SpringBoot线程池和Java线程池的用法和实现原理
使用默认的线程池
方式一：通过@Async注解调用
public class AsyncTest {
@Async
public void async(String name) throws InterruptedException {
System.out.println("async" + name + " " + Thread.currentThread().getName());
Thread.sleep(1000);
}
}启动类上需要添加@enableasync注解，否则不会生效。
@SpringBootApplication
//@EnableAsync
public class Test1Application {
public static void main(String[] args) throws InterruptedException {
ConfigurableApplicationContext run = SpringApplication.run(Test1Application.class, args);
AsyncTest bean = run.getBean(AsyncTest.class);
for(int index = 0; index <= 10; ++index){
bean.async(String.valueOf(index));
}
}
}方式二：直接注入 ThreadPoolTaskExecutor
此时可不加 @EnableAsync注解
@SpringBootTest
class Test1ApplicationTests {
@Resource
ThreadPoolTaskExecutor threadPoolTaskExecutor;
@Test
void contextLoads() {
Runnable runnable = () -> {
System.out.println(Thread.currentThread().getName());
};
for(int index = 0; index <= 10; ++index){
threadPoolTaskExecutor.submit(runnable);
}
}
}线程池默认配置信息
SpringBoot线程池的常见配置：
spring:
task:
execution:
pool:
core-size: 8
max-size: 16 # 默认是 Integer.MAX_VALUE
keep-alive: 60s # 当线程池中的线程数量大于 corePoolSize 时，如果某线程空闲时间超过keepAliveTime，线程将被终止
allow-core-thread-timeout: true # 是否允许核心线程超时,默认true
queue-capacity: 100 # 线程队列的大小,默认Integer.MAX_VALUE
shutdown:
await-termination: false # 线程关闭等待
thread-name-prefix: task- # 线程名称的前缀SpringBoot 线程池的实现原理
TaskExecutionAutoConfiguration 类中定义了 ThreadPoolTaskExecutor,该类的内部实现也是基于java原生的 ThreadPoolExecutor类。initializeExecutor()方法在其父类中被调用，但是在父类中 RejectedExecutionHandler 被定义为了 private RejectedExecutionHandler rejectedExecutionHandler = new ThreadPoolExecutor.AbortPolicy(); ，并通过initialize()方法将AbortPolicy传入initializeExecutor()中。
注意在TaskExecutionAutoConfiguration 类中，ThreadPoolTaskExecutor类的bean的名称为： applicationTaskExecutor 和 taskExecutor。
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// TaskExecutionAutoConfiguration#applicationTaskExecutor()
@Lazy
@Bean(name = { APPLICATION_TASK_EXECUTOR_BEAN_NAME,
AsyncAnnotationBeanPostProcessor.DEFAUL
T_TASK_EXECUTOR_BEAN_NAME })
@ConditionalOnMissingBean(Executor.class)
public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
return builder.build();
}// ThreadPoolTaskExecutor#initializeExecutor()
@Override
protected ExecutorService initializeExecutor(
ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);
ThreadPoolExecutor executor;
if (this.taskDecorator != null) {
executor = new ThreadPoolExecutor(
this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
queue, threadFactory, rejectedExecutionHandler) {
@Override
public void execute(Runnable command) {
Runnable decorated = taskDecorator.decorate(command);
if (decorated != command) {
decoratedTaskMap.put(decorated, command);
}
super.execute(decorated);
}
};
}
else {
executor = new ThreadPoolExecutor(
this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
queue, threadFactory, rejectedExecutionHandler);
}
if (this.allowCoreThreadTimeOut) {
executor.allowCoreThreadTimeOut(true);
}
this.threadPoolExecutor = executor;
return executor;
}// ExecutorConfigurationSupport#initialize()
public void initialize() {
if (logger.isInfoEnabled()) {
logger.info("Initializing ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
}
if (!this.threadNamePrefixSet && this.beanName != null) {
setThreadNamePrefix(this.beanName + "-");
}
this.executor = initializeExecutor(this.threadFactory, this.rejectedExecutionHandler);
}覆盖默认的线程池
覆盖默认的 taskExecutor对象，bean的返回类型可以是ThreadPoolTaskExecutor也可以是Executor。
@Configuration
public class ThreadPoolConfiguration {
@Bean("taskExecutor")
public ThreadPoolTaskExecutor taskExecutor() {
ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
//设置线程池参数信息
taskExecutor.setCorePoolSize(10);
taskExecutor.setMaxPoolSize(50);
taskExecutor.setQueueCapacity(200);
