并发编程（八）：线程安全策略

  通常我们保证线程安全策略的方式有以下几种：

  a、不可变对象

  b、线程封闭

  c、同步容器

  d、并发容器

  不可变对象

  

  可参考string类，可以采用的方式是将类声明为final，将所有成员都声明为私有的，对变量不提供set方法，将所有可变成员声明为final，通过构造器初始化所有成员，进行深度拷贝，在get方法中不直接返回对象本身，而是返回对象的拷贝。

  关于final，我们详细说明一下

  final-demo

@Slf4j
public class ImmutableExample1 {

    private final static Integer a = 1;
    private final static String b = "2";

    private final static Map<Integer, Integer> map = Maps.newHashMap();

    static {
        map.put(1, 2);
        map.put(3, 4);
        map.put(5, 6);
    }

    public static void main(String[] args) {

//        被final修饰的基本数据类型无法改变
//        a = 2;
//        b = "2";

//        引用对象，此引用无法指向别的对象，但可修改该对象的值
        map.put(1,3);
        log.info("{}", map.get(1));
    }

    //final可修饰传递进来的对象
    private void test(final int a) {

    }
}

  此demo需要我们注意的是，final修饰引用类型时，虽然不能将引用再指向别的对象，但可修改该对象的值；此外final还可修饰参数，这样传递进来的参数a无法被修改。此demo不是线程安全的

  除了final可以定义不可变对象，java提供的Collections类，也可定义不可变对象，Collections.unmodifiableXXX传入的对象一经初始化便无法修改,XXX可表示Collection、List、Set、Map等，谷歌提供的Guava类，也有类似的功能，ImmutableXXX，XXX同样可表示Collection、List、Set、Map等

  Collections-demo

@Slf4j
public class ImmutableExample2 {
    
    private  static Map<Integer, Integer> map = Maps.newHashMap();

    static {
        map.put(1, 2);
        map.put(3, 4);
        map.put(5, 6);
        //此处理后的map的值是不可以修改的
        map = Collections.unmodifiableMap(map);
    }

    public static void main(String[] args) {

//
        map.put(1,3);
        log.info("{}", map.get(1));
    }
}

输出如下：

  可见，用Collections.UnmodifiableMap修饰的对象是不可修改的，如果尝试修改对象的值，在程序运行时会抛出异常，此方法的实现可参考源码（其实就是将一个新的集合的所有更新方法变为抛出异常） 此demo是线程安全的。

  ImmutableSet-demo

public class ImmutableExample3 {

    //以下为不可变对象的集合
    private final static ImmutableList<Integer> list = ImmutableList.of(1, 2, 3);
    private final static ImmutableSet set = ImmutableSet.copyOf(list);

    private final static ImmutableMap<Integer, Integer> map = ImmutableMap.of(1,2,3,4);
    private final static ImmutableMap<Integer, Integer> map2 = ImmutableMap.<Integer, Integer>builder()
            .put(1, 2).put(3, 4).put(5, 6).build();

    public static void main(String[] args) {
//        此时不同意再添加新的元素
        map2.put(1, 3);
    }
}

输出如下：

  此demo是线程安全的，开发时如果我们的对象可以变为不可变对象，我们尽量将对象变为不可变对象，这样可以避免线程安全问题

  线程封闭

  线程封闭就是把对象封装到一个线程里，只有一个线程可以看到这个对象，这样就算这个对象不是线程也不会有线程安全问题

  关于堆栈封闭，我们自己定义的局部变量被多个线程访问时，每个局部变量都会被拷贝一份放到线程的栈中去，这样每个线程操作的对象相当于是不同的，所以不会有线程安全问题（全局变量容易引发并发问题）；ThreadLocal使用map实现了线程封闭，map的key是线程id，map的值是封闭的对象

  THreadLocal-demo

定义RequestHolder类来操作ThreadLocal：

public class RequestHolder {

    //只有当项目重新启动的时候，threadLocal中存储的值才会被释放
    private final static ThreadLocal<Long> requestHolder = new ThreadLocal<>();

    public static void add(Long id) {
        requestHolder.set(id);
    }

    public static Long getId() {
        return requestHolder.get();
    }
    public static void remove() {
        requestHolder.remove();
    }
}

  注意，如add方法，只需传入需要封闭的对象即可，key值会自动取线程id放入，get和remove方法类似

定义HttpFilter类处理请求：

@Slf4j
public class HttpFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) servletRequest;
        log.info("do filter,{},{}", Thread.currentThread().getId(), request.getServletPath());
        RequestHolder.add(Thread.currentThread().getId());
        //使请求继续被处理，不要拦住不动
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {

    }
}

配置启动类：

@SpringBootApplication
public class ConcurrencyApplication extends WebMvcConfigurerAdapter {

    public static void main(String[] args) {
        SpringApplication.run(ConcurrencyApplication.class, args);
    }

