長文慎入-探索Java併發編程與高併發解決方案

所有示例代碼,請見/下載於

https://github.com/Wasabi1234...

1 基本概念

1.1 併發

同時擁有兩個或者多個線程，如果程序在單核處理器上運行多個線程將交替地換入或者換出內存,這些線程是同時“存在"的，每個線程都處於執行過程中的某個狀態，如果運行在多核處理器上,此時，程序中的每個線程都將分配到一個處理器核上，因此可以同時運行.

1.2 高併發( High Concurrency)

互聯網分佈式系統架構設計中必須考慮的因素之一，通常是指，通過設計保證系統能夠同時並行處理很多請求.

1.3 區別與聯繫

• 併發: 多個線程操作相同的資源，保證線程安全，合理使用資源
• 高併發:服務能同時處理很多請求，提高程序性能

2 CPU

2.1 CPU 多級緩存

• 爲什麼需要CPU cache

CPU的頻率太快了，快到主存跟不上
如此,在處理器時鐘週期內，CPU常常需要等待主存，浪費資源。所以cache的出現，是爲了緩解CPU和內存之間速度的不匹配問題(結構:cpu-> cache-> memory ).

• CPU cache的意義
  1) 時間局部性
  如果某個數據被訪問，那麼在不久的將來它很可能被再次訪問
  2) 空間局部性

如果某個數據被訪問，那麼與它相鄰的數據很快也可能被訪問

2.2 緩存一致性(MESI)

用於保證多個 CPU cache 之間緩存共享數據的一致

• M-modified被修改

該緩存行只被緩存在該 CPU 的緩存中,並且是被修改過的,與主存中數據是不一致的,需在未來某個時間點寫回主存,該時間是允許在其他CPU 讀取主存中相應的內存之前,當這裏的值被寫入主存之後,該緩存行狀態變爲 E

• E-exclusive獨享

緩存行只被緩存在該 CPU 的緩存中,未被修改過,與主存中數據一致
可在任何時刻當被其他 CPU讀取該內存時變成 S 態,被修改時變爲 M態

• S-shared共享

該緩存行可被多個 CPU 緩存,與主存中數據一致

• I-invalid無效

• 亂序執行優化

處理器爲提高運算速度而做出違背代碼原有順序的優化

併發的優勢與風險

3 項目準備

3.1 項目初始化

3.2 併發模擬-Jmeter壓測

3.3 併發模擬-代碼

CountDownLatch

Semaphore(信號量)

以上二者通常和線程池搭配

下面開始做併發模擬

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 {

    /**
     * 請求總數
     */
    public static int clientTotal = 5000;

    /**
     * 同時併發執行的線程數
     */
    public static int threadTotal = 200;

    public static int count = 0;

    public static void main(String[] args) throws Exception {
        //定義線程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定義信號量,給出允許併發的線程數目
        final Semaphore semaphore = new Semaphore(threadTotal);
        //統計計數結果
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        //將請求放入線程池
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    //信號量的獲取
                    semaphore.acquire();
                    add();
                    //釋放
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        //關閉線程池
        executorService.shutdown();
        log.info("count:{}", count);
    }

    /**
     * 統計方法
     */
    private static void add() {
        count++;
    }
}

運行發現結果隨機,所以非線程安全

4線程安全性

4.1 線程安全性

當多個線程訪問某個類時，不管運行時環境採用何種調度方式或者這些進程將如何交替執行，並且在主調代碼中不需要任何額外的同步或協同，這個類都能表現出正確的行爲，那麼就稱這個類是線程安全的

4.2 原子性

4.2.1 Atomic 包

• AtomicXXX:CAS,Unsafe.compareAndSwapInt

提供了互斥訪問，同一時刻只能有一個線程來對它進行操作

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
@ThreadSafe
public class AtomicExample2 {

    /**
     * 請求總數
     */
    public static int clientTotal = 5000;

    /**
     * 同時併發執行的線程數
     */
    public static int threadTotal = 200;

    /**
     * 工作內存
     */
    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();
        //主內存
        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());
    }
}

• AtomicReference,AtomicReferenceFieldUpdater

• AtomicBoolean

• AtomicStampReference : CAS的 ABA 問題

4.2.2 鎖

synchronized:依賴 JVM

• 修飾代碼塊:大括號括起來的代碼，作用於調用的對象
• 修飾方法: 整個方法，作用於調用的對象

• 修飾靜態方法:整個靜態方法，作用於所有對象

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
@ThreadSafe
public class CountExample3 {

    /**
     * 請求總數
     */
    public static int clientTotal = 5000;

    /**
     * 同時併發執行的線程數
     */
    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 修正計數類方法

• 修飾類:括號括起來的部分，作用於所有對象

子類繼承父類的被 synchronized 修飾方法時,是沒有 synchronized 修飾的!!!

