Java里，synchronized和lock效率的简单比较 |
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通常而言，我们可能都倾向于使用Lock来实现线程的同步，但是synchronized和lock究竟哪个效率高呢？ 下面我们来做一个简单的对比实验，只比较最基础的部分，暂时不涉及等待和唤醒。 测试环境： JDK1.8 OS： win10 64bit 测试代码： public class T { Lock lock = new ReentrantLock(); public synchronized void f() { } public void g() { lock.lock(); lock.unlock(); } public static void main(String[] args) throws InterruptedException { int count = 1000000000, i; long time = 0; T t = new T(); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.f(); System.out.println("Sync Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.g(); System.out.println("Lock Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.f(); System.out.println("Sync Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.g(); System.out.println("Lock Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.f(); System.out.println("Sync Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); for (i = 0; i < count; i ++) t.g(); System.out.println("Lock Time:" + (System.currentTimeMillis() - time)); } }测试结果： Sync Time:4442 Lock Time:10824 Sync Time:2155 Lock Time:9592 Sync Time:2331 Lock Time:9550结论：synchronized的效率远远高于lock操作，而且随着实验的重复，synchronized会进行优化，效率大概是Lock的2-4倍。 另类结论： 从循环的次数可以看出，无论是synchronized或者Lock，本身的效率已经非常高了，10亿次的操作，只需要10秒不到的时间，加锁一次大概需要10ns而已，synchronized大概只需要2ns。所以，其差距基本上也可以忽略不计了。 下面我们来进行更复杂一点的测试，以考验多线程环境下的效率。 测试代码： public class T { Lock lock = new ReentrantLock(); public synchronized void f() { try { Thread.sleep(1); } catch (Exception e) { } finally { } } public void g() { lock.lock(); try { Thread.sleep(1); } catch (Exception e) { } finally { lock.unlock(); } } public static void main(String[] args) throws InterruptedException { int count = 10000, i; long time = 0; ExecutorService workers = null; T t = new T(); time = System.currentTimeMillis(); workers = Executors.newFixedThreadPool(10); for (i = 0; i < count; i ++) workers.submit(() -> t.f()); workers.shutdown(); workers.awaitTermination(100000, TimeUnit.DAYS); System.out.println("Sync Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); workers = Executors.newFixedThreadPool(10); for (i = 0; i < count; i ++) workers.submit(() -> t.g()); workers.shutdown(); workers.awaitTermination(100000, TimeUnit.DAYS); System.out.println("Lock Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); workers = Executors.newFixedThreadPool(10); for (i = 0; i < count; i ++) workers.submit(() -> t.f()); workers.shutdown(); workers.awaitTermination(100000, TimeUnit.DAYS); System.out.println("Sync Time:" + (System.currentTimeMillis() - time)); time = System.currentTimeMillis(); workers = Executors.newFixedThreadPool(10); for (i = 0; i < count; i ++) workers.submit(() -> t.g()); workers.shutdown(); workers.awaitTermination(100000, TimeUnit.DAYS); System.out.println("Lock Time:" + (System.currentTimeMillis() - time)); } }测试结果： Sync Time:10028 Lock Time:7511 Sync Time:6938 Lock Time:4332 结论：synchronized在初始化时需要耗费较多事件，总的来说，多线程环境下Lock的效率更高一些。 这和网络上的结论基本是一致的，即如果竞争复杂，则使用Lock，如果竞争简单，则可使用synchronized
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