Java中的锁之间的区别是什么？
synchronized 和 java.util.concurrent.lock.Lock 之间的区别
- 实现层面不一样。synchronized 是 Java 关键字，JVM层面 实现加锁和释放锁；Lock 是一个接口，在代码层面实现加锁和释放锁
- 是否自动释放锁。synchronized 在线程代码执行完或出现异常时自动释放锁；Lock 不会自动释放锁，需要在 finally {} 代码块显式地中释放锁
- 是否一直等待。synchronized 会导致线程拿不到锁一直等待；Lock 可以设置尝试获取锁或者获取锁失败一定时间超时
- 获取锁成功是否可知。synchronized 无法得知是否获取锁成功；Lock 可以通过 tryLock 获得加锁是否成功
- 功能复杂性。synchronized 加锁可重入、不可中断、非公平；Lock 可重入、可判断、可公平和不公平、细分读写锁提高效率
java.util.concurrent.lock.Lock 与 java.util.concurrent.lock.ReadWriteLock 之间的区别
- ReadWriteLock 定义了获取读锁和写锁的接口，读锁之间不互斥，非常适合读多、写少的场景
适用场景
- JDK 1.6 开始，对 synchronized 方式枷锁进行了优化，加入了偏向锁、轻量级锁和锁升级机制，性能得到了很大的提升。性能与 ReentrantLock 差不多
- 读多写少的情况下，考虑使用 ReadWriteLock
synchronized、ReentrantLock、ReentrantReadWriteLock 启动 990 个线程读共享变量，10 个线程写共享变量
package constxiong.concurrency.a020; import java.util.ArrayList; import java.util.List; import java.util.UUID; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantLock; import java.util.concurrent.locks.ReentrantReadWriteLock; /** * JDK 1.8 中锁性能的测试 * @author ConstXiong */ public class TestLockPerformance { public static Object obj = new Object();//用于 synchronized 获取锁 public static Lock lock = new ReentrantLock();//可重入锁 public static ReadWriteLock readWriteLock = new ReentrantReadWriteLock();//读写锁 public static final int READ = 0; public static final int WRITE = 1; // uuid，一个随机字符串 public static String uuid = UUID.randomUUID().toString(); public static void main(String[] args) throws InterruptedException { // testSynchronized(1000); testReentrantLock(1000); // testReadWriteLock(1000); } public static void testSynchronized(int threadNum) throws InterruptedException { long t1 = System.currentTimeMillis(); List<Thread> tList = new ArrayList<Thread>(); //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid for (int i = 0; i <threadNum; i++) { Thread t; if (i % 100 == 0) { t = new Thread(new WorkerSynchronized(WRITE)); } else { t = new Thread(new WorkerSynchronized(READ)); } t.start();//启动线程 tList.add(t); } for (Thread t : tList) { t.join(); } long t2 = System.currentTimeMillis(); System.out.println("testSynchronized 耗时：" + (t2 - t1)); } public static void testReentrantLock(int threadNum) throws InterruptedException { long t1 = System.currentTimeMillis(); List<Thread> tList = new ArrayList<Thread>(); //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid for (int i = 0; i <threadNum; i++) { Thread t; if (i % 100 == 0) { t = new Thread(new WorkerReentrantLock(WRITE)); } else { t = new Thread(new WorkerReentrantLock(READ)); } t.start();//启动线程 tList.add(t); } for (Thread t : tList) { t.join(); } long t2 = System.currentTimeMillis(); System.out.println("testReentrantLock 耗时：" + (t2 - t1)); } public static void testReadWriteLock(int threadNUm) throws InterruptedException { long t1 = System.currentTimeMillis(); List<Thread> tList = new ArrayList<Thread>(); //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid for (int i = 0; i <threadNUm; i++) { Thread t; if (i % 100 == 0) { t = new Thread(new WorkerReadWriteLock(WRITE)); } else { t = new Thread(new WorkerReadWriteLock(READ)); } t.start();//启动线程 tList.add(t); } for (Thread t : tList) { t.join(); } long t2 = System.currentTimeMillis(); System.out.println("testReadWriteLock 耗时：" + (t2 - t1)); } } //工作线程，使用 synchronized 关键字加锁 class WorkerSynchronized implements Runnable { //0-read;1-write private int type; WorkerSynchronized(int type) { this.type = type; } //加锁读 TestLockPerformance.uuid 变量，并打印 private void read() { synchronized (TestLockPerformance.obj) { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " read uuid = " + TestLockPerformance.uuid); } } //加锁写 TestLockPerformance.uuid 变量，并打印 private void write() { synchronized (TestLockPerformance.obj) { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } TestLockPerformance.uuid = UUID.randomUUID().toString(); System.out.println(Thread.currentThread().getName() + " write uuid = " + TestLockPerformance.uuid); } } @Override