并发编程（九）：线程不安全的类与写法
什么是线程不安全的类呢？
如果一个类的对象同时被多个线程访问，如果不做特殊的同步或并发处理，很容易表现出线程不安全的现象，比如抛出异常、逻辑处理错误等，这种类我们就称为线程不安全的类
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
@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
@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
@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
@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
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：
<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
@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
@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
@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。
