Java synchronized线程交替运行实现过程详解
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)
publicclassAlternateCover{ publicstaticvoidmain(String[]args){ finalchar[]arrLetter="ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray(); finalString[]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); } privatestaticvoidthreadRun(char[]arrLetter,String[]arrNumber){ finalObjectlock=newObject();//设置一个锁对象 //printarrNumber newThread(()->{ synchronized(lock){ for(Stringa: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(InterruptedExceptione){ e.printStackTrace(); } } lock.notify(); } },"arrNumber").start(); //printarrLetter newThread(()->{ synchronized(lock){//获取对象锁 for(chara: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(InterruptedExceptione){ 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 */ publicclassAlternateCover{ publicstaticvolatileBooleanflg=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 publicstaticvoidmain(String[]args){ finalchar[]arrLetter="ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray(); finalString[]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); } privatestaticvoidthreadRun(char[]arrLetter,String[]arrNumber){ finalObjectlock=newObject();//锁对象 //printarrLetter newThread(()->{ synchronized(lock){ if(!flg){//如果flg是false就将值设为true flg=true; } for(chara: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(InterruptedExceptione){ 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(); //printarrNumber newThread(()->{ 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(InterruptedExceptione){ e.printStackTrace(); } } for(Stringa:arrNumber){ System.out.print(a); try{ lock.notify(); lock.wait(); }catch(InterruptedExceptione){ 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 */ publicclassAlternateCover{ privatestaticCountDownLatchcount=newCountDownLatch(1);//计数器容量为1 publicstaticvoidmain(String[]args){ finalchar[]arrLetter="ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray(); finalString[]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); } privatestaticvoidthreadRun(char[]arrLetter,String[]arrNumber){ finalObjectlock=newObject(); //printarrLetter newThread(()->{ 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(chara: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(InterruptedExceptione){ 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(); //printarrNumber newThread(()->{ 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(InterruptedExceptione){ e.printStackTrace(); } for(Stringa: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(InterruptedExceptione){ e.printStackTrace(); } } lock.notify(); } },"arrNumber").start(); } }
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