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
- 為什麼需要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，是為了緩解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,當這裡的值被寫入主存之後,該緩存行狀態變為 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,被修改時變為 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
- 處理器為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為，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 shishushengbr/>*/
- @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 AtomicReferencecount = 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 shishushengbr/>*/
- @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 shishushengbr/>*/
- @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 shishushengbr/>*/
- @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
- */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
##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 shishushengbr/>*/
- @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 shishushengbr/>*/
- @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 shishushengbr/>*/
- @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
分享到:
閱讀更多 java高級程序猿 的文章
