线程不安全类与写法

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StringBuffer 与 StringBuilder
代码示例 1

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;

/**
 *
 * StringBuilder 线程不安全, 在一个方法内部定义局部变量进行使用的时候,
 * 属于堆栈封闭, 这个时候只有单个线程才能操作这个对象,这个时候就不涉及线程安全的问题.
 *  eg. 在方法内部定义一个StringBuilder 对象来操作 字符串
 */
@Slf4j
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 InterruptedException {

        ExecutorService executorService = Executors.newCachedThreadPool(); // 线程池
        final Semaphore semaphore = new Semaphore(threadTotal); //信号量
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); // 计数器闭锁
        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");
    }
}

运行结果是小于 5000 的

13:49:53.168 [main] INFO com.mmall.concurrency.example.commonUnsafe.StringExample1 - size:4994

代码示例 2

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;

/**
 *
 * StringBuffer 线程安全,其方法有 synchronized 关键字修饰, 每次只能让一个线程来使用, 这样势必就会在性能上有所损失
 */
@Slf4j
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 InterruptedException {

        ExecutorService executorService = Executors.newCachedThreadPool(); // 线程池
        final Semaphore semaphore = new Semaphore(threadTotal); //信号量
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); // 计数器闭锁
        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");
    }
}

SimpleDateFormat 与 JodaTime
代码示例 3

import lombok.extern.slf4j.Slf4j;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

/**
 * SimpleDateFormat 不是一个线程安全的对象
 */
@Slf4j
public class DateFormatExample1 {


    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");


    // 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 void main(String[] args) throws InterruptedException {

        ExecutorService executorService = Executors.newCachedThreadPool(); // 线程池
        final Semaphore semaphore = new Semaphore(threadTotal); //信号量
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); // 计数器闭锁
        for (int i = 0;i<clientTotal;i++) {
            executorService.execute(() ->{
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception ", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void add(){
        try {
            simpleDateFormat.parse("20180405");
        } catch (ParseException e) {
            log.error("parse Exception", e);
        }
    }
}

运行结果 , 可以看到, 当多线程并发执行的时候, 出现了很多的异常 , 所以 要使用SimpleDateFormat 必须声明它为局部变量才可以.

线程安全的 日期转换类 JodaTime , 它不是Java提供的 . 可以通过maven依赖加入

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.8.2</version>
</dependency>

代码示例 4

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;

/**
 * Created by Charles Date:2018/3/22
 */
@Slf4j
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 InterruptedException {

        ExecutorService executorService = Executors.newCachedThreadPool(); // 线程池
        final Semaphore semaphore = new Semaphore(threadTotal); //信号量
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); // 计数器闭锁
        for (int i = 0;i<clientTotal;i++) {
            executorService.execute(() ->{
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception ", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void add(){
        DateTime.parse("20180322", dateTimeFormatter).toDateTime();
        log.info("ee,{}",DateTime.parse("20180322", dateTimeFormatter).toDateTime());
    }
}