什么是线程安全性？如何线程安全
2020-12-13 05:19
标签：height ++ back cache src 内容 int port exit 什么是线程安全？ --当多个线程访问某个类时，不管运行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协同或者同步，这个类都能表现出正确的行为，那么这个类是线程安全的。 1.原子性(Aumic包) AutomicXXX类 ：采用CAS机，即 unsafe.compare.AndSwapInt 上面的代码是线程安全的，要是将AutomicInteger换成Integer，add方法也随着变成count++，线程将是不安全的。 incrementAndGet中的getAndAddInt是最为关键的代码。 var1 是传进来的对象（count）， 如果执行3+1操作，var2是当前的值3，var 4 是1 var5 是底层的值，也就是从主内存拿到的值，由于其他线程也在进行操作，工作内存一直想主内存进行存取操作，主内存的值一直在变化 CAS操作的核心也就是保证从主内存拿到的值，是预期获得的值并没有被其他线程更改，所以在上面的方法中，一直在进行while操作，比较var2 和 var 5, 一旦相同，才会进行相加的操作。主内存和工作内存的运行图如下： 11:42:20.273 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample2 - test2 1 - 0 c.SynchronizedExample2 - test2 2 - 3 Process finished with exit code 0 2。syhcnhronized修饰一个代码块和修饰一个方法，其原理大致一致，如果代码块修饰的整个方法的内容，那它们是一致的 11:52:20.619 [pool-1-thread-1] INFO com.justrun.threadthink.sync.SynchronizedExample1 - test1 1 - 0 什么是线程安全性？如何线程安全 标签：height ++ back cache src 内容 int port exit 原文地址：https://www.cnblogs.com/JTrun/p/11137500.html
public class AtomicControllerTest {
public static int clientTotal = 5000; //请求总数
public static int threadTotal = 200; //同时并发的线程数
public static AtomicInteger count = new AtomicInteger(0);
public static void main(String[] args) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool(); //创建线程池
final Semaphore semaphore = new Semaphore(threadTotal); //信号量（线程数）
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); //计数器 （把请求计数）
for(int i =0 ; i ){
executorService.execute(()->{
try {
semaphore.acquire();//信号量 判断进程是否执行
add();
semaphore.release();
} catch (Exception e){
log.error("Exception:", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await(); //当所有请求结束
executorService.shutdown();
log.info("count:{}", count);
}
private static void add(){
count.incrementAndGet();
}
}
synchronized：
1.修饰方法和修饰一个类时，效果一致，先获得资源的线程，先运行，其余访问改资源的线程需要等待package com.justrun.threadthink.sync;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Slf4j
public class SynchronizedExample2 {
// 修饰一个类
public static void test1(int j) {
synchronized (SynchronizedExample2.class) {
for (int i = 0; i ) {
log.info("test1 {} - {}", j, i);
}
}
}
// 修饰一个静态方法
public static synchronized void test2(int j) {
for (int i = 0; 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);
});
}
}
11:42:20.298 [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.synpackage com.justrun.threadthink.sync;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Slf4j
public class SynchronizedExample1 {
// 修饰一个代码块
public void test1(int j) {
synchronized (this) {
for (int i = 0; i ) {
log.info("test1 {} - {}", j, i);
}
}
log.info("test-----", j);
}
// 修饰一个方法
public synchronized void test2(int j) {
for (int i = 0; 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);
});
}
}
11:42:20.299 [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随机调度资源，每个线程都有机会得到资源
package com.justrun.threadthink.sync;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Slf4j
public class SynchronizedExample1 {
// 修饰一个代码块
public void test1(int j) {
synchronized (this) {
for (int i = 0; i ) {
log.info("test1 {} - {}", j, i);
}
}
log.info("test-----", j);
}
// 修饰一个方法
public synchronized void test2(int j) {
for (int i = 0; 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);
});
}
}
11:52:20.647 [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
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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-----