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

1. 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。
2. 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。
3. 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。