长文慎入-探索Java并发编程与高并发解决方案
本文共 15466 字，大约阅读时间需要 51 分钟。
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 cacheCPU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1) 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2) 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)
以上二者通常和线程池搭配
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++; }} 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
/**
- 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();}}
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()); }} 
- 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
/**
- 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++;}}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synchronizedJMM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)##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
/**
- 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 // Bif (instance == null) { // 同步锁synchronized (SingletonExample4.class) { if (instance == null) { // A - 3instance = new SingletonExample4(); }}}return instance;}}
#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###CountDownLatchpackage 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
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);}}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
-
*/
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);}}##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
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);}}
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
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);}}
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
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);}
}
#9 线程池##9.1 newCachedThreadPool##9.2 newFixedThreadPool##9.3 newSingleThreadExecutor看出是顺序执行的##9.4 newScheduledThreadPool#10 死锁
转载于:https://blog.51cto.com/13601128/2337470
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