系統啟動一個線程的成本是比較高的，因為它涉及到與操作系統的交互，使用線程池的好處是提高性能，當系統中包含大量並發的線程時，會導致系統性能劇烈下降，甚至導致JVM崩潰，而線程池的最大線程數參數可以控制系統中並發線程數不超過次數。
一、Executors 工廠類用來產生線程池，該工廠類包含以下幾個靜態工廠方法來創建對應的線程池。創建的線程池是一個ExecutorService對象，使用該對象的submit方法或者是execute方法執行相應的Runnable或者是Callable任務。線程池本身在不再需要的時候調用shutdown()方法停止線程池，調用該方法後，該線程池將不再允許任務添加進來，但是會直到已添加的所有任務執行完成後才死亡。
1、newCachedThreadPool()，創建一個具有緩存功能的線程池，提交到該線程池的任務（Runnable或Callable對象）創建的線程，如果執行完成，會被緩存到CachedThreadPool中，供後面需要執行的任務使用。
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();
//先添加三個任務到線程池
for(int i = 0 ; i < 3; i++) {
cacheThreadPool.execute(new Task());
}
//等三個線程執行完成後，再次添加三個任務到線程池
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
for(int i = 0 ; i < 3; i++) {
cacheThreadPool.execute(new Task());
}
}
}
執行結果如下：
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
線程池中的線程對象進行了緩存，當有新任務執行時進行了復用。但是如果有特別多的並發時，緩存線程池還是會創建很多個線程對象。
2、newFixedThreadPool(int nThreads) 創建一個指定線程個數，線程可復用的線程池。
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);
// 先添加三個任務到線程池
for (int i = 0; i < 5; i++) {
fixedThreadPool.execute(new Task());
}
// 等三個線程執行完成後，再次添加三個任務到線程池
try {
Thread.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < 3; i++) {
fixedThreadPool.execute(new Task());
}
}
}
執行結果：
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(),創建一個只有單線程的線程池，相當於調用newFixedThreadPool(1)
4、newSheduledThreadPool(int corePoolSize),創建指定線程數的線程池，它可以在指定延遲後執行線程。也可以以某一周期重復執行某一線程，知道調用shutdown()關閉線程池。
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();
}
}
運行結果如下：
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
由運行時間可看出，任務是按照5秒的周期執行的。
5、newSingleThreadScheduledExecutor() 創建一個只有一個線程的線程池，同調用newScheduledThreadPool(1)。
二、ForkJoinPool和ForkJoinTask
ForkJoinPool是ExecutorService的實現類，支持將一個任務劃分為多個小任務並行計算，在把多個小任務的計算結果合並成總的計算結果。它有兩個構造函數
ForkJoinPool(int parallelism)創建一個包含parallelism個並行線程的ForkJoinPool。
ForkJoinPool(),以Runtime.availableProcessors()方法返回值作為parallelism參數來創建ForkJoinPool。
ForkJoinTask 代表一個可以並行，合並的任務。它是實現了Future<T>接口的抽象類，它有兩個抽象子類，代表無返回值任務的RecuriveAction和有返回值的RecursiveTask。可根據具體需求繼承這兩個抽象類實現自己的對象，然後調用ForkJoinPool的submit 方法執行。
RecuriveAction 示例如下，實現並行輸出0-300的數字。
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();
}
}
在拆分小任務後，調用任務的fork（）方法，加入到ForkJoinPool中並行執行。
RecursiveTask示例，實現並行計算100個整數求和。拆分為每20個數求和後獲取結果，在最後合並為最後的結果。
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();
}
}
執行結果如下：
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
子任務執行完後，調用任務的join（）方法獲取子任務執行結果，再相加獲得最後的結果。