Java 线程池详解
系统启动一个线程的成本是比较高的,因为它涉及到与操作系统的交互,使用线程池的好处是提高性能,当系统中包含大量并发的线程时,会导致系统性能剧烈下降,甚至导致jvm崩溃,而线程池的最大线程数参数可以控制系统中并发线程数不超过次数。
一、executors 工厂类用来产生线程池,该工厂类包含以下几个静态工厂方法来创建对应的线程池。创建的线程池是一个executorservice对象,使用该对象的submit方法或者是execute方法执行相应的runnable或者是callable任务。线程池本身在不再需要的时候调用shutdown()方法停止线程池,调用该方法后,该线程池将不再允许任务添加进来,但是会直到已添加的所有任务执行完成后才死亡。
1、newcachedthreadpool(),创建一个具有缓存功能的线程池,提交到该线程池的任务(runnable或callable对象)创建的线程,如果执行完成,会被缓存到cachedthreadpool中,供后面需要执行的任务使用。
import java.util.concurrent.executorservice; import java.util.concurrent.executors; public class cachethreadpool { static class task implements runnable { @override public void run() { system.out.println(this + " " + thread.currentthread().getname() + " allstacktraces map size: " + thread.currentthread().getallstacktraces().size()); } } public static void main(string[] args) { executorservice cachethreadpool = executors.newcachedthreadpool(); //先添加三个任务到线程池 for(int i = 0 ; i < 3; i++) { cachethreadpool.execute(new task()); } //等三个线程执行完成后,再次添加三个任务到线程池 try { thread.sleep(3000); } catch (interruptedexception e) { e.printstacktrace(); } for(int i = 0 ; i < 3; i++) { cachethreadpool.execute(new task()); } } }
执行结果如下:
cachethreadpool$task@2d312eb9 pool-1-thread-1 allstacktraces map size: 7 cachethreadpool$task@59522b86 pool-1-thread-3 allstacktraces map size: 7 cachethreadpool$task@73dbb89f pool-1-thread-2 allstacktraces map size: 7 cachethreadpool$task@5795cedc pool-1-thread-3 allstacktraces map size: 7 cachethreadpool$task@256d5600 pool-1-thread-1 allstacktraces map size: 7 cachethreadpool$task@7d1c5894 pool-1-thread-2 allstacktraces map size: 7
线程池中的线程对象进行了缓存,当有新任务执行时进行了复用。但是如果有特别多的并发时,缓存线程池还是会创建很多个线程对象。
2、newfixedthreadpool(int nthreads) 创建一个指定线程个数,线程可复用的线程池。
import java.util.concurrent.executorservice; import java.util.concurrent.executors; public class fixedthreadpool { static class task implements runnable { @override public void run() { system.out.println(this + " " + thread.currentthread().getname() + " allstacktraces map size: " + thread.currentthread().getallstacktraces().size()); } } public static void main(string[] args) { executorservice fixedthreadpool = executors.newfixedthreadpool(3); // 先添加三个任务到线程池 for (int i = 0; i < 5; i++) { fixedthreadpool.execute(new task()); } // 等三个线程执行完成后,再次添加三个任务到线程池 try { thread.sleep(3); } catch (interruptedexception e) { e.printstacktrace(); } for (int i = 0; i < 3; i++) { fixedthreadpool.execute(new task()); } } }
执行结果:
fixedthreadpool$task@7045c12d pool-1-thread-2 allstacktraces map size: 7 fixedthreadpool$task@50fa0bef pool-1-thread-2 allstacktraces map size: 7 fixedthreadpool$task@ccb1870 pool-1-thread-2 allstacktraces map size: 7 fixedthreadpool$task@7392b4e3 pool-1-thread-1 allstacktraces map size: 7 fixedthreadpool$task@5bdeff18 pool-1-thread-2 allstacktraces map size: 7 fixedthreadpool$task@7d5554e1 pool-1-thread-1 allstacktraces map size: 7 fixedthreadpool$task@24468092 pool-1-thread-3 allstacktraces map size: 7 fixedthreadpool$task@fa7b978 pool-1-thread-2 allstacktraces map size: 7
3、newsinglethreadexecutor(),创建一个只有单线程的线程池,相当于调用newfixedthreadpool(1)
4、newsheduledthreadpool(int corepoolsize),创建指定线程数的线程池,它可以在指定延迟后执行线程。也可以以某一周期重复执行某一线程,知道调用shutdown()关闭线程池。
示例如下:
import java.util.concurrent.executors; import java.util.concurrent.scheduledexecutorservice; import java.util.concurrent.timeunit; public class scheduledthreadpool { static class task implements runnable { @override public void run() { system.out.println("time " + system.currenttimemillis() + " " + thread.currentthread().getname() + " allstacktraces map size: " + thread.currentthread().getallstacktraces().size()); } } public static void main(string[] args) { scheduledexecutorservice scheduledexecutorservice = executors.newscheduledthreadpool(3); scheduledexecutorservice.schedule(new task(), 3, timeunit.seconds); scheduledexecutorservice.scheduleatfixedrate(new task(), 3, 5, timeunit.seconds); try { thread.sleep(30 * 1000); } catch (interruptedexception e) { e.printstacktrace(); } scheduledexecutorservice.shutdown(); } }
