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do{  
 flag[i]=true;  
 while(flag[j]);  
 临界区；  
 flag[i]=false;  
 剩余区；  
}while(1)

Java实现代码如下：

package mutiple_thread;  
 
public class OS_SYN_A2{  
    public static  int flag[]=new int [3];  
    public static int cnt=0;  
    public static void main(String args[]){  
        class proo implements Runnable{  
            public proo(){  
                  
            }  
            @Override 
            public void run() {  
                // TODO Auto-generated method stub  
                while(true){  
                    flag[1]=1;  
                    while(flag[2]==1){  
                          
                    }  
                    if(cnt==5){  
                        flag[1]=0;  
                    }else{  
                        cnt++;  
                        System.out.println("pro ++! now id"+cnt);  
                        flag[1]=0;  
                    }  
                }  
            }  
              
        }  
 
        class conn implements Runnable{  
 
            @Override 
            public void run() {  
                // TODO Auto-generated method stub  
                while(true){  
                    flag[2]=1;  
                    while(flag[1]==1){  
                          
                    }  
                    //临界区  
                    if(cnt==0){  
                        flag[2]=0;  
                    }else{  
                        cnt--;  
                        System.out.println("con --! now id"+cnt);  
                        //退出临界区  
                        flag[2]=0;  
                    }  
                }  
            }  
        }  
        new Thread(new proo()).start();  
        new Thread(new conn()).start();  
    }  
      
}

这个算法的主要思路是通过设置flag来确定执行哪个线程，但是可能会造成饥饿，因此不行。

1.2 算法3

算法3通过共享两个变量 flag 和turn来实现同步。

package mutiple_thread;  
 
public class OS_SYN_A3{  
    public static  int flag[]=new int [3];  
    public static int turn=0;  
    public static int cnt=0;  
    public static void main(String args[]){  
        class proo implements Runnable{  
            public proo(){  
                  
            }  
            @Override 
            public void run() {  
                // TODO Auto-generated method stub  
                while(true){  
                    flag[1]=1;  
                    turn=2;  
                    while(flag[2]==1&&turn==2){  
                          
                    }  
                    if(cnt==5){  
                        flag[1]=0;  
                    }else{  
                        cnt++;  
                        System.out.println("pro ++! now id"+cnt);  
                        flag[1]=0;  
                    }  
                }  
            }  
              
        }  
 
        class conn implements Runnable{  
 
            @Override 
            public void run() {  
                // TODO Auto-generated method stub  
                while(true){  
                    flag[2]=1;  
                    turn=1;  
                    while(flag[1]==1&&turn==1){  
                          
                    }  
                    //临界区  
                    if(cnt==0){  
                        flag[2]=0;  
                    }else{  
                        cnt--;  
                        System.out.println("con --! now id"+cnt);  
                        //退出临界区  
                        flag[2]=0;  
                    }  
                }  
            }  
        }  
        new Thread(new proo()).start();  
        new Thread(new conn()).start();  
    }  
      
}

这是一种正确的软件实现方法。

2、经典同步问题的Java实现

2.1 读者写者问题

这里实现的读者优先的算法，使用了Java并发包的信号量来实现。

实现的伪代码如下：

读者进程：

while（1）{  
 wait(mutex)  
   count++;  
   if(readercount==1){  
   wait(writer);   
 }  
signal(mutex);  
do reading;  
wait(mutex);  
cnt--;  
if(cnt==0){  
  signal(writer);  
}  
signal(mutex);  
}  
}

算法通过共享writer和mutex两个信号量，来处理同步问题

package mutiple_thread;
import java.util.concurrent.Semaphore;
public class OS_Readerwriter{
static Semaphore sem=new Semaphore(1);
static Semaphore sem_wrt=new Semaphore(1);
static int readercount=0;
static String a="hahaha";
public static void main(String args[]){
class reader implements Runnable{
public reader(){
}
@Override
public void run() {
// TODO Auto-generated method stub
try {
sem.acquire();
readercount++;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if(readercount==1){
try {
sem_wrt.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
sem.release();
System.out.println("Reading "+a);
try {
sem.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
readercount--;
if(readercount==0){
sem_wrt.release();
}
sem.release();
}
}
class writer implements Runnable{
public writer(){
}
@Override
public void run() {
// TODO Auto-generated method stub
try {
sem_wrt.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
a=a+"abc";
System.out.println("Writing "+a);
sem_wrt.release();
}
}
for(int i=1;i<=10;i++){
new Thread(new writer()).start();
new Thread(new reader()).start();
}
}
}

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