Lock: 依賴特殊的 CPU 指令,代碼實現

4.2.3 對比

• synchronized: 不可中斷鎖，適合競爭不激烈，可讀性好
• Lock: 可中斷鎖，多樣化同步，競爭激烈時能維持常態
• Atomic: 競爭激烈時能維持常態，比Lock性能好; 只能同步一

個值

4.3 可見性

一個線程對主內存的修改可以及時的被其他線程觀察到

4.3.1 導致共享變量在線程間不可見的原因

• 線程交叉執行
• 重排序結合線程交叉執行
• 共享變量更新後的值沒有在工作內存與主存間及時更新

4.3.2 可見性之synchronized

JMM關於synchronized的規定

• 線程解鎖前，必須把共享變量的最新值刷新到主內存
• 線程加鎖時，將清空工作內存中共享變量的值，從而使

用共享變量時需要從主內存中重新讀取最新的值(加鎖與解鎖是同一把鎖)

4.3.3 可見性之volatile

通過加入內存屏障和禁止重排序優化來實現

• 對volatile變量寫操作時，會在寫操作後加入一條store

屏障指令，將本地內存中的共享變量值刷新到主內存

• 對volatile變量讀操作時，會在讀操作前加入一條load

屏障指令，從主內存中讀取共享變量

• volatile使用

volatile boolean inited = false;

//線程1:
context = loadContext();
inited= true;

// 線程2:
while( !inited ){
    sleep();
}
doSomethingWithConfig(context)

4.4 有序性

一個線程觀察其他線程中的指令執行順序，由於指令重排序的存在，該觀察結果一般雜亂無序

JMM允許編譯器和處理器對指令進行重排序，但是重排序過程不會影響到單線程程序的執行，卻會影響到多線程併發執行的正確性

4.4.1 happens-before 規則

5發佈對象

5.1 安全發佈對象

package com.mmall.concurrency.example.singleton;

import com.mmall.concurrency.annoations.NotThreadSafe;

/**
 * 懶漢模式 -》 雙重同步鎖單例模式
 * 單例實例在第一次使用時進行創建
 * @author shishusheng
 */
@NotThreadSafe
public class SingletonExample4 {

    /**
     * 私有構造函數
     */
    private SingletonExample4() {

    }

    // 1、memory = allocate() 分配對象的內存空間
    // 2、ctorInstance() 初始化對象
    // 3、instance = memory 設置instance指向剛分配的內存

    // JVM和cpu優化，發生了指令重排

    // 1、memory = allocate() 分配對象的內存空間
    // 3、instance = memory 設置instance指向剛分配的內存
    // 2、ctorInstance() 初始化對象

    /**
     * 單例對象
     */
    private static SingletonExample4 instance = null;

    /**
     * 靜態的工廠方法
     *
     * @return
     */
    public static SingletonExample4 getInstance() {
        // 雙重檢測機制 // B
        if (instance == null) {        
            // 同步鎖
            synchronized (SingletonExample4.class) { 
                if (instance == null) {
                    // A - 3
                    instance = new SingletonExample4(); 
                }
            }
        }
        return instance;
    }
}

7 AQS

7.1 介紹

• 使用Node實現FIFO隊列，可以用於構建鎖或者其他同步裝置的基礎框架
• 利用了一個int類型表示狀態
• 使用方法是繼承
• 子類通過繼承並通過實現它的方法管理其狀態{acquire 和release} 的方法操縱狀態
• 可以同時實現排它鎖和共享鎖模式(獨佔、共享)

同步組件

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
 */
@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;

/**
* 指定時間內處理任務
* 
* @author shishusheng 
* 
*/
@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

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
 */
@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);
    }
}

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
 */
@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);
    }
}

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
 */
@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 {
                    // 嘗試獲取一個許可
                    if (semaphore.tryAcquire()) {
                        test(threadNum);
                        // 釋放一個許可
                        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 線程池

9.1 newCachedThreadPool

9.2 newFixedThreadPool

9.3 newSingleThreadExecutor

看出是順序執行的

9.4 newScheduledThreadPool

10 死鎖