转载

长文慎入-探索Java并发编程与高并发解决方案

所有示例代码,请见/下载于

https://github.com/Wasabi1234/concurrency

#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
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
@NotThreadSafe
public 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
br/>*/

@Slf4j
@ThreadSafe

public 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();

}

}

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
@ThreadSafe
public class AtomicExample4 {

    private static AtomicReference<Integer> 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());
    }
}

长文慎入-探索Java并发编程与高并发解决方案

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
br/>*/

@Slf4j
@ThreadSafe

public 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++;

}

}
```
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)

##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
![](https://upload-images.jianshu.io/upload_images/4685968-ed313a1caed24223.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![发布对象](https://upload-images.jianshu.io/upload_images/4685968-b368f6fe5b350cbe.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![对象逸出](https://upload-images.jianshu.io/upload_images/4685968-88d207fcc6bf1866.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
##5.1 安全发布对象
![](https://upload-images.jianshu.io/upload_images/4685968-7400ab2abe1dbbfb.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![非线程安全的懒汉模式](https://upload-images.jianshu.io/upload_images/4685968-ba18bdbe3a3c4ed1.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![饿汉模式](https://upload-images.jianshu.io/upload_images/4685968-be2854c290143094.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![线程安全的懒汉模式](https://upload-images.jianshu.io/upload_images/4685968-e632243a5a97281a.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

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
br/>*/

@NotThreadSafe
public 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;
  }
  
  }
```
![](https://upload-images.jianshu.io/upload_images/4685968-823166cdf7936293.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/4685968-9002671b71096f6c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
#7 AQS
##7.1 介绍
![数据结构](https://upload-images.jianshu.io/upload_images/4685968-918dcaea77d556e9.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
- 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

br/>*/

@Slf4j

public 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);

}

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

br/>@Slf4j

public 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);

}

##Semaphore用法
![](https://upload-images.jianshu.io/upload_images/4685968-e6cbcd4254c642c5.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/4685968-dbefbf2c76ad5a2a.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/4685968-41f5f5a5fd135804.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
##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

br/>*/

@Slf4j

public 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);

}

![](https://upload-images.jianshu.io/upload_images/4685968-4fb51fa4926fd70e.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

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

br/>*/

@Slf4j

public 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);

}

![await 超时导致程序抛异常](https://upload-images.jianshu.io/upload_images/4685968-0f899c23531f8ee8.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

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

br/>*/

@Slf4j

public 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);

}

#9 线程池
##9.1 newCachedThreadPool
![](https://upload-images.jianshu.io/upload_images/4685968-1122da7a48223ba1.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
##9.2 newFixedThreadPool
![](https://upload-images.jianshu.io/upload_images/4685968-0ea942bf12e5210f.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
##9.3 newSingleThreadExecutor
看出是顺序执行的
![](https://upload-images.jianshu.io/upload_images/4685968-989d59429f589403.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
##9.4 newScheduledThreadPool
![](https://upload-images.jianshu.io/upload_images/4685968-f7536ec7a1cf6ecc.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/4685968-c90e09d5bfe707e6.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
#10 死锁
![](https://upload-images.jianshu.io/upload_images/4685968-461f6a4251ae8ca4.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/4685968-46d58773e597195f.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

原文 http://blog.51cto.com/13601128/2337470

正文到此结束