java多线程之Callable

Callable接口类似于Runnable接口的增强版,call()方法为线程执行体;

call()方法与run方法的区别:

(1)call()方法可以有返回值;

(2)call方法可以声明抛出异常;

Java5开始提供了Future接口来代表Callable接口里call()方法的返回值,并提供了一个FutureTask实现类.

FutureTask还实现了Runnable接口,因此可以作为Thread的target创建线程.

创建并启动有返回值的线程步骤:

(1)创建Callable接口,实现call()方法作为线程执行体.该call()方法有返回值.创建Callable实现类的实例;(2)使用FutureTask类来包装Callable对象.该FutureTask对象封装了该Callable对象的call()方法的返回值;(3)使用FutureTask对象作为Thread的对象的target创建并启动新线程;

(4)调用FutureTask对象的get()方法来获得子线程执行结束后的返回值.

仍然以火车售票系统为例:

import java.util.concurrent.Callable; import java.util.concurrent.FutureTask; public class SellTicket { public static void main(String[] args) throws InterruptedException, ExecutionException { Ticket tickets = new Ticket(10); TicketWindow tw = new TicketWindow(tickets); for(int i=1; i<4; i++){ FutureTask<Void> ft = new FutureTask<Void>(tw);//注意,每个线程都需要new一个新的FutureTask,否则会出现只有一个窗口卖票 Thread t = new Thread(ft,"TickWindow-" + i); t.start(); } } } class TicketWindow implements Callable<Void>{ //private int tickets = 10;//车票总量 private Ticket tickets; public TicketWindow(Ticket tickets){ this.tickets = tickets; } @Override public Void call(){ while(true){ synchronized (tickets) { int currentNo = tickets.getNum(); if(currentNo>0){ System.out.println(Thread.currentThread().getName() + "准备出票,剩余票数:" + currentNo + "张"); --currentNo; tickets.setNum(currentNo); System.out.println(Thread.currentThread().getName() + "卖出一张,剩余票数:" + currentNo + "张"); try { Thread.sleep(500);//出票成功后让当前售票窗口睡眠,以便让其他售票窗口卖票 } catch (InterruptedException e) { e.printStackTrace(); } //Thread.yield(); } else{ System.out.println(Thread.currentThread().getName() + "余票不足,停止售票!"); break; } } } return null; } } class Ticket{ private int num; public Ticket(int n){ this.num = n; } public void setNum(int n){ this.num = n; } public int getNum(){ return num; } }

输出:

TickWindow-1准备出票,剩余票数:10张 TickWindow-1卖出一张,剩余票数:9张 TickWindow-3准备出票,剩余票数:9张 TickWindow-3卖出一张,剩余票数:8张 TickWindow-3准备出票,剩余票数:8张 TickWindow-3卖出一张,剩余票数:7张 TickWindow-3准备出票,剩余票数:7张 TickWindow-3卖出一张,剩余票数:6张 TickWindow-3准备出票,剩余票数:6张 TickWindow-3卖出一张,剩余票数:5张 TickWindow-3准备出票,剩余票数:5张 TickWindow-3卖出一张,剩余票数:4张 TickWindow-3准备出票,剩余票数:4张 TickWindow-3卖出一张,剩余票数:3张 TickWindow-2准备出票,剩余票数:3张 TickWindow-2卖出一张,剩余票数:2张 TickWindow-2准备出票,剩余票数:2张 TickWindow-2卖出一张,剩余票数:1张 TickWindow-2准备出票,剩余票数:1张 TickWindow-2卖出一张,剩余票数:0张 TickWindow-2余票不足,停止售票! TickWindow-3余票不足,停止售票! TickWindow-1余票不足,停止售票!

对于实现了Callable接口的多线程类,还可以利用线程池方式提交任务.

