JDK并发包总结

本文主要介绍jdk中常用的同步控制工具以及并发容器, 其结构如下:

同步控制工具类

ReentrantLock

简而言之, 就是自由度更高的synchronized, 主要具备以下优点.

• 可重入: 单线程可以重复进入，但要重复退出
• 可中断: lock.lockInterruptibly()
• 可限时: 超时不能获得锁，就返回false，不会永久等待构成死锁
• 公平锁: 先来先得, public ReentrantLock(boolean fair), 默认锁不公平的, 根据线程优先级竞争.

示例 

 public class ReenterLock implements Runnable {
     public static ReentrantLock lock = new ReentrantLock();
     public static int i = 0;

     @Override
     public void run() {
         for (int j = 0; j < 10000; j++) {
             lock.lock();
              // 超时设置
 //            lock.tryLock(5, TimeUnit.SECONDS);
             try {
                 i++;
             } finally {
                 // 需要放在finally里释放, 如果上面lock了两次, 这边也要unlock两次
                 lock.unlock();
             }
         }
     }

     public static void main(String[] args) throws InterruptedException {
         ReenterLock tl = new ReenterLock();
         Thread t1 = new Thread(tl);
         Thread t2 = new Thread(tl);
         t1.start();
         t2.start();
         t1.join();
         t2.join();
         System.out.println(i);
     }
 }

中断死锁

线程1, 线程2分别去获取lock1, lock2, 触发死锁. 最终通过DeadlockChecker来触发线程中断.

 public class DeadLock implements Runnable{

     public static ReentrantLock lock1 = new ReentrantLock();
     public static ReentrantLock lock2 = new ReentrantLock();
     int lock;

     public DeadLock(int lock) {
         this.lock = lock;
     }

     @Override
     public void run() {
         try {
             if (lock == 1){
                 lock1.lockInterruptibly();
                 try {
                     Thread.sleep(500);
                 }catch (InterruptedException e){}
                 lock2.lockInterruptibly();

             }else {
                 lock2.lockInterruptibly();
                 try {
                     Thread.sleep(500);
                 }catch (InterruptedException e){}
                 lock1.lockInterruptibly();

             }
         }catch (InterruptedException e){
             e.printStackTrace();
         }finally {
             if (lock1.isHeldByCurrentThread())
                 lock1.unlock();
             if (lock2.isHeldByCurrentThread())
                 lock2.unlock();
             System.out.println(Thread.currentThread().getId() + "线程中断");
         }
     }

     public static void main(String[] args) throws InterruptedException {
         DeadLock deadLock1 = new DeadLock(1);
         DeadLock deadLock2 = new DeadLock(2);
         // 线程1, 线程2分别去获取lock1, lock2. 导致死锁
         Thread t1 = new Thread(deadLock1);
         Thread t2 = new Thread(deadLock2);
         t1.start();
         t2.start();
         Thread.sleep(1000);
         // 死锁检查, 触发中断
         DeadlockChecker.check();

     }
 }

 public class DeadlockChecker {
     private final static ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
     final static Runnable deadLockCheck = new Runnable() {
         @Override
         public void run() {
             while (true) {
                 long[] deadlockedThreadlds = mbean.findDeadlockedThreads();

                 if (deadlockedThreadlds != null) {
                     ThreadInfo[] threadInfos = mbean.getThreadInfo(deadlockedThreadlds);
                     for (Thread t : Thread.getAllStackTraces().keySet()) {
                         for (int i = 0; i < threadInfos.length; i++) {
                             if (t.getId() == threadInfos[i].getThreadId()) {
                                 t.interrupt();
                                 try {
                                     Thread.sleep(5000);
                                 } catch (InterruptedException e) {
                                 }
                             }
                         }
                     }
                 }
             }
         }
     };

     public static void check() {
         Thread t = new Thread(deadLockCheck);
         t.setDaemon(true);
         t.start();
     }
 }

Condition

类似于 Object.wait()和Object.notify(), 需要与ReentrantLock结合使用.

具体API如下:

     // await()方法会使当前线程等待，同时释放当前锁，当其他线程中使用signal()时或者signalAll()方法时，
     // 线程会重新获得锁并继续执行。或者当线程被中断时，也能跳出等待。这和Object.wait()方法很相似。
     void await() throws InterruptedException;
     // awaitUninterruptibly()方法与await()方法基本相同，但是它并不会再等待过程中响应中断。
     void awaitUninterruptibly();
     long awaitNanos(long nanosTimeout) throws InterruptedException;
     boolean await(long time, TimeUnit unit) throws InterruptedException;
     boolean awaitUntil(Date deadline) throws InterruptedException;
     // singal()方法用于唤醒一个在等待中的线程。相对的singalAll()方法会唤醒所有在等待中的线程。
     // 这和Obejct.notify()方法很类似。
     void signal();
     void signalAll();

