java线程不安全类与写法

线程不安全类

1.为什么java里要同时提供stringbuilder和stringbuffer两种字符串拼接类

2.simpledateformate是线程不安全的类，如果把它作为全局变量会有线程安全的问题，根据线程封闭原则，把它作为局部变量就可以了，或者用jodatime

jodatime使用示例

package com.alan.concurrency.example.commonUnsafe;

import com.alan.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

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

@Slf4j
@ThreadSafe
public class DateFormatExample3 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyyMMdd");

    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++) {
            final int count = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update(count);
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update(int i) {
        log.info("{}, {}", i, DateTime.parse("20180208", dateTimeFormatter).toDate());
    }
}

3.arraylist hashset hashmap

2.不安全的写法

1.先检查再执行 if(condition(a)){handle(a);} --要注意这个a是否会被多个线程共享修改，解决办法是锁+双重验证

2.vector并不是所有情况下都是线程安全的，例如两个线程，一个get，一个remove,如果同一下标的值是先remove,那么get时就会出错。

3.类的private方法会被隐式的修饰为final的