Java concurrency线程池之线程池原理(四)_动力节点Java学院整理
拒绝策略介绍
线程池的拒绝策略,是指当任务添加到线程池中被拒绝,而采取的处理措施。
当任务添加到线程池中之所以被拒绝,可能是由于:第一,线程池异常关闭。第二,任务数量超过线程池的最大限制。
线程池共包括4种拒绝策略,它们分别是:abortpolicy, callerrunspolicy, discardoldestpolicy和discardpolicy。
- abortpolicy -- 当任务添加到线程池中被拒绝时,它将抛出 rejectedexecutionexception 异常。
- callerrunspolicy -- 当任务添加到线程池中被拒绝时,会在线程池当前正在运行的thread线程池中处理被拒绝的任务。
- discardoldestpolicy -- 当任务添加到线程池中被拒绝时,线程池会放弃等待队列中最旧的未处理任务,然后将被拒绝的任务添加到等待队列中。
- discardpolicy -- 当任务添加到线程池中被拒绝时,线程池将丢弃被拒绝的任务。
线程池默认的处理策略是abortpolicy!
拒绝策略对比和示例
下面通过示例,分别演示线程池的4种拒绝策略。
1. discardpolicy 示例
import java.lang.reflect.field; import java.util.concurrent.arrayblockingqueue; import java.util.concurrent.threadpoolexecutor; import java.util.concurrent.timeunit; import java.util.concurrent.threadpoolexecutor.discardpolicy; public class discardpolicydemo { private static final int threads_size = 1; private static final int capacity = 1; public static void main(string[] args) throws exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size),"线程池"的阻塞队列容量为1(capacity)。 threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds, new arrayblockingqueue<runnable>(capacity)); // 设置线程池的拒绝策略为"丢弃" pool.setrejectedexecutionhandler(new threadpoolexecutor.discardpolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i < 10; i++) { runnable myrun = new myrunnable("task-"+i); pool.execute(myrun); } // 关闭线程池 pool.shutdown(); } } class myrunnable implements runnable { private string name; public myrunnable(string name) { this.name = name; } @override public void run() { try { system.out.println(this.name + " is running."); thread.sleep(100); } catch (exception e) { e.printstacktrace(); } } }
运行结果:
task-0 is running. task-1 is running.
结果说明:线程池pool的"最大池大小"和"核心池大小"都为1(threads_size),这意味着"线程池能同时运行的任务数量最大只能是1"。
线程池pool的阻塞队列是arrayblockingqueue,arrayblockingqueue是一个有界的阻塞队列,arrayblockingqueue的容量为1。这也意味着线程池的阻塞队列只能有一个线程池阻塞等待。
根据""中分析的execute()代码可知:线程池*运行了2个任务。第1个任务直接放到worker中,通过线程去执行;第2个任务放到阻塞队列中等待。其他的任务都被丢弃了!
2. discardoldestpolicy 示例
import java.lang.reflect.field; import java.util.concurrent.arrayblockingqueue; import java.util.concurrent.threadpoolexecutor; import java.util.concurrent.timeunit; import java.util.concurrent.threadpoolexecutor.discardoldestpolicy; public class discardoldestpolicydemo { private static final int threads_size = 1; private static final int capacity = 1; public static void main(string[] args) throws exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size),"线程池"的阻塞队列容量为1(capacity)。 threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds, new arrayblockingqueue<runnable>(capacity)); // 设置线程池的拒绝策略为"discardoldestpolicy" pool.setrejectedexecutionhandler(new threadpoolexecutor.discardoldestpolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i < 10; i++) { runnable myrun = new myrunnable("task-"+i); pool.execute(myrun); } // 关闭线程池 pool.shutdown(); } } class myrunnable implements runnable { private string name; public myrunnable(string name) { this.name = name; } @override public void run() { try { system.out.println(this.name + " is running."); thread.sleep(200); } catch (exception e) { e.printstacktrace(); } } }
运行结果:
task-0 is running. task-9 is running.
结果说明:将"线程池的拒绝策略"由discardpolicy修改为discardoldestpolicy之后,当有任务添加到线程池被拒绝时,线程池会丢弃阻塞队列中末尾的任务,然后将被拒绝的任务添加到末尾。
3. abortpolicy 示例
import java.lang.reflect.field; import java.util.concurrent.arrayblockingqueue; import java.util.concurrent.threadpoolexecutor; import java.util.concurrent.timeunit; import java.util.concurrent.threadpoolexecutor.abortpolicy; import java.util.concurrent.rejectedexecutionexception; public class abortpolicydemo { private static final int threads_size = 1; private static final int capacity = 1; public static void main(string[] args) throws exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size),"线程池"的阻塞队列容量为1(capacity)。 threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds, new arrayblockingqueue<runnable>(capacity)); // 设置线程池的拒绝策略为"抛出异常" pool.setrejectedexecutionhandler(new threadpoolexecutor.abortpolicy()); try { // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i < 10; i++) { runnable myrun = new myrunnable("task-"+i); pool.execute(myrun); } } catch (rejectedexecutionexception e) { e.printstacktrace(); // 关闭线程池 pool.shutdown(); } } } class myrunnable implements runnable { private string name; public myrunnable(string name) { this.name = name; } @override public void run() { try { system.out.println(this.name + " is running."); thread.sleep(200); } catch (exception e) { e.printstacktrace(); } } }
(某一次)运行结果:
java.util.concurrent.rejectedexecutionexception at java.util.concurrent.threadpoolexecutor$abortpolicy.rejectedexecution(threadpoolexecutor.java:1774) at java.util.concurrent.threadpoolexecutor.reject(threadpoolexecutor.java:768) at java.util.concurrent.threadpoolexecutor.execute(threadpoolexecutor.java:656) at abortpolicydemo.main(abortpolicydemo.java:27) task-0 is running. task-1 is running.
结果说明:将"线程池的拒绝策略"由discardpolicy修改为abortpolicy之后,当有任务添加到线程池被拒绝时,会抛出rejectedexecutionexception。
4. callerrunspolicy 示例
import java.lang.reflect.field; import java.util.concurrent.arrayblockingqueue; import java.util.concurrent.threadpoolexecutor; import java.util.concurrent.timeunit; import java.util.concurrent.threadpoolexecutor.callerrunspolicy; public class callerrunspolicydemo { private static final int threads_size = 1; private static final int capacity = 1; public static void main(string[] args) throws exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size),"线程池"的阻塞队列容量为1(capacity)。 threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds, new arrayblockingqueue<runnable>(capacity)); // 设置线程池的拒绝策略为"callerrunspolicy" pool.setrejectedexecutionhandler(new threadpoolexecutor.callerrunspolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i < 10; i++) { runnable myrun = new myrunnable("task-"+i); pool.execute(myrun); } // 关闭线程池 pool.shutdown(); } } class myrunnable implements runnable { private string name; public myrunnable(string name) { this.name = name; } @override public void run() { try { system.out.println(this.name + " is running."); thread.sleep(100); } catch (exception e) { e.printstacktrace(); } } }
(某一次)运行结果:
task-2 is running. task-3 is running. task-4 is running. task-5 is running. task-6 is running. task-7 is running. task-8 is running. task-9 is running. task-0 is running. task-1 is running.
结果说明:将"线程池的拒绝策略"由discardpolicy修改为callerrunspolicy之后,当有任务添加到线程池被拒绝时,线程池会将被拒绝的任务添加到"线程池正在运行的线程"中取运行
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