Future和Callable的使用總結(jié)

創(chuàng)始元靈6666 2022-06-17 發(fā)布于河北

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在默認的情況下，線程Thread對象不具有返回值的功能，如果在需要取得返回值的情況下是極為不方便的，但在Java1.5的并發(fā)包中可以使用Future和Callable來使線程具有返回值的功能。

1.Future和Callable的介紹

接口Callable與線程功能密不可分，但和Runnable的主要區(qū)別為:

(1) 接口Callable的call()方法可以有返回值，但Runnable接口的run()方法沒有返回值。

(2) Callable接口的call()方法可以聲明拋出異常，而Runnable接口的run()方法不可以聲明拋出異常。

執(zhí)行完Callable接口中的任務(wù)后，返回值是通過Future接口進行獲得的。

2.方法get()結(jié)合ExecutorService中的submit(Callable<T>)的使用

方法submit(Callable<T>)可以執(zhí)行參數(shù)為Callable的任務(wù)，方法get()用于獲得返回值，示例如下：

package mycallable;
import java.util.concurrent.Callable;
public class MyCallable implements Callable<String> {
	private int age;
	public MyCallable(int age) {
		super();
		this.age = age;
	}
	public String call() throws Exception {
		Thread.sleep(8000);
		return "返回值 年齡是：" + age;
	}
}

package test.run;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import mycallable.MyCallable;
public class Run {
	public static void main(String[] args) throws InterruptedException {
		try {
			MyCallable callable = new MyCallable(100);
			ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 3, 5L,
					TimeUnit.SECONDS, new LinkedBlockingDeque());
			Future<String> future = executor.submit(callable);
			System.out.println("main A " + System.currentTimeMillis());
			System.out.println(future.get());
			System.out.println("main B " + System.currentTimeMillis());
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

3.方法get()結(jié)合ExecutorService中的submit(Runnable)和isDone()的使用

方法submit()不僅可以傳入Callable對象，也可以傳入Runnable對象，說明submit方法支持有返回值和無返回值的功能。如果submit方法傳入Callable接口則可以有返回值，如果傳入Runnable則無返回值，打印的結(jié)果就是null。方法get()具有阻塞特性，而isDone()方法無阻塞特性。

package test.run;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
 * 如果submit方法傳入Callable接口則可以有返回值，如果傳入Runnable則無返回值，打印的結(jié)果就是null。方法get()具有阻塞特性，而isDone()方法無阻塞特性
 * @author linhaiy
 * @date 2019.03.06
 */
public class Run {
	public static void main(String[] args) {
		try {
			Runnable runnable = new Runnable() {
				@Override
				public void run() {
					System.out.println("打印的信息");
				}
			};
			ExecutorService executorRef = Executors.newCachedThreadPool();
			Future future = executorRef.submit(runnable);
			System.out.println(future.get() + " " + future.isDone());
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

4.使用ExecutorService接口中的方法submit(Runnable,T result)

方法submit(Runnable,T result)的第2個參數(shù)result可以作為執(zhí)行結(jié)果的返回值，而不需要使用get()方法來進行獲得。

package entity;
public class Userinfo {
	private String username;
	private String password;
	public Userinfo() {
		super();
	}
	public Userinfo(String username, String password) {
		super();
		this.username = username;
		this.password = password;
	}
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
}

package myrunnable;
import entity.Userinfo;
public class MyRunnable implements Runnable {
	private Userinfo userinfo;
	public MyRunnable(Userinfo userinfo) {
		super();
		this.userinfo = userinfo;
	}
	@Override
	public void run() {
		userinfo.setUsername("usernameValue");
		userinfo.setPassword("passwordValue");
	}
}

package test;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import myrunnable.MyRunnable;
import entity.Userinfo;
public class Test {
	FutureTask abc;   //接口Future的實現(xiàn)類
	public static void main(String[] args) {
		try {
			Userinfo userinfo = new Userinfo();
			MyRunnable myrunnable = new MyRunnable(userinfo);
			ThreadPoolExecutor pool = new ThreadPoolExecutor(10, 10, 10,
					TimeUnit.SECONDS, new LinkedBlockingDeque<Runnable>());
			Future<Userinfo> future = pool.submit(myrunnable, userinfo);
			System.out.println(future);
			System.out.println("begin time=" + System.currentTimeMillis());
			userinfo = future.get();
			System.out.println("get value " + userinfo.getUsername() + " "
					+ userinfo.getPassword());
			System.out.println("  end time=" + System.currentTimeMillis());
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

5.方法cancel(boolean mayInterruptIfRunning) 和isCancelled()的使用

方法cancel(boolean mayInterruptIfRunning)的參數(shù)mayInterruptIfRunning的作用是：如果線程正在運行則是否中斷正在運行的線程，在代碼中需要使用if(Thread.currentThread().isInterrupted())進行配合。

