线程，则会执行当前任务，如果没有则会创建一个新的线程来执任务。</p>
<p>可见，如果任务太多的话，依然会造成 OOM ，与 LinkedBlockingQueue 不同的是，LinkedBlockingQueue 是由于任务对象太多，导致 OOM，ThreadPoolExecutor 则是由于 线程数太多导致 OOM 。</p>
<h2 id="如何获取线程池中的返回结果？"><a href="#如何获取线程池中的返回结果？" class="headerlink" title="如何获取线程池中的返回结果？"></a>如何获取线程池中的返回结果？</h2><p>可以使用 <code>executor.submit(futureTask);</code>,提交一个 FutureTask。代码如下：</p>
<pre><code>自定义的线程
public class ThreadPoolCallable implements Callable<Integer> {
@Override
public Integer call() throws Exception {
Random random = new Random();
int number = random.nextInt(100);
System.out.println(Thread.currentThread().getName() + ":" + number);
return number;
}
}
使用：
public static void func5() throws ExecutionException, InterruptedException {
ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 3, 10,
TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.CallerRunsPolicy());
int result = 0;
for (int i = 0; i < 7; i++) {
Callable<Integer> callable = new ThreadPoolCallable();
FutureTask<Integer> futureTask = new FutureTask<>(callable);
executor.submit(futureTask);
result += futureTask.get();
}
executor.shutdown();
System.out.println("result = " + result);
}
执行结果：
pool-1-thread-1:41
pool-1-thread-2:83
pool-1-thread-1:15
pool-1-thread-2:21
pool-1-thread-1:26
pool-1-thread-2:38
pool-1-thread-1:98
result = 322
</code></pre>
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