taskExecutor.setKeepAliveSeconds(60);
taskExecutor.setThreadNamePrefix("myExecutor--");
taskExecutor.setWaitForTasksToCompleteOnShutdown(true);
taskExecutor.setAwaitTerminationSeconds(60);
//修改拒绝策略为使用当前线程执行
taskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
//初始化线程池
taskExecutor.initialize();
return taskExecutor;
}
}管理多个线程池
如果出现了多个线程池，例如再定义一个线程池 taskExecutor2，则直接执行会报错。此时需要指定bean的名称即可。
@Bean("taskExecutor2")
public ThreadPoolTaskExecutor taskExecutor2() {
ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
//设置线程池参数信息
taskExecutor.setCorePoolSize(10);
taskExecutor.setMaxPoolSize(50);
taskExecutor.setQueueCapacity(200);
taskExecutor.setKeepAliveSeconds(60);
taskExecutor.setThreadNamePrefix("myExecutor2--");
taskExecutor.setWaitForTasksToCompleteOnShutdown(true);
taskExecutor.setAwaitTerminationSeconds(60);
//修改拒绝策略为使用当前线程执行
taskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
//初始化线程池
taskExecutor.initialize();
return taskExecutor;
}引用线程池时，需要将变量名更改为bean的名称，这样会按照名称查找。
@Resource ThreadPoolTaskExecutor taskExecutor2;
对于使用@Async注解的多线程则在注解中指定bean的名字即可。
@Async("taskExecutor2")
public void async(String name) throws InterruptedException {
System.out.println("async" + name + " " + Thread.currentThread().getName());
Thread.sleep(1000);
}线程池的四种拒绝策略
JAVA常用的四种线程池
ThreadPoolExecutor 类的构造函数如下：
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}newCachedThreadPool
不限制最大线程数（maximumPoolSize=Integer.MAX_VALUE）,如果有空闲的线程超过需要，则回收，否则重用已有的线程。
new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());newFixedThreadPool
定长线程池，超出线程数的任务会在队列中等待。
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());newScheduledThreadPool
类似于newCachedThreadPool，线程数无上限，但是可以指定corePoolSize。可实现延迟执行、周期执行。
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}周期执行：
ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);
scheduledThreadPool.scheduleAtFixedRate(()->{
System.out.println("rate");
}, 1, 1, TimeUnit.SECONDS);延时执行：
scheduledThreadPool.schedule(()->{
System.out.println("delay 3 seconds");
}, 3, TimeUnit.SECONDS);newSingleThreadExecutor
单线程线程池，可以实现线程的顺序执行。
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}Java 线程池中的四种拒绝策略
CallerRunsPolicy：线程池让调用者去执行。AbortPolicy：如果线程池拒绝了任务，直接报错。DiscardPolicy：如果线程池拒绝了任务，直接丢弃。DiscardOldestPolicy：如果线程池拒绝了任务，直接将线程池中最旧的，未运行的任务丢弃，将新任务入队。
CallerRunsPolicy
直接在主线程中执行了run方法。
public static class CallerRunsPolicy implements RejectedExecutionHandler {
public CallerRunsPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
}效果类似于：
Runnable thread = ()->{
System.out.println(Thread.currentThread().getName());
try {
Thread.sleep(0);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
};
thread.run();AbortPolicy
直接抛出RejectedExecutionException异常，并指示任务的信息，线程池的信息。、
public static class AbortPolicy implements RejectedExecutionHandler {
public AbortPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}DiscardPolicy
什么也不做。
public static class DiscardPolicy implements RejectedExecutionHandler {
public DiscardPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}DiscardOldestPolicy
e.getQueue().poll(): 取出队列最旧的任务。e.execute(r): 当前任务入队。
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
public DiscardOldestPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}Java 线程复用的原理
java的线程池中保存的是 java.util.concurrent.ThreadPoolExecutor.Worker 对象，该对象在 被维护在private final HashSet<worker> workers = new HashSet<worker>();</worker></worker>。workQueue是保存待执行的任务的队列，线程池中加入新的任务时，会将任务加入到workQueue队列中。
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
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);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}work对象的执行依赖于 runWorker()，与我们平时写的线程不同，该线程处在一个循环中，并不断地从队列中获取新的任务执行。因此线程池中的线程才可以复用，而不是像我们平常使用的线程一样执行完毕就结束。
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);
}
} 