方法cancel()的返回值代表發(fā)送取消任務(wù)的命令是否成功完成。

6.方法get(long timeout,TimeUnit unit)的使用

方法get(long timeout,TimeUnit unit)的作用是在指定的最大時間內(nèi)等待獲得返回值。

package mycallable;
import java.util.concurrent.Callable;
public class MyCallable implements Callable<String> {
	public String call() throws Exception {     
		Thread.sleep(10000);                    //目的是為了在指定時間內(nèi)獲取不到返回值
		System.out.println("sleep 10秒執(zhí)行完了！");
		return "anyString";
	}
}

package test.run;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import mycallable.MyCallable;
public class Run {
	public static void main(String[] args) {
		try {
			MyCallable callable = new MyCallable();
			ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 3, 5L,
					TimeUnit.SECONDS, new LinkedBlockingDeque());
			System.out.println("begin " + System.currentTimeMillis());
			Future<String> future = executor.submit(callable);
			System.out.println("返回值" + future.get(5, TimeUnit.SECONDS));
			System.out.println("  end " + System.currentTimeMillis());
		} catch (InterruptedException e) {
			System.out.println("進入catch InterruptedException");
			e.printStackTrace();
		} catch (ExecutionException e) {
			System.out.println("進入catch ExecutionException");
			e.printStackTrace();
		} catch (TimeoutException e) {
			System.out.println("進入catch TimeoutException");
			e.printStackTrace();
		}
	}
}

7.自定義拒絕策略RejectedExecutionHandler接口的使用

接口RejectedExecutionHandler的主要作用是當線程池關(guān)閉后依然有任務(wù)要執(zhí)行時，可以實現(xiàn)一些處理。

package com.executionhandler;
import java.util.concurrent.FutureTask;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
public class MyRejectedExecutionHandler implements RejectedExecutionHandler {
	public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
		System.out.println(((FutureTask) r).toString() + " 被拒絕!");
	}
}

package com.executionhandler;
public class MyRunnable implements Runnable {
	private String username;
	public MyRunnable(String username) {
		super();
		this.username = username;
	}
	public void run() {
		System.out.println(username + " 在運行！");
	}
}

package com.executionhandler;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
public class Run {
	public static void main(String[] args) {
		ExecutorService service = Executors.newCachedThreadPool();
		ThreadPoolExecutor executor = (ThreadPoolExecutor) service;
		executor.setRejectedExecutionHandler(new MyRejectedExecutionHandler());
		service.submit(new MyRunnable("A"));
		service.submit(new MyRunnable("B"));
		service.submit(new MyRunnable("C"));
		executor.shutdown();
		service.submit(new MyRunnable("D"));
	}
}

8.方法execute()與submit()的區(qū)別

(1) 方法execute()沒有返回值，而submit()方法可以有返回值。

(2) 方法execute()在默認的情況下異常直接拋出，不能捕獲，但可以通過自定義ThreadFactory的方式進行捕獲，而submit()方法在默認的情況下，可以catch Execution-Exeception捕獲異常

9.驗證Future的缺點

package mycallable;
import java.util.concurrent.Callable;
public class MyCallable implements Callable<String> {
	private String username;
	private long sleepValue;
	public MyCallable(String username, long sleepValue) {
		super();
		this.username = username;
		this.sleepValue = sleepValue;
	}
	@Override
	public String call() throws Exception {
		System.out.println(username);
		Thread.sleep(sleepValue);
		return "return " + username;
	}
}

package test;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import mycallable.MyCallable;
public class Test {
	public static void main(String[] args) {
		try {
			MyCallable callable1 = new MyCallable("username1", 5000);
			MyCallable callable2 = new MyCallable("username2", 4000);
			MyCallable callable3 = new MyCallable("username3", 3000);
			MyCallable callable4 = new MyCallable("username4", 2000);
			MyCallable callable5 = new MyCallable("username5", 1000);
			List<Callable> callableList = new ArrayList<Callable>();
			callableList.add(callable1);
			callableList.add(callable2);
			callableList.add(callable3);
			callableList.add(callable4);
			callableList.add(callable5);
			List<Future> futureList = new ArrayList<Future>();
			ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 5, TimeUnit.SECONDS,
					new LinkedBlockingDeque<Runnable>());
			for (int i = 0; i < 5; i++) {
				futureList.add(executor.submit(callableList.get(i)));
			}
			System.out.println("run first time=  " + System.currentTimeMillis());
			for (int i = 0; i < 5; i++) {
				System.out.println(futureList.get(i).get() + " " + System.currentTimeMillis());
			}
			// 按順序打印的效果
			// 說明一個Future對應(yīng)指定的一個Callable
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

總結(jié)：Future和Callable這兩個接口的優(yōu)點就是從線程中返回數(shù)據(jù)以便進行后期的處理，但是FutureTask類也有其自身的缺點，就是阻塞性。

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