线程不安全类

• 如果一个类的对象同时可以被多个线程访问，如果不做特殊的同步和并发处理，那么这个类就很容易表现出线程不安全的现象，比如抛出异常，逻辑处理错误；

StringBuilder示例

• StringBuilder是线程不安全的；
• StringBuilder虽然线程不安全，但是性能更高，在方法中以局部变量的方式使用，可以做到线程封闭，从而避免了线程不

import com.example.concurrency.annotations.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;

@Slf4j
@NotThreadSafe
public class StringExample1 {

// 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 StringBuilder stringBuilder = new StringBuilder();

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();
update();
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", stringBuilder.length());
}

private static void update() {
stringBuilder.append("1");
}

}

输出：

• 不等于5000，线程不安全；

17:10:24.767 [main] INFO com.example.concurrency.example.commonunsafe.StringExample1 - size:4945

StringBuffer示例

• StringBuffer是线程安全的；

import com.example.concurrency.annotations.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;

@Slf4j
@ThreadSafe
public class StringExample2 {

// 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 StringBuffer stringBuffer = new StringBuffer();

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();
update();
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", stringBuffer.length());
}

private static void update() {
stringBuffer.append("1");
}

}

输出：

• 等于5000，线程安全；

17:11:48.784 [main] INFO com.example.concurrency.example.commonunsafe.StringExample2 - size:5000