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本文主要介绍jdk中常用的同步控制工具以及并发容器.

  • 同步控制工具类
    • ReentrantLock可重入锁
    • Condition
    • Semaphore信号量
    • ReadWriteLock读写分离锁
    • CountDownLatch倒数计时器
    • CyclicBarrier循环栅栏
    • LockSupport阻塞线程
  • 并发容器
    • Collections.synchronizedMap
    • ConcurrentHashMap
    • BlockingQueue
    • CopyOnWriteArrayList

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 

 1 public class ReenterLock implements Runnable {
 2     public static ReentrantLock lock = new ReentrantLock();
 3     public static int i = 0;
 4 
 5     @Override
 6     public void run() {
 7         for (int j = 0; j < 10000; j++) {
 8             lock.lock();
 9              // 超时设置
10 //            lock.tryLock(5, TimeUnit.SECONDS);
11             try {
12                 i++;
13             } finally {
14                 // 需要放在finally里释放, 如果上面lock了两次, 这边也要unlock两次
15                 lock.unlock();
16             }
17         }
18     }
19 
20     public static void main(String[] args) throws InterruptedException {
21         ReenterLock tl = new ReenterLock();
22         Thread t1 = new Thread(tl);
23         Thread t2 = new Thread(tl);
24         t1.start();
25         t2.start();
26         t1.join();
27         t2.join();
28         System.out.println(i);
29     }
30 }

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.

 1 public class DeadLock implements Runnable{
 2 
 3     public static ReentrantLock lock1 = new ReentrantLock();
 4     public static ReentrantLock lock2 = new ReentrantLock();
 5     int lock;
 6 
 7     public DeadLock(int lock) {
 8         this.lock = lock;
 9     }
10 
11     @Override
12     public void run() {
13         try {
14             if (lock == 1){
15                 lock1.lockInterruptibly();
16                 try {
17                     Thread.sleep(500);
18                 }catch (InterruptedException e){}
19                 lock2.lockInterruptibly();
20 
21             }else {
22                 lock2.lockInterruptibly();
23                 try {
24                     Thread.sleep(500);
25                 }catch (InterruptedException e){}
26                 lock1.lockInterruptibly();
27 
28             }
29         }catch (InterruptedException e){
30             e.printStackTrace();
31         }finally {
32             if (lock1.isHeldByCurrentThread())
33                 lock1.unlock();
34             if (lock2.isHeldByCurrentThread())
35                 lock2.unlock();
36             System.out.println(Thread.currentThread().getId() + "线程中断");
37         }
38     }
39 
40     public static void main(String[] args) throws InterruptedException {
41         DeadLock deadLock1 = new DeadLock(1);
42         DeadLock deadLock2 = new DeadLock(2);
43         // 线程1, 线程2分别去获取lock1, lock2. 导致死锁
44         Thread t1 = new Thread(deadLock1);
45         Thread t2 = new Thread(deadLock2);
46         t1.start();
47         t2.start();
48         Thread.sleep(1000);
49         // 死锁检查, 触发中断
50         DeadlockChecker.check();
51 
52     }
53 }
 1 public class DeadlockChecker {
 2     private final static ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
 3     final static Runnable deadLockCheck = new Runnable() {
 4         @Override
 5         public void run() {
 6             while (true) {
 7                 long[] deadlockedThreadlds = mbean.findDeadlockedThreads();
 8 
 9                 if (deadlockedThreadlds != null) {
10                     ThreadInfo[] threadInfos = mbean.getThreadInfo(deadlockedThreadlds);
11                     for (Thread t : Thread.getAllStackTraces().keySet()) {
12                         for (int i = 0; i < threadInfos.length; i++) {
13                             if (t.getId() == threadInfos[i].getThreadId()) {
14                                 t.interrupt();
15                                 try {
16                                     Thread.sleep(5000);
17                                 } catch (InterruptedException e) {
18                                 }
19                             }
20                         }
21                     }
22                 }
23             }
24         }
25     };
26 
27     public static void check() {
28         Thread t = new Thread(deadLockCheck);
29         t.setDaemon(true);
30         t.start();
31     }
32 }
View Code

