Executors.newFixedThreadPool和ArrayBlockingQueue一點使用心得
阿新 • • 發佈:2019-01-07
http://heipark.iteye.com/blog/1393847
newFixedThreadPool內部有個任務佇列,假設執行緒池裡有3個執行緒,提交了5個任務,那麼後兩個任務就放在任務隊列了,即使前3個任務sleep或者堵塞了,也不會執行後兩個任務,除非前三個任務有執行完的
newFixedThreadPool使用範例:
Java程式碼- import java.io.IOException;
- import java.util.concurrent.ExecutorService;
-
import java.util.concurrent.Executors;
- public class Test {
- public static void main(String[] args) throws IOException, InterruptedException {
- ExecutorService service = Executors.newFixedThreadPool(2);
- for (int i = 0; i < 6; i++) {
- final int index = i;
-
System.out.println("task: " + (i+1
- Runnable run = new Runnable() {
- @Override
- public void run() {
- System.out.println("thread start" + index);
- try {
- Thread.sleep(Long.MAX_VALUE);
-
} catch (InterruptedException e) {
- e.printStackTrace();
- }
- System.out.println("thread end" + index);
- }
- };
- service.execute(run);
- }
- }
- }
task: 2
thread start0
task: 3
task: 4
task: 5
task: 6
task: 7
thread start1
task: 8
task: 9
task: 10
task: 11
task: 12
task: 13
task: 14
task: 15
從例項可以看到for迴圈並沒有被固定的執行緒池阻塞住,也就是說所有的執行緒task都被提交到了ExecutorService中,檢視 Executors.newFixedThreadPool()如下:
public static ExecutorService newFixedThreadPool(int nThreads) {return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
可以看到task被提交都了LinkedBlockingQueue中。這裡有個問題,如果任務列表很大,一定會把記憶體撐爆,如何解決?看下面:
Java程式碼- import java.io.IOException;
- import java.util.concurrent.ArrayBlockingQueue;
- import java.util.concurrent.BlockingQueue;
- import java.util.concurrent.ThreadPoolExecutor;
- import java.util.concurrent.TimeUnit;
- public class Test {
- public static void main(String[] args) throws IOException, InterruptedException {
- BlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(3);
- ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 3, 1, TimeUnit.HOURS, queue, new ThreadPoolExecutor.CallerRunsPolicy());
- for (int i = 0; i < 10; i++) {
- final int index = i;
- System.out.println("task: " + (index+1));
- Runnable run = new Runnable() {
- @Override
- public void run() {
- System.out.println("thread start" + (index+1));
- try {
- Thread.sleep(Long.MAX_VALUE);
- } catch (InterruptedException e) {
- e.printStackTrace();
- }
- System.out.println("thread end" + (index+1));
- }
- };
- executor.execute(run);
- }
- }
- }
task: 2
thread start1
task: 3
task: 4
task: 5
task: 6
task: 7
thread start2
thread start7
thread start6
執行緒池最大值為4(??這裡我不明白為什麼是設定值+1,即3+1,而不是3),準備執行的任務佇列為3。可以看到for迴圈先處理4個task,然後把3個放到佇列。這樣就實現了自動阻塞佇列的效果。記得要使用ArrayBlockingQueue這個佇列,然後設定容量就OK了。