synchronized实现两个线程交替运行

淩亂°似流年 2022-12-18 09:56 139阅读 0赞

## 背景 ##

用两个线程交替输出A-Z和1-26，即一个线程输出A-Z，另一个线程输出1-26
    而且是交替形式
    
    线程1输出A——线程二输出1
    线程1输出B——线程二输出2
    线程1输出C——线程二输出3
    以此类推

## 分析 ##

主要考察线程之间的通信，思路就是创建两个线程

在一个线程输出一个内容之后，自己进入阻塞，去唤醒另一个线程

另一个线程同样，输出一个内容之后，自己进入阻塞，去唤醒另一个线程

## 代码实现（一） ##

public class AlternateCover { 
    
        public static void main(String[] args) { 
    
            final char[] arrLetter = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
            final String[] arrNumber = { "1","2","3","4","5","6","7","8","9","10","11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26"};
    
            threadRun(arrLetter, arrNumber);
        }
    
        private static void threadRun(char[] arrLetter,String[] arrNumber){ 
    
            final Object lock = new Object();// 设置一个锁对象
    
    
            // print arrNumber
            new Thread(() -> { 
                synchronized (lock) { 
                    for (String a : arrNumber) { 
                        System.out.print( a);
                        try { 
                            lock.notify();// 唤醒其他等待的线程 此处唤醒 arrLetter
                            lock.wait();// arrNumber自己进入等待 让出CPU资源和锁资源
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();
                }
            }, "arrNumber ").start();
    
    
            // print arrLetter
            new Thread(() -> { 
                synchronized (lock) { // 获取对象锁
                    for (char a : arrLetter) { 
                        System.out.print(a);
                        try { 
                            lock.notify();// 唤醒其他等待的线程 此处唤醒 arrNumber
                            lock.wait();// arrLetter自己进入等待 让出CPU资源和锁资源
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();// 最后那个等待的线程需要被唤醒，否则程序无法结束
                }
            }, "arrLetter ").start();
    
        }
    }

运行一下，确实实现了交替输出，但是多运行几次，就会发现问题

有时候是数字先输出，有时候是字母先输出

即两个线程谁先启动的顺序是不固定的

倘若试题中再加一句，必须要字母先输出，怎么办？

## 代码实现（二） ##

/** * 交替掩护 必须保证大写字母先输出 */
    public class AlternateCover { 
    
        public static volatile Boolean  flg = false;// 谁先开始的标志 volatile修饰目的是让该值修改对所有线程可见，且防止指令重排序
        public static void main(String[] args) { 
    
            final char[] arrLetter = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
            final String[] arrNumber = { "1","2","3","4","5","6","7","8","9","10","11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26"};
    
            threadRun(arrLetter, arrNumber);
        }
    
        private static void threadRun(char[] arrLetter,String[] arrNumber){ 
    
            final Object lock = new Object();// 锁对象
    
            // print arrLetter
            new Thread(() -> { 
                synchronized (lock) { 
                    if (!flg){  // 如果flg是false 就将值设为true
                        flg = true;
                    }
                    for (char a : arrLetter) { 
                        System.out.print(a);// 输出内容
                        try { 
                            lock.notify();// 唤醒在等待的其他线程中的一个（此处也只有另一个）
                            lock.wait();// 自己进入等待 让出CPU资源和锁资源
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();// 最后那个等待的线程需要被唤醒，否则程序无法结束
                }
            }, "arrLetter").start();
    
    
            // print arrNumber
            new Thread(() -> { 
                synchronized (lock) { 
                    if (!flg){ // 倘若是该线程先执行，那么flg次数还是false 就先等着
                        try { 
                            lock.wait();
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
    
                    for (String a : arrNumber) { 
                        System.out.print( a);
                        try { 
                            lock.notify();
                            lock.wait();
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();
                }
            }, "arrNumber").start();
    
        }
    
    
    }

如此问题可以得到解决，但有更优(装)雅（B）的解决办法

## CountDownLatch实现 ##

/** * 交替掩护 必须保证大写字母先输出 */
    public class AlternateCover { 
    
        private static CountDownLatch count = new CountDownLatch(1);// 计数器容量为1
        public static void main(String[] args) { 
    
            final char[] arrLetter = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
            final String[] arrNumber = { "1","2","3","4","5","6","7","8","9","10","11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26"};
    
            threadRun(arrLetter, arrNumber);
        }
    
        private static void threadRun(char[] arrLetter,String[] arrNumber){ 
    
            final Object lock = new Object();
    
            // print arrLetter
            new Thread(() -> { 
                synchronized (lock) { // 获取对象锁
                    count.countDown();// 对计数器进行递减1操作，当计数器递减至0时，当前线程会去唤醒阻塞队列里的所有线程（只针对count）
                    for (char a : arrLetter) { 
                        System.out.print(a);
                        try { 
                            lock.notify();// 唤醒其他等待的线程 此处唤醒 arrNumber
                            lock.wait();// arrLetter自己进入等待 让出CPU资源和锁资源
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();// 最后那个等待的线程需要被唤醒，否则程序无法结束
                }
            }, "arrLetter ").start();
    
    
            // print arrNumber
            new Thread(() -> { 
                synchronized (lock) { 
                    try { 
                        count.await();// 如果该线程先执行 阻塞当前线程，将当前线程加入阻塞队列
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                    for (String a : arrNumber) { 
                        System.out.print( a);
                        try { 
                            lock.notify();// 唤醒其他等待的线程 此处唤醒 arrLetter
                            lock.wait();// arrNumber自己进入等待 让出CPU资源和锁资源
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
                    }
                    lock.notify();
                }
            }, "arrNumber ").start();
    
        }
    
    
    }

以上就是synchronized实现两个线程交替运行的解法，欢迎留言交流