Condition

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

具体API如下:

 1     // 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,
 2     // 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。
 3     void await() throws InterruptedException;
 4     // 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。
 5     void awaitUninterruptibly();
 6     long awaitNanos(long nanosTimeout) throws InterruptedException;
 7     boolean await(long time, TimeUnit unit) throws InterruptedException;
 8     boolean awaitUntil(Date deadline) throws InterruptedException;
 9     // 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。
10     // 这和Obejct.notify()方法很类似。
11     void signal();
12     void signalAll();

示例

 1 public class ReenterLockCondition implements Runnable{
 2 
 3     public static ReentrantLock lock = new ReentrantLock();
 4     public static Condition condition = lock.newCondition();
 5 
 6     @Override
 7     public void run() {
 8         try {
 9             lock.lock();
10             condition.await();
11             System.out.println("Thread is going on");
12         } catch (InterruptedException e) {
13             e.printStackTrace();
14         } finally {
15             // 注意放到finally中释放
16             lock.unlock();
17         }
18     }
19 
20     public static void main(String[] args) throws InterruptedException {
21         ReenterLockCondition t1 = new ReenterLockCondition();
22         Thread tt = new Thread(t1);
23         tt.start();
24         Thread.sleep(2000);
25         System.out.println("after sleep, signal!");
26         // 通知线程tt继续执行. 唤醒同样需要重新获得锁
27         lock.lock();
28         condition.signal();
29         lock.unlock();
30     }
31 }

 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

1     public void acquire()
2     public void acquireUninterruptibly()
3     public boolean tryAcquire()
4     public boolean tryAcquire(long timeout, TimeUnit unit)
5     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.

 1 public class SemapDemo implements Runnable{
 2 
 3     // 设置5个许可
 4     final Semaphore semp = new Semaphore(5);
 5 
 6     @Override
 7     public void run() {
 8         try {
 9             semp.acquire();
10             // 模拟线程耗时操作
11             Thread.sleep(2000L);
12             System.out.println("Job done! " + Thread.currentThread().getId());
13         } catch (InterruptedException e) {
14             e.printStackTrace();
15         } finally {
16             semp.release();
17         }
18     }
19 
20     public static void main(String[] args){
21         ExecutorService service = Executors.newFixedThreadPool(20);
22         final SemapDemo demo = new SemapDemo();
23         for (int i = 0; i < 20; i++) {
24             service.submit(demo);
25         }
26     }
27 }

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.

1 private static ReentrantReadWriteLock readWriteLock=new ReentrantReadWriteLock();
2 private static Lock readLock = readWriteLock.readLock();
3 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

 1 public class CountDownLatchDemo implements Runnable{
 2     static final CountDownLatch end = new CountDownLatch(10);
 3     static final CountDownLatchDemo demo = new CountDownLatchDemo();
 4 
 5     @Override
 6     public void run() {
 7         try {
 8             Thread.sleep(new Random().nextInt(10) * 1000);
 9             System.out.println("check complete!");
10             end.countDown();
11         } catch (InterruptedException e) {
12             e.printStackTrace();
13         }
14     }
15 
16     public static void main(String[] args) throws InterruptedException {
17         ExecutorService service = Executors.newFixedThreadPool(10);
18         for (int i = 0; i < 10; i++) {
19             service.submit(demo);
20         }
21         // 等待检查
22         end.await();
23         // 所有线程检查完毕, 发射火箭.
24         System.out.println("fire");
25         service.shutdown();
26     }
27 }