示例

 public class ReenterLockCondition implements Runnable{

     public static ReentrantLock lock = new ReentrantLock();
     public static Condition condition = lock.newCondition();

     @Override
     public void run() {
         try {
             lock.lock();
             condition.await();
             System.out.println("Thread is going on");
         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             // 注意放到finally中释放
             lock.unlock();
         }
     }

     public static void main(String[] args) throws InterruptedException {
         ReenterLockCondition t1 = new ReenterLockCondition();
         Thread tt = new Thread(t1);
         tt.start();
         Thread.sleep(2000);
         System.out.println("after sleep, signal!");
         // 通知线程tt继续执行. 唤醒同样需要重新获得锁
         lock.lock();
         condition.signal();
         lock.unlock();
     }
 }

Semaphore信号量

锁一般都是互斥排他的, 而信号量可以认为是一个共享锁,

允许N个线程同时进入临界区, 但是超出许可范围的只能等待.

如果N = 1, 则类似于lock.

具体API如下, 通过acquire获取信号量, 通过release释放

     public void acquire()
     public void acquireUninterruptibly()
     public boolean tryAcquire()
     public boolean tryAcquire(long timeout, TimeUnit unit)
     public void release()

示例

模拟20个线程, 但是信号量只设置了5个许可.

因此线程是按序每2秒5个的打印job done.

 public class SemapDemo implements Runnable{

     // 设置5个许可
     final Semaphore semp = new Semaphore(5);

     @Override
     public void run() {
         try {
             semp.acquire();
             // 模拟线程耗时操作
             Thread.sleep(2000L);
             System.out.println("Job done! " + Thread.currentThread().getId());
         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             semp.release();
         }
     }

     public static void main(String[] args){
         ExecutorService service = Executors.newFixedThreadPool(20);
         final SemapDemo demo = new SemapDemo();
         for (int i = 0; i < 20; i++) {
             service.submit(demo);
         }
     }
 }

ReadWriteLock

读写分离锁, 可以大幅提升系统并行度.

• 读-读不互斥：读读之间不阻塞。
• 读-写互斥：读阻塞写，写也会阻塞读。
• 写-写互斥：写写阻塞。

示例

使用方法与ReentrantLock类似, 只是读写锁分离.

 private static ReentrantReadWriteLock readWriteLock=new ReentrantReadWriteLock();
 private static Lock readLock = readWriteLock.readLock();
 private static Lock writeLock = readWriteLock.writeLock();

CountDownLatch倒数计时器

一种典型的场景就是火箭发射。在火箭发射前，为了保证万无一失，往往还要进行各项设备、仪器的检查。

只有等所有检查完毕后，引擎才能点火。这种场景就非常适合使用CountDownLatch。它可以使得点火线程,

等待所有检查线程全部完工后，再执行.

示例

 public class CountDownLatchDemo implements Runnable{
     static final CountDownLatch end = new CountDownLatch(10);
     static final CountDownLatchDemo demo = new CountDownLatchDemo();

     @Override
     public void run() {
         try {
             Thread.sleep(new Random().nextInt(10) * 1000);
             System.out.println("check complete!");
             end.countDown();
         } catch (InterruptedException e) {
             e.printStackTrace();
         }
     }

     public static void main(String[] args) throws InterruptedException {
         ExecutorService service = Executors.newFixedThreadPool(10);
         for (int i = 0; i < 10; i++) {
             service.submit(demo);
         }
         // 等待检查
         end.await();
         // 所有线程检查完毕, 发射火箭.
         System.out.println("fire");
         service.shutdown();
     }
 }

CyclicBarrier循环栅栏

Cyclic意为循环，也就是说这个计数器可以反复使用。比如，假设我们将计数器设置为10。那么凑齐

第一批10个线程后，计数器就会归零，然后接着凑齐下一批10个线程.

示例

 public class CyclicBarrierDemo {

     public static class Soldier implements Runnable {

         private String soldier;
         private final CyclicBarrier cyclic;

         Soldier(CyclicBarrier cyclic, String soldier) {
             this.cyclic = cyclic;
             this.soldier = soldier;
         }

         @Override
         public void run() {
             try {
                 // 等待所有士兵到期
                 cyclic.await();
                 doWork();
                 // 等待所有士兵完成工作
                 cyclic.await();
             } catch (InterruptedException e) {
                 e.printStackTrace();
             } catch (BrokenBarrierException e) {
                 e.printStackTrace();
             }
         }

         void doWork() {
             try {
                 Thread.sleep(Math.abs(new Random().nextInt() % 10000));
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println(soldier + " 任务完成!");
         }
     }

     public static class BarrierRun implements Runnable {
         boolean flag;
         int N;

         public BarrierRun(boolean flag, int n) {
             this.flag = flag;
             N = n;
         }

