volatile的原理实现可以看这篇文章，真的是从硬件层面上说明了volatile怎样保证可见性。volatile有两个作用，一是保证变量的可见性，二是防止指令重排序。
下面这个实例，如果没有设置成volatile关键字，那么线程读的isRunning永远都是自己私有内存中的，线程将会一直在while循环中
public class RunThread extends Thread{
private volatile boolean isRunning = true;
private void setRunning(boolean isRunning){
this.isRunning = isRunning;
}
public void run(){
System.out.println("进入run方法..");
int i = 0;
while(isRunning == true){
//..
}
System.out.println("线程停止");
}
public static void main(String[] args) throws InterruptedException {
RunThread rt = new RunThread();
rt.start();
Thread.sleep(1000);
rt.setRunning(false);
System.out.println("isRunning的值已经被设置了false");
}
}
这是展示volatile虽然有可见性，但是没有原子性：
/**
* volatile关键字不具备synchronized关键字的原子性（同步）
* @author alienware
*
*/
public class VolatileNoAtomic extends Thread{
private static volatile int count = 0;
//这个被注释的代码可以保证结果正确
//private static AtomicInteger count = new AtomicInteger(0);
private static void addCount(){
for (int i = 0; i < 1000; i++) {
count++ ;
//这个被注释的代码可以保证结果正确
//count.incrementAndGet();
}
System.out.println(count);
}
public void run(){
addCount();
}
public static void main(String[] args) {
VolatileNoAtomic[] arr = new VolatileNoAtomic[100];
for (int i = 0; i < 10; i++) {
arr[i] = new VolatileNoAtomic();
}
for (int i = 0; i < 10; i++) {
arr[i].start();
}
}
}
这是使用atomic，保证原子性的代码：
public class AtomicUse {
private static AtomicInteger count = new AtomicInteger(0);
//多个addAndGet在一个方法内是非原子性的，需要加synchronized进行修饰，保证4个addAndGet整体原子性
/**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();
}
public static void main(String[] args) {
final AtomicUse au = new AtomicUse();
List<Thread> ts = new ArrayList<Thread>();
for (int i = 0; i < 100; i++) {
ts.add(new Thread(new Runnable() {
@Override
public void run() {
System.out.println(au.multiAdd());
}
}));
}
for(Thread t : ts){
t.start();
}
}
}
线程通信
ListAdd2.java，可以看出本来list已经到5了，那么t2应该出while循环抛异常，但是因为它执行了wait方法，释放锁了。而t1得到锁一直执行，虽然t1执行了notify方法，但是只是发出通知而已，只有它的方法执行完才释放锁让t2执行。
package com.bjsxt.base.conn008;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.LinkedBlockingQueue;
/**
* wait notfiy 方法，wait释放锁，notfiy不释放锁
* @author alienware
*
*/
public class ListAdd2 {
private volatile static List list = new ArrayList();
public void add(){
list.add("bjsxt");
}
public int size(){
return list.size();
}
public static void main(String[] args) {
final ListAdd2 list2 = new ListAdd2();
// 1 实例化出来一个 lock
// 当使用wait 和 notify 的时候 ， 一定要配合着synchronized关键字去使用
final Object lock = new Object();
// final CountDownLatch countDownLatch = new CountDownLatch(1);
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
try {
synchronized (lock) {
for(int i = 0; i <10; i++){
list2.add();
System.out.println("当前线程：" + Thread.currentThread().getName() + "添加了一个元素..");
Thread.sleep(500);
if(list2.size() == 5){
System.out.println("已经发出通知..");
// countDownLatch.countDown();
lock.notify();
}
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
if(list2.size() != 5){
try {
System.out.println("t2进入...");
lock.wait();
// countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("当前线程：" + Thread.currentThread().getName() + "收到通知线程停止..");
throw new RuntimeException();
}
}
}, "t2");
t2.start();
t1.start();
}
}
package com.xushu.multi;
import java.util.LinkedList;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
public class MyQueue {
private LinkedList<Object> list = new LinkedList<Object>();
private AtomicInteger count = new AtomicInteger(0);
private final int minSize = 0;
private final int maxSize;
public MyQueue(int size) {
this.maxSize = size;
}
private final Object lock = new Object();
public void put(Object obj) {
synchronized (lock) {
if (count.get() == this.maxSize) {
try {
lock.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
list.add(obj);
count.incrementAndGet();
lock.notify();
System.out.println("新加入的元素为：" + obj);
}
}
public Object take() {
Object ret = null;
synchronized (lock) {
if (count.get() == minSize) {
try {
lock.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
ret = list.removeFirst();
count.decrementAndGet();
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");
System.out.println("当前容器的长度:" + mq.getSize());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
mq.put("f");
mq.put("g");
mq.put("e");
}
}, "t1");
t1.start();
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
Object o1 = mq.take();
System.out.println("移除的元素为:" + o1);
Object o2 = mq.take();
System.out.println("移除的元素为:" + o2);
}
}, "t2");
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
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