开始先说下 原子变量和 锁的 比较
锁在高竞争的时候效率会高于原子变量,但是在中低竞争的时候原子变量的性能会好一些
类比:(交通拥堵时,交通信号灯能够实现更高的吞吐量,而在低拥堵时,环岛能实现更高的吞吐量)
知识点:
UnSafe的CAS操作
unsafe.compareAndSwapInt(this, valueOffset, oleVal, newVal)
UnSafe的实例化
1.JDK适用UnSafe是通过下面的代码获取的
private static final Unsafe unsafe = Unsafe.getUnsafe();
但是自定义使用UnSafe无法获取,会抛异常,原因如下代码。
public static Unsafe getUnsafe() { Class var0 = Reflection.getCallerClass(); if(var0.getClassLoader() != null) { throw new SecurityException("Unsafe"); } else { return theUnsafe; } }
2.构造函数私有化,无法new
3.么办法,只能反射了
try { Field field = Unsafe.class.getDeclaredField("theUnsafe"); field.setAccessible(true); unsafe = (Unsafe) field.get(Unsafe.class); } catch (Exception e) { e.printStackTrace(); }
CyclicBarrier(循环栅栏)
1.提供了两种初始化的构造方法
parties:到达栅栏的线程数目=parties,会释放栅栏
包含Runnable的构造方法,Runnable会在所有线程通过栅栏的时候触发
public CyclicBarrier(int parties) public CyclicBarrier(int parties, Runnable barrierAction)
2.CyclicBarrier可以循环使用
下面是代码,分两个部分,计数器类和测试类
import sun.misc.Unsafe; import java.io.Serializable; import java.lang.reflect.Field; public class CasCounter implements Serializable { private static Unsafe unsafe; private static long valueOffset; static { try { Field field = Unsafe.class.getDeclaredField("theUnsafe"); field.setAccessible(true); unsafe = (Unsafe) field.get(Unsafe.class); } catch (Exception e) { e.printStackTrace(); } try { valueOffset = unsafe.objectFieldOffset(CasCounter.class.getDeclaredField("value")); } catch (NoSuchFieldException e) { e.printStackTrace(); } } private volatile int value; public CasCounter() {value = 0;} public CasCounter(int initValue) {this.value = initValue;} public int getValue() { return value; } public int increment(int incrNum) {//线程安全 while (true) { int oleVal = value; int newVal = oleVal + incrNum; if (unsafe.compareAndSwapInt(this, valueOffset, oleVal, newVal)) { return newVal; } } } public int incrementNo(int incrNum) {//非线程安全 value += incrNum; return value; } }
import java.util.concurrent.BrokenBarrierException; import java.util.concurrent.CyclicBarrier; public class Test { public static void main(String[] args) throws BrokenBarrierException, InterruptedException { final CasCounter casCounter = new CasCounter(); final CyclicBarrier cb = new CyclicBarrier(1001, new Runnable() { public void run() { System.out.printf("finalVal : %s \n", casCounter.getValue()); } }); for (int i = 0; i < 1000; i++) { new Thread(new Runnable() { public void run() { try { cb.await(); } catch (InterruptedException e) { e.printStackTrace(); } catch (BrokenBarrierException e) { e.printStackTrace(); } for (int j = 0; j < 10; j++) { int curr = casCounter.increment(1); System.out.print("");//这行代码会增加冲突概率 } try { cb.await(); } catch (InterruptedException e) { e.printStackTrace(); } catch (BrokenBarrierException e) { e.printStackTrace(); } } }).start(); } cb.await(); cb.await(); } }
结果是1000*10=10000应该是正确的,在进行了上百次的测试的结果都是10000,程序应该没问题
finalVal : 0
finalVal : 10000