Java并发编程核心方法与框架-Executors的使用

合理利用线程池能够带来三个好处
降低资源消耗。通过重复利用已创建的线程降低线程创建和销毁造成的消耗。

提高响应速度。当任务到达时，任务可以不需要等到线程创建就能立即执行。

提高线程的可管理性。线程是稀缺资源，如果无限制的创建，不仅会消耗系统资源，还会降低系统的稳定性，使用线程池可以进行统一的分配，调优和监控。

官方建议使用Executors工厂类来创建线程池对象

使用newCachedThreadPool()方法创建无界线程池
使用Executors类的newCachedThreadPool()方法创建的是无界线程池，可以进行线程自动回收。所谓的“无界线程池”就是池中存放线程个数理论上是Integer.MAX_VALUE。

public class Run1 {
public static void main(String[] args) {
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.execute(new Runnable() {

@Override
public void run() {
try {
System.out.println("Runnable1 begin:" + System.currentTimeMillis());
Thread.sleep(1000);
System.out.println("A");
System.out.println("Runnable1 end:" + System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
executorService.execute(new Runnable() {

@Override
public void run() {
try {
System.out.println("Runnable2 begin:" + System.currentTimeMillis());
Thread.sleep(1000);
System.out.println("A");
System.out.println("Runnable2 end:" + System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
}

程序运行结果如下：

Runnable1 begin:1470228063076
Runnable2 begin:1470228063076
A
A
Runnable2 end:1470228064081
Runnable1 end:1470228064081

线程池的线程复用效果

public class MyRunnable implements Runnable {
private String username;
public MyRunnable(String username) {
super();
this.username = username;
}
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + " username=" + username + " begin:" + System.currentTimeMillis());
System.out.println(Thread.currentThread().getName() + " username=" + username + " end:" + System.currentTimeMillis());
}
}

public class Main {
public static void main(String[] args) throws InterruptedException {
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < 5; i++) {
executorService.execute(new MyRunnable("" + i));
}
Thread.sleep(2000);
System.out.println();
for (int i = 0; i < 5; i++) {
executorService.execute(new MyRunnable("" + i));
}
}
}

程序运行结果如下：

pool-1-thread-1 username=0 begin:1470229448635
pool-1-thread-4 username=3 begin:1470229448635
pool-1-thread-3 username=2 begin:1470229448635
pool-1-thread-2 username=1 begin:1470229448635
pool-1-thread-3 username=2 end:1470229448635
pool-1-thread-4 username=3 end:1470229448635
pool-1-thread-5 username=4 begin:1470229448635
pool-1-thread-1 username=0 end:1470229448635
pool-1-thread-5 username=4 end:1470229448636
pool-1-thread-2 username=1 end:1470229448635

pool-1-thread-1 username=2 begin:1470229450637
pool-1-thread-3 username=4 begin:1470229450637
pool-1-thread-3 username=4 end:1470229450637
pool-1-thread-2 username=0 begin:1470229450637
pool-1-thread-2 username=0 end:1470229450638
pool-1-thread-5 username=1 begin:1470229450637
pool-1-thread-4 username=3 begin:1470229450637
pool-1-thread-4 username=3 end:1470229450638
pool-1-thread-5 username=1 end:1470229450638
pool-1-thread-1 username=2 end:1470229450637

由打印结果可见，第一次for循环中创建了5个线程对象分别是pool-1-thread-1到pool-1-thread-5，第二次for循环中没有创建新的线程对象，复用了第一次for循环中创建的线程对象。

使用newCachedThreadPool(ThreadFactory)定制线程工厂

public class MyThreadFactory implements ThreadFactory {
@Override
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setName("定制池中线程对象的名称" + Math.random());
return thread;
}
}

public class Run {
public static void main(String[] args) {
MyThreadFactory myThreadFactory = new MyThreadFactory();
ExecutorService executorService = Executors.newCachedThreadPool(myThreadFactory);
executorService.execute(new Runnable() {

@Override
public void run() {
System.out.println(Thread.currentThread().getName() + "运行：" + System.currentTimeMillis());
}
});
}
}

