ThreadPoolExecutor中策略的选择与工作队列的选择（java线程池）

工作原理

1、线程池刚创建时，里面没有一个线程。任务队列是作为参数传进来的。不过，就算队列里面有任务，线程池也不会马上执行它们。

2、当调用 execute() 方法添加一个任务时，线程池会做如下判断：

a. 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；

　　 b. 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。

　　 c. 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；

　　 d. 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常，告诉调用者“我不能再接受任务了”。
3、当一个线程完成任务时，它会从队列中取下一个任务来执行。
4、当一个线程无事可做，超过一定的时间（keepAliveTime）时，线程池会判断，如果当前运行的线程数大于 corePoolSize，那么这个线程就被停掉。所以线程池的所有任务完成后，它最终会收缩到 corePoolSize 的大小。
　 　这样的过程说明，并不是先加入任务就一定会先执行。假设队列大小为 10，corePoolSize 为 3，maximumPoolSize 为 6，那么当加入 20 个任务时，执行的顺序就是这样的：首先执行任务 1、2、3，然后任务 4~13 被放入队列。这时候队列满了，任务 14、15、16 会被马上执行，而任务 17~20 则会抛出异常。最终顺序是：1、2、3、14、15、16、4、5、6、7、8、9、10、11、12、13。下面是一个线程池使用的例子：

排队

所有

BlockingQueue

都可用于传输和保持提交的任务。可以使用此队列与池大小进行交互：

如果运行的线程少于 corePoolSize，则 Executor 始终首选添加新的线程，而不进行排队。
如果运行的线程等于或多于 corePoolSize，则 Executor 始终首选将请求加入队列，而不添加新的线程。
如果无法将请求加入队列，则创建新的线程，除非创建此线程超出 maximumPoolSize，在这种情况下，任务将被拒绝。

排队有三种通用策略：

直接提交。工作队列的默认选项是

SynchronousQueue

，它将任务直接提交给线程而不保持它们。在此，如果不存在可用于立即运行任务的线程，则试图把任务加入队列将失败，因此会构造一个新的线程。此策略可以避免在处理可能具有内部依赖性的请求集时出现锁。直接提交通常要求无界 maximumPoolSizes 以避免拒绝新提交的任务。当命令以超过队列所能处理的平均数连续到达时，此策略允许无界线程具有增长的可能性。
无界队列。使用无界队列（例如，不具有预定义容量的

LinkedBlockingQueue

）将导致在所有 corePoolSize 线程都忙时新任务在队列中等待。这样，创建的线程就不会超过 corePoolSize。（因此，maximumPoolSize 的值也就无效了。）当每个任务完全独立于其他任务，即任务执行互不影响时，适合于使用无界队列；例如，在 Web 页服务器中。这种排队可用于处理瞬态突发请求，当命令以超过队列所能处理的平均数连续到达时，此策略允许无界线程具有增长的可能性。
有界队列。当使用有限的 maximumPoolSizes 时，有界队列（如

ArrayBlockingQueue

）有助于防止资源耗尽，但是可能较难调整和控制。队列大小和最大池大小可能需要相互折衷：使用大型队列和小型池可以最大限度地降低 CPU 使用率、操作系统资源和上下文切换开销，但是可能导致人工降低吞吐量。如果任务频繁阻塞（例如，如果它们是 I/O 边界），则系统可能为超过您许可的更多线程安排时间。使用小型队列通常要求较大的池大小，CPU 使用率较高，但是可能遇到不可接受的调度开销，这样也会降低吞吐量。

被拒绝的任务 当 Executor 已经关闭，并且 Executor 将有限边界用于最大线程和工作队列容量，且已经饱和时，在方法

execute(java.lang.Runnable)

中提交的新任务将被拒绝。在以上两种情况下，execute 方法都将调用其

RejectedExecutionHandler

的

RejectedExecutionHandler.rejectedExecution(java.lang.Runnable,
java.util.concurrent.ThreadPoolExecutor)

