（四）Java并发学习笔记--线程不安全类与写法

2022-04-13 14:02:43 浏览数 (1)

常见线程不安全的类有哪些呢

下图中，我们只画出了最常见的几种情况，我们常见的Collections集合都是线程不安全的

  • StringBuilder-demo:
代码语言：javascript复制
@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();

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

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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());

    }

}

我测试的时候输出为，4985（因为线程不安全，所以每次的输出可能是不同的），如果StringBuilder类为线程安全的话，输出应该为5000

  • StringBuffer-demo
代码语言：javascript复制
@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();

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

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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());

    }

}

输出为5000，且多次测试结果均为5000，证明StringBuffer类是线程安全的，通过看StringBuffer的实现可发现，其所有的实现都是加了synchronized关键字的，虽然可以保证线程安全但是性能是有损耗的，这也证明了StringBuilder的存在价值，如果定义StringBuilder为局部变量时是没有线程安全问题的，这就用到了上篇博客我们讲的堆栈封闭原理

  • simpleDateFormat-demo1
代码语言：javascript复制
@Slf4j
public class DateFormatExample1 {

    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");

    //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 void update() {
        try {
            simpleDateFormat.parse("20180208");
        } catch (ParseException e) {
            log.error("parse exception",e);
        }
    }

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();

    }
}

运行结果如下：

因为simpleDateFormat为线程不安全的类，所以在多线程访问的时候出现了异常

  • simpleDateFormat-demo2:
代码语言：javascript复制
@Slf4j
public class DateFormatExample2 {


    //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 void update() {
        try {
            //用堆栈封闭的方式
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
            simpleDateFormat.parse("20180208");
        } catch (ParseException e) {
            log.error("parse exception",e);
        }
    }

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();

    }
}

此demo为demo1的改进版，将SimpleDateFormat声明为局部变量，运用了堆栈封闭的方式保证了线程安全，运行此demo是没有异常抛出的

  • jodatime-demo
代码语言：javascript复制
import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

@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");


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

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();

    }
}

此demo引用了joda.time包，保证了线程安全，在实际的开发中，我们更推荐做日期转换的时候使用此包，这种处理方法不仅能保证线程安全，而且还有其它的优势。我导入的包如下：

代码语言：javascript复制
        <dependency>
          <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.9</version>
        </dependency>

以下我们做ArrayList，HashMap，HashSet的实例演示，它们都是线程不安全的，还好我们一般都将它们定义为局部变量（堆栈封闭），如果我们将它们定义为成员变量或static修饰的变量，在多个线程同时访问的时候就很容易出问题。

  • ArrayList-demo
代码语言：javascript复制
@Slf4j
public class ArrayListExample {

    //请求总数
    public static int clientTotal = 5000;
    //同时并发执行的线程数
    public static int threadTotal = 200;

    //arraylist是线程不安全的
    private static List<Integer> list = new ArrayList<>();


    private  static void update(int i) {
        list.add(i);
    }

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();
        log.info("size:{}", list.size());

    }
}

如果是线程安全的输出应该为5000，实际输出为4945，且每次运行输出的值可能不一样，所以它是线程不安全的

  • HashSet-demo
代码语言：javascript复制
@Slf4j
public class HashSetExample {

    //请求总数
    public static int clientTotal = 5000;
    //同时并发执行的线程数
    public static int threadTotal = 200;

    //HashSet是线程不安全的
    private static Set<Integer> set = new HashSet<>();


    private  static void update(int i) {
        set.add(i);
    }

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();
        log.info("size:{}", set.size());

    }
}

输出为4985，是线程不安全的(线程安全的话输出为5000)

  • HashMap-demo
代码语言：javascript复制
@Slf4j
public class HashMapExample {

    //请求总数
    public static int clientTotal = 5000;
    //同时并发执行的线程数
    public static int threadTotal = 200;

    //HashMap是线程不安全的
    private static Map<Integer,Integer> map = new HashMap<>();


    private  static void update(int i) {
        map.put(i,i);
    }

    public static void main(String[] args)throws Exception {

        //定义线程池
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁
        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();
        log.info("size:{}", map.size());

    }
}

输出为4886（且每次运行输出值可能不同），是线程不安全的（线程安全的话输出为5000）

线程不安全的写法

典型的线程不安全的写法是：先检查，再执行

if（condition（a））{handle（a）；} 就算a是一个线程安全的类所对应的对象，对a的处理handle（a）也是原子性的，但由于这两步之间的不是原子性的也会引发线程安全问题，如A、B两个线程都通过了a的判断条件，A线程执行handle（a）之后，a已经不符合condition（a）的判断条件了，可是此时B线程仍然要执行handle（a），这就引发了线程安全问题。

编程算法 hashmap java 安全

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