Java併發編程 - 線程不安全類與解決方案（String）

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 {

    // 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 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");
    }
}

// 輸出
size:4997

 

解決方案（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 {

    // 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 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");
    }
}

// 輸出
size:5000

 

• StringBuffer 源碼分析；StringBuffer 的大多數方法添加了 synchronized 關鍵字，是線程安全的；StringBuilder不是線程安全的。