所有示例代码,请见/下载于
1 基本概念
1.1 并发
同时拥有两个或者多个线程，如果程序在单核处理器上运行多个线程将交替地换入或者换出内存,这些线程是同时“存在"的，每个线程都处于执行过程中的某个状态，如果运行在多核处理器上,此时，程序中的每个线程都将分配到一个处理器核上，因此可以同时运行.
1.2 高并发( High Concurrency)
互联网分布式系统架构设计中必须考虑的因素之一，通常是指，通过设计保证系统能够同时并行处理很多请求.
1.3 区别与联系
- 并发: 多个线程操作相同的资源，保证线程安全，合理使用资源
- 高并发:服务能同时处理很多请求，提高程序性能
2 CPU
2.1 CPU 多级缓存
- 为什么需要CPU cache CPU的频率太快了，快到主存跟不上 如此,在处理器时钟周期内，CPU常常需要等待主存，浪费资源。所以cache的出现，是为了缓解CPU和内存之间速度的不匹配问题(结构:cpu-> cache-> memory ).
- CPU cache的意义
- 时间局部性 如果某个数据被访问，那么在不久的将来它很可能被再次访问
- 空间局部性 如果某个数据被访问，那么与它相邻的数据很快也可能被访问
2.2 缓存一致性(MESI)
用于保证多个 CPU cache 之间缓存共享数据的一致
- M-modified被修改 该缓存行只被缓存在该 CPU 的缓存中,并且是被修改过的,与主存中数据是不一致的,需在未来某个时间点写回主存,该时间是允许在其他CPU 读取主存中相应的内存之前,当这里的值被写入主存之后,该缓存行状态变为 E
- E-exclusive独享 缓存行只被缓存在该 CPU 的缓存中,未被修改过,与主存中数据一致 可在任何时刻当被其他 CPU读取该内存时变成 S 态,被修改时变为 M态
- S-shared共享 该缓存行可被多个 CPU 缓存,与主存中数据一致
- I-invalid无效
- 乱序执行优化 处理器为提高运算速度而做出违背代码原有顺序的优化 ##并发的优势与风险
3 项目准备
3.1 项目初始化
3.2 并发模拟-Jmeter压测
3.3 并发模拟-代码
CountDownLatch
Semaphore(信号量)
下面开始做并发模拟
package com.mmall.concurrency;import com.mmall.concurrency.annoations.NotThreadSafe;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author shishusheng * @date 18/4/1 */@Slf4j@NotThreadSafepublic class ConcurrencyTest { /** * 请求总数 */ public static int clientTotal = 5000; /** * 同时并发执行的线程数 */ public static int threadTotal = 200; public static int count = 0; public static void main(String[] args) throws Exception { //定义线程池 ExecutorService executorService = Executors.newCachedThreadPool(); //定义信号量,给出允许并发的线程数目 final Semaphore semaphore = new Semaphore(threadTotal); //统计计数结果 final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); //将请求放入线程池 for (int i = 0; i < clientTotal ; i++) { executorService.execute(() -> { try { //信号量的获取 semaphore.acquire(); add(); //释放 semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); //关闭线程池 executorService.shutdown(); log.info("count:{}", count); } /** * 统计方法 */ private static void add() { count++; }}复制代码 运行发现结果随机,所以非线程安全
4线程安全性
4.1 线程安全性
当多个线程访问某个类时，不管运行时环境采用何种调度方式或者这些进程将如何交替执行，并且在主调代码中不需要任何额外的同步或协同，这个类都能表现出正确的行为，那么就称这个类是线程安全的
4.2 原子性
4.2.1 Atomic 包
- AtomicXXX:CAS,Unsafe.compareAndSwapInt 提供了互斥访问，同一时刻只能有一个线程来对它进行操作
package com.mmall.concurrency.example.atomic;import com.mmall.concurrency.annoations.ThreadSafe;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;import java.util.concurrent.atomic.AtomicLong;/** * @author shishusheng */@Slf4j@ThreadSafepublic class AtomicExample2 { /** * 请求总数 */ public static int clientTotal = 5000; /** * 同时并发执行的线程数 */ public static int threadTotal = 200; /** * 工作内存 */ public static AtomicLong count = new AtomicLong(0); public static void main(String[] args) throws Exception { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal ; i++) { executorService.execute(() -> { try { System.out.println(); semaphore.acquire(); add(); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); //主内存 log.info("count:{}", count.get()); } private static void add() { count.incrementAndGet(); // count.getAndIncrement(); }}复制代码 package com.mmall.concurrency.example.atomic;import com.mmall.concurrency.annoations.ThreadSafe;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.atomic.AtomicReference;/** * @author shishusheng * @date 18/4/3 */@Slf4j@ThreadSafepublic class AtomicExample4 { private static AtomicReference count = new AtomicReference<>(0); public static void main(String[] args) { // 2 count.compareAndSet(0, 2); // no count.compareAndSet(0, 1); // no count.compareAndSet(1, 3); // 4 count.compareAndSet(2, 4); // no count.compareAndSet(3, 5); log.info("count:{}", count.get()); }}复制代码 -
