問題背景：多個線程對一個共享的資源進(jìn)行讀寫訪問。寫線程之間需要互斥，讀線程跟寫線程需要互斥，讀線程之間不用互斥。

早些時候聽張sir的課，講述java5中增強并發(fā)的功能。用java.util.concurrent.locks中ReadWriteLock 可以輕松解決讀寫鎖問題。我在思考如果沒有ReadWriteLock，單靠synchronized可以怎樣做呢? 的確，比較麻煩。

1.結(jié)合張sir傳授的面向?qū)ο蟮脑O(shè)計思想，首先設(shè)計一個業(yè)務(wù)類Business作為共享資源，封裝write跟read方法。

2.因為write必定互斥，所以直接定義synchronized。

3.read之間不要互斥 所以read 不能直接定義synchronized的 但是 write跟read 需要互斥 如何控制 我想到的一個方法是在read里 加入synchronized(this){} 同時定義readThreads計數(shù)器作為信號量 我試想下會出現(xiàn)下面幾種情況：

read[m]表示某個線程的read方法 。

write[n] 同上

1>read[m]中執(zhí)行到synchronized(this){readThreads++;}時 write[n]來了 write[n] 會被自身的synchronized阻塞。

2>read[m]在do something(此時無鎖)時 write[n] 來了 因為 readThreads!=0 而被迫wait。

3> 每次read[m]結(jié)束時 wait中的write[n]會被notify 但如果發(fā)現(xiàn)還有其他的read的話 write[n] 只能無奈地再次wait。

4>當(dāng)readThreads==0并且調(diào)用notifyAll 時 read[m] 和 write[n] 會競爭cpu 如果write[n]再次落敗，則會出現(xiàn)1>或3> ; 如果成了，則如下：

5> 如果write[n] wait中醒來占鎖，read[m]被阻塞synchronized(this){readThreads++;}之上。

6>如果被阻塞的write[n]占鎖，read[m]被阻塞synchronized(this){readThreads++;}之上。

從以上看來read 和 write 是互斥的。

4.實現(xiàn)細(xì)節(jié)如下：<如有錯誤歡迎指出交流>

package communication;  
import java.util.Random;  
 
public class ReadWriteLockTest {  
        public static void main(String[] args){  
                final Business business = new Business();  
                  
                //啟動4線程 2讀 2寫  
                for(int i=1;i<=2;i++){  
                                new Thread(new Runnable(){  
                                        public void run() {  
                                                for(int j=1;j<1000;j++){  
                                                        business.read();  
                                                        try {  
                                                                Thread.sleep(900);  
                                                        } catch (InterruptedException e) {  
                                                                e.printStackTrace();  
                                                        }                                                          
                                                }                                          
                                        }                                  
                                }).start();  
                                  
                                new Thread(new Runnable(){  
                                        public void run() {  
                                                Random r = new Random();  
                                                for(int j=1;j<1000;j++){  
                                                        int i = r.nextInt(100);  
                                                        business.write(i);  
                                                        try {  
                                                                Thread.sleep(1000);  
                                                        } catch (InterruptedException e) {  
                                                                e.printStackTrace();  
                                                        }  
                                                }                                          
                                        }                  
                                }).start();  
                }  
                  
        }  
          
}  
//封裝的業(yè)務(wù)類  
class Business{  
        private int data=0; //共享資源屬性  
        private int readThreads = 0; //讀線程數(shù)  
        //private boolean isWriting  = false;   
        //是否執(zhí)行寫 后來發(fā)現(xiàn)不需要 當(dāng)write搶占鎖時 所有的read 都被擋在synchronized (this){}之上 無機(jī)會執(zhí)行wait  
        public void read(){  
                synchronized (this) {          
                        /*while(isWriting){  
                                try {                          
                                        this.wait();                                          
                                } catch (InterruptedException e) {  
                                        // TODO Auto-generated catch block  
                                        e.printStackTrace();  
                                }  
                        }*/ 
                        //readThreads不被鎖的話 會出現(xiàn)read和write不互斥的小概率事件 導(dǎo)致線程不安全  
                        readThreads++;  
                          
                }  
                  
                System.out.println(Thread.currentThread().getName()+" read begin");  
                System.out.println(Thread.currentThread().getName()+" read:"+data);  
                System.out.println(Thread.currentThread().getName()+" read finish");  
                          
                synchronized (this) {  
                        readThreads--;  
                        this.notifyAll();  
                }          
        }  
          
        public synchronized void write(int i){  
                while(readThreads != 0){//當(dāng)read 正處于do something狀態(tài)時 來個write 那就只有等等先了  
                        try {  
                                this.wait();  
                        } catch (InterruptedException e) {  
                                e.printStackTrace();  
                        }  
                }  
                //isWriting = true;  
                System.out.println(Thread.currentThread().getName()+" write start");  
                data = i;  
                System.out.println(Thread.currentThread().getName()+" write:"+i);  
                System.out.println(Thread.currentThread().getName()+" write over");  
                //isWriting = false;  
                this.notifyAll();  
        }  
} 

思考中：

5.當(dāng)讀頻繁時 readThreads會長時間!= 0 寫線程會餓死 這個可以如何解決？

原文鏈接：http://www.cnblogs.com/hottea4Goodspeed/archive/2012/03/06/2381257.html

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