按序打印——wait的再理解

Posted 2022-11-27 无赖H4

按序打印

• 加锁
• volatile
• wait
• oj

我们提供了一个类：

public class Foo
public void first() print(“first”);
public void second() print(“second”);
public void third() print(“third”);

三个不同的线程 A、B、C 将会共用一个 Foo 实例。

一个将会调用 first() 方法
一个将会调用 second() 方法
还有一个将会调用 third() 方法
请设计修改程序，以确保 second() 方法在 first() 方法之后被执行，third() 方法在 second() 方法之后被执行。
LeetCode按序打印

思路：
通过一个静态属性保留上一次打印的数字，然后加锁判断保证按序打印

加锁

 
public class Demo2 
    private static int lastPrinted = 3;


    static class A extends Thread 
        @Override
        public void run() 
            while (true) 
                while (true) 
                    synchronized (Demo2.class) 
                        if (lastPrinted == 3) 
                            break;
                        
                    
                
                System.out.println(1);
                lastPrinted = 1;
            
        
    

    static class B extends Thread 
        @Override
        public void run() 
            while (true) 
                while (true) 
                    synchronized (Demo2.class) 
                        if (lastPrinted == 1) 
                            break;
                        
                    
                
                System.out.println(2);
                lastPrinted = 2;
            
        
    

    static class C extends Thread 
        @Override
        public void run() 
            while (true) 
                while (true) 
                    synchronized (Demo2.class) 
                        if (lastPrinted == 2) 
                            break;
                        
                    
                
                System.out.println(3);
                lastPrinted = 3;
            
        
    


    public static void main(String[] args) 
        Thread a = new Demo2.A();
        Thread b = new Demo2.B();
        Thread c = new Demo2.C();
        a.start();
        b.start();
        c.start();
    

volatile

通过volatile修饰，避免因为内存可见性出现的问题（缓存问题），
但是还是会造成cpu空转

public class Demo1 
    private static volatile int lastPrinted = 3;


    static class A extends Thread 
        @Override
        public void run() 
            while (true) 
                while (lastPrinted != 3) 
                
                System.out.println(1);
                lastPrinted = 1;
            
        
    

    static class B extends Thread 
        @Override
        public void run() 
            while (true) 
                while (lastPrinted == 1) 
                
                System.out.println(2);
                lastPrinted = 2;
            
        
    

    static class C extends Thread 
        @Override
        public void run() 
            while (true) 
                while (lastPrinted == 2) 
                
                System.out.println(3);
                lastPrinted = 3;
            
        
    


    public static void main(String[] args) 
        Thread a = new Demo2.A();
        Thread b = new Demo2.B();
        Thread c = new Demo2.C();
        a.start();
        b.start();
        c.start();
    

wait

package 按序打印;


import timer.Demo;

public class Demo3 
    private static volatile int lastPrinted = 3;


    static class A extends Thread 
        @Override
        public void run() 
            try 
                while (true) 
                    while (lastPrinted != 3) 
                        synchronized (Demo3.class) 
                            if (lastPrinted != 3) 
                                Demo3.class.wait();
                            
                        
                    
                    System.out.println(1);
                    lastPrinted = 1;
                    synchronized (Demo.class) 
                        Demo3.class.notifyAll();
                    
                
             catch (InterruptedException e) 
                e.printStackTrace();
            
        
    

    static class B extends Thread 
        @Override
        public void run() 
            try 
                while (true) 
                    while (lastPrinted != 1) 
                        synchronized (Demo3.class) 
                            if (lastPrinted != 1) 
                                Demo3.class.wait();
                            
                        
                    
                    System.out.println(2);
                    lastPrinted = 2;
                    synchronized (Demo.class) 
                        Demo3.class.notifyAll();
                    
                
             catch (InterruptedException e) 
                e.printStackTrace();
            
        
    

    static class C extends Thread 
        @Override
        public void run() 
            try 
                while (true) 
                    while (lastPrinted != 2) 
                        synchronized (Demo3.class) 
                            if (lastPrinted != 2) 
                                Demo3.class.wait();
                            
                        
                    
                    System.out.println(3);
                    lastPrinted = 3;
                    synchronized (Demo.class) 
                        Demo3.class.notifyAll();
                    
                
             catch (InterruptedException e) 
                e.printStackTrace();
            
        
    


    public static void main(String[] args) 
        Thread a = new Demo2.A();
        Thread b = new Demo2.B();
        Thread c = new Demo2.C();
        a.start();
        b.start();
        c.start();
    

oj

package 按序打印;

/**
 *
 */
public class Foo 

    private int step = 3;

    public Foo() 
    

    public void first(Runnable printFirst) throws InterruptedException 

        while (step != 3) 
            synchronized (this) 
                if (step != 3) 
                    wait();
                
            
        
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        step = 1;
        synchronized (this) 
            notifyAll();
        
    

    public void second(Runnable printSecond) throws InterruptedException 
        while (step != 1) 
            synchronized (this) 
                if (step != 1) 
                    wait();
                
            
        
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        step = 2;
        synchronized (this) 
            notifyAll();
        
    

    public void third(Runnable printThird) throws InterruptedException 
        while (step != 2) 
            synchronized (this) 
                if (step != 2) 
                    wait();
                
            
        
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
        step = 3;
        synchronized (this) 
            notifyAll();