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

 1 public class CyclicBarrierDemo {
 2 
 3     public static class Soldier implements Runnable {
 4 
 5         private String soldier;
 6         private final CyclicBarrier cyclic;
 7 
 8         Soldier(CyclicBarrier cyclic, String soldier) {
 9             this.cyclic = cyclic;
10             this.soldier = soldier;
11         }
12 
13         @Override
14         public void run() {
15             try {
16                 // 等待所有士兵到期
17                 cyclic.await();
18                 doWork();
19                 // 等待所有士兵完成工作
20                 cyclic.await();
21             } catch (InterruptedException e) {
22                 e.printStackTrace();
23             } catch (BrokenBarrierException e) {
24                 e.printStackTrace();
25             }
26         }
27 
28         void doWork() {
29             try {
30                 Thread.sleep(Math.abs(new Random().nextInt() % 10000));
31             } catch (InterruptedException e) {
32                 e.printStackTrace();
33             }
34             System.out.println(soldier + " 任务完成!");
35         }
36     }
37 
38     public static class BarrierRun implements Runnable {
39         boolean flag;
40         int N;
41 
42         public BarrierRun(boolean flag, int n) {
43             this.flag = flag;
44             N = n;
45         }
46 
47         @Override
48         public void run() {
49             if (flag) {
50                 System.out.println("士兵:" + N + "个, 任务完成!");
51             } else {
52                 System.out.println("士兵:" + N + "个, 集合完毕!");
53                 flag = true;
54             }
55         }
56     }
57 
58     public static void main(String[] args){
59         final int N = 5;
60         Thread[] allSoldier = new Thread[N];
61         boolean flag = false;
62         CyclicBarrier cyclic = new CyclicBarrier(N, new BarrierRun(flag, N));
63         // 设置屏障点, 主要为了执行这个方法.
64         System.out.println("集合任务!");
65         for (int i = 0; i < N; i++) {
66             System.out.println("士兵" + i + " 报到!");
67             allSoldier[i] = new Thread(new Soldier(cyclic, "士兵" + i));
68             allSoldier[i].start();
69         }
70 
71     }
72

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

1 LockSupport.park();
2 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

 1 public class LockSupportDemo {
 2     public static Object u = new Object();
 3     static ChangeObjectThread t1 = new ChangeObjectThread("t1");
 4     static ChangeObjectThread t2 = new ChangeObjectThread("t2");
 5     public static class ChangeObjectThread extends Thread {
 6 
 7         public ChangeObjectThread(String name) {
 8             super(name);
 9         }
10 
11         @Override
12         public void run() {
13             synchronized (u) {
14                 System.out.println("in " + getName());
15                 LockSupport.park();
16             }
17         }
18     }
19 
20     public static void main(String[] args) throws InterruptedException {
21         t1.start();
22         Thread.sleep(100);
23         t2.start();
24         LockSupport.unpark(t1);
25         LockSupport.unpark(t2);
26         t1.join();
27         t2.join();
28     }
29 }

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

支持高并发的HashMap. 通过将一个大的hashmap切割成无数个小的分区hashmap(Segment<K,V>).

执行put的时候把key映射到其中一个小的分区中, 假如有十几个线程, 那么可能就会对应十几个分区.

 1     public V put(K key, V value) {
 2         ConcurrentHashMap.Segment<K,V> s;
 3         if (value == null)
 4             throw new NullPointerException();
 5         int hash = hash(key);
 6         int j = (hash >>> segmentShift) & segmentMask;
 7         // 通过unsafe对j进行偏移来寻找key所对应的分区
 8         if ((s = (ConcurrentHashMap.Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
 9                 (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
10             // 如果分区不存在, 则创建新的分区
11             s = ensureSegment(j);
12         // kv放到分区中
13         return s.put(key, hash, value, false);
14     }