AtomicReference,AtomicReferenceFieldUpdater
-
AtomicBoolean
-
AtomicStampReference : CAS的 ABA 问题
4.2.2 锁
synchronized:依赖 JVM
- 修饰代码块:大括号括起来的代码，作用于调用的对象
- 修饰方法: 整个方法，作用于调用的对象
- 修饰静态方法:整个静态方法，作用于所有对象
package com.mmall.concurrency.example.count;import com.mmall.concurrency.annoations.ThreadSafe;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author shishusheng */@Slf4j@ThreadSafepublic class CountExample3 { /** * 请求总数 */ public static int clientTotal = 5000; /** * 同时并发执行的线程数 */ public static int threadTotal = 200; public static int count = 0; public static void main(String[] args) throws Exception { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal ; i++) { executorService.execute(() -> { try { semaphore.acquire(); add(); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("count:{}", count); } private synchronized static void add() { count++; }}复制代码 synchronized 修正计数类方法
- 修饰类:括号括起来的部分，作用于所有对象 子类继承父类的被 synchronized 修饰方法时,是没有 synchronized 修饰的!!!
Lock: 依赖特殊的 CPU 指令,代码实现
4.2.3 对比
- synchronized: 不可中断锁，适合竞争不激烈，可读性好
- Lock: 可中断锁，多样化同步，竞争激烈时能维持常态
- Atomic: 竞争激烈时能维持常态，比Lock性能好; 只能同步一 个值
4.3 可见性
一个线程对主内存的修改可以及时的被其他线程观察到
4.3.1 导致共享变量在线程间不可见的原因
- 线程交叉执行
- 重排序结合线程交叉执行
- 共享变量更新后的值没有在工作内存与主存间及时更新
4.3.2 可见性之synchronized
JMM关于synchronized的规定
- 线程解锁前，必须把共享变量的最新值刷新到主内存
- 线程加锁时，将清空工作内存中共享变量的值，从而使 用共享变量时需要从主内存中重新读取最新的值(
加锁与解锁是同一把锁)
4.3.3 可见性之volatile
通过加入内存屏障和禁止重排序优化来实现
- 对volatile变量写操作时，会在写操作后加入一条store 屏障指令，将本地内存中的共享变量值刷新到主内存
- 对volatile变量读操作时，会在读操作前加入一条load 屏障指令，从主内存中读取共享变量
- volatile使用
volatile boolean inited = false;//线程1:context = loadContext();inited= true;// 线程2:while( !inited ){ sleep();}doSomethingWithConfig(context)复制代码 4.4 有序性
一个线程观察其他线程中的指令执行顺序，由于指令重排序的存在，该观察结果一般杂乱无序
JMM允许编译器和处理器对指令进行重排序，但是重排序过程不会影响到单线程程序的执行，却会影响到多线程并发执行的正确性
4.4.1 happens-before 规则
5发布对象
5.1 安全发布对象
package com.mmall.concurrency.example.singleton;import com.mmall.concurrency.annoations.NotThreadSafe;/** * 懒汉模式 -》 双重同步锁单例模式 * 单例实例在第一次使用时进行创建 * @author shishusheng */@NotThreadSafepublic class SingletonExample4 { /** * 私有构造函数 */ private SingletonExample4() { } // 1、memory = allocate() 分配对象的内存空间 // 2、ctorInstance() 初始化对象 // 3、instance = memory 设置instance指向刚分配的内存 // JVM和cpu优化，发生了指令重排 // 1、memory = allocate() 分配对象的内存空间 // 3、instance = memory 设置instance指向刚分配的内存 // 2、ctorInstance() 初始化对象 /** * 单例对象 */ private static SingletonExample4 instance = null; /** * 静态的工厂方法 * * @return */ public static SingletonExample4 getInstance() { // 双重检测机制 // B if (instance == null) { // 同步锁 synchronized (SingletonExample4.class) { if (instance == null) { // A - 3 instance = new SingletonExample4(); } } } return instance; }}复制代码 7 AQS
7.1 介绍
- 使用Node实现FIFO队列，可以用于构建锁或者其他同步装置的基础框架
- 利用了一个int类型表示状态
- 使用方法是继承
- 子类通过继承并通过实现它的方法管理其状态{acquire 和release} 的方法操纵状态
- 可以同时实现排它锁和共享锁模式(独占、共享) 同步组件
CountDownLatch
