關于讀寫鎖算法的Java實現(xiàn)及思考是怎么樣的

本篇文章給大家分享的是有關關于讀寫鎖算法的Java實現(xiàn)及思考是怎么樣的，小編覺得挺實用的，因此分享給大家學習，希望大家閱讀完這篇文章后可以有所收獲，話不多說，跟著小編一起來看看吧。

問題背景：多個線程對一個共享的資源進行讀寫訪問。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計思想，首先設計一個業(yè)務類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write搶占鎖時 所有的read 都被擋在synchronized (this){}之上 無機會執(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.當讀頻繁時 readThreads會長時間!= 0 寫線程會餓死 這個可以如何解決？

以上就是關于讀寫鎖算法的Java實現(xiàn)及思考是怎么樣的，小編相信有部分知識點可能是我們?nèi)粘９ぷ鲿姷交蛴玫降?。希望你能通過這篇文章學到更多知識。更多詳情敬請關注億速云行業(yè)資訊頻道。