运行结果如下:
time 1458921795240 pool-1-thread-1 allstacktraces map size: 6 time 1458921795241 pool-1-thread-2 allstacktraces map size: 6 time 1458921800240 pool-1-thread-1 allstacktraces map size: 7 time 1458921805240 pool-1-thread-1 allstacktraces map size: 7 time 1458921810240 pool-1-thread-1 allstacktraces map size: 7 time 1458921815240 pool-1-thread-1 allstacktraces map size: 7 time 1458921820240 pool-1-thread-1 allstacktraces map size: 7
由运行时间可看出,任务是按照5秒的周期执行的。
5、newsinglethreadscheduledexecutor() 创建一个只有一个线程的线程池,同调用newscheduledthreadpool(1)。
二、forkjoinpool和forkjointask
forkjoinpool是executorservice的实现类,支持将一个任务划分为多个小任务并行计算,在把多个小任务的计算结果合并成总的计算结果。它有两个构造函数
forkjoinpool(int parallelism)创建一个包含parallelism个并行线程的forkjoinpool。
forkjoinpool(),以runtime.availableprocessors()方法返回值作为parallelism参数来创建forkjoinpool。
forkjointask 代表一个可以并行,合并的任务。它是实现了future<t>接口的抽象类,它有两个抽象子类,代表无返回值任务的recuriveaction和有返回值的recursivetask。可根据具体需求继承这两个抽象类实现自己的对象,然后调用forkjoinpool的submit 方法执行。
recuriveaction 示例如下,实现并行输出0-300的数字。
import java.util.concurrent.forkjoinpool; import java.util.concurrent.recursiveaction; import java.util.concurrent.timeunit; public class actionforkjointask { static class printtask extends recursiveaction { private static final int threshold = 50; private int start; private int end; public printtask(int start, int end) { this.start = start; this.end = end; } @override protected void compute() { if (end - start < threshold) { for(int i = start; i < end; i++) { system.out.println(thread.currentthread().getname() + " " + i); } } else { int middle = (start + end) / 2; printtask left = new printtask(start, middle); printtask right = new printtask(middle, end); left.fork(); right.fork(); } } } public static void main(string[] args) { forkjoinpool pool = new forkjoinpool(); pool.submit(new printtask(0, 300)); try { pool.awaittermination(2, timeunit.seconds); } catch (interruptedexception e) { e.printstacktrace(); } pool.shutdown(); } }
在拆分小任务后,调用任务的fork()方法,加入到forkjoinpool中并行执行。
recursivetask示例,实现并行计算100个整数求和。拆分为每20个数求和后获取结果,在最后合并为最后的结果。
import java.util.random; import java.util.concurrent.executionexception; import java.util.concurrent.forkjoinpool; import java.util.concurrent.future; import java.util.concurrent.recursivetask; public class taskforkjointask { static class caltask extends recursivetask<integer> { private static final int threshold = 20; private int arr[]; private int start; private int end; public caltask(int[] arr, int start, int end) { this.arr = arr; this.start = start; this.end = end; } @override protected integer compute() { int sum = 0; if (end - start < threshold) { for (int i = start; i < end; i++) { sum += arr[i]; } system.out.println(thread.currentthread().getname() + " sum:" + sum); return sum; } else { int middle = (start + end) / 2; caltask left = new caltask(arr, start, middle); caltask right = new caltask(arr, middle, end); left.fork(); right.fork(); return left.join() + right.join(); } } } public static void main(string[] args) { int arr[] = new int[100]; random random = new random(); int total = 0; for (int i = 0; i < arr.length; i++) { int tmp = random.nextint(20); total += (arr[i] = tmp); } system.out.println("total " + total); forkjoinpool pool = new forkjoinpool(4); future<integer> future = pool.submit(new caltask(arr, 0, arr.length)); try { system.out.println("cal result: " + future.get()); } catch (interruptedexception e) { e.printstacktrace(); } catch (executionexception e) { e.printstacktrace(); } pool.shutdown(); } }
执行结果如下:
total 912 forkjoinpool-1-worker-2 sum:82 forkjoinpool-1-worker-2 sum:123 forkjoinpool-1-worker-2 sum:144 forkjoinpool-1-worker-3 sum:119 forkjoinpool-1-worker-2 sum:106 forkjoinpool-1-worker-2 sum:128 forkjoinpool-1-worker-2 sum:121 forkjoinpool-1-worker-3 sum:89 cal result: 912
子任务执行完后,调用任务的join()方法获取子任务执行结果,再相加获得最后的结果。
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