你真得会写java多线程手撕代码题吗？

秒速五厘米 2023-07-25 06:23 9阅读 0赞

## 你真得会写java多线程手撕代码题吗？ ##

> 多线程部分是java中最重要的一部分之一，所以也是面试官经常问的部分。
> 
> 头条面试的时候，经常会让你手撕一些多线程代码题。下面介绍几个经典
> 
> 得面试题

### 两个线程，线程1打印A，线程2打印B,两个线程同时并发，要求保证先打印A,后打印B ###

#### 使用`synchronized`\+ `wait、notify` ####

public class Main{ 
        static class Number{ 
            public int num = 1;
            Number(){ }
        }
        public static  Number number = new Number();
        public static void main(String []args) { 
            Thread a = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    synchronized(number){ 
                        try{ 
                        	//避免假唤醒
                            while(number.num == 2) number.wait();
                        }catch(Exception e) { 
                            e.printStackTrace();
                        }
                        System.out.println("A");
                        number.num = 2;
                        number.notify();
                    }
                }
            });
    
            Thread b = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    synchronized(number) { 
                        try{ 
                            while(number.num == 1) number.wait();
                        }catch(Exception e){ 
                            e.printStackTrace();
                        }
                        System.out.println("B");
                        number.num = 1;
                        number.notify();
                    }
                }
            });
            b.start();
            try{ 
                Thread.sleep(1000);
            }catch(Exception e) { 
                e.printStackTrace();
            }
            a.start();
        }
    
    }

#### 使用ReetrantLock+Condition ####

import java.util.concurrent.locks.*;
    public class Main{ 
        public static  ReentrantLock lock = new ReentrantLock();
        public static Condition condition = lock.newCondition();
        public static int num = 1;
        public static void main(String []args) { 
            Thread a = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    lock.lock();
                    try{ 
                        while(num == 2) condition.await();
                    }catch(Exception e) { 
                        e.printStackTrace();
                    }
                    System.out.println("A");
                    num = 2;
                    condition.signal();
                    lock.unlock();
                }
            });
    
            Thread b = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    lock.lock();
                    try{ 
                        while(num == 1) condition.await();
                    }catch(Exception e) { 
                        e.printStackTrace();
                    }
                    System.out.println("B");
                    num = 1;
                    condition.signal();
                    lock.unlock();
                }
            });
            b.start(); 
            try{ 
                Thread.sleep(1000);
            }catch(Exception e){ 
                e.printStackTrace();
            };
            a.start();
        }
    
    }

### 两个线程，一个线程打印奇数，一个线程打印偶数，控制先一个奇数后一个偶数这种顺序.使用ReetrantLock+Condition ###

import java.util.concurrent.locks.*;
    public class Main{ 
        public static  ReentrantLock lock = new ReentrantLock();
        public static Condition condition = lock.newCondition();
        public static int num = 2;
        public static void main(String []args) { 
            Thread a = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    try{ 
                        for(int i = 2; i <= 10; i=i+2) { 
                            lock.lock();
                            while(num == 2) condition.await();
                            System.out.print(i);
                            num = 2; condition.signal();
                            lock.unlock();
                        }
                    }catch(Exception e) { 
                        e.printStackTrace();
                    }
                }
            });
    
            Thread b = new Thread(new Runnable(){ 
                @Override
                public void run() { 
                    try{ 
                        for(int i = 1; i <= 10; i = i+2){ 
                            lock.lock();
                            while(num == 1) condition.await();
                            System.out.print(i);
                            num = 1; condition.signal();
                            lock.unlock();
                        }
                    }catch(Exception e) { 
                        e.printStackTrace();
                    }
                }
            });
            a.start(); 
            try{ 
                Thread.sleep(10);
            }catch(Exception e){ 
                e.printStackTrace();
            };
            b.start();
        }
    }

### 参考文献 ###

[JAVA线程间通信的几种方式][JAVA]

[JAVA]: https://blog.csdn.net/u011514810/article/details/77131296