Segment.put源码

 1     Segment(float lf, int threshold, ConcurrentHashMap.HashEntry<K,V>[] tab) {
 2         this.loadFactor = lf;
 3         this.threshold = threshold;
 4         this.table = tab;
 5     }
 6 
 7     final V put(K key, int hash, V value, boolean onlyIfAbsent) {
 8         // tryLock通过无锁cas操作尝试获取锁(无等待), 继承自ReentrantLock.
 9         // 如果成功则, node = null
10         // 如果不成功, 则可能其他线程已经在插入数据了,
11         // 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
12         ConcurrentHashMap.HashEntry<K,V> node = tryLock() ? null :
13                 scanAndLockForPut(key, hash, value);
14         V oldValue;
15         try {
16             ConcurrentHashMap.HashEntry<K,V>[] tab = table;
17             // 获取分区中哪一个entry链的index
18             int index = (tab.length - 1) & hash;
19             // 获取第一个entry
20             ConcurrentHashMap.HashEntry<K,V> first = entryAt(tab, index);
21             for (ConcurrentHashMap.HashEntry<K,V> e = first;;) {
22                 // e != null , 存在hash冲突, 把他加到当前链表中
23                 if (e != null) {
24                     K k;
25                     if ((k = e.key) == key ||
26                             (e.hash == hash && key.equals(k))) {
27                         oldValue = e.value;
28                         if (!onlyIfAbsent) {
29                             e.value = value;
30                             ++modCount;
31                         }
32                         break;
33                     }
34                     e = e.next;
35                 }
36                 else {
37                     // 无hash冲突, new entry
38                     if (node != null)
39                         node.setNext(first);
40                     else
41                         node = new ConcurrentHashMap.HashEntry<K,V>(hash, key, value, first);
42                     int c = count + 1;
43                     // 空间大小超出阈值, 需要rehash, 翻倍空间.
44                     if (c > threshold && tab.length < MAXIMUM_CAPACITY)
45                         rehash(node);
46                     else
47                         //放到分区中
48                         setEntryAt(tab, index, node);
49                     ++modCount;
50                     count = c;
51                     oldValue = null;
52                     break;
53                 }
54             }
55         } finally {
56             unlock();
57         }
58         return oldValue;
59     }

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.

 1     public void put(E e) throws InterruptedException {
 2         checkNotNull(e);
 3         final ReentrantLock lock = this.lock;
 4         lock.lockInterruptibly();
 5         try {
 6             // 队列满了, 写进入等待
 7             while (count == items.length)
 8                 notFull.await();
 9             insert(e);
10         } finally {
11             lock.unlock();
12         }
13     }
14 
15     public E take() throws InterruptedException {
16         final ReentrantLock lock = this.lock;
17         lock.lockInterruptibly();
18         try {
19             // 队列空的, 读进入等待
20             while (count == 0)
21                 notEmpty.await();
22             return extract();
23         } finally {
24             lock.unlock();
25         }
26     }

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

 1     public boolean add(E e) {
 2         final ReentrantLock lock = this.lock;
 3         // 写数据的锁
 4         lock.lock();
 5         try {
 6             Object[] elements = getArray();
 7             int len = elements.length;
 8             // 复制到新的数组
 9             Object[] newElements = Arrays.copyOf(elements, len + 1);
10             // 加入新元素
11             newElements[len] = e;
12             // 修改引用
13             setArray(newElements);
14             return true;
15         } finally {
16             lock.unlock();
17         }
18     }
19 
20     final void setArray(Object[] a) {
21         array = a;
22     }
23 
24     // 读的时候无锁
25     public E get(int index) {
26         return get(getArray(), index);
27     }

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.

 1 public class CopyOnWriteListDemo {
 2 
 3     static List<UUID> list = new CopyOnWriteArrayList<UUID>();
 4 //    static List<UUID> list = new ArrayList<UUID>();
 5 
 6     // 往list中写数据
 7     public static class AddThread implements Runnable {
 8 
 9         @Override
10         public void run() {
11             UUID uuid = UUID.randomUUID();
12             list.add(uuid);
13             System.out.println("++Add uuid : " + uuid);
14 
15         }
16     }
17 
18     // 从list中读数据
19     public static class ReadThread implements Runnable {
20 
21         @Override
22         public void run() {
23             System.out.println("start read size: " + list.size() + " thread : " + Thread.currentThread().getName());
24             for (UUID uuid : list) {
25                 System.out.println("Read uuid : " + uuid + " size : " + list.size() + "thread: " + Thread.currentThread().getName());
26             }
27         }
28     }
29 
30 
31     public static void main(String[] args) throws InterruptedException {
32         initThread(new AddThread(), 10);
33         initThread(new ReadThread(), 100);
34     }
35 
36     private static void initThread(Runnable runnable, int maxNum) throws InterruptedException {
37         Thread[] ts = new Thread[maxNum];
38         for (int k = 0; k < maxNum; k++) {
39             ts[k] = new Thread(runnable);
40         }
41         for (int k = 0; k < maxNum; k++) {
42             ts[k].start();
43         }
44     }
45 }

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