         @Override
         public void run() {
             if (flag) {
                 System.out.println("士兵:" + N + "个, 任务完成!");
             } else {
                 System.out.println("士兵:" + N + "个, 集合完毕!");
                 flag = true;
             }
         }
     }

     public static void main(String[] args){
         final int N = 5;
         Thread[] allSoldier = new Thread[N];
         boolean flag = false;
         CyclicBarrier cyclic = new CyclicBarrier(N, new BarrierRun(flag, N));
         // 设置屏障点, 主要为了执行这个方法.
         System.out.println("集合任务!");
         for (int i = 0; i < N; i++) {
             System.out.println("士兵" + i + " 报到!");
             allSoldier[i] = new Thread(new Soldier(cyclic, "士兵" + i));
             allSoldier[i].start();
         }

     }
 } 

结果

集合任务!
士兵0 报到!
士兵1 报到!
士兵2 报到!
士兵3 报到!
士兵4 报到!
士兵:5个, 集合完毕!
士兵3 任务完成!
士兵1 任务完成!
士兵0 任务完成!
士兵4 任务完成!
士兵2 任务完成!
士兵:5个, 任务完成!

LockSupport

一个线程阻塞工具, 可以在任意位置让线程阻塞.

与suspend()比较, 如果unpark发生在park之前, 并不会导致线程冻结, 也不需要获取锁.

API

 LockSupport.park();
 LockSupport.unpark(t1);

中断响应

能够响应中断，但不抛出异常。

中断响应的结果是，park()函数的返回，可以从Thread.interrupted()得到中断标志

 public class LockSupportDemo {
     public static Object u = new Object();
     static ChangeObjectThread t1 = new ChangeObjectThread("t1");
     static ChangeObjectThread t2 = new ChangeObjectThread("t2");
     public static class ChangeObjectThread extends Thread {

         public ChangeObjectThread(String name) {
             super(name);
         }

         @Override
         public void run() {
             synchronized (u) {
                 System.out.println("in " + getName());
                 LockSupport.park();
             }
         }
     }

     public static void main(String[] args) throws InterruptedException {
         t1.start();
         Thread.sleep(100);
         t2.start();
         LockSupport.unpark(t1);
         LockSupport.unpark(t2);
         t1.join();
         t2.join();
     }
 }

并发容器

Collections.synchronizedMap

其本质是在读写map操作上都加了锁, 因此不推荐在高并发场景使用.

ConcurrentHashMap

内部使用分区Segment来表示不同的部分, 每个分区其实就是一个小的hashtable. 各自有自己的锁.

只要多个修改发生在不同的分区, 他们就可以并发的进行. 把一个整体分成了16个Segment, 最高支持16个线程并发修改.

代码中运用了很多volatile声明共享变量, 第一时间获取修改的内容, 性能较好.

     public V put(K key, V value) {
         ConcurrentHashMap.Segment<K,V> s;
         if (value == null)
             throw new NullPointerException();
         int hash = hash(key);
         int j = (hash >>> segmentShift) & segmentMask;
         // 通过unsafe对j进行偏移来寻找key所对应的分区
         if ((s = (ConcurrentHashMap.Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
                 (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
             // 如果分区不存在, 则创建新的分区
             s = ensureSegment(j);
         // kv放到分区中
         return s.put(key, hash, value, false);
     }

Segment.put源码

     Segment(float lf, int threshold, ConcurrentHashMap.HashEntry<K,V>[] tab) {
         this.loadFactor = lf;
         this.threshold = threshold;
         this.table = tab;
     }

     final V put(K key, int hash, V value, boolean onlyIfAbsent) {
         // tryLock通过无锁cas操作尝试获取锁(无等待), 继承自ReentrantLock.
         // 如果成功则, node = null
         // 如果不成功, 则可能其他线程已经在插入数据了,
         // 此时会尝试继续获取锁tryLock, 自旋MAX_SCAN_RETRIES次, 若还是拿不到锁才开始lock
         ConcurrentHashMap.HashEntry<K,V> node = tryLock() ? null :
                 scanAndLockForPut(key, hash, value);
         V oldValue;
         try {
             ConcurrentHashMap.HashEntry<K,V>[] tab = table;
             // 获取分区中哪一个entry链的index
             int index = (tab.length - 1) & hash;
             // 获取第一个entry
             ConcurrentHashMap.HashEntry<K,V> first = entryAt(tab, index);
             for (ConcurrentHashMap.HashEntry<K,V> e = first;;) {
                 // e != null , 存在hash冲突, 把他加到当前链表中
                 if (e != null) {
                     K k;
                     if ((k = e.key) == key ||
                             (e.hash == hash && key.equals(k))) {
                         oldValue = e.value;
                         if (!onlyIfAbsent) {
                             e.value = value;
                             ++modCount;
                         }
                         break;
                     }
                     e = e.next;
                 }
                 else {
                     // 无hash冲突, new entry
                     if (node != null)
                         node.setNext(first);
                     else
                         node = new ConcurrentHashMap.HashEntry<K,V>(hash, key, value, first);
                     int c = count + 1;
                     // 空间大小超出阈值, 需要rehash, 翻倍空间.
                     if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                         rehash(node);
                     else
                         //放到分区中
                         setEntryAt(tab, index, node);
                     ++modCount;
                     count = c;
                     oldValue = null;
                     break;
                 }
             }
         } finally {
             unlock();
         }
         return oldValue;
     }