方法。下面提供了四种预定义的处理程序策略：

在默认的

ThreadPoolExecutor.AbortPolicy

中，处理程序遭到拒绝将抛出运行时

RejectedExecutionException

。
在

ThreadPoolExecutor.CallerRunsPolicy

中，线程调用运行该任务的execute 本身。此策略提供简单的反馈控制机制，能够减缓新任务的提交速度。
在

ThreadPoolExecutor.DiscardPolicy

中，不能执行的任务将被删除。
在

ThreadPoolExecutor.DiscardOldestPolicy

中，如果执行程序尚未关闭，则位于工作队列头部的任务将被删除，然后重试执行程序（如果再次失败，则重复此过程）。

定义和使用其他种类的

RejectedExecutionHandler

类也是可能的，但这样做需要非常小心，尤其是当策略仅用于特定容量或排队策略时。

公用的类ThreadPoolTask

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public class ThreadPoolTask implements Runnable {
// 保存任务所需要的数据
private Object threadPoolTaskData;
private static int consumerTaskSleepTime = 2000;

ThreadPoolTask(Object tasks) {
this.threadPoolTaskData = tasks;
}

public void run() {
// 处理一个任务，这里的处理方式太简单了，仅仅是一个打印语句
System.out.println("start .." + threadPoolTaskData);
try {
//便于观察，等待一段时间
Thread.sleep(consumerTaskSleepTime);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("finish " + threadPoolTaskData);
threadPoolTaskData = null;
}

}

public class ThreadPoolTask implements Runnable {
// 保存任务所需要的数据
private Object threadPoolTaskData;
private static int consumerTaskSleepTime = 2000;

ThreadPoolTask(Object tasks) {
this.threadPoolTaskData = tasks;
}

public void run() {
// 处理一个任务，这里的处理方式太简单了，仅仅是一个打印语句
System.out.println("start .." + threadPoolTaskData);
try {
//便于观察，等待一段时间
Thread.sleep(consumerTaskSleepTime);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("finish " + threadPoolTaskData);
threadPoolTaskData = null;
}

}

ThreadPoolExecutor.AbortPolicy

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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

运行结果

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put task@ 1 put task@ 2 start ..task@ 1 put task@ 3 put task@ 4 start ..task@ 2 put task@ 5 put task@ 6 put task@ 7 start ..task@ 6 put task@ 8 start ..task@ 7 java.util.concurrent.RejectedExecutionException at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source) at ThreadPool.main(ThreadPool.java:22) put task@ 9 java.util.concurrent.RejectedExecutionException at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source) at ThreadPool.main(ThreadPool.java:22) put task@ 10 java.util.concurrent.RejectedExecutionException at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source) at ThreadPool.main(ThreadPool.java:22) finish task@ 2 finish task@ 6 finish task@ 7 start ..task@ 4 start ..task@ 3 finish task@ 1 start ..task@ 5 finish task@ 4 finish task@ 3 finish task@ 5

put task@ 1
put task@ 2
start ..task@ 1
put task@ 3
put task@ 4
start ..task@ 2
put task@ 5
put task@ 6
put task@ 7
start ..task@ 6
put task@ 8
start ..task@ 7
java.util.concurrent.RejectedExecutionException
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source)
at ThreadPool.main(ThreadPool.java:22)
put task@ 9
java.util.concurrent.RejectedExecutionException
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source)
at ThreadPool.main(ThreadPool.java:22)
put task@ 10
java.util.concurrent.RejectedExecutionException
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source)
at ThreadPool.main(ThreadPool.java:22)
finish task@ 2
finish task@ 6
finish task@ 7
start ..task@ 4
start ..task@ 3
finish task@ 1
start ..task@ 5
finish task@ 4
finish task@ 3
finish task@ 5

ThreadPoolExecutor.CallerRunsPolicy

[java]
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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