    @Bean
    public FilterRegistrationBean httpFilter() {
        FilterRegistrationBean registrationBean = new FilterRegistrationBean();
        registrationBean.setFilter(new HttpFilter());
        registrationBean.addUrlPatterns("/threadLocal/*");
        return registrationBean;
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new HttpInterceptor()).addPathPatterns("/*");
    }
}

  拦截以threadLocal开头的url，并利用Interceptor拦截所有的接口

定义Interceptor

@Slf4j
public class HttpInterceptor extends HandlerInterceptorAdapter {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        log.info("preHandle");
        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        RequestHolder.remove();
        log.info("afterCompletion");
        return;
    }
}

  此Interceptor的作用是当接口处理后，移除ThreadLocal中对应的值

定义Controller来进行验证

@Controller
@RequestMapping("/threadLocal")
public class ThreadLocalController {

    @RequestMapping("/test")
    @ResponseBody
    public Long test() {
        return RequestHolder.getId();
    }
}

  同步容器

  我们都知道ArrayList、HashMap等为线程不安全的，上图标识了它们对应的同步处理的容器

  Vector-demo1

@Slf4j
public class VectorExample1 {

    //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 Vector<Integer> list = new Vector<>();


    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());

    }
}

  执行结果为5000，但是并不能说是线程安全的，同步容器不能保证在所有的情景下都保证线程安全，可参考Vector-demo2

  Vector-demo2

public class VectorExample2 {

    private static Vector<Integer> vector = new Vector<>();


    public static void main(String[] args) {


        while (true) {
            for (int i = 0; i < vector.size(); i++) {
                vector.add(i);
            }

            Thread thread1 = new Thread() {
                public void run() {
                    for (int i = 0; i < vector.size(); i++) {
                        vector.remove(i);
                    }
                }
            };

            Thread thread2 = new Thread() {
                public void run() {
                    for (int i = 0; i < 10; i++) {
                        vector.get(i);
                    }
                }
            };

            thread1.start();
            thread2.start();
        }

    }

}

输出如下：

    表明某一线程访问的数据，可能被其他线程remove掉，导致出现下标越界异常，此demo是线程不安全的

  HashTable-demo1

@Slf4j
public class HashTableExample {

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

    //Hashtable是线程安全的
    private static Map<Integer,Integer> map = new Hashtable<>();


    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());

    }
}

  运行结果为5000，此demo为线程安全的

  Collections-List-demo1

@Slf4j
public class CollectionsExample1 {

    //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 = Collections.synchronizedList(Lists.newArrayList());


    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());

    }
}

  运行结果为5000，是线程安全的

  Collections-Set-demo1

@Slf4j
public class CollectionsExample2 {

    //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 Set<Integer> set = Collections.synchronizedSet(Sets.newHashSet());


    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());

    }
}

  返回结果为5000，是线程安全的

  Collections-Map-demo1

@Slf4j
public class CollectionsExample3 {

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

    //Hashtable是线程安全的
    private static Map<Integer, Integer> map = Collections.synchronizedMap(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());

    }
}

  返回结果为5000，是线程安全的

  并发容器J.U.C

  以上是常见的不安全的容器类，对应的并发容器类，我们以demo的方式进行演示

  CopyOnWriteArrayList、CopyOnWriteArraySet因为需要copy数组，需要消耗内存，可能引发yonggc胡哦哦这fullgc，并且不能做到实时性，适合读多写少的情景

  ConcurrentSkipListSet 支持自然排序，并且可以在构造的时候自己定义比较器，可以保证每一次的操作是原子性的，比如add（）、remove等，但是对于批量操作，如addAll()等并不能保证原子性（需要自己手动做同步操作，如加锁等）

  ConcurrentHashMap针对读操作做了大量的优化，这个类具有特别高的并发性，高并发场景下有特别好的表现

  ConcurrentSkipListMap与ConcurrentHashMap相比的key是有序的，它支持更高的并发，它的存取时间和线程数是没有关系的，在一定的数据量下，并发的线程越多ConcurrentSkipListMap越等体现出它的优势来

  CopyOnWriteArrayList-demo

public class CopyOnWriteArrayListExample {

    //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 CopyOnWriteArrayList<>();


    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());

    }
}

  运行结果为5000，是线程安全的

  CopyOnWriteArraySet-demo

@Slf4j
public class CopyOnWriteArraySetExample {

    //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 Set<Integer> set = new CopyOnWriteArraySet<>();


    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());

    }
}

  运行结果为5000，是线程安全的

  ConcurrentSkipListSet-demo

@Slf4j
public class ConcurrentSkipListSetExample {

    //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 Set<Integer> set = new ConcurrentSkipListSet<>();


    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5000，是线程安全的

  ConcurrentSkipListMap-demo

@Slf4j
public class ConcurrentSkipListMapExample {

    //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 ConcurrentSkipListMap<>();


    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5000，是线程安全的

  ConcurrentHashMap-demo

@Slf4j
@ThreadSafe
public class ConcurrentHashMapExample {

    //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 ConcurrentHashMap<>();


    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5000，是线程安全的