如果想要对ConcurrentHashMap排序, 则可以使用ConcurrentSkipListMap,

他支持并发排序, 是一个线程安全的类似TreeMap的实现.

BlockingQueue

阻塞队列, 主要用于多线程之间共享数据.

当一个线程读取数据时, 如果队列是空的, 则当前线程会进入等待状态.

如果队列满了, 当一个线程尝试写入数据时, 同样会进入等待状态.

适用于生产消费者模型.

其源码实现也相对简单.

     public void put(E e) throws InterruptedException {
         checkNotNull(e);
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             // 队列满了, 写进入等待
             while (count == items.length)
                 notFull.await();
             insert(e);
         } finally {
             lock.unlock();
         }
     }

     public E take() throws InterruptedException {
         final ReentrantLock lock = this.lock;
         lock.lockInterruptibly();
         try {
             // 队列空的, 读进入等待
             while (count == 0)
                 notEmpty.await();
             return extract();
         } finally {
             lock.unlock();
         }
     }

因为BlockingQueue在put take等操作有锁, 因此非高性能容器,

如果需要高并发支持的队列, 则可以使用ConcurrentLinkedQueue. 他内部也是运用了大量无锁操作.

CopyOnWriteArrayList

CopyOnWriteArrayList通过在新增元素时, 复制一份新的数组出来, 并在其中写入数据, 之后将原数组引用指向到新数组.

其Add操作是在内部通过ReentrantLock进行锁保护, 防止多线程场景复制多份数组.

而Read操作内部无锁, 直接返回数组引用, 并发下效率高, 因此适用于读多写少的场景.

源码

     public boolean add(E e) {
         final ReentrantLock lock = this.lock;
         // 写数据的锁
         lock.lock();
         try {
             Object[] elements = getArray();
             int len = elements.length;
             // 复制到新的数组
             Object[] newElements = Arrays.copyOf(elements, len + 1);
             // 加入新元素
             newElements[len] = e;
             // 修改引用
             setArray(newElements);
             return true;
         } finally {
             lock.unlock();
         }
     }

     final void setArray(Object[] a) {
         array = a;
     }

     // 读的时候无锁
     public E get(int index) {
         return get(getArray(), index);
     }

示例

使用10个读线程, 100个写线程. 如果使用ArrayList实现, 那么有可能是在运行过程中抛出ConcurrentModificationException.

原因很简单, ArrayList在遍历的时候会check modCount是否发生变化, 如果一边读一边写就会抛异常.

 public class CopyOnWriteListDemo {

     static List<UUID> list = new CopyOnWriteArrayList<UUID>();
 //    static List<UUID> list = new ArrayList<UUID>();

     // 往list中写数据
     public static class AddThread implements Runnable {

         @Override
         public void run() {
             UUID uuid = UUID.randomUUID();
             list.add(uuid);
             System.out.println("++Add uuid : " + uuid);

         }
     }

     // 从list中读数据
     public static class ReadThread implements Runnable {

         @Override
         public void run() {
             System.out.println("start read size: " + list.size() + " thread : " + Thread.currentThread().getName());
             for (UUID uuid : list) {
                 System.out.println("Read uuid : " + uuid + " size : " + list.size() + "thread: " + Thread.currentThread().getName());
             }
         }
     }

     public static void main(String[] args) throws InterruptedException {
         initThread(new AddThread(), 10);
         initThread(new ReadThread(), 100);
     }

     private static void initThread(Runnable runnable, int maxNum) throws InterruptedException {
         Thread[] ts = new Thread[maxNum];
         for (int k = 0; k < maxNum; k++) {
             ts[k] = new Thread(runnable);
         }
         for (int k = 0; k < maxNum; k++) {
             ts[k].start();
         }
     }
 }

下图运行结果中可以看出来, 同一个线程, 即使在读的过程中发生了size变化, 也不会抛出ConcurrentModificationException