volatile關鍵字的做用是變量在多個線程之間可見
volatile的做用是強制線程到主內存(共享內存)裏讀取變量,而不是線程工做內存區裏去讀取變量,從而實現了多個線程之間的變量可見,也就是知足線程安全的可見性。java
private volatile boolean isRunning = true; private void setRunning(boolean isRunning){ this.isRunning = isRunning; } public void run(){ System.out.println("進入run方法.."); int i = 0; while(isRunning == true){ //.. } System.out.println("線程中止"); }
volatile 關鍵字雖然擁有多個線程之間的可見性,可是卻不具有原子性
volatile關鍵字用於針對多個線程可變的變量操做,可是不能替代synchronized關鍵字的同步功能。小程序
atomic類支持原子性操做安全
private static AtomicInteger count = new AtomicInteger(0); /**synchronized*/ public synchronized int multiAdd(){ try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } count.addAndGet(1); count.addAndGet(2); count.addAndGet(3); count.addAndGet(4); //+10 return count.get(); }
多個addAndGet在一個方法內是非原子性的,須要加synchronized進行修飾,保證4個addAndGet總體原子性多線程
線程通訊
使用wait和notify能夠實現線程之間的通訊併發
// final Object lock = new Object(); final CountDownLatch countDownLatch = new CountDownLatch(1); Thread t1 = new Thread(new Runnable() { public void run() { //synchronized (lock) { try { //countDownLatch.countDown(); //countDownLatch.awat(); //lock.notify(); //lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } //} } }, "t1");
使用wait/nofity的缺點是沒法實現實時的通訊 推薦使用countDownLatch 來實現實時的交互ide
使用wait/notify模擬Queue性能
public class MyQueue { //1 須要一個承裝元素的集合 private LinkedList<Object> list = new LinkedList<Object>(); //2 須要一個計數器 private AtomicInteger count = new AtomicInteger(0); //3 須要制定上限和下限 private final int minSize = 0; private final int maxSize ; //4 構造方法 public MyQueue(int size){ this.maxSize = size; } //5 初始化一個對象 用於加鎖 private final Object lock = new Object(); //put(anObject): 把anObject加到BlockingQueue裏, //若是BlockQueue沒有空間,則調用此方法的線程被阻斷, //直到BlockingQueue裏面有空間再繼續. public void put(Object obj){ synchronized (lock) { while(count.get() == this.maxSize){ try { lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } //1 加入元素 list.add(obj); //2 計數器累加 count.incrementAndGet(); //3 通知另一個線程(喚醒) lock.notify(); System.out.println("新加入的元素爲:" + obj); } } //take: 取走BlockingQueue裏排在首位的對象, //若BlockingQueue爲空, //阻斷進入等待狀態直到BlockingQueue有新的數據被加入. public Object take(){ Object ret = null; synchronized (lock) { while(count.get() == this.minSize){ try { lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } //1 作移除元素操做 ret = list.removeFirst(); //2 計數器遞減 count.decrementAndGet(); //3 喚醒另一個線程 lock.notify(); } return ret; } public int getSize(){ return this.count.get(); } public static void main(String[] args) { final MyQueue mq = new MyQueue(5); mq.put("a"); mq.put("b"); mq.put("c"); mq.put("d"); mq.put("e"); System.out.println("當前容器的長度:" + mq.getSize()); Thread t1 = new Thread(new Runnable() { @Override public void run() { mq.put("f"); mq.put("g"); } },"t1"); t1.start(); Thread t2 = new Thread(new Runnable() { @Override public void run() { Object o1 = mq.take(); System.out.println("移除的元素爲:" + o1); Object o2 = mq.take(); System.out.println("移除的元素爲:" + o2); } },"t2"); try { //代替Thread.sleep(1000); TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); } t2.start(); } }
TimeUnit是java.util.concurrent包下面的一個類,TimeUnit提供了可讀性更好的線程暫停操做,一般用來替換Thread.sleep()this
每次寫一個具備設計意義的小程序 考驗的是分析能力atom
ThreadLocal:
ThreadLocal 是線程局部變量 是一種多線程間 併發訪問變量 的解決方案
ThreadLocal 徹底不提供鎖,以空間換時間的方式 爲每個線程提供變量的獨立副本 以保障線程安全線程
private static ThreadLocal<String> th = new ThreadLocal();
在併發量不高 的時候 ,加鎖的性能會更好
座位一套無鎖的線程安全解決方案 使用ThreadLocal能夠減小所競爭
單例模式+多線程
在提升性能的時候 有保證了線程安全
public class DubbleSingleton { private static DubbleSingleton ds; public static DubbleSingleton getDs(){ if(ds == null){ try { //模擬初始化對象的準備時間... Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (DubbleSingleton.class) { if(ds == null){ ds = new DubbleSingleton(); } } } return ds; } }
public class Singleton { private static class InnerSingleton { private static Singleton single = new Singleton(); } public static Singleton getInstance(){ return InnerSingleton.single; } }