package com.mmall.concurrency.example.aqs;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * @author shishusheng */@Slf4jpublic class CountDownLatchExample1 { private final static int threadCount = 200; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final CountDownLatch countDownLatch = new CountDownLatch(threadCount); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { test(threadNum); } catch (Exception e) { log.error("exception", e); } finally { countDownLatch.countDown(); } }); } countDownLatch.await(); log.info("finish"); exec.shutdown(); } private static void test(int threadNum) throws Exception { Thread.sleep(100); log.info("{}", threadNum); Thread.sleep(100); }}复制代码 package com.mmall.concurrency.example.aqs;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;/*** 指定时间内处理任务* * @author shishusheng * */@Slf4jpublic class CountDownLatchExample2 { private final static int threadCount = 200; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final CountDownLatch countDownLatch = new CountDownLatch(threadCount); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { test(threadNum); } catch (Exception e) { log.error("exception", e); } finally { countDownLatch.countDown(); } }); } countDownLatch.await(10, TimeUnit.MILLISECONDS); log.info("finish"); exec.shutdown(); } private static void test(int threadNum) throws Exception { Thread.sleep(100); log.info("{}", threadNum); }}复制代码 Semaphore用法
CycliBarrier
package com.mmall.concurrency.example.aqs;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * @author shishusheng */@Slf4jpublic class CyclicBarrierExample1 { private static CyclicBarrier barrier = new CyclicBarrier(5); public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int threadNum = i; Thread.sleep(1000); executor.execute(() -> { try { race(threadNum); } catch (Exception e) { log.error("exception", e); } }); } executor.shutdown(); } private static void race(int threadNum) throws Exception { Thread.sleep(1000); log.info("{} is ready", threadNum); barrier.await(); log.info("{} continue", threadNum); }}复制代码 package com.mmall.concurrency.example.aqs;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;/** * @author shishusheng */@Slf4jpublic class CyclicBarrierExample2 { private static CyclicBarrier barrier = new CyclicBarrier(5); public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int threadNum = i; Thread.sleep(1000); executor.execute(() -> { try { race(threadNum); } catch (Exception e) { log.error("exception", e); } }); } executor.shutdown(); } private static void race(int threadNum) throws Exception { Thread.sleep(1000); log.info("{} is ready", threadNum); try { barrier.await(2000, TimeUnit.MILLISECONDS); } catch (Exception e) { log.warn("BarrierException", e); } log.info("{} continue", threadNum); }}复制代码 package com.mmall.concurrency.example.aqs;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author shishusheng */@Slf4jpublic class SemaphoreExample3 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(3); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { // 尝试获取一个许可 if (semaphore.tryAcquire()) { test(threadNum); // 释放一个许可 semaphore.release(); } } catch (Exception e) { log.error("exception", e); } }); } exec.shutdown(); } private static void test(int threadNum) throws Exception { log.info("{}", threadNum); Thread.sleep(1000); }}复制代码 9 线程池
9.1 newCachedThreadPool
9.2 newFixedThreadPool
9.3 newSingleThreadExecutor
看出是顺序执行的