public void run() { //type = 0,线程读 TestLockPerformance.uuid 变量 if (type == 0) { read(); //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量 } else { write(); } } } //工作线程，使用 ReentrantLock 加锁 class WorkerReentrantLock implements Runnable { //0-read;1-write private int type; WorkerReentrantLock(int type) { this.type = type; } //加锁读 TestLockPerformance.uuid 变量，并打印 private void read() { TestLockPerformance.lock.lock(); try { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " read uuid = " + TestLockPerformance.uuid); } finally { TestLockPerformance.lock.unlock(); } } //加锁写 TestLockPerformance.uuid 变量，并打印 private void write() { TestLockPerformance.lock.lock(); try { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } TestLockPerformance.uuid = UUID.randomUUID().toString(); System.out.println(Thread.currentThread().getName() + " write uuid = " + TestLockPerformance.uuid); } finally { TestLockPerformance.lock.unlock(); } } @Override public void run() { //type = 0,线程读 TestLockPerformance.uuid 变量 if (type == 0) { read(); //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量 } else { write(); } } } //工作线程，使用 ReentrantReadWriteLock 关键字加锁 class WorkerReadWriteLock implements Runnable { //0-read;1-write private int type; WorkerReadWriteLock(int type) { this.type = type; } //加锁读 TestLockPerformance.uuid 变量，并打印 private void read() { TestLockPerformance.readWriteLock.readLock().lock(); try { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " read uuid = " + TestLockPerformance.uuid); } finally { TestLockPerformance.readWriteLock.readLock().unlock(); } } //加锁写 TestLockPerformance.uuid 变量，并打印 private void write() { TestLockPerformance.readWriteLock.writeLock().lock(); try { //休眠 20 毫秒，模拟任务执行耗时 try { Thread.sleep(20); } catch (InterruptedException e) { e.printStackTrace(); } TestLockPerformance.uuid = UUID.randomUUID().toString(); System.out.println(Thread.currentThread().getName() + " write uuid = " + TestLockPerformance.uuid); } finally { TestLockPerformance.readWriteLock.writeLock().unlock(); } } @Override public void run() { //type = 0,线程读 TestLockPerformance.uuid 变量 if (type == 0) { read(); //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量 } else { write(); } } }
调用测试方法
testSynchronized(1000);耗时
Thread-0 write uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
Thread-252 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
Thread-251 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
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Thread-255 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
Thread-254 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
Thread-253 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
testSynchronized 耗时：22991
调用测试方法
testReentrantLock(1000);耗时
Thread-0 write uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
Thread-1 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
Thread-485 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
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Thread-997 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
Thread-998 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
Thread-999 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
testReentrantLock 耗时：22935
调用测试方法
testReadWriteLock(1000);耗时
Thread-0 write uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
Thread-277 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
Thread-278 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
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Thread-975 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
Thread-971 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
Thread-964 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
testReadWriteLock 耗时：543
通过耗时测试可以看出，使用 synchronized 和 ReentrantLock 耗时相近；但是由于 990 个线程读，10 个线程写，使用 ReentrantReadWriteLock 耗时 543 毫秒。
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