StringBuilder
package com.mmall.concurrency.example.commonUnsafe;
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;
@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
size:4997
解决方案(StringBuffer)
package com.mmall.concurrency.example.commonUnsafe;
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;
@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
size:5000
- StringBuffer 源码分析;StringBuffer 的大多数方法添加了 synchronized 关键字,是线程安全的;StringBuilder不是线程安全的。