主要步骤如下,

(1)通过Executors.newFixedThreadPool(int nThreads)等方法创建线程池对象;

(2)创建Callable接口的实现类,作为线程池对象的submit提交对象;

(3)使用Future对象来获得Callable对象的call()方法的返回值;

ExecutorService executorService = Executors.newFixedThreadPool(nThreads); someClassImplimentCallable someClass = new someClassImplimentCallable(); Future<T> future = executorService.submit(someClass);

这种方式模拟的火车售票系统如下:

import java.util.concurrent.Callable; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; public class SellTicket { public static void main(String[] args) throws InterruptedException, ExecutionException { Ticket tickets = new Ticket(10); ExecutorService executorService = Executors.newFixedThreadPool(3); for(int i=1; i<4; i++){ TicketWindow tw = new TicketWindow(tickets); Future<Void> future = executorService.submit(tw); } executorService.shutdown(); } } class TicketWindow implements Callable<Void>{ //private int tickets = 10;//车票总量 private Ticket tickets; public TicketWindow(Ticket tickets){ this.tickets = tickets; } @Override public Void call(){ while(true){ synchronized (tickets) { int currentNo = tickets.getNum(); if(currentNo>0){ System.out.println(Thread.currentThread().getName() + "准备出票,剩余票数:" + currentNo + "张"); --currentNo; tickets.setNum(currentNo); System.out.println(Thread.currentThread().getName() + "卖出一张,剩余票数:" + currentNo + "张"); try { Thread.sleep(500);//出票成功后让当前售票窗口睡眠,以便让其他售票窗口卖票 } catch (InterruptedException e) { e.printStackTrace(); } //Thread.yield(); } else{ System.out.println(Thread.currentThread().getName() + "余票不足,停止售票!"); break; } } } return null; } } class Ticket{ private int num; public Ticket(int n){ this.num = n; } public void setNum(int n){ this.num = n; } public int getNum(){ return num; } }

Java5开始通过显式定义同步锁对象来实现同步-Lock对象

接口Lock互斥锁ReadWriteLock 读写锁实现类ReentrantLock 可重入锁ReentrantReadWriteLockJDK8新增 StampedLock

ReentrantLock较为常用,格式如下:

class Example{ //定义锁对象 private final ReentrantLock lock = new ReentrantLock(); //定义需要保证线程安全的方法 public void function(){ //加锁 lock.lock(); try{ //需要保证线程安全的代码; }finally{ lock.unlock(); } } }

使用同步锁实现的多线程模拟火车售票系统:

import java.util.concurrent.Callable; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; import java.util.concurrent.FutureTask; import java.util.concurrent.locks.ReentrantLock; public class SellTicket { public static void main(String[] args) throws InterruptedException, ExecutionException { //方法1,创建Thread子类进行提交,结果是一个窗口卖票? Ticket tickets = new Ticket(10); TicketWindow tw = new TicketWindow(tickets); for(int i=1; i<4; i++){ FutureTask<Void> ft = new FutureTask<Void>(tw); Thread t = new Thread(ft,"TickWindow-" + i); t.start(); } } } class TicketWindow implements Callable<Void>{ //private int tickets = 10;//车票总量 private Ticket tickets; private final ReentrantLock lock = new ReentrantLock(); public TicketWindow(Ticket tickets){ this.tickets = tickets; } @Override public Void call(){ while(true){ lock.lock(); try{ int currentNo = tickets.getNum(); if(currentNo>0){ System.out.println(Thread.currentThread().getName() + "准备出票,剩余票数:" + currentNo + "张"); --currentNo; tickets.setNum(currentNo); System.out.println(Thread.currentThread().getName() + "卖出一张,剩余票数:" + currentNo + "张"); try { Thread.sleep(500);//出票成功后让当前售票窗口睡眠,以便让其他售票窗口卖票 } catch (InterruptedException e) { e.printStackTrace(); } } else{ System.out.println(Thread.currentThread().getName() + "余票不足,停止售票!"); break; } }finally{ lock.unlock(); } } return null; } } class Ticket{ private int num; public Ticket(int n){ this.num = n; } public void setNum(int n){ this.num = n; } public int getNum(){ return num; } }

本文参考:

https://blog.csdn.net/pengych_321/article/details/50664749

https://my.oschina.net/u/566591/blog/1576410

http://blog.cnsyear.com/2018/03/12/1520865594911.html

https://www.cnblogs.com/dolphin0520/p/3949310.html

http://www.cnblogs.com/felixzh/p/6036074.html