Java線程安全 關於原子性與volatile的試驗
阿新 • • 發佈:2017-05-14
har length rgs 無法 over pub boolean 即使 dex
1. 變量遞增試驗
1 static /*volatile*/ int shared=0;//volatile也無法保證++操作的原子性 2 static synchronized int incrShared(){//不加synchronized的話,shared最終結果值小於預期 3 return ++shared; 4 } 5 public static void testIncrShared(String[] args) throws InterruptedException { 6 shared=0; 7 Thread[] thrds = newThread[20]; 8 for(int j=0;j<thrds.length;j++){ 9 thrds[j] = new Thread(new Runnable() { 10 @Override 11 public void run() { 12 for(int k=0;k<1000;k++){ 13 System.out.println(incrShared()); 14 }15 } 16 }); 17 } 18 for(int j=0;j<thrds.length;j++){ 19 thrds[j].start(); 20 } 21 for(int j=0;j<thrds.length;j++){ 22 thrds[j].join(); 23 } 24 System.out.println(shared); 25 }
2. volatile試驗
1static /*volatile*/ int a0,a1;//這裏加volatile的話,可以避免r0==r1==0的結果 2 static /*volatile*/ int r0,r1;//這裏即使加volatile,也無法避免r0==r1==0的結果 3 public static void testVolatile(String[] args) throws InterruptedException { 4 int[] a=new int[2]; 5 int[] r=new int[2]; 6 final int SLEEP=10; 7 final Object lock=new Object(); 8 Runnable run1=new Runnable() { 9 @Override 10 public void run() { 11 try { 12 Thread.sleep(SLEEP); 13 } catch (InterruptedException e) { 14 e.printStackTrace(); 15 } 16 //synchronized (lock) {//加鎖也可以建立happens-before關系,避免r0==r1==0的結果 17 a0=1; 18 //} 19 r1=a1; 20 } 21 }; 22 Runnable run2=new Runnable() { 23 @Override 24 public void run() { 25 try { 26 Thread.sleep(SLEEP); 27 } catch (InterruptedException e) { 28 e.printStackTrace(); 29 } 30 //synchronized (lock) { 31 a1=1; 32 //} 33 r0=a0; 34 } 35 }; 36 Thread thrd1; 37 Thread thrd2; 38 39 int i; 40 int[][] acnt=new int[2][2]; 41 int[][] rcnt=new int[2][2]; 42 for(i=0;i<10000;i++){ 43 a0=a1=0; 44 r0=r1=0; 45 thrd1 = new Thread(run1); 46 thrd2 = new Thread(run2); 47 48 thrd1.start(); 49 thrd2.start(); 50 51 thrd1.join(); 52 thrd2.join(); 53 54 a[0]=a0; 55 a[1]=a1; 56 r[0]=r0; 57 r[1]=r1; 58 System.out.println(i); 59 System.out.println(Arrays.toString(a)); 60 System.out.println(Arrays.toString(r)); 61 acnt[a[0]][a[1]]++; 62 rcnt[r[0]][r[1]]++; 63 } 64 System.out.println(Arrays.deepToString(acnt)); 65 System.out.println(Arrays.deepToString(rcnt)); 66 }
3. volatile試驗2
1 static boolean shouldStop=false; 2 public static void testVolatile2(String[] args) throws InterruptedException { 3 shouldStop=false; 4 Runnable run1=new Runnable() { 5 @Override 6 public void run() { 7 int i=0; 8 while (!shouldStop) {//無法讀取到線程2修改後的shouldStop值,導致無限循環 9 i++; 10 //System.out.println(i);//如果調用其他函數的話,就又可以讀取到shouldStop的最新值了 11 } 12 } 13 }; 14 Runnable run2=new Runnable() { 15 @Override 16 public void run() { 17 try { 18 Thread.sleep(10); 19 } catch (InterruptedException e) { 20 e.printStackTrace(); 21 } 22 shouldStop=true; 23 } 24 }; 25 Thread thrd1; 26 Thread thrd2; 27 28 int i; 29 int[][] acnt=new int[2][2]; 30 int[][] rcnt=new int[2][2]; 31 for(i=0;i<100;i++){ 32 thrd1 = new Thread(run1); 33 thrd2 = new Thread(run2); 34 35 thrd1.start(); 36 thrd2.start(); 37 38 thrd2.join(); 39 System.out.println(i); 40 System.out.println(shouldStop); 41 thrd1.join(); 42 43 } 44 }
Java線程安全 關於原子性與volatile的試驗