运行结果

[java]
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put task@ 1 put task@ 2 start ..task@ 1 put task@ 3 start ..task@ 2 put task@ 4 put task@ 5 put task@ 6 put task@ 7 start ..task@ 6 put task@ 8 start ..task@ 8 start ..task@ 7 finish task@ 2 finish task@ 1 start ..task@ 3 start ..task@ 4 finish task@ 6 start ..task@ 5 finish task@ 7 finish task@ 8 put task@ 9 put task@ 10 start ..task@ 9 finish task@ 4 finish task@ 3 start ..task@ 10 finish task@ 9 finish task@ 5 finish task@ 10

put task@ 1
put task@ 2
start ..task@ 1
put task@ 3
start ..task@ 2
put task@ 4
put task@ 5
put task@ 6
put task@ 7
start ..task@ 6
put task@ 8
start ..task@ 8
start ..task@ 7
finish task@ 2
finish task@ 1
start ..task@ 3
start ..task@ 4
finish task@ 6
start ..task@ 5
finish task@ 7
finish task@ 8
put task@ 9
put task@ 10
start ..task@ 9
finish task@ 4
finish task@ 3
start ..task@ 10
finish task@ 9
finish task@ 5
finish task@ 10

ThreadPoolExecutor.DiscardPolicy

[java]
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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

运行结果

[java]
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put task@ 1 put task@ 2 start ..task@ 1 put task@ 3 start ..task@ 2 put task@ 4 put task@ 5 put task@ 6 put task@ 7 start ..task@ 6 put task@ 8 put task@ 9 start ..task@ 7 put task@ 10 finish task@ 2 finish task@ 1 start ..task@ 3 start ..task@ 4 finish task@ 7 finish task@ 6 start ..task@ 5 finish task@ 3 finish task@ 4 finish task@ 5

put task@ 1
put task@ 2
start ..task@ 1
put task@ 3
start ..task@ 2
put task@ 4
put task@ 5
put task@ 6
put task@ 7
start ..task@ 6
put task@ 8
put task@ 9
start ..task@ 7
put task@ 10
finish task@ 2
finish task@ 1
start ..task@ 3
start ..task@ 4
finish task@ 7
finish task@ 6
start ..task@ 5
finish task@ 3
finish task@ 4
finish task@ 5

ThreadPoolExecutor.DiscardOldestPolicy

[java]
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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadPool {

//让可执行程序休息一下
private static int executePrograms = 0;
private static int produceTaskMaxNumber = 10;

public static void main(String[] args) {
// 构造一个线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3,
TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
new ThreadPoolExecutor.CallerRunsPolicy());

for (int i = 1; i <= produceTaskMaxNumber; i++) {
try {
String task = "task@ " + i;
System.out.println("put " + task);
threadPool.execute(new ThreadPoolTask(task));

// 便于观察，等待一段时间
Thread.sleep(executePrograms);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

运行结果：

[java]
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put task@ 1 put task@ 2 start ..task@ 1 put task@ 3 start ..task@ 2 put task@ 4 put task@ 5 put task@ 6 put task@ 7 start ..task@ 6 put task@ 8 start ..task@ 7 put task@ 9 put task@ 10 finish task@ 6 finish task@ 7 start ..task@ 8 finish task@ 1 start ..task@ 9 finish task@ 2 start ..task@ 10 finish task@ 8 finish task@ 9 finish task@ 10

put task@ 1
put task@ 2
start ..task@ 1
put task@ 3
start ..task@ 2
put task@ 4
put task@ 5
put task@ 6
put task@ 7
start ..task@ 6
put task@ 8
start ..task@ 7
put task@ 9
put task@ 10
finish task@ 6
finish task@ 7
start ..task@ 8
finish task@ 1
start ..task@ 9
finish task@ 2
start ..task@ 10
finish task@ 8
finish task@ 9
finish task@ 10