程序运行结果如下：

定制池中线程对象的名称0.2671917944865071运行：1470230269473

使用newFixedThreadPool(int)方法创建有界线程池

public class MyRunnable implements Runnable {
String username;
public MyRunnable(String username) {
super();
this.username = username;
}
@Override
public void run() {
try {
System.out.println(Thread.currentThread().getName() + " username=" + username + " begin:" + System.currentTimeMillis());
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " username=" + username + " end:" + System.currentTimeMillis());
} catch (Exception e) {
e.printStackTrace();
}
}
}

public class Run {
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(3);
for (int i = 0; i < 3; i++) {
executorService.execute(new MyRunnable("" + i));
}
for (int i = 0; i < 3; i++) {
executorService.execute(new MyRunnable("" + i));
}
}
}

程序运行结果如下：

pool-1-thread-1 username=0 begin:1470230865037
pool-1-thread-3 username=2 begin:1470230865037
pool-1-thread-2 username=1 begin:1470230865037
pool-1-thread-3 username=2 end:1470230867043
pool-1-thread-1 username=0 end:1470230867042
pool-1-thread-3 username=0 begin:1470230867043
pool-1-thread-1 username=1 begin:1470230867043
pool-1-thread-2 username=1 end:1470230867043
pool-1-thread-2 username=2 begin:1470230867043
pool-1-thread-3 username=0 end:1470230869047
pool-1-thread-1 username=1 end:1470230869047
pool-1-thread-2 username=2 end:1470230869047

此时线程池中最多有三个线程。

使用newFixedThreadPool(int, ThreadFactory)定制线程工厂

public class MyThreadFactory implements ThreadFactory {
@Override
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setName("定制池中线程对象的名称" + Math.random());
return thread;
}
}

public class Run {
public static void main(String[] args) {
MyThreadFactory threadFactory = new MyThreadFactory();
ExecutorService executorService = Executors.newFixedThreadPool(2, threadFactory);
Runnable runnable = new Runnable() {

@Override
public void run() {
try {
System.out.println("begin 我在运行" + System.currentTimeMillis() + " " + Thread.currentThread().getName());
Thread.sleep(2000);
System.out.println("end 我在运行" + System.currentTimeMillis() + " " + Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
}

}
};
executorService.execute(runnable);
executorService.execute(runnable);
executorService.execute(runnable);
}
}

程序运行结果如下：

begin 我在运行1470231214770 定制池中线程对象的名称0.07643716796315236
begin 我在运行1470231214770 定制池中线程对象的名称0.5060032203497645
end 我在运行1470231216773 定制池中线程对象的名称0.5060032203497645
end 我在运行1470231216773 定制池中线程对象的名称0.07643716796315236
begin 我在运行1470231216773 定制池中线程对象的名称0.5060032203497645
end 我在运行1470231218774 定制池中线程对象的名称0.5060032203497645

使用newSingleThreadExecutor()方法创建单一线程池

public class MyRunnable implements Runnable {
String username;
public MyRunnable(String username) {
super();
this.username = username;
}
@Override
public void run() {
try {
System.out.println(Thread.currentThread().getName() + " username=" + username + " begin:" + System.currentTimeMillis());
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " username=" + username + " end:" + System.currentTimeMillis());
} catch (Exception e) {
e.printStackTrace();
}
}
}

public class Run {
public static void main(String[] args) {
ExecutorService executorService = Executors.newSingleThreadExecutor();
for (int i = 0; i < 3; i++) {
executorService.execute(new MyRunnable("" + i));
}
}
}

程序运行结果如下：

pool-1-thread-1 username=0 begin:1470231470978
pool-1-thread-1 username=0 end:1470231472978
pool-1-thread-1 username=1 begin:1470231472978
pool-1-thread-1 username=1 end:1470231474982
pool-1-thread-1 username=2 begin:1470231474982
pool-1-thread-1 username=2 end:1470231476984

此时线程池中只有一个线程。

newSingleThreadExecutor(ThreadFactory) 使用方法与前面的大体一致。