什么是線程安全性？如何線程安全

什么是線程安全？

 --當多個線程訪問某個類時，不管運行環境采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協同或者同步，這個類都能表現出正確的行為，那么這個類是線程安全的。

 

 

 

 1.原子性(Aumic包)

AutomicXXX類  ：采用CAS機，即 unsafe.compare.AndSwapInt

public class AtomicControllerTest {


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

    public static int threadTotal = 200;  //同時並發的線程數

    public static AtomicInteger count =  new AtomicInteger(0);

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();  //創建線程池
        final Semaphore semaphore = new Semaphore(threadTotal);  //信號量（線程數）
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);  //計數器  （把請求計數）
        for(int i =0 ; i < clientTotal ; i++){
            executorService.execute(()->{
                try {
                    semaphore.acquire();//信號量  判斷進程是否執行
                    add();
                    semaphore.release();
                } catch (Exception e){
                    log.error("Exception:", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();  //當所有請求結束
        executorService.shutdown();
        log.info("count:{}", count);

    }
    private static void add(){
        count.incrementAndGet();
    }
}

上面的代碼是線程安全的，要是將AutomicInteger換成Integer，add方法也隨着變成count++，線程將是不安全的。

incrementAndGet中的getAndAddInt是最為關鍵的代碼。

 

var1 是傳進來的對象（count）， 如果執行3+1操作，var2是當前的值3，var 4 是1

var5 是底層的值，也就是從主內存拿到的值，由於其他線程也在進行操作，工作內存一直想主內存進行存取操作，主內存的值一直在變化

CAS操作的核心也就是保證從主內存拿到的值，是預期獲得的值並沒有被其他線程更改，所以在上面的方法中，一直在進行while操作，比較var2 和 var 5,

一旦相同，才會進行相加的操作。主內存和工作內存的運行圖如下：

 

 

 

synchronized：
1.修飾方法和修飾一個類時，效果一致，先獲得資源的線程，先運行，其余訪問改資源的線程需要等待

package com.justrun.threadthink.sync;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class SynchronizedExample2 {

    // 修飾一個類
    public static void test1(int j) {
        synchronized (SynchronizedExample2.class) {
            for (int i = 0; i < 10; i++) {
                log.info("test1 {} - {}", j, i);
            }
        }
    }

    // 修飾一個靜態方法
    public static synchronized void test2(int j) {
        for (int i = 0; i < 10; i++) {
            log.info("test2 {} - {}", j, i);
        }
    }

    public static void main(String[] args) {
        SynchronizedExample2 example1 = new SynchronizedExample2();
        SynchronizedExample2 example2 = new SynchronizedExample2();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(() -> {
            example1.test2(1);
        });
        executorService.execute(() -> {
            example2.test2(2);
        });
    }
}

11:42:20.273 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 0
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 1
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 2
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 3
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 4
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 5
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 6
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 7
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 8
11:42:20.298 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 9
11:42:20.298 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 0
11:42:20.298 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 1
11:42:20.298 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 2
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.syn

package com.justrun.threadthink.sync;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class SynchronizedExample1 {

    // 修飾一個代碼塊
    public void test1(int j) {
        synchronized (this) {
            for (int i = 0; i < 10; i++) {
                log.info("test1 {} - {}", j, i);
            }
        }
        log.info("test-----", j);
    }

    // 修飾一個方法
    public synchronized void test2(int j) {
        for (int i = 0; i < 10; i++) {
            log.info("test2 {} - {}", j, i);
        }
    }

    public static void main(String[] args) {
        SynchronizedExample1 example1 = new SynchronizedExample1();
        SynchronizedExample1 example2 = new SynchronizedExample1();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(() -> {
            example1.test1(1);
        });
        executorService.execute(() -> {
            example2.test1(2);
        });
    }
}

 

c.SynchronizedExample2 - test2 2 - 3
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 4
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 5
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 6
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 7
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 8
11:42:20.299 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 2 - 9

Process finished with exit code 0

 

2。syhcnhronized修飾一個代碼塊和修飾一個方法，其原理大致一致，如果代碼塊修飾的整個方法的內容，那它們是一致的

隨機調度資源，每個線程都有機會得到資源

package com.justrun.threadthink.sync;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class SynchronizedExample1 {

    // 修飾一個代碼塊
    public void test1(int j) {
        synchronized (this) {
            for (int i = 0; i < 10; i++) {
                log.info("test1 {} - {}", j, i);
            }
        }
        log.info("test-----", j);
    }

    // 修飾一個方法
    public synchronized void test2(int j) {
        for (int i = 0; i < 10; i++) {
            log.info("test2 {} - {}", j, i);
        }
    }

    public static void main(String[] args) {
        SynchronizedExample1 example1 = new SynchronizedExample1();
        SynchronizedExample1 example2 = new SynchronizedExample1();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(() -> {
            example1.test1(1);
        });
        executorService.execute(() -> {
            example2.test1(2);
        });
    }
}

11:52:20.619 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 0
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 1
11:52:20.619 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 0
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 2
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 1
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 3
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 2
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 4
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 3
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 5
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 6
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 7
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 8
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 9
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 4
11:52:20.647 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test-----
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 5
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 6
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 7
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 8
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 2 - 9
11:52:20.647 [pool-1-thread-2] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test-----

volatile

 

volatile不適合存取操作，雖然能夠獲得主存中的最新的值，但無法保證更新后的值得唯一性