转 https://www.oschina.net/code/snippet_111708_25438

这个问题挺经典，我这个解法的本质在于将问题抽象为生产者消费者模型，但是是一个特殊的生产者消费者模型，有两点要求：

1、缓冲区大小为1（用一个布尔变量表示就可以了）

2、缓冲区初始为空

再具体点可以将其想象为一个一次只能放一张纸打印的打印机，放纸的线程是A，打印的线程是B。初始状态打印机没有纸。

// 打印机类

public

class

Printer {

     

    

private

boolean

hasBufferToPrint =

false

;  

// 打印缓冲区是否有内容可以打印

 

    

// 打印A：相当于生产者，放一张纸

    

public

synchronized

void

printA() {

        

while

(hasBufferToPrint) {  

// 缓冲区还有内容

            

try

{

                

wait();

            

}

catch

(InterruptedException e) {

                

e.printStackTrace();

            

}

        

}

         

        

System.out.print(

"A"

);

        

hasBufferToPrint =

true

;

         

        

notify();  

// 唤醒打印B的线程

    

}

     

    

// 打印B：相当于消费者，消耗缓冲区中的纸，打印纸张

    

public

synchronized

void

printB() {

        

while

(!hasBufferToPrint) {

            

try

{

                

wait();

            

}

catch

(InterruptedException e) {

                

e.printStackTrace();

            

}

        

}

         

        

System.out.print(

"B"

);

        

hasBufferToPrint =

false

;

         

        

notify();  

// 唤醒打印A的线程

    

}

 

    

static

class

ThreadA

extends

Thread {

        

private

Printer printer;

 

        

public

ThreadA(Printer printer) {

            

this

.printer = printer;

        

}

 

        

public

void

run() {

            

for

(

int

i =

0

; i <

10

; i++) {

                

printer.printA();

            

}

        

}

    

}

 

    

static

class

ThreadB

extends

Thread {

        

private

Printer printer;

         

        

public

ThreadB(Printer printer) {

            

this

.printer = printer;

        

}

         

        

public

void

run() {

            

for

(

int

i =

0

; i <

10

; i++) {

                

printer.printB();

            

}

        

}

    

}

 

    

public

static

void

main(String args[]) {

        

Printer printer =

new

Printer();  

// A、B线程共享同一个打印机

        

Thread a =

new

ThreadA(printer);

        

Thread b =

new

ThreadB(printer);

         

        

a.start();

        

b.start();

    

}

}